Research - Assignment Essays

write research on this I have attached the outline you did last time there are 3 sources already and i have attached those sources too take help, and there should be 5 total sources you can include any 2 trusted online article of your choice.

In work please include things like, things that would affect aviation, the global crisis aspect of electric aviation, and also include the opposing view as well.

Annotated Bibliography

Schäfer, A. W., Barrett, S. R., Doyme, K., Dray, L. M., Gnadt, A. R., Self, R., O’Sullivan, A., Synodinos, A. P., & Torija, A. J. (2019). Technological, economic and environmental prospects of all-electric aircraft.
Nature Energy,
4(2), 160–166. https://doi.org/10.1038/s41560-018-0294-x.

In this research article, authors perform a comprehensive evaluation of the potential for all-electric aircraft to become a viable alternative to hydrocarbon fuel-powered commercial aviation. They consider factors such as the energy, economic, and environmental implications of such a transition. The authors argue that for all-electric aircraft to be viable, batteries need to have a higher specific energy and lower costs, as well as a reduction in the costs and CO2 intensity of electricity. I chose this article for my research because it provides a thorough analysis of the technical, economic, and environmental prospects of all-electric aircraft. The authors consider both the potential benefits and drawbacks of such a transition and provide a realistic assessment of the necessary conditions for all-electric aircraft to become a viable alternative to hydrocarbon-fueled aviation. One text citation from this source is: “All-electric aircraft could greatly reduce the environmental impact of aviation” (Schäfer et al. 2019, 164).

Tahir, M., Khan, S. A., Khan, T., Waseem, M., Khan, D., & Annuk, A. (2022).
More electric aircraft challenges: A study on 270 V/90 V interleaved bidirectional DC–DC converter. Energy Reports. Retrieved February 6, 2023, from

https://www.sciencedirect.com/science/article/pii/S2352484722012240

The article focuses on the challenges associated with the implementation of a more electric aircraft (MEA) concept. The MEA is seen as a way to make aircraft more energy efficient and reduce their carbon footprint. The authors focus on the design of a dc-dc converter that interfaces between 270 VDC and 28 VDC bus bars in aircraft applications. They argue that while the Dual Active Bridge topology is well-known and promising, the presence of current and voltage ripples can be detrimental to the life of the battery and filtering components. To reduce the voltage gain requirements and minimize ripple, the authors propose a two-stage bidirectional converter that combines a three-phase interleaved buck-boost converter with the Dual Active Bridge. I chose this article for my research because it provides a detailed and practical examination of one of the technical challenges associated with the implementation of the MEA concept. One text citation from this source is: “An efficient and reliable bidirectional dc–dc converter is an indispensable part of More electric aircraft that is energy efficient and playing a curial role in meeting environmental goals” (Tahir et al. 2022, 1139).

Thalin, P., Taubert, S., Mare, J.-C., & Rajamani, R. (2018).
Fundamentals of Electrical Aircraft. SAE International. “Fundamentals of Electric Aircraft”

This is text that offers an objective view of the electric aircraft and the innovative technologies enabling aircraft electrification. The book provides insight into the paradigm shift across different aircraft segments from General Aviation to Large Aircraft through case studies. The authors, who are industry veterans, address design constraints, timelines, and performance enhancements of electric aircraft. They analyze how fuel burn savings may bring more value for money with the delivery of new electric technologies. The publication balances futuristic approaches with the operational realities of the business and provides a general view of the progress made so far and what to expect in the future. I chose this book for my research because of its comprehensive coverage of the topic and its focus on the fundamental concepts and technologies behind electric aviation. One text citation from this source is: “From an environmental perspective, a lot of progress has been made thanks to the more-electric aircraft in service, such as the Boeing 787.” (Thalin et al., 2018, 226)

References

Tahir, M., Khan, S. A., Khan, T., Waseem, M., Khan, D., & Annuk, A. (2022, July 2).
More electric aircraft challenges: A study on 270 V/90 V interleaved bidirectional DC–DC converter. Energy Reports. Retrieved February 6, 2023, from https://www.sciencedirect.com/science/article/pii/S2352484722012240

Thalin, P., Taubert, S., Mare, J.-C., & Rajamani, R. (2018).
Fundamentals of Electrical Aircraft. SAE International.

AnAlysis
https://doi.org/10.1038/s41560-018-0294-x

1Air Transportation Systems Laboratory, UCL Energy Institute, University College London, Central House, London, UK. 2Laboratory for Aviation and
the Environment, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA, USA. 3Institute of Sound and
Vibration Research, Engineering and the Environment, University of Southampton, Highfield, Southampton, UK. *e-mail: a.schafer@ucl.ac.uk

Owing to their high energy content per unit weight and vol-
ume, easy handling, global availability and manageable
costs, liquid hydrocarbons have been a key enabler of com-

mercial flight over the past century. In 2015, the global aircraft fleet
consumed 276 million tonnes of jet fuel—7% of global oil products1.

However, reliance on oil products comes at an environmen-
tal cost. Aircraft CO2 emissions, owing to combustion of jet fuel,
comprise 2.7% of energy-use-related CO2 emissions1,2. It is also esti-
mated that the non-CO2 warming impacts of aircraft are of the same
magnitude as aircraft CO2 emissions, thus approximately doubling
aviation’s contribution to climate change3–5. The single largest non-
CO2 contributor to warming may be the formation of contrails and
contrail cirrus clouds3. In addition, aviation combustion emissions
that affect air quality, such as NOx, are set to rise substantially6. This
may increase the estimated 16,000 premature mortalities per year
attributable to aviation emissions globally7. There is also growing
evidence that noise from aircraft results in adverse health impacts
and premature mortality among affected populations8.

Various options exist for reducing CO2 emissions from aircraft.
For example, fuel burn per revenue passenger kilometre (RPK) of
the US narrow-body aircraft fleet could be reduced by around 2%
per year at no cost through 20509, whereas reductions obtainable
for wide-body, long-distance aircraft would probably be smaller.
However, these rates will be outpaced by the anticipated global
aviation demand growth of around 4.5% per year10,11. In contrast
to fuel-efficiency improvements, low-carbon fuels (for example,
biofuels) could partially decouple CO2 emissions from aviation
growth, although these options face cost and scale limitations and
do not much help with non-CO2 impacts12,13, except for a potential
thinning of contrails (the effect of which has an uncertain sign)14,15.
Similarly, liquid hydrogen16 and liquified natural gas17 could greatly
reduce direct CO2 emissions, but these fuels’ higher hydrogen con-
tent would result in enhanced contrail and cirrus cloud formation.

Until recently, energy carriers that do not entail in-flight com-
bustion have not been considered. This work focuses on all-electric

aircraft that have the potential to eliminate both direct CO2 emis-
sions and direct non-CO2 impacts, although the net impact will
depend on the power generation mix and associated emissions.
However, exploiting these unparalleled benefits requires substantial
technological advances, especially inbattery performance and cost.

Technology trajectories towards all-electric aircraft
Two broad technology trajectories appear to lead to all-electric air-
craft. The first trajectory builds upon the incremental electrification
of jet engines. This class of hybrid-electric aircraft includes designs
without batteries (that is, turbo-electric aircraft), in which the elec-
tric propulsion system serves to increase propulsive efficiency and
provide for some degree of boundary-layer ingestion (in which
ingestion and re-energizing of the aircraft boundary layer improves
efficiency)18,19. The extent of fuel burn reductions is then the net
effect of the increased propulsive efficiency and the additional
weight of the electrical components. Hybrid-electric aircraft with
batteries are also being considered, where the batteries may provide
for additional power or regeneration at limited specific operating
conditions. Although hybrid-electric aircraft with batteries would
entail direct combustion emissions for the majority of flights, they
could provide for reduced or eliminated emissions during particu-
larly sensitive parts of a flight—such as flying through ice super-
saturated parts of the atmosphere (to reduce contrails) or during
takeoff and landing (to reduce near-airport emissions). With suf-
ficient advancements in battery technology, however, the ultimate
design is an all-electric aircraft, which would have no direct com-
bustion emissions and thus have the potential to remove aviation-
specific non-CO2 impacts and reduce CO2 emissions depending
on the source of the electricity. In contrast, the second technology
trajectory builds upon scaling up all-electric air taxis. Ref. 20 reports
55 such air vehicle designs, 80% of which are already all-electric.
Progress in battery technology, especially specific energy, would
then enable the scaling up of all-electric designs to larger vehicles,
first to regional jets and then to narrow-body aircraft.

Technological, economic and environmental
prospects of all-electric aircraft
Andreas W. Schäfer 1*, Steven R. H. Barrett2, Khan Doyme1, Lynnette M. Dray1, Albert R. Gnadt 2,
Rod Self3, Aidan O’Sullivan1, Athanasios P. Synodinos3 and Antonio J. Torija3

Ever since the Wright brothers’ first powered flight in 1903, commercial aircraft have relied on liquid hydrocarbon fuels. However,
the need for greenhouse gas emission reductions along with recent progress in battery technology for automobiles has gener-
ated strong interest in electric propulsion in aviation. This Analysis provides a first-order assessment of the energy, economic
and environmental implications of all-electric aircraft. We show that batteries with significantly higher specific energy and
lower cost, coupled with further reductions of costs and CO2 intensity of electricity, are necessary for exploiting the full range
of economic and environmental benefits provided by all-electric aircraft. A global fleet of all-electric aircraft serving all flights
up to a distance of 400–600 nautical miles (741–1,111 km) would demand an equivalent of 0.6–1.7% of worldwide electricity
consumption in 2015. Although lifecycle CO2 emissions of all-electric aircraft depend on the power generation mix, all direct
combustion emissions and thus direct air pollutants and direct non-CO2 warming impacts would be eliminated.

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mailto:a.schafer@ucl.ac.uk

http://orcid.org/0000-0002-0301-664

http://orcid.org/0000-0003-3480-113

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AnAlysisNature eNergy

All-electric aircraft energy use
Aircraft energy use (E) per RPK during cruise flight can be described
conveniently by the Breguet range equation21,22. Rearranged for
energy intensity, equations (1) and (2) report energy use per RPK
for jet engine aircraft (JEA) and all-electric aircraft (AEA), with
PAX being the number of passengers transported, L/D the lift-to-
drag ratio, ηtotal the total (tank-to-wake) efficiency of the jet engine
or electric propulsion system, and W the weight of either fuel, the jet
engine aircraft at the beginning (i) or the end (f) of the mission, or
of the all-electric aircraft at any point during the mission.

η=E PAX L D W W W/ RPK 1 / ( ( / )) / ln( / ) (1)JEA total,JEA fuel i f

η=E PAX L D W/ RPK 1 / ( ( / )) (2)AEA total, AEA AEA

Assuming the same passenger count and lift-to-drag ratio
between the jet engine and all-electric aircraft, equations 1 and 2
differ by only the propulsion system efficiencies and the weight fac-
tor. The latter is about 50–100% larger for all-electric aircraft as a
consequence of the relatively low-specific energy batteries23,24. For
narrow-body jet engine aircraft, Wi/Wf is typically 1.1–1.3; with Wfuel
accounting for typically 10–30% of a narrow-body aircraft takeoff
weight, the weight factor then roughly corresponds to the narrow-
body aircraft takeoff weight. The resulting 50–100% higher energy
intensity of all-electric aircraft is mitigated by the roughly two-fold
tank-to-wake efficiency of electric propulsion systems compared to
their jet engine counterparts22,25. We note that this calculation does
not include the energy use associated with takeoff and climb, nor
does it account for the upstream efficiency losses associated pri-
marily with electricity generation. The latter strongly depend on the
power generation technology and accounting practices for renew-
able energy.

A key enabler of electric flight and a critical determinant of
energy intensity is the battery-pack specific energy. This variable
enters the energy intensity of all-electric aircraft in equation (2) via
the aircraft weight. If the on-board battery energy supply is kept
constant, a higher specific energy leads to a lower all-electric air-
craft weight and thus a lower aircraft energy use per RPK, which, in
turn yields a longer range. In addition, a lighter aircraft would allow
the downsizing of other components, such as landing gear, motor
power, and so on, which yield additional energy intensity reduc-
tions and range gains.

Today’s best available Li-ion battery cells have a specific energy
of around 250 Wh kg−1 (refs 26,27). Assuming a packing efficiency of
80%, which is at the lower end of projected future levels28 and below
that of the recently developed Airbus E-Fan29, the pack-specific
energy would result in roughly 200 Wh kg−1 and 1.7% of the jet fuel
energy content. This battery would be capable of powering electric
air taxis with 1–4 passengers over a distance of around 100 km20.
However, short-range electric aircraft demand battery-pack specific
energies of 750–2,000 Wh kg−1, which translates into 6–17% of the
jet fuel energy content, depending on aircraft size and range22–24,30,31.
Much of the required 4–10-fold increase in battery-pack specific
energy could potentially be achieved with advanced Li–S technol-
ogy, although Li-air chemistry may ultimately be required for the
higher end of that range. Both of these battery technologies have
low specific power, so an additional, high-power battery or another
means of augmenting power may be required for takeoff and climb.

The historical long-term rate of increase in specific energy of the
major battery chemistries has been around 3% per year, a doubling
every 23 years32,33, although since 2000, specific energy has increased
at a rate of 4% per year33. Although there is no ‘Moore’s Law’ equiva-
lent for batteries—since significant advances require entirely new
battery chemistries to be made practicable before incremental
improvement can occur—this historical observation does suggest

that the timescale for such progress to be made could be of the order
of decades. On the basis of a continuation of the historical increase
in specific energy, current levels of specific energy of 250 Wh kg−1
for advanced Li-ion battery cells, and a packing efficiency of 80%,
a battery-pack specific energy of 800 Wh kg−1 could potentially be
reached at around mid-century. This is consistent with the times-
cale of change in the aviation industry—for both the infrastructure
and aircraft design lifecycles. For the purposes of this work we take
the lower end of the above battery-pack specific energy range of
800 Wh kg−1 that is required for Airbus A320/Boeing 737-sized air-
craft to be capable of up to 600 nautical miles (1,111 km) missions,
depending on the specific layout and amount of batteries carried23.

In addition to battery pack specific energy, all-electric aircraft
weight is determined by the power-to-weight ratio of the motors
and the supporting infrastructure, consisting mainly of cables and
power electronics. Whereas regional jets with about 50 seats are
likely to require significantly improved mainstream technology,
narrow-body aircraft with 100 seats and above may depend upon
lightweight high-temperature superconducting electric motors due
to the intrinsically high weight of conventional electric motors and
the difficulty in providing cooling34.

environmental impacts
All-electric aircraft would completely eliminate direct combus-
tion emissions and thus remove associated direct CO2 and non-
CO2 warming. The lifecycle CO2 intensity of all-electric aircraft is
determined by the CO2 intensity of electricity used, losses associ-
ated with battery charging and electricity transmission/distribution,
and the specific aircraft design and operation. Figure 1 depicts the
warming intensity of a first-generation 180-seat, 150-passenger,
all-electric aircraft over a mission of 400 nautical miles (741 km),
which is projected to consume 180 Wh per RPK for a battery-pack
specific energy of 800 Wh kg−1 (ref. 23). Using the 2015 average
US grid CO2 intensity of 456 g of CO2 per kWh, this all-electric
aircraft would generate 91 g of CO2 per RPK, if losses associated
with electricity transmission/distribution and battery charging are
included. This value is about 20% higher than the lifecycle CO2
intensity of its modern, jet engine counterparts (the ‘US’ dashed line
in Fig. 1). However, if non-CO2 impacts are taken into account (by
way of a factor of two3–5), the overall warming per RPK would be
reduced by around 30%. The lifecycle CO2 intensity of all-electric
aircraft would further decline with improved aircraft and battery
technology and the potential transition of the grid towards renew-
able energy. Conversely, a longer range capability would result in a
higher energy and thus CO2 intensity owing to the additional bat-
tery weight, as visible from equation (2). Note that CO2 emissions
and non-CO2 impacts (such as cooling related to sulphur emissions
from coal-fired power stations35) may still occur depending on the
power generation mix.

If greenhouse gas emissions from battery production were taken
into account, the warming intensity of all-electric aircraft shown in
Fig. 1 would be slightly larger. Based on Li-ion battery studies, the
increase in warming intensity would be 2–10 g of CO2 equivalent
per RPK, depending upon the underlying assumptions36. However,
employing end-of-economic-life high-performance batteries in sta-
tionary applications would significantly reduce these emission lev-
els, as would the enhanced use of renewable electricity for battery
production (see Methods).

In addition to removing direct non-CO2 impacts, all-electric air-
craft would also eliminate direct air pollution. While indirect air
pollution may occur depending on the power generation technolo-
gies employed, there is greater potential for emissions control of
ground-based power generation compared to in-flight combustion.

Noise impacts of all-electric aircraft may be better or worse than
conventional aircraft, depending on the design decisions made.
Assuming a conventional tube and wing configuration, which

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AnAlysis Nature eNergy

does not take advantage of the design flexibility offered by electric
propulsion, we estimated an overall improved noise performance
of all-electric aircraft relying on a battery-pack specific energy of
800 Wh kg−1 compared to best-in-class current-generation short-
haul aircraft. Considering both takeoff and landing operations, a
36% reduction in noise contour area is estimated, compared to the
best-in-class aircraft (see Methods). This could allow extended air-
port operation hours, thus increasing aircraft utilization and airport
capacity. During takeoff, aircraft noise is mainly determined by the
thrust of the engines required. Owing to lower fan pressure ratios
and the absence of combustion noise, we anticipate a more than 50%
reduction in takeoff noise contour area. In contrast, during landing,
the higher weight of all-electric aircraft means that the determi-
nants of noise (principally lift, drag and landing speed) will result
in a 15% larger noise contour area compared to those of best-in-
class narrow-body aircraft. Higher battery-pack specific energy and
future aircraft designs would provide the opportunity for reduced
noise through novel aircraft design concepts and changes in opera-
tional procedures (such as highly distributed propulsion and steep
approaches with propulsors in generating mode).

All-electric aircraft economics
Compared to gas turbine engine aircraft, all-electric aircraft will
have a different operating cost structure. Over its lifetime, an all-

electric aircraft may require several generations of potentially
expensive batteries, a factor that contributes to upfront investments
(via the first set of batteries) and maintenance costs (via replace-
ment batteries). In addition, its higher weight could increase the
maintenance requirements of landing gear components. On the
other hand, all-electric aircraft may also experience cost savings.
For example, they would not require a fuel system or an additional
gas turbine (auxiliary power unit) for generating electricity, engine
starting, and so on. In addition, there may be potential for reduc-
tions in engine maintenance costs owing to the relative mechanical
simplicity of electric motors, although this is uncertain for narrow-
body aircraft due to the challenges of cooling high-temperature
superconducting electric motors.

Taking into account only the differences in the largest-expendi-
ture items between an all-electric aircraft and a jet engine aircraft
in terms of capital costs (energy storage and propulsion system)
and maintenance costs (landing gear and battery replacement),
Fig. 2 depicts the potential range of breakeven electricity prices for
a first-generation Airbus A320/Boeing 737-sized all-electric air-
craft with a range of 400 nautical miles (741 km). Two sets of lines
are shown with different levels of specific energy. Each set repre-
sents battery costs of US$ 100 kWh−1 and US$ 200 kWh−1, which
reflect the target and 2017 level of Li-ion batteries37. At the 2015

Batte
ry-

sp
ecif

ic
energy:

1,200 W
h/kg

$100 per

k
W

$200 per k
W

$100 per k
Wh

$200 per k
Wh

Batte
ry-specific

energy:

00 W

h/kg
6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

B
re

ak
ev

en
e

le
ct

ric
ity

p
ric

e
(C

p
er

k
W

)

Jet fuel price (US$ per gallon)

Thickness of breakeven cost
lines reflects uncertainty of
electric propulsor costs

Range of 2015 US electricity
price: industry (lower end) –
residential (upper end)

2015 +$100/tCO2

Fig. 2 | Break-even electricity price for a first-generation all-electric
aircraft. The reference jet engine aircraft is an Airbus A320neo. The
all-electric aircraft has batteries with a specific energy of 800 Wh kg−1
(grey lines) or 1,200 Wh kg−1 (blue lines), each with battery costs of US$
100 kWh−1 or US$ 200 kWh−1. On the basis of a battery-pack specific
energy of 800 Wh kg−1, jet fuel prices would need to be at least US$ 2.3
or 2.8 per gallon (US$ 97 or US$ 118 per barrel)—depending on the cost
of the battery—in order to achieve cost effectiveness relative to jet engine
aircraft in light of the 2015 US electricity end-use prices. Whereas the 2015
US jet fuel price of US$ 1.8 per gallon would lead to breakeven prices below
the range of the observed end-use electricity prices in the USA, a CO2 price
of US$ 100 per tonne of tCO2 (US$ 0.97 per gallon of jet fuel) would lead to
breakeven electricity prices within the range of observed end-use electricity
prices (provided electricity is produced on a carbon-neutral basis). If
taking into account non-CO2 impacts on the basis of an ‘uplift factor’ of 2,
corresponding to a doubling of the greenhouse gasemissions price (here,
to US$ 200 per tonne of CO2 equivalent), the cost effectiveness would
increase further. It is apparent that battery costs would need to be around
US$ 100 kWh−1 or less to achieve cost effectiveness over the longer term.
About the same battery cost target exists for automobiles, albeit at a much
lower specific energy, to achieve cost parity with internal combustion
engine vehicles37. More advanced batteries with a higher specific energy,
more advanced aircraft designs, and repurposing end-of-life batteries for
use in other sectors would improve the economics of all-electric aircraft.
Costs in the graph are in 2015 US$.

