Can we accelerate the improvement of energy efﬁciency in aircraft systems?

Joosung J. Lee

College of Engineering, Yonsei University, Seoul 120-749, Republic of Korea

a r t i c l e

i n f o

Article history:
Received 30 April 2008
Received in revised form 5 February 2009
Accepted 8 September 2009
Available online 6 October 2009

Keywords:
Aviation energy efﬁciency
Environmental impact
Technological innovation
Sustainable air transport
Social demand articulation

a b s t r a c t

An aircraft is composed of systems that convert fuel energy to mechanical energy in order to perform
work—the movement of people and cargo. Today, the fast-growing demand for air travel has outpaced
the rate of improvement in the energy efﬁciency of aircraft systems. The increase in the total energy con-
sumption and environmental impact of aviation necessitates a strategy to induce further technological
and operational innovations to mitigate the increase in aircraft energy use and environmental effects.
However, the uncertainty associated with the climate effects of jet engine emissions hinders further
improvement to the energy efﬁciency of aircraft systems. Also the unique characteristics (e.g., trade-
off between emissions species) of aircraft systems make it difﬁcult to focus on abatement efforts. Based
on a short review of how aircraft technology and operations relate to energy use and the future outlook
for aircraft performance, energy use, and environmental impact, the key technology and policy issues
related to improving the energy efﬁciency of aircraft systems are presented. Then, the drivers of techno-
logical change in aircraft systems are examined. Government regulation effects and industry character-
istics as they relate to improvement of energy use are also presented. Based on these discussions, this
paper provides insights on how to accelerate the induction of energy efﬁcient, environmentally friendly
innovations.

1. Introduction

There has been fast growth in aviation as a form of mobility, and

consequently there has been signiﬁcant growth in energy use. In
2005, aviation accounted for 3.7 trillion revenue passenger-kilo-
meters (RPK’s), which is approximately 10% of world RPK’s traveled
on all modes of transportation and 40% of the value of world freight
shipments

[1]

. Demand for air travel has grown fastest among all

modes of transport. Subsequent to the events of September 11,
2001, total RPKs fell by 8% and fuel burn fell by 16% based on 2-
year averages before and after. The percentage of the commercial
ﬂeet that is parked increased from 6% to 13%

[2]

. However, future

projections estimate a resumption of the long-term growth trend
within the next several years. Worldwide growth is anticipated
to continue at around 4–5% per year

[3]

Aviation fuel consumption today corresponds to between 2%

and 3% of the total fossil fuel use worldwide, more than 80% of
which is used by civil aviation operations

[4,5]

. According to Sau-

sen and Schumann

[6]

, aviation fuel consumption contributes to

the increase in atmospheric CO

concentration, which is computed

to be on the order of 1.7% due to past aviation fuel consumption

through 1995. Furthermore, it may increase to about 3% in 2050.
The evolution of transportation demand also suggests an increase
in per-person energy use for transportation. Minimizing energy
use has always been a fundamental design goal for commercial air-
craft. Airlines have lost over US $35 billion during the period 2001–
2005, and the high price of fuel will continue to challenge airline
proﬁtability

[7]

. At the same time, the growth of air transportation

applies increasing pressures to improve technology and opera-
tional efﬁciency to limit environmental impacts. A balanced ap-
proach is necessary to ensure sustaining growth in an aviation
industry where cost reduction is critical and environmental perfor-
mance is improved.

Much still remains uncertain about climate change and the role

played by aviation, although climate scientists express particular
concern about jet engine emissions in the upper atmosphere,
where the warming effect of some pollutants is ampliﬁed

[8]

. Ex-

perts also point out that it is more difﬁcult to reduce carbon diox-
ide emissions from aviation, as today’s aircraft engines are already
energy efﬁcient

[9]

. In addition to carbon dioxide, jet engines emit

many pollutants into the atmosphere, including nitrogen oxides,
sulfur oxides, soot, and even water vapor. Thus, current studies
show that there are many uncertainties about both the physical
processes and economic impacts of climate change, and that it is
difﬁcult to improve the environmental performance of aircraft sys-
tems to offset the fast growth of aviation and the resultant impact
on climate with more proactive actions.

0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:

10.1016/j.enconman.2009.09.011

Address: 262 Seongsan-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea.

Tel.: +82 2 2123 5724; fax: +82 2 364 7807.

E-mail address:

JSL@yonsei.ac.kr

Energy Conversion and Management 51 (2010) 189–196

Contents lists available at

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Energy Conversion and Management

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n c o n m a n

The objectives of this paper are to (1) highlight the bottlenecks

in the energy efﬁciency improvement of aircraft systems and (2)
provide insights for accelerating energy efﬁcient, environmentally
friendly innovations for the aviation industry. In particular, this
paper demonstrates an effective process to induce aircraft manu-
facturers and airlines to articulate social demands such as environ-
mental protection into their product development and operating
strategies.

Section

examines trends in air transport demand, energy use,

and the associated environmental impacts. It focuses on current
literature to reveal the uncertainties in the atmospheric effects of
jet engine emissions. The existence of large uncertainties makes
it difﬁcult to direct intensive efforts to reduce emissions. Section

thus emphasizes the need to advance the scientiﬁc aspects of

the impacts of aviation emissions on the global climate, and sug-
gests speciﬁc areas for further research. Section

introduces air-

craft systems from the perspective of energy conversion, and
discusses key performance parameters of aircraft energy efﬁciency.
A technological and operational outlook for reduced aircraft energy
use is also presented. From this, the limits of aircraft energy efﬁ-
ciency improvement are discussed. Section

further demonstrates

that historically, energy prices are the main driver of energy efﬁ-
ciency improvement for aircraft systems. Based on a comparative
analysis with the case of aircraft noise reduction, this paper pre-
sents a way to expedite improvements in energy efﬁciency