100

120

140

160

180

200

0 200 400 600 800 1,000

A
irc

ra
ft

w
ar

m
in

g
in

te
ns

ity
(

g
C

O
2

eq
ui

v.
p

er
R

P
K

)

Carbon intensity of electricity (g CO2 per kW

Jet engine aircraft: CO2 and non-CO2

Jet engine aircraft: CO2 only

Brazil EU-28 World China

Coal-based electricity

Oil-based electricity

Gas-based electricity

100

200

300

400

500

600

700

800

1970 1980 1990 2000 2010 2020

World average

C
ar

bo
n

in
te

ns
ity

(
g

C
O

2
pe

r
kW

Fig. 1 | Warming intensity of a projected first-generation all-electric
aircraft and of a current-generation jet engine aircraft versus carbon
intensity of electricity. The mission length is 400 nautical miles (741 km).
The lifecycle CO2 intensity of the all-electric aircraft with an electricity
intensity of 180 Wh per RPK is based on a design in ref. 23 and takes into
account efficiencies of 95% for battery charging and 95% for electricity
transmission or distribution. In contrast, the lifecycle CO2 intensity of the
A320neo of 75 g of CO2 per RPK is based on an energy intensity of 0.9
MJ per RPK, calculated with the aircraft performance model Piano-X61,
and a well-to-tank efficiency of 88%62; its warming intensity corresponds
to two times its direct CO2 emissions. The shaded areas represent the
interquartile range of the CO2 intensity of coal-based (limited to 1,000 g
of CO2 per kWh), oil-based and natural gas-based electricity on a country
basis in 20152. The 2015 electricity fuel mix in Brazil, the EU-28, the US and
the world average would lead to a lower warming intensity of all-electric
aircraft compared to jet engine aircraft (two times the CO2 intensity), as
exemplified by the dashed red arrows for the US. If we consider only long-
lived CO2 emissions, the CO2 intensity of all-electric aircraft would be below
that of their jet engine counterparts for the 2015 EU-28 and Brazilian fuel
mix, but larger in the US, China and the world as a whole. Meeting the Paris
Agreement on climate requires significantly stronger reductions in the CO2
intensity of electricity as experienced historically (see inset), which would
lead to a proportional decline in the CO2 intensity of all-electric aircraft.

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US jet fuel price of US$ 1.8 per gallon, the breakeven electricity
prices of only the all-electric aircraft with a battery-pack specific
energy of 1,200 Wh kg−1 and battery costs of US$ 100 kWh−1 would
fall within the 2015 US electricity price range of 6.9–12.7 cents
per kWh, depending on the end-use sector38. In contrast, a first-
generation all-electric aircraft with a battery-pack specific energy
of 800 Wh kg−1 and a range of 400 nautical miles (741 km) would
be economically viable only with battery costs of around US$
100 kWh−1 or less and policies that result in significant reductions
in electricity prices or increases in jet fuel prices. A carbon tax of
US$ 100 per tonne of CO2, which translates into US$ 0.97 per gal-
lon of jet fuel, would increase the break-even electricity price of
the first-generation all-electric aircraft with a battery-pack specific
energy of 800 Wh kg−1 to levels observed within the USA, if elec-
tricity is produced from renewable sources. This suggests that poli-
cies that support both low-carbon electricity and the introduction
of a carbon tax may be central prerequisites for introducing all-
electric aircraft if today’s market conditions prevail until all-elec-
tric aviation becomes technically feasible. However, as battery-pack
specific energy increases and costs of renewable power decline, the
cost-effectiveness of all-electric aircraft will improve and the need
for supportive policies will diminish. The conditions required for
cost parity with jet engine aircraft are also more relaxed for shorter
missions but more stringent for longer missions, primarily owing
to the extra battery weight and its impact on energy use.

All-electric aircraft adoption potenti

Since advanced batteries with 5–10 times the pack specific energy
of today’s Li-ion batteries would still contain only 8–17% of the
energy content per unit weight of jet fuel (although this does not
credit electrochemical storage with the higher energy conversion
efficiency compared to gas turbines), all-electric aircraft would be
constrained to short-range missions, at least initially. The limitation
to short-distance operations of all-electric aircraft can be seen in
Fig. 3, which depicts the global air transportation network in 2015
by distance band. The range of 600 nautical miles (1,111 km) (yel-
low trajectories) could be covered with all-electric aircraft relying
on a battery-pack specific energy of 800 Wh kg−1 (ref. 23). Although

a higher battery pack specific energy could lead to a more integrated
flight network, there are technological limits.

Operating beyond distances of 1,200 nautical miles (2,222 km)
in a single-stage flight would require a battery- pack specific energy
of at least 1,600 Wh kg−1 (ref. 23), which may remain a significant
technological challenge for decades to come. From today’s perspec-
tive, the only way to further expand the all-electric aircraft network
by operating over flight distances longer than 1,200 nautical miles
would be via multistage flights with at least one intermediate stop.
(This, of course, is contingent on achieving a battery-pack specific
energy of 800 Wh kg−1). However, this strategy would probably
lead to reduced travel demand owing to the associated increase in
travel time. In addition, multistage flights may be limited by airport
capacity and noise regulations. Thus, all-electric aircraft operations
would probably remain limited to intra-continental traffic, in the
absence of notable breakthroughs in battery technology or changes
in consumer behaviour.

Yet a short-range all-electric aircraft market can generate large-
scale impacts. As shown in Fig. 4, an all-electric aircraft fleet with a
useful range of 600 nautical miles (1,111 km) could substitute up to
15% of global RPK and up to half of global departures. In addition,
it could substitute almost 15% of commercial aircraft fuel use and
eliminate around 40% of global landing-and-takeoff-related NOx
emissions.

Impact on electricity generation
Using the aircraft performance characteristics specified by ref. 23,
we simulate the electricity demand of a hypothetical, all-electric
aircraft fleet operating within the global 2015 flight network. This
analysis, using the AIM2015 integrated model39, suggests that the
energy demand by all-electric narrow-body aircraft operating
at flight distances up to 400–600 nautical miles (741–1,111 km)
would correspond to 112–344 TWh or 0.6–1.7% of 2015 global
electricity consumption (see Methods). This percentage range
reflects the global average of variable country-level data, culminat-
ing in slightly higher percentages within the industrialized world
of 0.6–2.2% of total US electricity consumption and 1.3–3.7% for
the UK.

Flights/day

5 10 15 20

Distance

< 600 nmi < 1,200 nmi < 2,400 nmi < 4,800 nmi > 4,800 nmi

Fig. 3 | Global flight network in 2015 by distance band. Initially, all-electric aircraft operations would be limited to short distances. The range of 600
nautical miles (1,111 km), feasible with an all-electric aircraft employing a battery with a specific energy of 800 Wh kg−1 (ref. 23), would result in one or
more local networks per continent. With rising battery-pack specific energy and flight distances, individual continental flight networks would begin to
consolidate. However, from today’s perspective, it is questionable whether all-electric aircraft will be capable of operating over distances of 1,200 nautical
miles (2,222 km) or more with a single-stage flight, as this would require a battery-pack specific energy of at least 1,600 Wh kg−1 (ref. 23). This implies that
all-electric aircraft would mostly operate on intra-continental routes rather than the long-distance transatlantic or transpacific routes.

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Assuming that the aircraft batteries for each first morning flight
would be charged overnight, around 85% of recharging would occur
over the course of a day. This would lead to extra power genera-
tion capacity requirements of 1.2–3.6 GW in the UK, 6.6–27 GW in
the US and 31–118 GW globally for aircraft operating ranges of
400–600 nautical miles, assuming a 35% capacity factor as typical
for renewable power systems. If world population and income levels
follow the IPCC SSP2 ‘Middle-of-the-Road’ scenario, the resulting
increase in air travel demand would imply that electricity require-
ments triple by 2050.

Discussion
All-electric aircraft could greatly reduce the environmental impact
of aviation. Most importantly, they could eliminate direct CO2
and non-CO2 warming, in addition to removing all air pollutants.
Moreover, all-electric aircraft have the potential to mitigate noise,
especially during takeoff. The extent to which these benefits can
be exploited from the global aircraft fleet will depend critically
upon battery-pack specific energy. All-electric aircraft with bat-
tery packs of 800 Wh kg−1, enabling a range up to 600 nautical miles
(1,111 km), could replace half of all aircraft departures, mitigate
airport area NOx emissions by 40%, and reduce fuel use and direct
CO2 emissions by 15%. Assuming strong progress in battery tech-
nology, aircraft with the two-fold endurance leading to a range of
1,200 nautical miles (2,222 km) could replace more than 80% of all
aircraft departures, mitigate airport area NOx emissions by more
than 60%, and reduce fuel use and direct CO2 emissions by around
40%. Although a realization of these prospects may fall well into the
second half of this century, they seem too large to ignore.

This analysis has shown that future, first-generation all-electric
narrow-body aircraft may not be economically competitive to jet
engine aircraft under today’s market conditions. To reach cost effec-
tiveness with conventional aircraft, jet fuel prices would need to be
in excess of US$ 100 per barrel. Conversely, if jet fuel prices remain
at their 2015 level, end-use electricity prices would need to be below
4–6 cents per kWh, depending on battery costs, to ensure the eco-
nomic competitiveness of all-electric aircraft. In addition, today’s
CO2 intensity of electricity would lead typically to higher lifecycle
CO2 emission levels compared to jet engine aircraft over the same
mission, although the total warming impact may be reduced in
most parts of the world.

Since timescales of mutually reinforcing technologies are mea-
sured in decades (that is, new aircraft design, battery development,

electricity grid decarbonization and sufficiently strong decline
in electricity prices from renewable power to increase cost effec-
tiveness), research and development of critical all-electric aircraft
components would need to start immediately in order to exploit
the opportunities provided by an all-electric aircraft system in
the decades to come. A potential path of manageable risk would
be the development first of turbo-electric and then hybrid-electric
technology, with the possible exception of all-electric regional air-
craft, which can rely on less stringent requirements for battery-pack
specific energy and power and may not require high-temperature
superconducting technology. Although these transition technolo-
gies will not result in significant reductions of greenhouse gas
emissions, they are critical enablers of and technology milestones
towards an all-electric aircraft system.

Methods
Distribution of passenger kilometres and fuel burn by distance. Departures
and fuel burn by distance is derived from flight schedules and passenger numbers
from the Sabre Market Intelligence Database40, assuming great circle routing. To
estimate fuel burn and landing- and- takeoff NOx emissions, we use the aircraft
performance model from the Aviation Integrated Model AIM201541, the updated
version of AIM42.

Electric aircraft noise assessment. The impact of aircraft noise on communities
near airports depends not only on noise levels from the aircraft but also on its
operational characteristics. Quantification of this impact is usually mapped using
noise contours, which, in turn, depend upon the noise–power–distance (NPD)
curves of the aircraft. For existing aircraft, NPD curves are publicly available43 but
for novel aircraft, they need to be estimated.

In the present study, the all-electric aircraft NPD curves have been derived
from those of a baseline A320-232 aircraft using a method that accounts for both
operational and technological variations of the aircraft from the baseline case44–47.
The all-electric aircraft airframe and propulsor fans are assumed to behave
acoustically in a similar manner to their conventional equivalents. Propulsor
weight is estimated based on the method of ref. 48. Together with nacelle drag
and an estimation of battery and cabling weight, the NPD curves for a number of
distributed propulsion configurations and missions can be calculated49. In these
calculations, airframe, fan and jet mixing noise are considered but motor noise has
been ignored. Based on predictions by ref. 50, motor noise can be presumed to be
negligible compared to fan and jet mixing noise contributions. From the NPDs,
aircraft noise contours have been calculated using a method known as RANE
(rapid airport noise estimation) that has been benchmarked against the US Federal
Aviation Administration’s Integrated Noise Model (INM 7.0c)51. Typical results are
illustrated in the Supplementary Information.

Aircraft warming impact of battery production. The warming intensity in Fig. 1
excludes greenhouse gas emissions associated with battery production. According
to ref. 36, the literature-based values range from 39–196 kg of CO2 equivalent
per kWh, depending on the methodological approach, the method for imputing
missing data, the carbon intensity of electricity and other factors. Given a battery
capacity of 64,000 kWh23, the amount of greenhouse gasemissions due to battery
production would result in 2,500–12,500 tonnes of CO2 equivalent. Assuming
an average of 150 passengers per aircraft, a block speed of 800 km per hour,
an average utilization of 10 h per day, and a battery lifetime of 3 years, battery-
production-related greenhouse gasemissions would result in 2–10 g of CO2
equivalent per RPK or 2–10% of the warming intensity of an all-electric aircraft,
provided the carbon intensity of electricity corresponds to the world average of
around 500 g of CO2 per kWh. Note that this range represents an upper limit,
because end-of-life high-performance batteries will probably experience a second
life in stationary applications. In addition, a lower carbon intensity of electricity
will result in further reductions52.

Cost-effectiveness of all-electric aircraft. The key difference between the
A320neo reference aircraft and the derived all-electric aircraft is the energy storage
and propulsion system. Our all-electric aircraft capital cost estimate (referring only
to recurring costs) is based upon the reference aircraft average retail price of US$
46 million, which includes the price of two gas turbine engines at US$ 5.5 million,
after a whole-aircraft discount of 57%53. Not taking into account the credit for
the obsolete fuel system and auxiliary power unit, we add the cost of batteries at
US$ 100 kWh−1 and US$ 200 kWh−1. These numbers reflect the projected future
and current costs of Li-ion batteries. Given the projected battery capacity of 28
MWh (21 MWh) for first-generation all-electric aircraft with a battery specific
energy of 800 Wh kg−1 (1,200 Wh kg−1), the total cost of batteries results in US$
2.8 million (US$ 2.1 million) and US$ 5.6 million (US$ 4.2 million), respectively.
The replacement costs of the batteries after their useful life of 5,000 cycles is then
accounted for in the maintenance costs.

0.2

0.4

0.6

0.8

1.0

P
ro

po
rt

io
n

of
g

lo
ba

l t
ot

0 1,000 2,000 3,000 4,000 5,000 6,000

Distance (nautical miles)

Departures
LTO NOx
RPK
Fuel

Fig. 4 | Cumulative distributions of key operational variables by the global
commercial aircraft fleet in 2015. The variables consist of departures,
NOx emissions at landing and takeoff (LTO), RPK and fuel consumed. The
flight distances of multiples of 600 nautical miles (1,111 km) are shown in
terms of shaded areas. Full adoption of an all-electric aircraft with a range
of 600 nautical miles would account for half of all aircraft departures and
for 15% of all RPK. It would eliminate one-third of all narrow-body-related
landing and takeoff NOx emissions and 15% of global narrow-body jet fuel
use. Extending the range to 1,200 nautical miles (2,222 km) would greatly
increase the impact. All numbers were derived with the Aviation Integrated
Model AIM201539.

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AnAlysisNature eNergy

Our estimate of the cost range of the electric propulsion system is based
upon two limiting cases. The lower-end estimate assumes electric propulsor costs
without high-temperature superconducting motors. It is based upon electric
propulsion system costs of US$ 8 kW−1, which corresponds to the 2022 US
Department of Energy target for electric motors plus inverters for automobile
applications54. Based upon a maximum power requirement of 12.5 MW for each
of the four propulsion units during take-off for the aircraft with a battery specific
energy of 800 Wh kg−1, the cost of one electric motor plus inverter amounts to US$
100,000. These costs exclude the fan, which costs about 15% of the cost of a gas
turbine engine55 or US$ 410,000. Hence, the cost of one propulsion system totals
US$ 510,000, which translates into around US$ 2 million for the four units. For the
aircraft with a battery specific energy of 1,200 Wh kg−1, the lower maximum power
requirement leads to propulsion system costs that are only around half as high.

The higher-end cost case accounts for a high-temperature super-conducting
electric propulsion system. Perhaps conservatively, it corresponds to the cost of
two jet engines, or US$ 5.5 million. Subtracting the costs of four fans would lead
to motor plus power electronics costs of US$ 3.9 million. In light of the maximum
aircraft power requirement of 50 MW, these costs would then translate into US$
78 kW−1. The latter are within the range of the high-temperature super-conducting
motor costs cited by Hoelzen et al.56. However, with progress in high-temperature
super-conducting wire technology, especially, and increases in production
scale, high-temperature super-conducting motor costs are expected to decline
drastically57,58.

Although estimating the cost of all-electric aircraft propulsors in decades is
highly uncertain, these numbers may be indicative of the order-of-magnitude cost.
The results imply (see Fig. 2) that the uncertainty in the electric propulsion system
costs is unimportant relative to the uncertainty in battery cost or overall aircraft
performance, even if propulsion system costs are a factor of two or more greater
than our higher case.

In addition to capital costs, the cost-effectiveness analysis takes into account
maintenance costs and energy costs. Expenditures for crew and airport/airspace
were assumed to be identical for the two competing aircraft types. Maintenance
costs of the A320neo were computed with data from Aircraft Commerce on the
basis of the A320–20059 and resulted in US$ 960 per flight hour. This number
compares well with US Form 41 data60. In contrast, the maintenance costs of
the all-electric aircraft range from US$ 1,170 per flight hour for batteries with a
specific energy of 1,200 Wh kg−1 and costs of US$ 100 kWh−1 to US$ 1,500 per
flight hour for batteries with a specific energy of 800 Wh kg−1 and costs of US$
200 kWh−1. The higher maintenance costs of all-electric aircraft can be attributed
mainly to battery maintenance. Using a discount rate of 7%, maintenance costs
for the aircraft with 800 Wh kg−1 batteries result in US$ 265 and US$ 530 per
flight hour for the US$ 100 and US$ 200 kWh−1 battery costs, respectively. Owing
to the required smaller battery capacity, maintenance costs for the aircraft with
1,200 Wh kg−1 batteries result in US$ 205 and US$ 410 per flight hour.

Impact on electricity generation. The hypothetical year-2015 and year-2050
electricity demand projections are obtained using the global aviation systems
model AIM39. For 2015, we take the baseline global network as represented in
AIM, which is obtained from a global scheduled passenger and flight database for
201540. For each flight segment up to an assumed range of 400–600 nautical miles,
we calculate the electricity demand under the assumption that all passengers are
carried on all-electric narrow-body aircraft of the type and size specified in ref. 23.
We use a performance model fitted to the electricity demand of an all-electric
aircraft with a battery specific energy of 800 Wh kg−1, a 400- or 600- nautical-mile
design range, and different passenger load factors, assuming passenger load factors
similar to those historically flown on each segment. This procedure provides an
estimate of the electricity demand per airport.

We use the central SSP2 reference case from ref. 39 to project demand by flight
segment in 2050. The mid-range trends for future socioeconomic characteristics
underlying this projection results in 2017–2037 demand growth rates consistent
to those from the most recent Airbus and Boeing forecasts10,11. Total RPK in 2050
is around 3.7 times the value in 2015. The same procedure as for 2015 is used to
estimate electricity demand; the increase in electricity demand is lower compared
to total RPK because of a shift towards longer-haul flights, which cannot be served
by all-electric aircraft.

Data availability
The data that support the plots within this paper and other findings of this study
are available from the corresponding author upon reasonable request.

Received: 8 November 2017; Accepted: 1 November 2018;
Published online: 10 December 2018

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Acknowledgements
Research underlying this work was made possible by the UK Engineering and Physical
Sciences Research Council (EP/P511262/1) and the National Science Foundation
Graduate Research Fellowship (grant number 1122374). We thank M. Schofield, J. Sabnis
and R. Gardner for discussions and K. Al Zayat for early contributions to this work.

Author contributions
A.W.S. led the overall study, the analysis of the results and the preparation of the
manuscript. S.R.H.B. led the all-electric aircraft performance study and contributed
to the analysis of the results and to the preparation of the manuscript. R.S. led the all-
electric aircraft noise study and contributed to the preparation of the manuscript. A.R.G.
carried out the all-electric aircraft performance simulations and contributed to the
preparation of the manuscript. L.M.D. carried out the analysis of the results. K.D. and
A.O’S. contributed to the analysis of the results. A.P.S. and A.J.T. contributed to the all-
electric aircraft noise study.

Competing interests
The authors declare no competing interests.

Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/
s41560-018-0294-x.

Reprints and permissions information is available at www.nature.com/reprints.

Correspondence and requests for materials should be addressed to A.W.S.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.© The Author(s), under exclusive licence to
Springer Nature Limited 2018

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http://www.nature.com/reprints

http://www.nature.com/natureenergy

Reproduced with permission of copyright owner. Further reproduction
prohibited without permission.

Technological, economic and environmental prospects of all-electric aircraft

Technology trajectories towards all-electric aircraft

All-electric aircraft energy use

Environmental impacts

All-electric aircraft economics

All-electric aircraft adoption potential

Impact on electricity generation

Discussion

Methods

Distribution of passenger kilometres and fuel burn by distance

Electric aircraft noise assessment

Aircraft warming impact of battery production

Cost-effectiveness of all-electric aircraft

Impact on electricity generation

Acknowledgements

Fig. 1 Warming intensity of a projected first-generation all-electric aircraft and of a current-generation jet engine aircraft versus carbon intensity of electricity.

Fig. 2 Break-even electricity price for a first-generation all-electric aircraft.

Fig. 3 Global flight network in 2015 by distance band.

Fig. 4 Cumulative distributions of key operational variables by the global commercial aircraft fleet in 2015.

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2022 The 4th International Conference on Clean Energy and Electrical Systems (CEES 2022),
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More electric aircraft challenges: A study on 270 V/90 V interleaved
bidirectional DC–DC converter

Mustafa Tahira,∗, Shoaib Ahmed Khana, Tahir Khana, Muhammad Waseema,∗,
Danish Khana, Andres Annukb

a College of Electrical Engineering, Zhejiang University, No. 38 Zheda Road, Xihu District, Hangzhou 310027, China
b Chair of Energy Application Engineering, Institute of Forestry and Engineering, Estonian University of Life Sciences, 51006 Tartu, Estonia

Received 29 May 2022; accepted 25 June 2022
Available online 2 July 2022

Abstract

The concept of more electric aircraft (MEA) has gained increased importance in the aviation industry to make the aircraft
nergy efficient and reduce carbon footprints. MEA facilitates transition towards clean energy and accomplishing the United
ations’ net-zero emission goal by 2050. An efficient and reliable dc–dc converter is an indispensable part of the MEA that

nterfaces 270VDC and 28VDC bus bars in aircraft applications. Dual Active Bridge is a well-known and promising topology for
his application; however, the problem of current and voltage ripples deteriorates the life of the battery and filtering components.
n addition, the mentioned voltage levels demand high voltage gain, which makes design challenging. To reduce the voltage
ain requirement along with ripple minimization, a two-stage bidirectional converter is proposed by combing a three-phase
nterleaved buck-boost converter with Dual Active Bridge. The three-phase interleaved bidirectional converter is modelled and
nalysed using the state-space averaging method, and a controller is established. Simulation results demonstrate the efficacy
f the design. The research carried out in this paper can serve as a reference and offer future research directions using the
roposed idea, which might assist other researchers in developing an efficient design, eventually facilitating the MEA and
arbon footprint reduction.
2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

http://creativecommons.org/licenses/by-nc-nd/4.0/).

eer-review under responsibility of the scientific committee of the 4th International Conference on Clean Energy and Electrical Systems, CEES, 2022.

eywords: More electric aircraft (MEA); Interleaved bidirectional converter (IBC); Electrification; Clean energy; Green energy

1. Introduction

Climate change has drastic effects on human life. The aerospace industry is responsible for 2% of total CO2
emission, and the percentage will increase to 3% by 2050 [1,2] since air passenger traffic is increasing at an
average rate of 7% annually [3]. Advisory Council for Aeronautics Research has set a goal to reduce 50% in
CO2 emission and 80% in NOx emission [3]. The salient challenges in the aerospace industry involve improving

∗ Corresponding authors.
E-mail addresses: mustafatahir@zju.edu.cn (M. Tahir), mwaseem@zju.edu.cn (M. Waseem).

ttps://doi.org/10.1016/j.egyr.2022.06.08

352-4847/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:
/creativecommons.org/licenses/by-nc-nd/4.0/).

eer-review under responsibility of the scientific committee of the 4th International Conference on Clean Energy and Electrical Systems,
EES, 2022.

http://www.elsevier.com/locate/egyr

https://doi.org/10.1016/j.egyr.2022.06.084

http://www.elsevier.com/locate/egyr

http://crossmark.crossref.org/dialog/?doi=10.1016/j.egyr.2022.06.084&domain=pdf

http://creativecommons.org/licenses/by-nc-nd/4.0/

mailto:mustafatahir@zju.edu.cn

mailto:mwaseem@zju.edu.cn

https://doi.org/10.1016/j.egyr.2022.06.084

http://creativecommons.org/licenses/by-nc-nd/4.0/

M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–

1140

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emission, reducing fuel consumption and cost [4,5]. Hydraulic, pneumatic and mechanical power sources were used
in aerospace applications [6]; nonetheless, such power sources were bulky with higher maintenance cost [7]. NAS

completed a study that shows 10% reduction in empty aircraft weight and 9% reduction in fuel consumption can be
achieved by using more electrical technologies in aircraft [8]. In [9], authors showed that replacing the pneumatic
system with an electrical system in Boeing-787 has reduced 20% fuel consumption and CO2 emission compared
o the conventional Boeing-767 aircraft. Therefore, the concept of MEA is trendy and electrical power demand
n modern aircraft has increased. The Airbus 380 and the Boeing 787 have bigger electrical systems than their
redecessors. In an aircraft electrical system, there are diverse types of loads that require a variety of input voltage
o operate. As a result, there is a need for reliable and efficient power electronics equipment [7].

Fig. 1. Layout of aircraft distribution network.

In MEA, 270 V and 28 V are typical bus voltage levels [4]. As shown in Fig. 1 [10], a bidirectional dc–dc
onverter is indispensable for power flow between two bus bars. The bidirectional dc–dc converter works in boost
ode (28 V to 270 V) to support the startup of power generation and back up the critical load using a battery storage

ystem in case of generator failure. During regular aircraft operation, a bidirectional dc–dc converter works in buck
ode (270 V to 28 V) to power up the low voltage loads along with battery charging. In technical literature, both

solated and non-isolated dc–dc converters can be found. In MEA, galvanic isolation is an essential requirement
o prevent fault propagation between bus bars [4,11]. Therefore, researchers have devoted more efforts to isolated
opologies and Dual Active Bridge (DAB) is one of the most attractive topologies. Currently, the systems are moving
owards higher power density with better performance by using wideband gap devices [12], Zero Voltage Switching
ZVS) and Zero Current Switching (ZCS) [13–15]. Research on the combination of different topologies has gained
mportance as it enables increased power density design with improved efficiency. Researchers in [14] combined the
LC and DAB converters to maximize the performance. This paper proposes a two-stage topology where the design
nd modelling of a three-phase interleaved bidirectional converter are emphasized since many studies have already
nvestigated the DAB. The content structure of the paper is as follows: The proposed idea is presented in Section 2.
ection 3 explains the working modes of the interleaved bidirectional converter. Section 4 provides mathematical
odelling and controller design. Discussion and analysis of simulation results are covered in Section 5. Concluding

emarks are given in Section 6.

. Proposed idea

The main drawback of most topologies in avionic applications is enormous current ripple stress in output/input
apacitors [4]. In addition, for MEA applications, the voltage gain requirement is pretty high. To get an efficient
nd reliable performance with minimum input/output current and voltage ripples, a combination of three-phase
nterleaved buck-boost converter with DAB converter is proposed to produce the best possible dc–dc converter.

esides, the two-stage approach can streamline the design by easing voltage gain requirements. The first stage

1134

M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–1140

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d
c

t
T

s
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i

g
d

offers minimum ripples in current and voltage, while the second stage (DAB) is efficient owing to soft switching
and provides galvanic isolation, a mandatory requirement in MEA.

In the proposed structure, an interleaved bidirectional converter (IBC) is connected between 270 V (high voltage
c bus) and 90 V (Intermediate dc-link). Dual Active Bridge (DAB) is employed between 90 V (Intermediate
c-link) and 28 V (Low voltage bus), as depicted in Fig. 2. This paper focuses on the interleaved bidirectional
onverter (IBC) employed between 270 V and 90 V.

Fig. 2. Proposed two-stage bidirectional dc–dc converter.

3. Analysis of IBC modes

The proposed topology works by combing IBC with DAB to charge and discharge the battery. In the proposed
topology, IBC is responsible for power flow between High Voltage (HV) dc-bus and intermediate dc link. The circuit
diagram of IBC is shown in Fig. 4, where 1st phase (La, Sa1, Sa2, Da1, Da2), the second phase (Lb, Sb1, Sb2, Db1, Db2)
and third phase (Lc, Sc1, Sc2, Dc1, Dc2) are in parallel. CHV is the HV dc bus capacitor and CLV is the intermediate
dc-link capacitor. Besides, UHV and ULV are HV dc bus voltage and intermediate dc-link voltage, respectively.

3.1. Boost mode

Under boost mode, IBC takes part in battery discharging as a front-end converter. Lower parallel switches Sa2,
Sb2, Sc2 and upper parallel diodes Da1, Db1, Dc1 take part in boost mode. The duty cycle in boost can be calculated
using (1).

D = 1 −
ULV

UH V
(1)

Lower switches (Sa2, Sb2, Sc2) have the same duty cycle with a 120◦ phase shift. There are six modes of operations
n continuous conduction mode (CCM) when duty cycle >0.6Ts, as shown in Fig. 3.

Fig. 3. Working waveforms of IBC in boost mode.

Modes 1, 3, 5: Operation during modes 1, 3 and 5 is the same. The switches Sa2, Sb2, Sc2 are turned on during
hese modes, and the current in all inductors increases linearly. At this time while CHV provides energy to load.
he equivalent electrical circuit for these modes can be seen in Fig. 4(a).

Mode 2: In the first mode, all inductors were being charged. In the second mode, Sb2 is turned off, and the
tored energy in Lb gets transferred to the load through the Db1. Switches Sa2 and Sc2 remain turned on during the
econd mode, and current across La and Lc keep increasing linearly. The equivalent electrical circuit for mode 2 is
llustrated in Fig. 4(b).

Mode 4: In mode 4, Sa2 and Sb2 are turned on and Sc2 is turned off. The current across La and Lb increases
radually and Lc transfers energy to the load through Dc1. The equivalent electrical circuit during this mode is

emonstrated in Fig. 4(c).

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M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–1140

i
c

m
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i
i
e
T

Fig. 4. Equivalent electrical circuits of three-phase interleaved converter in boost mode.

Mode 6: Sb2 and Sc2 are turned on in the last mode, and Lb and Lc get into the charging state. Meanwhile, Sa2

s turned off. As a result, the stored energy in La gets transferred to the load through Da1. The equivalent electrical
ircuit for mode 6 can be seen in Fig. 4(d).

. Modelling and control

.1. Modelling

Interleaved bidirectional converter (IBC) is a nonlinear system. To analyse static and dynamic performance, a
athematical model is crucial. IBC has two modes (buck/boost). For that, boost mode state-space modelling is

arried out in this paper and that of buck mode follows the same methodology. There are six modes of operation
n boost and their equivalent circuits are given in Section 3. The mathematical modelling in all modes of operation
s carried out to get a state-space model. The converter is considered in CCM, and conduction losses are not
ncompassed in modelling. To model the system, inductor currents and bus voltage are taken as state variables.
he intermediate dc-link voltage is taken as the input variable as illustrated in (2).

x = [iLa(t), iLb(t), iLc(t), u H V (t), ]T u = uLV (t) (2)

During boost mode, lower active switches are controlled by duty cycles D1, D2 and D3. Applying KCL and KVL
n Fig. 4(b), circuit behaviour is expressed in

(3)

In mode 2 ((1 − D2) Ts):⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

La
diLa(t)

dt
= uLV (t)

Lb
diLb(t)

dt
= uLV (t) − u H V (t)

Lc
diLc(t)

dt
= uLV (t)

CH V
duH V (t)

= iLb(t) −
u H V (t)

(3)

dt Rload

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M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–1140

w
i

f
c
o
P
a
G

Using (3), the state-space matrix for mode two is defined in (4).

{ .
x = A1x + B1u

y = C1x
A1

⎛⎜⎜⎜⎜⎜⎝

0 0 0 0

0 0 0 −

1/Lb

0 0 0 0

0 1/CH V 0 −1/(RloadCH V )

⎞⎟⎟⎟⎟⎟⎠ B1 =

⎛⎜⎜⎜⎜⎜⎝
1/La

1/Lb

1/Lc

⎞⎟⎟⎟⎟⎟⎠ (4)

By circuit analysis of remaining equivalent circuits of boost mode in Section 3, state-space matrices for the other
odes of operation are obtained and given in (5)–(7).
In mode 4: ((1 − D3) Ts):

A2 =

⎛⎜⎜⎜⎜⎜⎝
0 0 0 0

0 0 0 0

0 0 0 −1/Lc

0 0 1/CH V −1/(RloadCH V )

⎞⎟⎟⎟⎟⎟⎠ B2 =

⎛⎜⎜⎜⎜⎜⎝
1/La

1/Lb

1/Lc

⎞⎟⎟⎟⎟⎟⎠ (5)

In mode 6 ((1 − D1) Ts):

A3 =

⎛⎜⎜⎜⎜⎜⎝
0 0 0 −1/La

0 0 0 0

1/CH V 0 0 −1/(RloadCH V )

⎞⎟⎟⎟⎟⎟⎠ B3 =

⎛⎜⎜⎜⎜⎜⎝
1/La

1/Lb

1/Lc

⎞⎟⎟⎟⎟⎟⎠ (6)

In mode 1, 3, and 5 ((D1 + D2 + D3 − 2) T s):

A4 =

⎛⎜⎜⎜⎜⎜⎝
0 0 0 0

0 0 0 0

0 0 0 −1/(RloadCH V )

⎞⎟⎟⎟⎟⎟⎠ B4 =

⎛⎜⎜⎜⎜⎜⎝
1/La

1/Lb

1/Lc

⎞⎟⎟⎟⎟⎟⎠ (7)

Using state-space averaging and superposition theorem, the transfer function for inductor current to duty cycle
s expressed in (8).

G iLi d j (s) =

îLi (s)

d̂ j (s)

⏐⏐⏐⏐⏐
ûc(s)=0

UH V CH V s + (1 − D j )ILi +

UH V
Rload

L i CH V s2 +
L i

Rload
s + (1 − D j )2

(8)

here i = a, b, c. and j = 1, 2, 3. Correspondingly, the transfer function voltage to inductor current is expressed
n (9).

Gu H V iLi (s) =
ûH V (s)

îLi (s)

⏐⏐⏐⏐⏐
ûLV (s)=0

−ILi L i CH V s2
+

[
(1 − D j )UH V CH V −

ILi L i
Rload

]
s +

(1−D j )UH V
Rload

(CH V s +
1

Rload
)[UH V CH V s + (1 − D j )ILi +

UH V
Rload

]
(9)

.2. Controller

For both (boost & buck) modes, dual-loop control is implemented. The block diagram of the control system
or boost mode with inner current loop and outer voltage loop is illustrated in Fig. 5. Three independent current
ontrollers are employed to realize equal current sharing among interleaved inductors which is crucial for reliable
peration. Duty cycles are generated by current loops and reference current is derived from sampled dc bus voltage.
I controller is used because of its excellent performance, feasibility and ease of implementation. GPI−La, GPI−Lb
nd GPI−Lc are current controller transfer functions. Likewise, GPI−U is voltage controller transfer function and

(s), G (s) and G (s) are modulation transfer functions. Carrier phase-shifted SPWM is employed to reduce

M1 M2 M3

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M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–1140

Table 1. Design parameters of the interleaved converter.

Parameter Symbol Value

Power P 6.6 kW
HV side voltage UHV 270 V
DC-link voltage ULV 90 V
Inductor La Lb Lc 200 µH
Frequency fs 100 kHz
DC link filter capacitance CLV 30 µF
DC bus filter capacitance CHV 150 µF

Fig. 5. Dual loop control block scheme for three-phase interleaved converter.

the current ripple. Using (8), (9) and parameters in Table 1, transfer functions of the system can be obtained.
After adding a controller into the system, the crossover frequency is much smaller than the switching frequency.
Frequency domain analysis of the system is completed using the bode plot. The phase margin value is set at 82
degree, and the PI controller is tuned to neutralize the small-signal disturbance for voltage and current controllers.
The controller gain values are the same for all three current controllers. In boost mode, Uref is set equal to dc bus
voltage and open-loop system analysis is completed in the frequency domain for the voltage controller. Phase and
gain margins are observed. Considering the margins, the gain values for the controllers are tuned to achieve optimal
performance.

5. Simulation results

To verify the efficacy of system, three phase IBC is simulated in MATLAB with a dual loop control at rated
power of 6.6 kW. In boost mode, dc-link voltage acts as input voltage (90 V), beside bus voltage (270 V) is the
output voltage and vice versa. Ripple compassion of the three-phase IBC converter and its steady-state and dynamic
performance is validated through the simulation results.

Ripple reduction in the output voltage of three-phase IBC compared with single phase counterpart during the
boost and buck mode is shown in Fig. 6(a–b) and (c), respectively. In addition, the output current ripple reduction in
the buck mode is illustrated in Fig. 6(d). Likewise, Fig. 6(e) shows that the input current ripple of IBC in the boost
mode is minimal. In addition, the total input current is the sum of inductor currents and equal current distribution
(ILa = ILb = ILc) among inductors is achieved, which is crucial for converter reliability. Hence, results show that
IBC has a much smaller ripple than its conventional counterpart.

To investigate the dynamic performance, load is increased from 3 kW to 6 kW and decreased back to 3 kW at
0.4 s and 0.8 s, respectively, for both buck/boost modes. For boost mode, it can be seen in Fig. 6(f) that bus voltage
stabilized rapidly and overshoot is restrained within 3%. Moreover, all inductor currents trace the load mutation
effectively, as depicted in Fig. 6(g). A similar response of current and voltage curves for buck mode is shown in
Fig. 6(h). A drop in the input voltage (90 V to 80 V) is also investigated in boost mode. Fig. 6(i) reveals that the
controller quickly stabilizes dc bus voltage during input voltage mutation, which shows that the designed controller
provides the desired performance. Hence, results validate the performance of the designed converter for both modes
of operations.

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M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–1140

Fig. 6. Simulation results.

6. Conclusion

An efficient and reliable bidirectional dc–dc converter is an indispensable part of More electric aircraft that
is energy efficient and playing a curial role in meeting environmental goals. This paper proposes a two-stage
bidirectional converter, which can infuse flexibility in topology, easing voltage gain and outrageous ripple reduction.
An in-depth analysis of three-phase interleaved bidirectional converter is carried out. Firstly, working modes of BIC
are analysed in detail, and then by using the state-space averaging method, modelling is completed, and expressions
of transfer functions are derived. Steady-state and dynamic analysis is carried out using open-loop transfer functions,
and a dual loop controller is established. Besides, current balancing among inductor currents is also considered as
it is crucial for reliable converter operation. The authenticity of the proposed design is verified by simulation,
which shows that the designed converter has much smaller ripples in output/input current and voltage. This study
encompasses the in-depth analysis of the IBC stage, and combined analysis of both stages is a part of future work.
In addition, the potential of the two-stage idea can be unleashed by exploiting the wideband gap devices that enable
high efficiency and high power density design. The advanced control techniques can also be investigated on the
proposed two-stage converter to unfold its benefits. Furthermore, the analysed IBC is suitable for a modular design
approach to extend power ratings. The research carried out in this paper can serve as a reference for designing,
modelling, and controlling dc–dc converter for other applications requiring low ripples.

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M. Tahir, S. Ahmed Khan, T. Khan et al. Energy Reports 8 (2022) 1133–1140

S
t

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could
ave appeared to influence the work reported in this paper.

cknowledgements

The authors extend their appreciation to the Estonian Centre of Excellence in Zero Energy and Resource Efficient
mart Buildings and Districts, ZEBE, grant TK146, funded by the European Regional Development Fund to support

his research.

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More electric aircraft challenges: A study on 270 V/90 V interleaved bidirectional DC–DC converter

Introduction

Proposed idea

Analysis of IBC modes

Boost Mode

Modelling and control

Modelling

Controller

Simulation results

Conclusion

Declaration of Competing Interest

Acknowledgements

References

233

As the reader may have found, Fundamentals of Electric Aircraft provides basic knowledge
of how the aviation industry is getting impacted by the electrification trend

This paradigm
streamlines conventional aircraft using several types of energy (fuel, hydraulic, pneumatic,
mechanical) into ones that parlay the enhanced efficiency of a single type of energy, namely,
electric energy. Scaled-up energy storage on an electric aircraft, sometimes in combination
with other sources, is poised to supply power to electrified loads, ranging from a variety of
systems to propulsion itself.

The automotive sector has made significant strides in the field of electrification.
Nevertheless, the efficiency of carbon fuel is difficult to beat. Market penetration has been
slow due to higher production costs linked to smaller volumes, limitations in range perfor-
mance, and the cost of energy storage. But that is changing every day and the tipping point
for massive ramp-up is just a couple of years ahead, and technologies developed will certainly
prevail in other sectors.

In fact, leveraging the latest advances in battery technology, especially with regard to
specific energy, a bigger number of electric general aviation aircraft are poised to take to
the skies at the turn of the decade. From there, concerning larger aircraft for short- and
long-haul flights, it will probably be at least a decade or two before mature and power-dense
electrical architectures get to acceptable levels. Even then, these aircraft might probably
wind up being hybrid-electric only.

Global investments and research are in full swing, ushering in breakthroughs that will
help reshape the landscape for greener travel with the electric aircraft. Replacing carbon
fuel in aviation with an alternative energy source is not that easy. We may wonder if the
weight of centuries is not what bestows kerosene with its unmatchable specific energy, but
it would take reasonable time for any alternative cost-effective energy-storage technology
to completely unseat its predominance.

What can be seen is that electrification, whether via fuel cells or batteries, is slowly
outpacing carbon fuels and is already an enabler for autonomous travel in the automotive
sector. The same kind of situation is likely to be encountered in aviation for urban mobility
concepts, even if autonomous air travel is not for tomorrow.

We all know how paramount safety is for aircraft design and operations. That means,
by no way, safety rules may end up getting transgressed. Icarus, in Greek mythology, would
probably have survived his flight to freedom if only he had heeded Daedalus’ cautionary
warnings for “safe flight”. But, he went too high in altitude and dangerously close to the
sun, and faced the tragedy of his own loss. If only his wings had featured the right level of
autonomy and authority, they would have prevented him from doing what he did and saved
his life. Reassuring though, lessons learned from ancient times along with more recent ones
remain so hardwired in the minds of aviation experts that safety is never a second thought.
When designing future air transport solutions, that wisdom of hindsight will help ensure

conclusion

Thalin, Pascal, et al. Fundamentals of Electric Aircraft, SAE International, 2018. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/dcccd-ebooks/detail.action?docID=28983708.
Created from dcccd-ebooks on 2023-02-05 20:17:44.

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234 Conclusion

compliance to regulations with no compromise. Otherwise, a third rail will undoubtedly
be hit with certification authorities, penalizing entry into market.

As the electric aircraft advances, more and more technologies will deliver but will
also set their own limitations depending on when and how they are implemented and
operated. Regarding timescales, the reliance on kerosene may gradually decrease thanks
to electric and hybrid-electric aircraft developments down the line, but its ultimate
phasing out may happen only in the long run.

Future urban and general aviation, regional and short- or long-haul aircraft are
poised to be greener and quieter while maximizing passenger mobility and flight expe-
rience in a sustainable manner. Electric aircraft are the mainstay of this inexorable push.
Cities over which they may fly, and communities close to airports or “vertiports” they
would operate from, would ultimately enjoy relief from air traffic pollution and noise.

Pascal Thalin

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This chapter summarizes scenarios of how an upcoming more electric aircraft and especially
an upcoming electric aircraft would impact current processes and regulations for aircraft
operations

Therefore, the following sections address both ground operations and in-flight
operations. As these topics are introduced within a mid- to long-term timeframe and as
some key questions defining the most plausible path remain unanswered, the following
discussion is an attempt to envision future air transport operations with some caveats
though.

9.1 Ground Operations
9.1.1 Maintenance—State of the Art
Maintenance in aviation covers all tasks ensuring the compliance with Airworthiness
Directives including Service Bulletins. These tasks are highly regulated and mainly specified
by the Original Equipment Manufacturers (OEMs) in order to guarantee a safe use of every
component. The regulations are supervised by national regulation agencies like the Federal
Aviation Administration (FAA), Civil Aviation Authority (CAA), or European Aviation

Maintainability and
Operational Overview
Sven Taubert
Lufthansa Technik

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182 CHAPTER 9 Maintainability and Operational Overview

Safety Agency (EASA) and internationally coordinated by bodies like the International
Civil Aviation Organization(ICAO). To ensure the compliance with these regulations,
every maintenance task and every appointed staff has to be licensed. Maintenance is
also part of the certification process—during type certificate (TC) or amendments to
TC, the maintenance schedule is documented and approved by the certifying authority.

The maintenance market itself is very fragmented. Most of the larger airlines are
performing at least some of these tasks themselves. The rest would be subcontracted to
either independent Maintenance, Repair, and Overhaul (MRO) companies like Lufthansa
Technik, among many others, or the component/aircraft OEMs themselves.

9.1.1.1 Maintenance Planning: Currently, maintenance tasks are often planned far
in advance. Most checks are repeated in periodic intervals depending either on the flight
hours or cycles. One cycle describes one takeoff and one landing. The main categorization
is done in A, B, C, and D checks.

A and B checks are performed in line maintenance, and C and D checks performed
in base maintenance. Line maintenance includes minor checks and unscheduled inci-
dences, solvable within a few days or which would cause the aircraft to lose its airwor-
thiness. Loss of airworthiness or “Aircraft on Ground” (AOG) prohibits flying with
immediate effect.

Planned checks are mainly done during night shifts. During daytime, between
flights for aircraft in service, the main task for line maintenance is to service and repair
“on-wing” as may be necessary. Base maintenance includes major upgrades like installing
a satellite communication system, cabin refurbishment, larger inspections, and almost
all “off-wing” repairs.

9.1.1.1.1 A Check. The A check subscribes the smallest planned interval. It has to
be performed every 400-600 flight hours or every 200-300 cycles, depending on the
aircraft type and the components needing maintenance, and can take up to 50-80
man-hours. Normally, airlines will try to complete an A check during one night shift in
a hangar (6h-10h). Some of the tasks can also be postponed or even done earlier to ensure
a maximized utilization time of the aircraft by a guaranteed safety level. As there are
thousands of components in each aircraft, an ingenious maintenance schedule can easily
save millions of dollars.

9.1.1.1.2 B Check. The B check has to be performed every 6-8 months and takes 150-200
man-hours. Again, these estimations depend on the type of aircraft and equipment
conditions. All tasks can be done within a downtime of 1-3 days. This is widely considered
as the maximum time for line maintenance. Using better planning possibilities, more
and more MROs divide B checks into several A checks (Checks A-1 through A-10) to
avoid daytime downtime and maximize aircraft utilization time.

9.1.1.1.3 C Check. The C check has to be performed every 20-24 months, depending
on either actual flight hours (or cycles) or manufacturer Service Bulletins. This check
grounds the aircraft for 1-2 weeks and involves up to 6000 man-hours. This includes
inspections of a majority of the aircraft systems and components. The C check cannot
be subdivided into several B checks, as a lot of components have to be shipped to off-site

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CHAPTER 9 Maintainability and Operational Overview 183

suppliers. Due to the long downtime, the C check can be performed at specialized MRO
facilities, which are not necessarily near the airline’s hubs. Most airlines make use of the
check to perform upgrades like cabin modifications, installation of connectivity equip-
ment, or avionic upgrades. For the Boeing 747-400, the C check itself may cost somewhere
between $0.7 million and $1.5 million.

9.1.1.1.4 3C Check. The 3C check, sometimes known as “Intermediate Layover” (IL),
is necessary on some aircraft to check structural parts and some high-load parts for
corrosion. Similar to the C check it can be combined with major cabin upgrades like
class changes with new seats or monuments. The combination of these tasks saves again
downtime of the aircraft. The 3C check can be incorporated in several C checks or into
one D check. The main reason for that incorporation is an improvement of the reliability
due to better corrosion protection.

9.1.1.1.5 D Check. The D check, sometimes known as “Heavy Maintenance Visit”
(HMV), has to be performed every 6-10 years. It is by far the largest maintenance check
and takes 2-3 months. During the 50,000 man-hours more or less necessary, the whole
aircraft is disassembled. All parts have to be checked. For some structural visual inspec-
tions, even the aircraft paint has to be removed. As this check is very time consuming
and workload intensive, it is often performed in countries with low labor and hangar
costs. Such a check may easily cost several million dollars. For example, a Boeing 747-400
has a D check every 72 months (6 years) costing around $5 million. Due to the long
downtime, D checks are planned far in advance. It is common to move the inspection
ahead in time to perform it during the winter flight plan utilizing a smaller number of
aircraft. As the costs are significant, an upcoming D check can cause the phase-out of
an aircraft because its residual value may be lower than the check costs. Most of the
commercial aircraft undergo three D checks overall.

9.1.1.2 Maintenance Prediction—Condition Monitoring: The advent of
computerized control systems gave rise to the concept of condition monitoring, and
computing capacity is a key enabler. As the “condition monitoring” term indicates,
maintenance tasks here are not based on fixed flight hours, cycles, or time anymore, but
on the actual condition of the components. The condition is mainly determined by
sensors. Often, the condition has to be defined by multiple indicators, including:

• Pressure
• Vibration and sound
• Temperature and heat transfer rates
• Speed (axial and rotation)
• Power consumption, current, and voltage
• Stress, pressure, and shock
• Overall position
• Computer outputs

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Thanks to increased availability of real-time data today, even non-critical systems
can feature condition monitoring. The big advantage of such systems is the reduction
in maintenance costs they bring. The MRO is able to focus on inspecting and/or changing
components with actual defects or find issues much faster, reducing aircraft downtime.

The challenge of condition monitoring is to find the economic balance between the
system costs and expected savings. As condition monitoring systems need advanced
instrumentation, the initial costs can be significant. Legacy components often lack data
or even a power interface, making retrofit scenarios very unlikely. Wireless data commu-
nication may ease the situation. An example for this technology is vibration sensing of
rotating parts like a turbine shaft or a pump motor.

Analyzing more complex components requires data fusion. The challenge here is
to use the right data combination and interpretation to determine the defective compo-
nents to be replaced.

One of the hazards of condition-based maintenance (CBM) is the rising complexity
of the instrumentation itself. If the sensors are faulty themselves, they may indicate
component failures that are inexistent, increasing again the maintenance costs.

All and even more electric aircraft implement electrical systems in lieu of hydraulic
and pneumatic systems; all-electric systems also eliminate traditional propulsion
systems. Most of the electrified systems require active power control enabled by sensors.
So the hardware requirements for condition monitoring may be met. But the difficulty
remains in finding what kind of data indicates what kind of failure. This is why data
interpretation algorithms often get mature only months or even years down the line
thanks to cross-checks of sensor readings with actual maintenance findings.

9.1.1.2.1 Condition-Based Maintenance (CBM) and Predictive Maintenance
(PdM). The terms Condition Based Maintenance (CBM) and Predictive Maintenance
(PdM) are often used as synonyms. Actually, there is a slight difference, or to be more
precise, there is a considerable overlap. PdM uses often the tools and methods of CBM
but could also use other data sources to predict life of parts.

CBM should only be performed if the component shows any indication of failure
or unacceptable degraded performance. PdM has the goal to predict the failure of a
component so that it could be changed or repaired preemptively even if the part is not
showing any signs of degradation. Therefore, big data analytics based on statistical
principles are used.

As a simple example, let’s take an armrest of an economy class aircraft seat. CBM could
monitor the stress curve through the part. If this curve changes to a certain shape, this
would be interpreted as reduced performance and the part would be changed. Predictive
maintenance would use the same information in the same manner, but it could also consider,
for instance, the number of passengers having taken that seat. The PdM model predicts that
every 30,000 passengers the armrest has to be changed due to scratches. Scratches cannot
be detected by any sensor, making it impossible to track the actual condition. An experi-
ence-based model, on the other hand, would give a very good indication when a change
should be considered. The classic maintenance schedule is based on “simple” experi-
ence-based models. The essential difference with modern predictive maintenance models
is combining the experience-based data with live data. This extensive data may reveal
patterns indicating failure root causes which were unknown or undetected before.

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CHAPTER 9 Maintainability and Operational Overview 185

Another very important aspect of CBM is the “on-wing” capability. Most inspection
technologies perform sensing on components during their operation, making a compli-
cated disassembly unnecessary or at least far less mandatory. This is an important factor
for cost savings.

Table 9.1 shows the advantages and disadvantages of CBM.
To reap the full benefits of a PdM approach, the dataset of each and every aircraft

of an airline has to be connected to a so-called computerized maintenance management
system (CMMS). It manages the condition of all components and triggers maintenance
tasks and schedules. Otherwise, a fleet with hundreds of aircraft, each with millions of
parts, cannot be run efficiently.

To achieve this, several inspection technologies are needed. A part’s condition is
nondestructively analyzed by sensors measuring different light spectrums, acoustics,
vibration, temperature, speed, power, stress, liquid composition, or computer outputs.
Often, not one but several indicators are utilized to determine the condition, both
analyzing the equipment and its immediate environment. Especially wireless commu-
nication technology kicked off the implementation of sensor networks.

In the following, some of the mentioned technologies are described in further detail:

Visual inspection
Visual inspections have been used in aerospace industry for decades. For

example, black light and contrast liquids are used to detect cracks, folds, and
corrosion. Endoscopes, mirrors, and lenses help maintenance staff to detect
failures on hidden and not easily accessible spots like in the engines. Most of
these inspections are performed during maintenance checks by MRO staff, but
recent developments in camera sensor technology enabled automated live data
collection. As visible light camera sensors need at least 300 lumens to deliver
good results, infrared monitoring is utilized for more and more inspections. Its
versatile utilization possibilities allow detection of mechanical and electrical
failures (Figure 9.1). Due to its low price it is considered as one of the most cost-
efficient inspection methods.

Acoustics
Sonic and ultrasonic real-time analyses are mainly used for moving parts like

shafts. Sonic monitoring is used in applications that need less accuracy. It is less
expensive and can also be detected by trained staff by intent listening with their
bare ears. Ultrasound has enough acoustic resolution to inspect even machines

TABLE 9.1 Pros and cons of condition-based monitoring

Advantages Disadvantages
Increase of utilization time Increase of CBM-enabled component cost

Improvement of component reliability Risk of increased maintenance tasks due to
sensor failureMinimizing AOG time

maintenance cost reduction via optimized
maintenance schedules

CBM/PdM is non-invasive. Equipment can
be operated during inspection

May generate considerable retrofit effort

Increase of safety levels Unforeseen failures in case of false analytics

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186 CHAPTER 9 Maintainability and Operational Overview

rotating with high frequency like jet engines or turbo pumps. Ultrasonic
microphones can “hear” the difference between smooth operation of a rotating
machine and one with too much friction or stress by comparing sound profiles.
Each type of machine has its own sound profile, similar to an acoustic
fingerprint. Changes like friction, stress, or deformation generate additional
distinguished sounds visible in the upper ultrasound spectrum. That effect
detects an abnormal wear far before visible inspection could. The only
comparable method would be vibration analytics. But, unlike the ultrasonic
method, there are instances whereby vibration monitoring may fail to
distinguish the faulty component from the healthy one (Figure 9.2).

Aircraft OEMs outlined that predictive maintenance based on data analytics and
CBM could eliminate all AOG events within the 2025-2035 timeframe. This timeline
allows for such advances to be taken into account in future design configurations of the
electric aircraft. Meanwhile, implementation of more electric systems, until there is
finally a full electric aircraft, will not only benefit the environment but also the main-
tenance effort. Until now, modern conventional aircraft have decreased fuel consumption
and weight at the cost of more system complexity. This trend of more and more complex
system architectures could be totally reversed on an all-electric aircraft, electrification
allowing for the removal of the pneumatic, hydraulic, and fuel distribution systems
along with traditional propulsion systems.

FIGURE 9.1 Infrared monitoring for failure detection.

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FIGURE 9.2 Vibration monitoring for failure detection (method limitation shown).

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CHAPTER 9 Maintainability and Operational Overview 187

9.1.2 Changes for More Electric Aircraft
As a first step, a more electric aircraft may substitute hydraulic and pneumatic systems
with electrical ones. Hydraulic systems can distribute large forces over the whole aircraft
through cumbersome piping pumps and valves with a huge number of fittings and
couplings. However, due to redundancy built into architectures, it is very robust.
Unfortunately, it also poses some challenges concerning maintenance. Because of its
complexity, leaks are often very difficult to locate. In the meantime the leaking fluid
could cause a cascade of other failures. This makes the maintenance of the hydraulic but
also pneumatic systems time consuming and expensive.

Hydraulic and pneumatic systems were used because it meant that the actuators
supplied by them could be built less complex, lighter, and smaller. As electrical motors
made enormous progress in these areas over the past years, this advantage is almost
leveled out. Electrical cabling, on the other hand, is far less complex then piping.
The functions of today’s hydraulic, pneumatic, and power systems could all be inte-
grated into one power distribution network. Instead of maintaining three systems there
would be just one. Moreover, cable breakages are much easier to detect than pipe leaks.
Wire fault location uses a technology called reflectometry. This consists in sending a
discontinued signal into the damaged cable and the break or short would reflect the
signal and send it back. The time required for the reflection indicates the distance to
the problem.

In general, electric systems are much easier to monitor for health and system
status. Even on a more electric aircraft, the reduction in system parts by moving to a
primarily electric architecture may be significant. A case in point is the more electric
Boeing 787 where the bleedless architecture allows reduction of overall mechanical
systems complexity by more than 50% compared to a conventional 767; the elimination
of pneumatic systems is a major contributor [9.1]. As a consequence of this reduction
in mechanical systems’ complexity, airline operations may get less maintenance inten-
sive and more reliable than with conventional systems of bleed architectures, even
though the complexity on the electrical side goes up. In fact, the move to electric
systems helps in cutting significantly the schedule interruptions compared to a conven-
tional aircraft for the systems affected by the no-bleed/more electric architecture.
Other benefits include improved health monitoring and greater fault tolerance.

Moreover, the 787 features greatly expanded and improved systems monitoring
capability coupled with an advanced onboard maintenance computing system. This
capability combined with e-enabling technologies, which make real-time ground-based
monitoring possible, significantly aid in rapid, accurate troubleshooting of the 787.
Airplane systems information used in conjunction with fully integrated support products
help maintenance and engineering organizations quickly isolate failed components and
reduce return-to-service times. No-fault-found (NFF) removals are also drastically
reduced compared to conventional aircraft, reducing yet another major cost driver for
operators. Therefore, thanks to the extensive system electrification on the Boeing 787,
even though hydraulics have not been totally removed, airlines have far less maintenance
to perform less often than they are used to. This leads to extended check intervals
compared to conventional aircraft as shown in Figure 9.3.

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9.1.3 Changes for an Electric Aircraft
With an electric aircraft, the complexity of the engine decreases by orders of magnitude.
A current jet engine is one of the most advanced combustion machines ever developed.
It consists of up to 30,000 parts. The combustion chamber reaches up to 2000°C and
even the exhaust gas can still reach 500°C. Therefore, dozens of blades have to be made
of ceramics or single-crystal metal or have advanced cooling outlets for cool air
shielding. All this makes it not only the most expensive aircraft system at the moment
of purchase but also in terms of maintenance. Current jet engines and their support
systems have hundreds of thousands of years of service history, while the electric aircraft
is still in its infancy state and will take some learning process to get to the current
existing aircraft state.

An electric engine for aerospace applications would in contrast probably have only
around 250-300 parts. Also, temperature should not be in the same order of magnitude.
So, it is obvious that electric engines will have a much lower share of total cost then
today’s jet engines.

The electric aircraft also adds a new system: a solid power source. While batteries
are already used, the dimension needed for an electric aircraft escalates to an entirely
new level. If we imagine batteries, which can store enough energy to use them as primary
power source, they would have to be maintained unlike jet fuel today. Battery state of
charge and state of health are key measures to ensure safe operations and full battery
utilization. That means that it would be mandatory to devise an approach to maintain
them. Replacing them after some hundred cycles comparable to today’s smartphones
or even cars would not be ecologically and financially sustainable. Furthermore, a lot
of current battery technology is based on rare earth materials. How that maintenance
service could be offered and by whom is currently not clear. Decisive factors are tech-
nical expertise, global capacity, price, but also upcoming regulations.

FIGURE 9.3 C-check interval – long range aircraft [9.2].

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CHAPTER 9 Maintainability and Operational Overview 189

9.1.4 Airport Operations
Next to maintenance, airport operations form the other part of ground operations.
In the coming sections, topics like infrastructure, aircraft turnaround, and emergency
processes will be addressed.

Airports are currently the foundation of aerospace business. Apart from decentral-
ized helicopter landing areas (helipads), every aerospace mission starts and ends at an
airport. Its infrastructure includes runways, taxiways, aprons, terminals, hangars, traffic
control centers, and support infrastructure like fuel reservoirs. International airports
facilitate border control and customs checks as well.

9.1.4.1 Infrastructure: One fundamental question remains: which future aircraft
concepts need what kind of infrastructure? Looking at long- and short-range more
electric and electric aircraft, this will not change. Larger aircraft will always need a
certain support infrastructure, especially if international travel is involved. The next
section on aircraft handling will go into further detail on this.

Urban air mobility concepts like the Volocopter shown in Figure 9.4 will need
a much smaller scale of infrastructure. However, the core functions of an airport
have to be covered somehow. A combination of helipads and “vertiports” could be a
solution.

Assuming that urban air mobility concepts are more competitive concerning
pricing than current helicopter services, the number of missions would be much
higher. Therefore, a simple and seamless boarding and disembarking process is manda-
tory. Many studies show that security control is currently the pain point number one
at airports. Automated or remote-controlled advanced scanner technology in combi-
nation with a kind of background pre-screening similar to systems like ESTA in the
United States could minimize the effort. As this process should be done within
minutes, a large waiting area like in current terminals would not be needed.

The vehicle itself would need a refueling or recharging
facility and maybe a small line maintenance station.
Depending on the success rate of predictive analytics, the
maintenance could be even coupled with similar services
at the airport. While urban air transportation is probably
offered in larger cities that are already equipped with at least
one full-scale airport, the question whether traffic control
of low altitude urban mobility would be managed by local
airport(s) is still open. A lot of urban air mobility concepts
reckon with the fact that air traffic management principles
and its responsibility are yet be addressed. On top of the
focus on autonomous operation, navigation, sense, and
avoid are also key technologies.

However, that would mean that urban air mobility
concepts would be mainly point-to-point connections
between localities featuring so-called rooftop “vertihubs”
or “vertistops” integrated to roads. First test cases support
that prediction.

FIGURE 9.4 Example of urban air mobility vehicle
(Volocopter) [9.3].

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Future smart ground transportation systems may also be poised to compete directly
with urban air mobility concepts. In fact, most of the current ground mobility business
models focus on the pain point of gridlocks found in many urban metropolitan areas.
Therefore, if autonomous taxi or car sharing concepts were to become reality, with their
own traffic management in place, congestion issues would be resolved, and a shadow
would be cast on the appeal of air mobility.

From an energy consumption point of view, aerospace applications will always
be more demanding and thereby more expensive. Also old applications will have to
merge with the new ones seamlessly, which is quite a challenge. In addition, door-to-
door transportation depends on ground-based systems to a certain extent. Even if urban
air mobility concepts save their customers some time, ground-based solutions offer not
only lower prices but also more comfort and onboard infrastructure allowing better
utilization of travel time. A similar situation can be currently observed with trains versus
aircraft on short-range flights, especially in Europe and Asia.

9.1.4.2 Aircraft Handling: Focusing again on larger electric aircraft concepts, these
vehicles would need similar aircraft handling services like today with two exceptions—
Refueling/Recharging and Pushback/Taxiing.

9.1.4.3 Refueling/Recharging: As one of the biggest changes of an electric aircraft
would be the replacement of the main energy source, the refueling process would have
to be completely redesigned. If the new energy source is liquid or gaseous, current
processes might not differ much.

Assuming a kind of battery system, there are normally two solutions—recharging or
replacing. Recharging would have the main challenge of time. Today, a turnaround of an
A320 is between 30 to 60 min. Refueling is only permitted if no passengers are on board or
a fire truck is on standby next to the aircraft. But predicting that recharging would be allowed
during the whole turnaround time, it is still technically very challenging to recharge the
batteries within that timeframe without losing too much battery life. In addition, battery
technology would have to improve a lot from a specific energy standpoint.

If the specific energy challenge could be solved but the recharging timeframe could
not, a battery swap would be the most likely solution. Some car manufacturers showed
already such concepts of fast and automated battery swapping systems.

Both solutions have one common challenge for the airport—standardization.
Today, all aircraft f ly with the same fuel. Looking at electric vehicles, there are a
multitude of power plug standards. Each car manufacturer defined its own plug
with specific standards for voltage and current. For aircraft ground support equip-
ment (GSE), there is a standard connector which is the Euro plug. Nevertheless, for
future electric aircraft batteries, the existing Euro connectors may not be sufficient.
This may require standardization bodies to develop standards for the battery
recharging process. Moreover, depending on the aircraft system architecture,
different OEMs are likely to come up with totally different power architectures. So,
it will be essential for airports to define a very f lexible, but also scalable (up and
down), power grid architecture.

Battery pack shapes are mainly driven by the geometry of the powered device/
vehicle. Predicting a variety of configurations could also mean that current geometry

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standards are not useful anymore. For example, the cargo
LD-X standard container (Figure 9.5) fits into the cargo
hold of almost every long haul aircraft—A380, A350,
A330/A340, B747, B777, B787 …. A f lying wing config-
uration may waste a lot of space by adapting to that
standard, leading to a new type of container.

Accordingly, if all electric aircraft were to have very
diverse configurations, airports would have to store large
stocks of different kinds of batteries. This would not just
mean a storage space challenge but also consequential costs.

This is why standardization bodies like SAE International
are already discussing possible solutions with all impacted
industry stakeholders to avoid that. The objective is to
produce a standard that would minimize variability and
provide guidance for standardized design, production, and
testing.

Likewise, fuel cell utilization at airports requires storage and/or distribution of
hydrogen to airport facilities for aircraft refueling. This requires modifications to infra-
structure addressing hazard prevention and safety concerns.

9.1.4.4 Pushback/Taxiing: The increasing awareness and attention to environ-
mental concerns drives innovation in aeronautics. One of the issues to be addressed is
aircraft maneuvering on ground using jet engines, thus causing a variety of undesired
emissions. The general working principle is to use electric vehicles to move aircraft
from the gate to the runway and vice versa. For several years, research and demonstra-
tions have been carried out in the short-range segment to validate electric taxiing
achieved by the aircraft on its own. This solution consists of integrating electric wheel
motors in the landing gear which draw electrical power from the APU (or fuel cell) in
combination with batteries, if necessary. With the APU being more fuel-efficient than
a turbofan engine, this aircraft-based electric taxiing allows for reductions in fuel burn
and emissions. Moreover, with this solution, retrofit on legacy aircraft may be possible.
Chapter 7 provides a deep insight into this incremental electrification approach of
conventional aircraft.

9.2 In-Flight Operations
This section discusses both cockpit and cabin implications of electric aircraft concepts,
bearing in mind that far more changes are expected for the flight deck rather than the
cabin. Starting from pilot licenses, an overview of controversial topics such as single
pilot operations, autonomous flight, future urban mobility with pilots akin to drone
operators, etc. is openly presented. As a general comment, not all these topics are
directly linked to the electric aircraft. But this is the future, so when reading through
this visioning exercise, remember that the future is predictable only with limited
certainty.

FIGURE 9.5 2 LD-3 containers get loaded into a
Lufthansa A380 aircraft.

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192 CHAPTER 9 Maintainability and Operational Overview

9.2.1 Flight Deck Operations
With the introduction of electric aircraft there will be at least some changes in the cockpit.

9.2.1.1 Complex Configurations/Licenses: Similar to aircraft certification chal-
lenges, pilot licensing will be a topic for electric aircraft operation. Today, pilots need a
certificate issued by the Civil Aviation Authority of the country where they want to
operate. Despite several attempts, the licensing process is still different from country to
country. The most known agencies regulating pilot certification are the Federal Aviation
Administration (FAA) in the United States and the European Aviation Safety Agency
(EASA) for the European Union and Switzerland.

As there are different aircraft categories and classes, there are different levels of
licenses, further distinguished by ratings:

Private and Commercial Pilot License (PPL and CPL)
For non-commercial and commercial operation of small aircraft, respectively.

Depending on the country, different sub-ratings are available, for example,
complex aircraft, single-engine/multi-engine aircraft (maximum takeoff weight
(MTOW) of 2t), etc.

Airline Transport Pilot License (ATPL)
This license is required in order to fly commercial planes in the range of those

usually operated by airlines. ATPL is the highest level license covering PPL and
CPL. For each aircraft type flown, an additional type rating is required.

Drone operators
Some countries start regulating drone operations by requiring a Drone Operator

License. Especially commercial drone operations are covered by these initiatives.
However, a structured international standard is currently not available.

Depending on electric aircraft configuration and its TC, the corresponding pilot
license is derived. The biggest gap here is within the urban air mobility concepts. These
vehicles are often a combination of ultra-light aircraft, helicopters, and drones. There
is, for instance, an urban air mobility prototype which is currently applying for an EASA
ultra-light certification. The derived ultra-light pilot license would be just valid for Europe
and not for commercial use.

The bigger electric aircraft concepts for commercial short-range applications would
be certified under CFR Part 25 so that the normal ATP license with a specific type rating
would probably be sufficient.

9.2.2 Single Pilot Operations
A topic that is currently controversially discussed is large commercial aircraft single
pilot operations. It would mark a first step towards autonomous flight. Both automotive
and aerospace vehicles powered by fossil fuel would be capable to integrate the necessary
technology for (greater) autonomy. However, as the topic of autonomous driving is by
public perception linked to electrically powered cars, this impression is transferred as

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CHAPTER 9 Maintainability and Operational Overview 193

well to aerospace. Nevertheless, there are some arguments why an electric aircraft
program could foster the implementation of this concept.

As many concepts for even large short-range applications envision multi-impellor
or multi-propeller with multi-power sources, thrust control in case of failure is getting
much more complex. Moreover, these engines not only produce thrust but also lift or
steer the vehicle. It is presumable that, similar to modern unstable fighter jets, these
aircraft are not maneuverable without significant computing support. So, as the computer
is anyway responsible for a significant portion of flight operation, the question is how
effective a human pilot would be interfering in case of a failure. There might be the
conclusion that the most effective operation in terms of cost but also safety is an advanced
autopilot system supporting one pilot.

The biggest issue in this scenario would be how to compensate the loss of a single
pilot in case of medical emergency, for instance. For this particular case, initial studies
support a remote pilot solution. As a side effect, these remote pilots could also support
aircraft with a twin pilot operation if one of the pilots gets suddenly unfit during flight.

9.2.3 Autonomous Flight
Of course, autonomous flight is discussed with even more controversy. Similar to single
pilot operation of commercial flights, autonomous flight is not directly related to electric
aircraft but is often mentioned in the same breath.

In contrast to single pilot operation, autonomous operation today is mainly driven
by smaller electric aircraft concepts like civil drone applications and urban air mobility
vehicles. Here, the business case rests on the assumption that autonomous flight is both
technically possible and socially acceptable, and is cost-efficient. Urban air mobility may
also need autonomous operation to close the business case just because there won’t
be enough pilots.

In commercial aviation, autopilot systems have been the norm for years. Automatic
landing systems are in place, but the standard procedure is still for the pilot to land the
plane manually. Fully autonomous flights were also demonstrated multiple times. In
1988, as one of the most famous examples, Russian spaceship Buran, which looks quite
similar to the American Space Shuttle, performed a fully autonomous start, flight in
space, and landing. In military drone operations,
autonomous or semi-autonomous flights are increas-
ingly becoming the standard procedure. With the
Northrop Grumman X-47B Unmanned Combat Air
Vehicle (UCAV), even automated air-to-air refueling
and landing on an aircraft carrier were demonstrated
(Figure 9.6).

Note that these flights have been performed on
vehicles with no passengers on board, but the tech-
nology is mature. Yet, some industry actors, especially
airline related, have suggested that autonomous flight
in commercial aviation will take more than one gener-
ation to become reality. The Aircraft OEMs Boeing and

FIGURE 9.6 UCAV autoland on aircraft carrier [9.4].

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194 CHAPTER 9 Maintainability and Operational Overview

Airbus are more optimistic and want to start trials within months not decades. Both sides
have valid arguments.

One of the main arguments against autonomous flight concerns safety. The issue is
less on the computing power but more on the generation of intelligent decision trees. In
unforeseen events, even Artificial Intelligence (AI) isn’t capable to come up with “inno-
vative” ideas to solve the issue. Furthermore, AI needs vast amounts of data to train
machine learning for the resolution of problems. It is currently not proven that the available
data from, for example, simulator emergency situations is sufficient for that approach.

On the other hand, autonomous driving has gained remarkable traction in its devel-
opment, while a car operates in a far less predictable environment than an aircraft. Therefore,
problem-solving algorithms from the automotive industry could also help aviation overcome
technical challenges. The near future will show if the OEMs can reach mandatory safety
levels and prove that autonomous flight is technically feasible on a commercial aircraft. The
outlined and published timelines by various startups and OEMs must be treated with some
reservation. In the 1980s, visions of the future also predicted that autonomous flights would
become reality no later than the year 2000. Like single pilot operation, the development of
a complete new aircraft, especially an electric aircraft redefining current standards, would
boost technical development and, therefore, the chances of implementation.

Another aspect is the social acceptance of autonomous flight. Whether or not passen-
gers would board a self-flying plane is just as hotly debated as technical questions. Similar
to trains or nowadays cars and trucks, some people believe in autonomy and others do
not. History shows that, while it will take some time, people will get used to the new
technology. In several cities, like in Paris, for instance, fully automated subways have
been in service for years and passengers readily ride them, often without noticing the
absence of a driver. Nevertheless, there are still public transportation providers who are
convinced that their customers would not accept a driverless train. Self-driving cars
triggered an even bigger discussion. That this debate is not always driven by logical
arguments becomes obvious when accidents do happen, especially when there are fatal-
ities. Most of these technology demonstrators have theoretically proven that they have
much higher reliability compared to human drivers, and the social debate will certainly
heat up around whether to adopt the concept or not.

On the other hand, cars with anti-lock braking systems (ABS) or Electronic Stability
Control (ESC or ESP) are considered safer, even if this means that a computer overrides the
human driver impulse and manages autonomous braking. So, when higher safety levels of
an autonomous system compared to a human operator is statistically proven, and the general
public is convinced, it could even become a must-have item. Given the emotional rather than
logic development of public opinion, it is very hard to predict if or when that will happen.

9.2.4 Pilots as Drone Operators
In unmanned military applications, pilots working as single or even multiple drone operators
is something which has already been implemented over the years, but for commercial aviation
it is still considered as disruptive. The straightforward reason behind this is that commercial
aircraft carry passengers, and as such have to comply with stringent safety requirements.

In the early 2000s, the United States developed weaponized drones from a prototype
status to what has come to be known as UCAVs. Combat drone operations are becoming

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CHAPTER 9 Maintainability and Operational Overview 195

increasingly commonplace regardless of where around the globe the theater of operation
is located. Incidentally, most of the pilots are remotely stationed in Europe (Figure 9.7)
or in the United States, sometimes several thousand miles away from combat zones. This
distance causes a signal delay of up to a couple of seconds between the time a command
is issued to the drone and the time when it is received.

Due to that delay, time critical operations like start and landing are performed by
drone operators on location. When the drones reach their operational altitude, there is
a handover to the remotely located pilots. As most of these operations are highly auto-
mated observation missions, drones operate semi-autonomously, that is, fly without
human interaction unless a potential finding requires an operator’s action. This is why
drone operators have the capacity to “fly” more than one drone.

Future combat drones may leverage hybrid-electric propulsion for stealthier VTOL
operations like with the LightningStrike from Aurora, a Boeing company, wherein
vertical takeoff and landing would be possible without a runway. This would stave off
the handover constraint making UCAVs operable remotely right from the onset.

Anyway, the handover principle between remote and local control stations used in
current UCAV operations could also be adapted for future commercial aviation or
delivery drones. For instance, urban air mobility concepts using electric propulsion like
CityAirbus shown in Figure 9.8 could operate autonomously until a situation arises

FIGURE 9.7 Remote and local Ground Control Stations part of the Predator
Unmanned Aircraft System (UAS) [9.5].

FIGURE 9.8 CityAirbus urban mobility concept vehicle rooftop landing (rendering) [9.6].

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196 CHAPTER 9 Maintainability and Operational Overview

where the system would need human assistance. This would give the concept the possi-
bility to utilize all four seats for passengers alone and benefit as well from advanced
safety measures. Depending on the upcoming scenarios, there could even be a similar
split between local operators and centralized facilities like flight control radio stations.
Of course, this infrastructure would also have to support single pilot operation or auton-
omous flight of larger airliners.

9.2.5 Cabin Operations
The other part of in-flight operations is performed on the other side of the cockpit door:
cabin operations. Cabin operations consist of both passenger service—a very important
brand experience touch point for airlines—and passenger safety, the more important
aspect from a regulatory standpoint.

Passenger service processes will slightly differ depending on cabin geometry and
flight profile. However, it is one of the top priorities of every airline to offer a consistent
service product. The customer should know what baseline to expect, enhanced by the
personal touch of every crew member. The current processes are updated regularly based
on experience and driven by changing customer expectations. A more electric or electric
aircraft is not likely to have a major impact in this area.

Beyond service, flying personnel are required to guarantee the safety of passengers.
An electric aircraft with a different power distribution system or even a different main
power source could impact today’s procedures. Today’s procedures require aircraft and
ground power supplies to be properly grounded in order to ensure their protection
against electrical hazards (static electricity and failures) and that of crew, passengers,
and maintenance personnel. These procedures have to be revisited with the advent of
more electric or electric aircraft due to the presence of more hazardous high voltage
within power systems and energy storage.

One of the questions currently open is how to indicate that the aircraft is electrically
grounded. This issue could impact the safety of the crew, passengers, but also first
responders in case of an emergency landing. The latter case involves potential hazards
unique to onboard electrical systems such as electrocution, fire, and battery electrolyte
spillage.

A potentially higher explosion risk of batteries could lead to a shorter
maximum evacuation time. Therefore, new ways of disembarking would have to
be developed.

Looking at urban air mobility concepts, safety in case of emergency landing could
also become a key topic. As most concepts plan for autonomously operated vehicles,
there is no trained crew to help passengers leave the vehicle in case of emergency. Digital
information technology could potentially compensate portions of these tasks, but the
physical support assisting injured or movement-restricted passengers is technically
challenging to compensate.

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CHAPTER 9 Maintainability and Operational Overview 197

References
[9.1] Sinnett, M., “787 No-Bleed Systems—Saving Fuel and Enhancing Operational

Efficiencies,” AERO Magazine Q4, 2007, published by Boeing.
[9.2] Boeing, “Airline Economics,” 2016 Airline Planning Workshop, Airports Council

International (ACI), North America, USA, 2016.
[9.3] https://www.volocopter.com/en/product/, accessed May 5, 2018.
[9.4] https://www.usnews.com/news/articles/2013/06/11/new-military-uav-may-lead-to-

commercial-drone-flights, accessed May 4, 2018.
[9.5] https://www.uasvision.com/2011/07/15/view-inside-a-predator-ground-control-

station/, accessed May 5, 2018.
[9.6] https://airbus-h.assetsadobe2.com/is/image/content/dam/corporate-topics/

publications/press-release/CityAirbus-01 ?wid=3626&fit=constrain, accessed May 4,
2018.

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https://www.volocopter.com/en/product/

https://www.usnews.com/news/articles/2013/06/11/new-military-uav-may-lead-to-commercial-drone-flights

https://airbus-h.assetsadobe2.com/is/image/content/dam/corporate-topics/publications/press-release/CityAirbus-01 ?wid=3626%26fit=constrain

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199

As discussed in Chapter 2, fuel consumption is a major cost driver in aircraft operations

In order to quantify its impact, understanding the cost structure of airlines is fundamental.

10.1 Airline Cost Structure
When airlines operate their fleet, operating costs are incurred, such as fuel, maintenance,
flight personnel, and aircraft leasing/ownership, among others. Additionally, other charges
borne on ground have to be considered, such as servicing and ticketing. Finally, airlines
have to pick up the tab for so-called system operating costs. Generally speaking, the
following breakdown gives a rough order of magnitude of the cost segments at play:

FLIGHT (DIRECT) OPERATING COSTS (DOC) = 5

• All costs related to aircraft flying operations
• Includes pilots, flight crew, fuel, maintenance, and aircraft ownership

GROUND OPERATING COSTS =

30%

• Servicing of passengers and aircraft at airport stations
• Includes aircraft landing fees and reservations/sales charges

SYSTEM OPERATING COSTS =

20%

• Marketing, administrative, and general overhead items
• Includes in-flight services and ground equipment ownership

Performance and Business
Value of Electric Aircraft
Pascal Thalin
Chair and Member – SAE Electric Aircraft Steering Group

C H A P T E R

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200 CHAPTER 10 Performance and Business Value of Electric Aircraft

For relative comparisons, Figure 10.1 shows a rough breakdown of airline direct
operating costs (DOC).

As shown, fuel consumption could account for up to one-third of typical airline
DOC. Simply put, any reduction made possible in this expense could potentially benefit
airline profitability.

Figure 10.2 portrays International Air Transport Association (IATA) data on fuel
costs over time of the airline industry. Just to give an idea, it was forecasted that
worldwide fuel expenses in 2017 would top out at above a staggering USD 131 billion.

FIGURE 10.1 Airline direct operating costs [10.1].

FIGURE 10.2 Worldwide airline industry fuel costs (USD) [10.2].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 201

The chart in Figure 10.3 shows the price variation over time of jet fuel used to operate
conventional aircraft. Actually, jet fuel price variations mirror the price variations of crude
oil price. Unfortunately, airlines have no way to influence these price fluctuations.

Now, let us take a closer look at the share of fuel costs in the total airline operating
costs. Figure 10.4 shows how fuel-related costs of the airline industry tend to vary over
time. The evolution of the ratio of fuel costs to total operating costs is also
highlighted.

As the chart shows, from 1997 to 2001, with relatively low fuel prices of around USD
0.6/gallon, the overall fuel cost on average was a mere 12% of the airline operating cost.

FIGURE 10.3 Jet fuel cost (USD/gallon) [10.3].

FIGURE 10.4 Worldwide fuel expense (USD) and ratio (%) [10.4, 10.5, 10.6].

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202 CHAPTER 10 Performance and Business Value of Electric Aircraft

But in 2008, at the height of skyrocketing fuel prices (USD 3.8/gallon), the ratio of fuel
cost to airline total operating expense topped out at 36%. Ever since, following two dips
in fuel price, the same ratio returned to 19% in 2016.

Now, with regard to DOC, Figure 10.5 also helps us understand the orders of magni-
tude at the airline level. The chart uses two industry metrics, namely, the block hour and
the Cost per Available Seat Mile (CASM). It is worthwhile defining them here. The block
hour is the industry standard measure for aircraft utilization. For a given flight, block
hours take into account the time from the moment the aircraft door closes at departure
until the moment the aircraft door opens at the arrival gate following its landing. The
CASM is a common unit of measurement used to compare the efficiency of various airlines.
It is obtained by dividing the operating costs of an airline by available seat miles (ASM).
Generally, the lower the CASM the more profitable and efficient the airline.

10.2 Aircraft Fuel Costs
Obviously, there is a direct link between the type and size of aircraft being operated and
fuel costs incurred. This also holds true for the ratio of fuel costs to DOC which include
fuel, maintenance, crew, and aircraft ownership/leasing costs.

The variation of fuel costs across different aircraft segments is illustrated in
Figure 10.6, a snapshot for the year 2013 of large certified U.S. passenger air carriers.
Costs are expressed in USD per block hour of operation.

Looking at the proportion of fuel in the aircraft DOC, it can be concluded that the
larger the aircraft size the more expensive the fuel bill and higher the ratio between fuel
costs and operating costs.

This chart shows that, in 2013, for the wide-body aircraft segment, for instance, the
fuel ratio was above half of the total costs, just when oil prices stagnated at extremely
high levels.

FIGURE 10.5 Direct operating costs across aircraft segments (U.S. carriers—2013) [10.7].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 203

10.3 Airline Fuel Efficiency
Fuel costs of an airline are of course directly linked to jet fuel price in the first place, but
there is only so much an airline can do about this. Secondly, fuel costs are tied to the
fuel efficiency inherent to the type and age of aircraft making up the fleet, besides the
actual efficiency of airline operations. Finally, fuel consumption takes a hit from airspace
and airport inefficiencies in the form of tarmac delays and holding delays for instance.
But, once again, airlines cannot control these situations.

The impact of jet fuel price could be allayed if the airline opts for shrewd hedging
strategies to minimize exposure to price fluctuations. Better operational practices could
also help reduce fuel costs further. Last but not least, fuel cost mitigation can be achieved
by introducing newer and more fuel-efficient aircraft into the fleet.

Let us see what is behind the fuel efficiency of an aircraft and how it has evolved
over time from an airline perspective. For a given type of aircraft and a set of parameters
(payload, range, speed), fuel burn is directly linked to the compounded efficiencies of
the following:

• Propulsion
• Systems
• Airframe

Figure 10.7 shows for the commercial jet aircraft segment how, notwithstanding
fuel price fluctuations, the introduction of new aircraft has helped cut back on fuel
consumption. Additional fuel efficiency improvements are expected over the short term

10%

20%

30%

40%

50%

60%

70%

80%

2000

4000

6000

8000

10000

12000

14000

16000

0 1 2 3 4 5 6

Fuel Cost (USD/BH) (LHS)

Total Cost (USD/BH) (LHS)

Fuel (% Total Cost) (RHS)

Turboprop Regional Jet Narrow Body Wide Body Wide Body
(20 – 60 seats) (> 60 seats) (> 160 seats) (< 300 seats) (> 300 seats)

FIGURE 10.6 Fuel cost variation across aircraft segments in 2013 (U.S. carriers) [10.8].

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204 CHAPTER 10 Performance and Business Value of Electric Aircraft

with the entry into service of re-engined versions of conventional aircraft (e.g., Airbus
A320neo and Boeing 737 MAX/777X).

One other advantage of aircraft fuel-efficiency gains resides in the improved range
performance, that is, the ability of the aircraft to fly farther over longer distances without
having to stop to refuel.

Figure 10.8 retraces range improvement of new aircraft over the span of the past
40 years [10.9]. Since 1988, the range has increased by 40%, while operating empty weight

FIGURE 10.7 Fuel burn for new aircraft and fuel prices [10.9].

FIGURE 10.8 Structural efficiency and range for new aircraft [10.9].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 205

per unit aircraft floor area increased modestly, about 6%. Normally, on a given mission,
the longer the aircraft range the lower the fuel efficiency because of more weight. Hence,
for the range increase over time shown in Figure 10.8, the fuel efficiency improvement,
rather than degradation, is attributable to airframe technology advances (e.g., composite)
having been able to offset weight penalties, as noted in ICAO’s 2010 fuel efficiency tech-
nology review.

All in all, fuel prices alone may not provide a consistent, long-term motivation for
fuel efficiency improvements in the aviation sector. The drive to curb carbon emissions
through the reduction of fuel consumption has become another key industry driver.
Having no handle on fuel price fluctuations, faced with highly demanding environmental
targets, global research in the aerospace industry is focusing on an array of solutions.

Simply put, decreasing fuel, maintenance, and other costs with no compromise on
performance at the very least is the minimum expectation.

Thankfully, there are solutions ranging from incremental enhancements to all-out
aircraft redesigns. An out-of-the-box approach parlaying advancements in architectures
and technologies, leveraging innovations drawn from other industries such as automo-
tive, and including cutbacks in operational costs combined with better environ-
ment-friendly performance and services could be delivered in the coming years.

The push towards sustainable aviation ushering in cleaner and quieter aircraft
technology entails drastic performance requirements that even the most advanced
technology on conventional aircraft can hardly cope with. Therefore, rethinking the
overall aircraft design and a departure from conventional architectures, technologies,
and integration is unavoidable. The move towards the electric aircraft becomes all the
more relevant in this context, given the significant added value it could bring in key
performance drivers such as fuel efficiency, carbon footprint, and noise while enhancing
overall cost effectiveness for the airlines.

In this technology race, industry players are spearheading research to get the picture
straight as to how to make the electric aircraft a reality and meet the aggressive challenges
of air transport facing the industry.

10.4 Business Aviation
Previous analyses show that major benefits could stem from the electrification of ice
protection provided weights are not altered, or (even better) reduced, at the aircraft level.
In order to see the benefits of electrification, let us take a look at how some conventional
systems perform on a business jet, and what lost performance can be retrieved through
electrification and the strings attached to it [10.10].

The conventional pneumatic ice protection system operates from hot bleed air
derived from the engines. However, this process impacts engine thrust and the aircraft
performance gets negatively affected. Figure 10.9 shows the degradation of engine thrust
with the conventional system.

Switching on bleed air decreases thrust by 5%-7% when all engines are operating, and
10%-12% in the case where one engine is inoperative. This thrust degradation not only

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206 CHAPTER 10 Performance and Business Value of Electric Aircraft

needs additional fuel but, in the latter most critical case, ends
up restricting aircraft range performance (Figure 10.10).

On a Cessna Citation Jet 2 type of business aircraft with
pneumatic ice protection, in order to comply with the
minimal climb gradient (2.5% net) of the standard instru-
ment procedure in icing conditions with one engine inop-
erative, the aircraft takeoff weight has to be reduced by
removing almost half of the mission fuel. This drastically
degrades range.

Electric ice protection could thwart this drawback and
bring real benefit by recovering range performance lost with
the conventional system (Figure 10.10). Actually, compared

to pneumatic ice protection, the electric version would produce only about one-fifth as
much decrease (2%) in thrust.

It is clear that the advantage of electric ice protection lies in lower thrust degradation
that allows climbout with more fuel on board. Therefore, in icing conditions, an electric ice
protection system can double the usable range of a business jet, compared to a bleed air system.

Let us now consider another pneumatic system, the Environmental Control System
(ECS). On business jets, compared to the baseline pneumatic version, although the electric
version draws less power from the engines, the positive impact on thrust remains only
marginal. This suggests that the greatest energy-saving opportunity of electric ECS, as
opposed to the case of ice protection, might not lie in the energy consumed by the system,
but rather in the weight reduction of downsizing or eliminating the bleed air components.
Reduction in life cycle costs is another area where electrification can add value, thanks
to reductions in part count enabling savings on both inventory and maintenance.

On top of these examples, significant performance enhancement could only stem
from system weight reductions within electrical architectures, when transitioning from
conventional non-electrical systems to their electrified versions.

FIGURE 10.10 Business jet ice protection—range impact [10.10].

FIGURE 10.9 Thrust impact of conventional ice
protection [10.10].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 207

Weight reductions on an aircraft can allow additional fuel to be carried, and therefore
extend the range capability. For instance, on the three-passenger Cessna Citation Jet 2, a
100 lb systems weight reduction can take the aircraft farther by around 60 nm.

It is also interesting to check the fuel-burn reduction that may be obtained by
parlaying system weight gains into a scaled-down (wing, tail, and engine) redesign
to obtain a more fuel-efficient aircraft. In other words, since the aircraft now has 100
lbs less payload, the takeoff thrust requirement is lower, which can translate into
smaller tail structure, etc. In this approach, the same 100 lb systems weight reduction,
for instance, could help shave 300 lbs off the max takeoff weight on a downsized
Citation Jet 2 design, therefore helping reduce the fuel load by around 1.6% on a
1700 nm flight.

But bringing down the weight of systems that undergo electrification is hardly an
easy task due to the fact that ongoing research is yet to reach the weight-reduction targets.
As explained in Chapter 2, on a more electric version of this type of aircraft, a drastic
reduction in the global engine power offtake for systems is possible, even though the
electric part is drastically increased. Without improvements in the state-of-the-art
electrical components, the overall weight of the airplane can go up, negating any benefit
brought upon by electrification. This added weight would then cause a fuel burn penalty
or decrease the aircraft’s range.

Incidentally, any system-level weight increase gets amplified at the aircraft level. In fact,
carrying additional system weight onboard comes down to additional fuel load and volume
to be carried up to the destination. This in turn calls for extra fuel, fuel reserves, and space,
equating to an overall amplified weight increase. Consequently,
the fuel burn also gets amplified in the same manner. This ampli-
fication effect is called “spiral” or “snowball effect.”

The hurdles behind weight reduction are not trivial. Let
us consider a larger, longer-range legacy business jet such as
the Falcon 2000 manufactured by Dassault Aviation [10.11].
The analysis in [10.11] uses scaled-up systems and does not
take into account weight-optimized engines and systems.
Figures 10.11 and 10.12 compare the conventional Falcon
2000 to its more-electric and all-electric versions: partial elec-
trification whereby only hydraulic systems are switched to
electrical ones and total electrification of all systems. In
Figure 10.11, it can be seen that system weight increase for the
all-electric aircraft total is north of 300 kg, which at the
aircraft level gets amplified to above a ton due to the snowball
effect (Figure 10.12).

These weight penalties hurt fuel efficiency and also the
range performance.

In the partial electrification case, even though suppressing
hydraulic systems reduces the fuel consumption needed for
system power delivery, the reaped benefit is hardly sufficient
to counter the overconsumption from weight increase with
the electrical replacement.

FIGURE 10.11 System weight [10.11].

FIGURE 10.12 Max takeoff weight [10.11].

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208 CHAPTER 10 Performance and Business Value of Electric Aircraft

The all-electric version, for a given “engine cycle,” does not bring efficiency improve-
ments and, therefore, leads to an impressive fuel burn increase compared to the conven-
tional baseline.

Even in the case of partial aircraft electrification (hydraulic systems going electrical),
capable of better fuel efficiency and cost of ownership with only a minor range penalty,
fuel savings get drowned by the fuel penalty of additional system weight.

In summary, based on the above analysis of the Falcon 2000, fuel penalties stemming
from weight increases during electrification may get exacerbated by the snowball effect, and
end up compromising the expected performance and cost benefits of electrification.

This view shows that the stakes are high in the research of power-dense and fuel-
efficient solutions when transitioning from legacy aircraft to their electrified versions,
whether incrementally or through a complete redesign. The power-to-weight ratio has
to be drastically enhanced in order to make the electric business aircraft competitive.

When sticking to conventionally designed fixed-wing aircraft and turbofan engines, the
way forward proposed by several business jet manufacturers is to develop newer and more
efficient engines and systems and implement well-thought-out integration strategies.

For instance, engine integration of upsized electric power offtake could be dealt
with either by embedding power generators into the engine core or, for that matter, by
encapsulating electric power generators into the gearboxes that drive them. Another
approach could involve sharing oil cooling functions between the engine and the gener-
ator (s) installed on them.

One other strategy could consist of seeking alternatives to the conventional sourcing
of electric power from the engines. To this end, resorting to fuel-efficient and power-
dense sources available on board may alleviate the burden on the engines while bringing
economies of scale in fuel consumption and helping resolve the weight penalty issue.
When retooled to that purpose, complementary sources such as the Auxiliary Power
Unit (APU) could be up to the job and allow the aircraft to benefit from better fuel
efficiencies and power-to-weight ratios.

With regard to the electrical network, 28 VDC or 115 VAC electrical networks are
found on many legacy business jets for the power supply of systems. If intensive system
electrification is carried out, legacy networks ought to be transformed into high-voltage
networks. In fact, with high-voltage DC, the levels of current carried by cables and used
by various equipment are lower; thereby weight reduction is possible through the down-
sizing of cabling onboard. Moreover, the fact that high voltage is centrally created and
distributed to the end systems downstream relieves the burden on these systems to
locally create their own HVDC bus. Lastly, moving up in speed, the power generation
for a HVDC network is more weight-optimized compared to legacy networks.

As recent research on business aircraft suggests [10.12], the legacy electrical network
can be switched to a lighter 270 DC network thanks to high-speed starter-generators and
power-dense power conversion equipment. From thereon, the electrification of pneumatic
systems helps garner estimated weight savings of more than 100 kg. As explained previ-
ously, this could be translated into either range enhancement or fuel cost savings.

As developed in Chapter 2, let us bear in mind that on an electric aircraft the power
electronics needed for the power conversion and motor control constitute a large portion
of the overall weight. In the process of electrification, there is a tendency to dedicate one

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CHAPTER 10 Performance and Business Value of Electric Aircraft 209

motor controller to each system. Unfortunately, this worsens the dead weight being carried
by the aircraft because of the dead time during which controllers do not have to operate.

Logically, multi-purpose motor controllers could be the panacea for reducing dead
time. During flight, such a controller can be switched from one system to another when
these systems are operated in a sequential manner. But their coverage usually is limited
to two dedicated applications.

Taking this approach one step further, dead time and dead weight can be drastically
reduced by using standardized power electronic modules addressing multiple systems,
as explained in Chapter 2. This modular paradigm, on top of the versatility it brings,
helps eliminate unnecessary margins that have to be built into dedicated individual or
multi-purpose motor controllers.

10.5 Short-Range Aircraft
Similar to the analysis conducted on the Falcon 2000 business jet [10.11] and developed
in the previous paragraph, let us now consider another study [10.13] performed on a
short-range aircraft.

The benchmark aircraft for this comparative study is a conventional 165-passenger,
short-range (3500 nm) twin jet, with a baseline A320 design but including technologies
derived from programs not too far in the past like A318, A380, and A350. In this aircraft,
pneumatic systems are a mainstay, whereas hydraulic systems are limitedly used.

The study compares the benchmark to a more-electric version based on the removal
of pneumatic systems, and corresponding engine offtakes while sticking to reduced
usage of hydraulic power. The turbofan engine technology and performance are kept
unchanged between the two versions. The benefits of “more-electric” systems have also
been assessed at the aircraft level from the perspectives of maintenance and environ-
mental impact based on fuel consumption.

The weight breakdown of systems analyzed on the benchmark aircraft is reflected
in Figure 10.13. This can be compared to the weight picture shown in Figure 10.14 for

FIGURE 10.13 Systems weight (benchmark) [10.13]. FIGURE 10.14 Systems weight (more-electric) [10.13].

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210 CHAPTER 10 Performance and Business Value of Electric Aircraft

the more-electric version. In short, apart from the bleed air part of the pneumatic systems,
a general weight increase is noticed on the more-electric aircraft.

The weight assessment of systems shows that the more-electric short-range aircraft
is heavier than the reference short-range aircraft. As shown in Figure 10.15, when shifting
to the more-electric aircraft by way of system electrification, the weight reduction
benefits, reaped thanks to several systems, are “outweighed” by the weight penalties
encountered in the remaining systems. As illustrated in Figure 10.15, major penalties
trouncing weight benefits are attributable to the following consequences of electrification,
their contributors, and key impacting factors:

• Scale-up of electrical power needs on board: electrical power generation/
distribution technology and power density

• Replacement of conventional systems with electrical versions: ice protection and
ECS technology and power density as well as additional cooling technologies and
their power density

• Integrating electrified systems: weight- and volume-related additional impacts on
airframe linked to the above contributors

Let us focus on how electrification may impact aircraft performance from a drag
perspective. On the reference aircraft, fresh air supply needed for the operation of the
ECS is ensured by the engines thanks to the engine bleed system. On the contrary, the
same engines when fitted to the more-electric aircraft cannot deliver the air required by
the ECS because the bleed system is removed in the electrification process. Therefore,
alternative air supply has to be sourced from the outside of the aircraft, requiring the
implementation of the following design features:

• Cabin air re-circulation: supply of fresh cabin air from outside the aircraft using scoops
• Electrical ECS and cooling system: ram air from outside the aircraft

FIGURE 10.15 Aircraft weight (more-electric vs. conventional) [10.13].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 211

As a result, compared to the reference aircraft, these additional implementations
on the more-electric aircraft wind up creating a drag penalty. Nevertheless, research of
possible mitigation means could help alleviate this drawback.

Fuel burn comparisons in [10.13] are based on a 500 nm mission. Firstly, the
more-electric aircraft has reduced engine-specific fuel consumption compared to the
reference aircraft. This stems from engine efficiency gains brought by the removal of bleed
components.

Based on this and after further computation, notwithstanding the weight disad-
vantage of the more electric aircraft, the study in [10.13] arrives to the conclusion that
there is quite no difference in fuel burn between the short-range reference aircraft and
its more-electric version.

In addition to the fuel burn assessment, the study evaluates the Direct Maintenance
Cost (DMC) for a system perimeter limited to the APU, electric, bleed, ECS, and cooling
systems.

The result shows a slight decrease in DMC in favor of the short-range more-electric
aircraft.

Ever since the study was performed, leveraging latest breakthroughs, the aerospace
industry has developed and tested a full suite of technologies that could be readily inte-
grated on a more-electric aircraft, provided the required maturity levels are reached. Even
though system weight and integration remain an outstanding challenge, conceptual design
studies conclude that the more-electric aircraft may readily deliver airplane benefits in
terms of maintenance, operational flexibility, and technology growth potential without,
at the very least, any fuel burn penalty. Better, system simplification and weight savings
could lead to potential fuel efficiency enhancement. Furthermore, as more-electric tech-
nologies are friendlier to the environment than conventional solutions, they appear as
key enablers for sustainable growth of the aerospace industry.

But on the flip side, due to its smaller size, the weight and integration stakes facing
the short-range more-electric aircraft are far more challenging compared to a long-range
more-electric aircraft.

In summary, compared to its more-electric version, the conventional metallic narrow
body has a head start from a weight standpoint, whereas both versions are on a par when
it comes to fuel efficiency. Therefore, weight reductions on the more-electric version could
make it more competitive by reducing fuel costs. Hence the deep focus in ongoing research
on power-dense solutions. Yet, the situation is not that negative. Global research being
carried out targets not only the alleviation of system weight penalties found in initial
studies, but also addresses weight cutbacks with respect to conventional systems.

To make the case for the more-electric aircraft, one may think of resorting to a
lighter composite airframe. But, keep in mind that weighty corrections would have to
be implemented in order to render such an airframe compatible with the consequences
of system electrification, such as incorporating missing features like noise barrier and
lightning protection that come free with a metallic airframe. Once again, the size of the
aircraft influences the level of optimization that composites could offer.

While the industry supplier base is scrambling to get power-dense more-electric
system options on the table, aircraft manufacturers took the approach of investigating
the potential of quick-win incremental changes on existing aircraft. In order to deliver

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212 CHAPTER 10 Performance and Business Value of Electric Aircraft

step reductions in fuel burn, they continued to work with system suppliers, and also
turned to engine manufacturers and put the onus on them as well.

Actually, while the industry waited for technology promises to spring up on the
systems side, engine manufacturers invested heavily in research to improve drastically
the fuel efficiency of turbofan engines. Ultimately, engine manufacturers ended up
offering fuel-efficient turbofan engine alternatives very close in form, fit, and function
to legacy ones. On a secondary level, rework of some systems and structural areas allowed
extra efficiency gains.

The re-engining approach also became compelling in the face of sky-high fuel prices.
Aircraft manufacturers heeded market calls and resorted to incremental re-engining
developments on existing platforms, allowing lower risks and costs and shorter time to
market compared to a full-blown aircraft development. The re-engining solution for
legacy short-range aircraft platforms, despite a higher price tag, was readily endorsed
by airlines under pressure from hurting fuel prices. Moreover, in such an approach,
airlines continue to benefit from proven maturity in service for all the other parts of the
re-engined aircraft except, of course, the engines themselves.

In such newly developed turbofan engines, still comprising the bleed system, tremen-
dous improvement in fuel efficiency has been possible thanks to leapfrog technologies
and materials in the engine redesign, based on following two competing architectures
with comparable performance:

• Traditional turbofan architecture on the LEAP engine from CFM International
• Disruptive turbofan called “Geared Turbofan (GTF)” on the PurePower® engine

from Pratt & Whitney

Both developments have materialized into ready-for-service engines that can
be purchased by airlines as replacements for engines hitherto offered by aircraft
manufacturers. They offer drastic fuel-burn reductions at aircraft level. Figure 10.16 shows

FIGURE 10.16 Aircraft fuel burn (re-engined vs. conventional) [10.14].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 213

how, compared to the original aircraft, on the re-engined version the various fuel efficiency
gains stack up.

All in all, simply re-engining the short-range aircraft brings a steep 15% reduction
in aircraft fuel burn.

Helped by the technology overhaul that conventional turbofan engines have gone
through in the re-engining strategy, fuel efficiency wise the situation tips in favor of the
re-engined version of the conventional short-range aircraft. Logically, this puts into
question the performance assumptions, mainly fuel efficiency, behind the reference
aircraft used for benchmarking its more-electric version.

Nothing to do with a more electric aircraft, the advent of re-engining options has all but
challenged the development of narrow-body more-electric aircraft by raising the bar higher
with regard to the reference aircraft against which the more-electric version has to compete.

Factoring in the newly available engine performance into the conventional aircraft
architecture serving as the benchmark widens the fuel burn gap that the more-electric
aircraft would have to offset first, and prior to offering additional benefits in its own right.

Therefore, making the case for the more-electric short-range aircraft gets trickier
from a timeline and technology perspective. This is the main reason why decisions on
the more-electric versions of the short-range aircraft segment are ever more tied to
power-to-weight and efficiency improvements that aircraft systems research can deliver.

In the meantime, out-of-the-box approaches have gone full swing in the research
of alternatives to both more-electric and re-engined versions of the conventional short-
range aircraft. When observing the trends behind, both power-dense electrified systems
and electric propulsion, whether hybrid-electric or totally electric, appear to be mainstays
in the next paradigm shift in electric aircraft design.

As discussed previously, hybrid-electric propulsion will require onboard recharge-
able energy storage devices such as battery packs. Global leapfrog research advances are
speeding up large-scale shrinking of batteries while making them as energy dense as
called for by the electric aircraft design. In the meantime, sweet spots are under inves-
tigation using downsized conventional turbine engines, matched and operated hand in
hand with a certain degree of electric propulsion. This gives birth to the concept of
hybrid-electric propulsion, wherein a parallel may be drawn with HEV powertrains
already commonplace in the automotive industry.

Isikveren [10.15] provides performance comparison
between a future medium-range 180-passenger reference
aircraft with advanced electrical systems and its hybrid-electric
version using 1500 Wh/kg batteries. This means a tenfold
increase in specific energy, compared to the most recent battery
technology available on the Tesla electric car. Results obtained
for a range of 1100 nm show that hybrid-electric propulsion,
utilizing electric energy during 50% of cruise, could theoretically
wind up delivering a 20% reduction in fuel burn compared to
the reference aircraft fitted with turbine engines.

Hornung [10.16] analyzes hybrid-electric propulsion using
2000 Wh/kg batteries on a reference 189-passenger short-range
aircraft with a self-trimming nonplanar C-wing (Figure 10.17).

FIGURE 10.17 C-wing hybrid-electric aircraft [10.16].

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214 CHAPTER 10 Performance and Business Value of Electric Aircraft

Moreover, aircraft systems are also fully electric and powered solely by batteries.
Batteries, lodged in cargo containers, would not require recharge during turnaround
because used battery containers would simply get swapped with pre-charged ones during
turnaround. The electric propulsion system comprises large high-temperature super-
conducting electric motors with an integrated cryocooler and high-voltage power
electronics dedicated to motor control. On the systems side, high-voltage DC is the
adopted standard for the electrical network supplying loads via power electronics or
power supplies.

For a range of 900 nm, Figure 10.18 shows the performance of such an aircraft
depending on the degree (or ratio) of propulsion electrification. A ratio of “1” stands for
fully electric propulsion, a ratio of “0” applies to the most recent bleedless turbofans;
any value in between pertains to hybrid-electric operations using both fuel and battery
energy.

Because no jet fuel is used by the fully electric aircraft, its operation entails zero
emissions. When the electrification ratio is set to 50%, efficiency gains are around 30%,
even though aircraft weight is increased by around 25%. This weight increase is mainly
due to the following factors:

• Battery system, despite a challenging specific energy assumption of 2000 Wh/kg.
• Wing redesign to keep wing loading constant. In fact, wing structural weight

increases when compensating for lower wing bending moment relief from lower
quantity of fuel onboard.

• Lower amount of fuel mass burn-off during mission.

Yet, still sticking to conventional “wing-and-tube” aircraft shapes, another study
in [10.17] shows that a tri-fan morphology, comprising two under-wing podded gas-
turbines (GT) and one aft-fuselage mounted serially configured motor (M) driven by

FIGURE 10.18 Hybrid-electric aircraft performance vs. degree of electrification [10.16].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 215

batteries, could be an appropriate choice for a 180-passenger
hybrid-electric aircraft (Figure 10.19).

The study concludes that, even though resorting to a
high-temperature superconducting motor could deliver 2%-4%
reduction in aircraft weights, a normal conducting motor is
deemed to be a pragmatic choice. In the latter case, the tri-fan
morphology motor would have to deliver 8.5 MW of shaft
power for a 180-passenger aircraft. The reference aircraft used
for performance comparisons is a year 2000 A320-200 aircraft
with evolutionary technologies: advanced ultra-high bypass
(~20) geared turbofan, all-electric systems, high wing aspect
ratio (~12%), reduced zero-lift drag, and advanced structural
materials. It achieves a 39% reduction in block fuel, compared
to the A320-200, demonstrating the high level of optimization
taken into account for the reference.

Over a range of 1100 nm, Figure 10.20 shows that in order for the hybrid-electric
aircraft to achieve 15% block fuel reduction compared to the reference aircraft, batteries
with 940 Wh/kg specific energy would be necessary. Likewise, a 20% block fuel reduction
would be possible over ranges of 900, 1100, and 1300 nm, with a battery specific energy
of 920, 1100, and 1290 Wh/kg, respectively.

Let us consider the operating costs, excluding certain costs linked to cost of ownership
(depreciation, interests, and insurance) and including additional noise and emissions-
related charges. From the perspective of operating costs, the hybrid-electric aircraft is ~10%
more expensive than the reference aircraft. Concerning energy costs, that is, fuel and
electricity, they go up by ~6%. These figures were estimated using a price of USD 3.30/g

FIGURE 10.19 “Wing-and-tube” hybrid-electric
aircraft [10.17].

FIGURE 10.20 Hybrid-electric aircraft fuel savings vs. range and battery specific
energy [10.17].

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216 CHAPTER 10 Performance and Business Value of Electric Aircraft

for kerosene and USD 0.1109/kWh for electricity required for battery charging. Fuel price
fluctuations considered in the sensitivity analysis show that with USD 2/g, the operating
costs go up by 4-5%, compared to the reference architecture, whereas the same costs are
cut back by 5%-7% with USD 6/g. Given these cost variations, achieving a cost-neutral
situation, thanks to further optimizations, is deemed realistic. For a 1100 nm mission,
using 940 Wh/kg batteries for hybrid cruise, the operating cost breakdown for both the
reference and the hybrid-electric aircraft is shown in Figure 10.21 [10.17].

Now, building from a baseline Boeing 737 type of aircraft fitted with CFM56 engines
(not the LEAP-1B ones), the study in [10.18] has devised a reference aircraft design called
SUGAR High operating solely from fuel. From a structural standpoint, the reference
design incorporates various improvements based on high-span truss-braced tube and
wing morphology allowing high L/D ratio. The engines of the reference aircraft incorporate
engine technology enhancements, spanning from the baseline CFM56 to the latest avail-
able engine options for the short-range aircraft segment.

The study goes further by checking the potential of
hybrid-electric propulsion combined with 750 Wh/kg batteries
on the SUGAR High reference aircraft leading to its hybrid-elec-
tric version called SUGAR Volt (Figure 10.22).

Results obtained in [10.18] for a 900 nm mission demon-
strate in the first place that the future reference aircraft would
per se deliver block fuel reduction of up to 54% compared to
the baseline 737 (Figure 10.23). Secondly, the hybrid-electric
variant brings a fuel-burn reduction of 14% compared to the
reference which in reality amounts to a 60% reduction when
compared to the conventional short range with baseline turbofan
engines (not including the most recent advances available on
the 737 MAX or the A320neo). These conclusions are illustrated
in Figure 10.23 showing comparisons with respect to the baseline
737 of the future all-fuel reference (SUGAR High) on the one
hand and, on the other, its hybrid-electric variants. The latter
concerns two different electric motor sizings (1380 and 1750 hp),

FIGURE 10.21 Trip operating cost breakdown—reference vs. hybrid-electric
aircraft [10.17].

FIGURE 10.22 High-span truss-braced tube
and wing hybrid-electric aircraft [10.19].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 217

both being utilized in balanced hybrid operation throughout
the duration of the mission.

For hybrid designs, it is important to look at reductions in
fuel, but also reductions in total energy use in order to take into
account energy expended in charging the batteries if they are
charged on ground prior to flight. In this way, and by examining
the costs of each type of energy source, the airliners can make
informed choices. For emissions benefits, one must look at the
fuel burned while flying and also go deeper and look at any
carbon expended during the manufacturing of batteries and
production of the electricity used to charge them.

10.6 Long-Range Aircraft
As discussed, re-engined versions of short range are already
in service. With regard to legacy long-range aircraft,
re- engining with minor structural and system modifications
is also in the making.

But, compared to what happened with the electrification
efforts on short-range aircraft, the story has been quite different
on the long-range aircraft. In fact, prior to the race towards power-
dense systems solutions and the re-engining strategy, cleansheet
more-electric versions have been successfully brought to market
through the Boeing 787 and Airbus A350 long-range aircraft developments. These programs
also benefited from more efficient engine developments and, in the particular case of the Boeing
787, were afforded a bleedless version to match the large scale of system electrification.

On the Boeing 787, the bleedless system architecture brings fuel burn reduction in the
order of 3%. The bleedless engines allow for circa 15% reduction in specific fuel consumption.
Structural changes bring their share of efficiency gains thanks to decreased weight and lower
maintenance. In fact, massive usage of lighter composite structures helps leverage these benefits
at the aircraft level, and also compensate weight penalties caused by system electrification.

In essence, economies of scale in the operation of the more-electric Boeing 787
could only be delivered through the balanced combination of system electrification,
bleedless engines, and composite airframe structures.

When targeting better operational efficiency with the more-electric design, there
are differences in the relative constraints of the short-range and long-range aircraft, the
latter being largely helped by its bigger size, longer range, and larger engines:

• Larger size facilitates the integration and thermal management of large-scale
electrification.

• Longer flight cycle durations allow better fuel optimization.
• Larger engines are more fuel efficient than smaller ones.

Once again, without resorting to composite materials on the structural side, these
benefits may not have materialized.

FIGURE 10.23 Hybrid-electric aircraft performance
vs. reference and baseline 737 aircraft [10.18].

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218 CHAPTER 10 Performance and Business Value of Electric Aircraft

In summary, with a long-range more-electric composite aircraft with bleedless
systems architecture and turbofan engines, such as the Boeing 787, airlines may expect
fuel burn reduction in the order of 15%-20%, depending on route distances. Lower costs
for base checks, thanks to the composite structure and its monitoring, among other
factors, allow for at least 15% reduction in maintenance costs. All this contributes to the
Boeing 787’s ~20% operating cost advantage over similar legacy aircraft.

With this new type of more-electric long-range aircraft, airlines expect to cash in
on the opportunity to reduce the fuel and maintenance portions of the operating costs.
But, for a given trip, the total trip cost has to take into account aircraft financing charges
as well. However, airlines often still find that aircraft financing charges could make their
total unit costs higher than the older aircraft they are replacing. Thanks to carbon fiber
technology, composite aircraft life is expected to double, and so its financing terms and
lease rate may be lower than for conventional aircraft. Luckily, newly purchased aircraft
enjoy maintenance cost write-offs during the initial years of operation thanks to manu-
facturer warranties helping bring down operational costs. All in all, when using
more-electric aircraft to replace older conventional aircraft, financing cost burden could
be allayed by large-enough reductions in cash operating costs [10.20].

The cost performance estimations presented in Figure 10.24 corroborate the
competitive edge of the more-electric Boeing 787 over other legacy Boeing aircraft in
the long-range segment. The comparison is based on a 4000 nm mission, like on a trans-
atlantic Paris-New York trip, for a fuel cost of USD 2/gallon.

From an operational standpoint, this analysis of Boeing aircraft shows that the baseline
more-electric B787-8 stacks up against the conventional B767-200 with a steep 24% cut in
the unit cost per available seat-mile (CASM) for a barely higher trip cost (+3.3%). Similarly,
when comparing the larger B787-9 to the legacy B767-400ER, the CASM plummets by
17% along with a 4% dip in the trip cost. Incidentally, with the B787, airlines not only
benefit from the operating savings but could potentially offer new routes where the B767
doesn’t go and where the B777 is deemed too big. Cases in point for such ultra-long-haul

FIGURE 10.24 Trip cost and CASM (more-electric vs. conventional) [10.21].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 219

operations are longer transatlantic (e.g., London-San Francisco or Europe–South America),
trans-Pacific or Europe–Asia Pacific flights with an average range of 5500 nm.

Therefore, it is obvious that the long-range more-electric aircraft sets a new standard
in operating economics by providing airlines with an aircraft that has a significantly
improved CASM compared to their current fleet. In addition, thanks to its ultra-long-
range capability, it provides the bonus of an opportunity to transform their long-haul
networks.

The more-electric Boeing 787-8 has been in service since 2011. Airline operational
data for the year ending in 2013 [10.7] shows that the DOC per seat per mile for this
aircraft is ¢7.2. This is almost 8% below the average ¢7.8/unit cost registered for the
overall long-range aircraft segment.

Revamped system architectures, more efficient engines relieved of system constraints
(bleedless), and composite airframes have been the most recent key enablers in the
design of more-electric aircraft all the way up to entry into service in the long-range
market. Depending on the aircraft segment, when shifting conventional systems to the
electric domain, performance could either be leveraged as shown above or get under-
mined by weight issues stemming from electrification in the absence of design solutions
alleviating them.

Economies of scale possible on a large aircraft may get watered down when the
aircraft size goes down. The more-electric short-range or business aircraft requires ever
more stringent optimizations and ups the ante when it comes to integration. While
power-dense system research is in full swing to meet these expectations, engine manu-
facturers have been able to drastically improve turbine engine performance. This allows
existing aircraft to be re-engined at reduced development costs within a shorter time-
to-market, thereby undercutting fuel cost impacts on operational economics.

Nevertheless, the re-engining wild card may have reached its limits. Any further
attempt to sizably enhance fuel efficiency of turbine engines might prove unsuccessful,
for all their potential and margins may already have been squeezed out.

Therefore, aircraft and engine manufacturers may have run out of viable incremental
optimization opportunities on existing aircraft platforms. Even though operational cost
and carbon footprint reductions do make the case for more efficient conventional aircraft
and engines, they still come with noise and greenhouse gas emissions. Industry consensus
for business growth, competitiveness, and environmental friendliness is grounded in
the prospects of radically different aircraft designs resorting to alternatives to jet fuel.
This longer-term approach simultaneously targets steep cutbacks in operational costs,
driven not only by fuel efficiency but also by the cost of ownership, and environmental
impacts of aircraft design and operations.

Taking stock of the performance ceiling reached with turbine engines using jet fuel,
research is doubling down on the electric propulsion paradigm, breaking new ground
in energy efficiency and emissions by getting rid of the reliance on fuel altogether. This
opens new avenues for innovative configurations where energy is transmitted advanta-
geously around the aircraft to achieve aerodynamic advantages, and reduce cost of
operations and opens up avenues for using other sources of ground power.

This entails the replacement of fuel by an alternative electric energy source or storage
carried on board. When the replacement is carried out only partially, we end up having

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220 CHAPTER 10 Performance and Business Value of Electric Aircraft

a hybrid-electric aircraft powered by hybrid-electric engines running on both fuel and
electricity. A direct parallel can be made with the hybrid-electric vehicle (HEV) revo-
lutionizing the automotive industry. In this case, the original fuel load gets split into
two parts, a downsized fuel part and another part made up of batteries. The hybrid-elec-
tric engine can be schematized as a downsized turbine engine working in tandem with
a highly efficient electric motor. Similar to the more-electric aircraft, the electric and
hybrid-electric aircraft systems come in their electrified versions.

It is important to note that though electric motors are more efficient than turbofans,
the aircraft system also has to take into account how the energy sources are stored on
board: jet fuel versus battery cells, typically. Similar as well to the power-to-weight ratio
in systems electrification, gravimetric energy density (Wh/kg) and volumetric energy
density (Wh/L), commonly named “mass specific energy” and “volume specific energy,”
respectively, are key characteristics of energy storage on electric or hybrid-electric aircraft.
Unfortunately, these parameters for even today’s most advanced battery storage systems
fall well short of those obtained with kerosene: by a factor of 18 for the volume ratio and
a factor of 60 for the weight ratio. This core problem tends to restrict the extent to which
batteries may be utilized for powering systems and engines.

Space and volume issues previously raised on smaller-sized aircraft may come in
the way of battery implementation. Nevertheless, the lower volumetric ratio may be less
critical as long as the aircraft is not limited in space available for integration. Otherwise,
the aircraft would require larger wings, fuselage, or additional external “energy pods”
which would lead to losses in overall aircraft efficiency due to larger wetted surface. Ideas
such as tightly integrating batteries and other storage systems with structural elements,
like in the Tesla electric vehicle, are also coming through in the aerospace sector.

In summary, electric propulsion could help ratchet up overall aircraft performance,
provided the specific energy of batteries replacing fuel is up to the challenge. Therefore, at
the aircraft level, efficiency of turbine engines using jet fuel, though with highly competitive
specific energy, may be outstripped by novel hybrid-electric or electric propulsion architec-
tures powered by a new generation of batteries with dramatically increased specific energy.

10.7 Regional Aircraft
Regional aircraft are powered by either turbofan or more fuel-efficient turbo-propeller
engines. Let us see how battery supplied electric propulsion stacks up against the latter
type of fuel-dependent turbine engine.

The Dornier 328, a regional aircraft fitted with turboprop engines, can transport
32 passengers over a range of 1200 km. On such an aircraft, if fuel is simply replaced by
baseline battery technology (180 Wh/kg), and engines are switched to electric ones,
range capability would plummet to 202 km [10.22] for the same payload. But, if the
mass-specific energy of batteries were to be pushed higher to, for instance, 720 kW/h,
range performance would get extended to 800 km, but still fall short of the conventional
aircraft capability of 1200 km.

Figure 10.25 portrays how, depending on the specific energy of batteries, payload
variations impact range performance for both the conventional aircraft (Dornier 328 TP)

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CHAPTER 10 Performance and Business Value of Electric Aircraft 221

and its electrical version (Dornier 328 E) with drop-in battery and electric propulsion
replacement of fuel and turboprop engines.

Due to the comparatively higher specific energy of kerosene, from Figure 10.25, it
can be concluded that when it comes down to range performance, trading payload for
fuel is much more beneficial than trading payload for batteries, the payload-range
gradient depending on the specific energy of batteries.

Thereupon, without taking advantage from worthwhile structural modifications,
the battery-powered aircraft might seem less flexible. Nevertheless, payload-range capa-
bility equivalent to that of the baseline aircraft is reached when batteries with specific
energy exceeding about 1500 Wh/kg are used.

This puts the bar very high for battery performance when resorting to electric
propulsion on large aircraft. In order to power large aircraft, a dramatic improvement
in battery technology would be required. Hence, the success of scaled-up electric aircraft
hinges on lofty levels of specific energy for battery technology that ongoing research is
yet to deliver. To attract commercial interest for larger (regional) aircraft, the specific
energy of today’s battery technology, in the range of 150-200 Wh/kg, would have to
be increased by a factor of around ten.

Nevertheless, practically speaking, the specific energy of current battery technology
is such that airplanes powered by electric propulsion are today limited in size to small
planes carrying up to two passengers over rather short ranges and limited endurance.
Neglecting costs, the current technology is suitable for small ultra-light aircraft, but not
yet for commercial aviation.

10.8 General Aviation
As with previously discussed aircraft segments, the drive for electrification in general
aviation is once again motivated by technical and cost performance enhancements.
Reducing carbon emissions through transformative aircraft and airspace operations is
among the industry goals, together with the alleviation of specific general aviation’s

FIGURE 10.25 Regional aircraft performance (electric vs. conventional) [10.22].

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222 CHAPTER 10 Performance and Business Value of Electric Aircraft

concerns like the far higher accident rate compared to other aircraft segments and road
transportation.

Electric propulsion offers a threefold improvement in efficiency compared to turbine
engines, whereas the leap in energy efficiency can top out at 4% compared to piston
engines commonly used in general aviation. Incidentally, high efficiency of electric
propulsion is achieved across more than half of the operating speed range. Additionally,
with electric motors being six times more power-dense compared to piston engines,
electric propulsion could deliver power-to-weight ratings drastically improved by more
than 500%. All the above add up to lower energy consumed and incurred costs. Electric
propulsion in general aviation, thanks to the lower passenger count, size, and range,
may help cut costs by up to 10%.

Electric propulsion, be it hybrid-electric or totally electric, offers lower community
noise. Since recourse to air breathing is either alleviated or totally suppressed, greenhouse
gas emissions are either cut back or, better, reduced to zero. Moreover, operational pitfalls
of general aviation such as power lapses with altitude or hot weather conditions, directly
related to the reliance on air-breathing, may also be circumvented by electric propulsion.

Fewer moving parts with electric propulsion wind up offering more reliable designs.
Also, the inherent integration benefits of electric propulsion allow compact aircraft
sizing applicable to all aircraft segments, including the small scales in play in general
aviation. On top of this, when Distributed Electric Propulsion (DEP) is implemented,
the additional integration benefits enable closely coupled synergies across aerodynamics,
propulsion, control, acoustics, and structures.

Conventional general aviation aircraft are only aerodynamically efficient at low speeds
in cruise due to the wing oversizing necessary to meet constraints related to stall conditions
and airfield lengths. This unfortunately compromises the lift-to-drag ratio. When
converting to an electric aircraft, wing downsizing in conjunction with distributed-electric
propulsion offers better wing loading, more resilient aerodynamics, lower drag, and higher
lift; therefore allowing higher speeds during cruise (Figure 10.26) [10.23].

In summary, to the extent that required battery specific energy and cost reductions
are made viable, with the help of bespoke certification and standardization procedures
yet to come, electric aircraft in general aviation may become a reality in the near future.
Ongoing research targets both Conventional TakeOff and Landing (CTOL) and Vertical
Takeoff and Landing (VTOL) types of aircraft, carrying 4-9 passengers and one or more
passengers, respectively, with tentative entry into service slated for 2025.

FIGURE 10.26 Conventional GA aircraft redesigned into an electric aircraft with DEP [10.23].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 223

Although aircraft range performance is independent of speed, up to now many
general aviation flight demonstrators have concentrated on low speeds only. Nevertheless,
research focus is now being devoted to the proof of concept of higher speed electric
aircraft, paving the way for upward scalability in electric aircraft design. Therefore, the
general aviation electric aircraft is poised to become the stepping stone for scaled-up
aircraft designs, thereby allowing the advent of larger electric-aircraft platforms in the
future (Figure 10.27) [10.23].

A tentative entry into service timeline of CTOL electric aircraft could be drawn up
as follows:

2020: General aviation
2025: Commuters (~9 passengers)
2030: Regional airliners
2035: Large aviation
Distributed electric propulsion when applied to vertical flight (VTOL) could

dramatically improve key performance characteristics, thereby allowing attractive
value propositions (Figure 10.28) for mobility solutions including future autonomous
air travel.

Just like with CTOL, the VTOL aircraft incurs low marginal costs of operation by
cutting energy costs by a factor of more than ten, and alleviating maintenance costs by
more than half. The increase in speed by more than half allows quicker travel times and
high productivity at fleet level, thanks to high-utilization rates (>1500 h/year).

FIGURE 10.27 Upward scalability of CTOL electric aircraft design [10.23].

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224 CHAPTER 10 Performance and Business Value of Electric Aircraft

The expected total vehicle cost per mile for distributed electric propulsion applied
to vertical flight is compared across various on-demand transportation choices in
Figure 10.29.

Enabled by its quieter operation, the electric VTOL aircraft may therefore open
new markets for more competitive on-demand urban air travel, compared to current
ground transportation services offered by networks such as Uber or Lyft. Nevertheless,
such an air travel solution cannot become a reality without bespoke air traffic control
and infrastructure yet to come. Provided adequate air traffic management is in place
ensuring safe, seamless, fluid, and quick turnarounds, VTOL urban air travel may

FIGURE 10.28 Distributed electric propulsion VTOL vs. conventional aircraft [10.23].

FIGURE 10.29 Total vehicle operating cost per mile vs. cruise speed [10.23].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 225

offer quicker and hassle-free journeys by circumventing road congestions, totally
upending travel experience with zero emissions over the city.

10.9 Cost of Ownership
On top of trip cost and CASM comparisons, airlines use the metric of cost of ownership
to evaluate the economics of newly marketed aircraft. This parameter allows the bench-
marking of all costs incurred by this or that aircraft an airline intends to purchase. This
information can be fed into the value analyses it usually performs in parallel. The partic-
ularity of cost of ownership, also going by the name of “Life Cycle Cost (LCC),” as
opposed to trip cost or CASM, resides in the fact that it is estimated not just at a given
point of time but over the lifetime of an aircraft. It is based on cost projections starting
from entry into service until the disposal of the aircraft.

Obviously, the electric aircraft, which puts together more expensive newly developed
technologies and materials, comes with a higher price tag when it enters the market.
This is similar, absent the government incentives attached, to what is observed in the
automotive sector.

As discussed previously, maintenance cost savings can potentially contribute to an
overall reduction in cost of ownership. The more the aircraft gets electrified the more
its maintenance costs could be cut back, especially when health monitoring is imple-
mented on board. Nevertheless, prior to the fruition of the benefits of the latter down
the line, a certain amount of time lag is induced by factors such as an initial learning
period, recoup time of health monitoring overheads, etc. As presented in [10.11],
Figure 10.30 compares the health monitoring and aircraft downtime costs of a
conventional, more-electric, and an all-electric business jet, whereas Figure 10.31
displays the relative break even times of the cost of ownership index.

FIGURE 10.30 Health Monitoring (HM) and downtime cost comparison
(business jet) [10.11].

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226 CHAPTER 10 Performance and Business Value of Electric Aircraft

10.10 Environmental Footprint
From an environmental perspective, a lot of progress has been made thanks to the
more-electric aircraft in service, such as the Boeing 787.

The Boeing 787 bleedless systems, more fuel-efficient and easier to maintain, are
also good news for passengers. Since bleed air, used in the air-conditioning of conven-
tional aircraft, is a byproduct of the engines, it might still contain traces of combustion
products which at very, very low concentrations don’t pose a health risk. On the contrary,
thanks to its electrified air-conditioning system without reliance on engine bleed air,
the Boeing 787 uses cleaner outside air that has never been in contact with the engines.

Noise is also a major concern, whether perceived by passengers seated inside the
aircraft or by communities along the flight routes.

Passenger aircraft noise is hardly attributable to engines only, for it is also generated
by the aerodynamics of some structural and system components. During approach
conditions, with landing gears deployed and flaps extended, airframe noise predomi-
nates; whereas during other flight phases, the engines are to be blamed for the majority
of the noise.

On conventional aircraft, bleed air is also used for de-icing: hot air is simply blown
over the wings via exhaust holes but create an unpleasant “whistling” noise. In the
bleedless system, an “electric blanket” is built directly into the wings, which heats the
wings to keep them free from ice. This is a completely silent process, providing a nicer
experience for passengers, but also for communities living near flight routes.

A lot more of these “more electric” optimizations are possible by replacing other
pneumatic and hydraulic systems with electromechanical ones. Pneumatics and hydrau-
lics are responsible for a lot of the weird noises that passengers hear during a flight; the
switch to electrical solutions would result in quieter operations.

As concluded by the study in [10.18], considering a 154-passenger short-range
aircraft, the total aircraft noise footprint hardly changes between the reference aircraft
and its hybrid-electric variant. This is no surprise considering that similar noise perfor-
mance is obtained for both reference turbine engines and their hybrid-electric variants.

FIGURE 10.31 Cost of Ownership index comparison (business jet) [10.11].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 227

Nevertheless, there is still room for further noise reductions by tackling the root causes
of dominant subcomponents in noise generation. As it turns out, this approach comes
down to rethinking the design of landing gear and flap systems on the airframe side,
along with jet and fan optimizations on the engine side.

All-electric aircraft allow outright elimination of combustion engine noises offering
drastic noise reductions. Figure 10.32 shows how, in the case of general aviation, distrib-
uted electric propulsion helps alleviate noise issues by enabling sound reduction methods
such as “reduced propulsor tip speed” and “spread spectrum,” among others [10.25].

Let us now consider the situation concerning the nitrous oxide emissions (NOx).
Bradley and Droney [10.18] assessed NOx emissions for hybrid-electric engine variants
designed to power the “high span truss-braced” short-range aircraft reference described
in Section 10.5. Results obtained show that during takeoff and landing phases,
hybrid-electric engines could achieve, thanks to propulsive and thermal efficiency gains
in combination with improved thrust lapse characteristics, diminished NOx emissions
in comparison with the CFM-56, the baseline turbofan powering the Boeing 737. So
much so that NOx levels achieved by hybrid-electric engines, estimated in the range
of 7.5% to 11% of CAEP1/6 levels, literally outperform the “not-to-exceed” goal of 20%
of the same levels.

As far as cruise conditions are considered, for the hybrid-electric aircraft with engines
operating in the “balanced” hybrid mode, wherein both turbine engine and electric motor
sections are operated in a balanced manner throughout the mission, NOx emissions are
close to the 80% reduction goal. When resorting to a hybrid-electric engine capable of
the “core shutdown” mode, meaning that the turbine engine section gets cut off at a certain
point during the mission leaving the electric motor to operate on its own, the aircraft has
essentially no NOx emissions over approximately 50% of the cruise segment of a 900 nm
mission.

As of 2010, direct greenhouse gas emissions from aviation account for more than
2% of global emissions. Nevertheless, this hardly represents a significant share of the
14% of global emissions attributable to the entire transportation sector.

1 Committee on Aviation Environmental Protection of ICAO.

FIGURE 10.32 General Aviation noise performance (turboprop vs. DEP) [10.25].

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228 CHAPTER 10 Performance and Business Value of Electric Aircraft

Climate change is a global issue that needs to be tackled from many fronts. In the
aerospace industry aircraft and engine manufacturers, their supply chain, airlines,
airports, air traffic management services, research institutes, and civil aviation author-
ities have been working towards a common objective of reducing the overall impact of
aviation on the environment.

In order to reach ambitious environmental goals in a limited timeframe, government
policy [10.26] and funding have been concentrated on research in this area. As such,
Europe is at the forefront of atmospheric research and has taken the lead in the formulation
of a prioritized environmental action plan, and the establishment of global environmental
standards.

In 2011, a European group of experts set out a vision of European aviation with the
publication of Flightpath 2050. In response to this, the Advisory Council for Aviation
Research and Innovation in Europe (ACARE) produced a Strategic Research and
Innovation Agenda (SRIA) in 2012 that defined the path to reach these ambitious goals.
This vision would draw upon airframe, engine, system, ATM/infrastructure, and airline
operation optimizations. To spur this into action, the ACARE council set the following
steps for the industry to follow (Figure 10.33) [10.27]:

• An average improvement of 1.5% per year in terms of fuel efficiency to reach a
carbon-neutral situation by 2020 in the first place as an intermediate goal.

• From 2020 onwards, ensure a carbon-neutral growth, assisted by economic measures,
to ultimately achieve in 2050 the following reductions compared to 2005 levels:

■ 50% reduction in CO2 (Figure 10.33)
■ 80% reduction in NOx emissions
■ 50% reduction in noise

FIGURE 10.33 Initial aviation carbon emissions reduction roadmap [10.27].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 229

Recently the European Commission has established a 2050 target goal in its
highly ambitious Flightpath 2050 Vision for Aviation. Pushing the environmental
agenda even more aggressively further down the line, Flightpath 2050 sets forth
steeper industry goals for 2050 in comparison to capabilities of typical new aircraft
available in 2000:

• 75% reduction in CO2 emissions per passenger kilometer
• 90% reduction in NOx emissions
• 65% reduction in perceived noise emissions of flying aircraft

This sets the bar very high on additional new-generation technology to come on top
of advances in existing technology including significant shifts in the design approach
of both aircraft and engine systems.

Also included are goals calling for aircraft movements to be emission-free when
taxiing.

As discussed in Chapter 9, taxiing is an area where manufacturers have already
been looking to increase fuel efficiency and reduce emissions and noise through elec-
trification. Currently, aircraft use their main engines to move around on the tarmac. As
you can imagine, this consumes a lot of fuel. By adding electric motors to the wheels of
the plane and powering them from the onboard Auxiliary Power Unit (APU), significant
fuel burn reduction can be achieved on ground operations. This allows an aircraft to
be able to taxi without using its main engines, offering an attractive fuel saving of around
~3% on ground operations for a short-/medium-haul aircraft.

Sometimes, green solutions do come with economic benefits, but this may be tied
to the type of energy used and the related price levels. One such instance is the electric
green taxiing system, an incremental electrification solution. Unluckily, relatively low
oil prices are not helping the entry into service of such solutions. Indeed, the development
of the electric taxiing system described above is currently on hold, despite wide industry
support, the reason being that oil prices are so low that it doesn’t make economic sense,
at least for now.

On one hand, airlines may undeniably get more profitable in case of oil price drops.
But on the other hand, low oil prices weaken the economic prospects of incremental
developments based on conventional aircraft platforms, irrespective of whether they use
electric or conventional technology, as long as their focus is on fuel-burn reduction.
In line with that rationale, in the face of low oil prices, re-engined aircraft platforms
may suffer weakened sales growth due to their higher price tag.

On conventional aircraft, as fuel is a major cost driver, bringing fuel consumption
down helps indirectly to reduce gas emission levels. But we can’t rely on the industry’s
natural desire to decrease fuel consumption in order to reduce emissions, because
during periods of low fuel prices the impetus may slacken, or worse, simply grind to
a halt.

For the future of aviation, the electric aircraft, whether fully electric or in a
hybrid-electric version, is the centerpiece in finding the sweet spot between reining in
emissions, boosting energy efficiency, and offering a compelling business case.

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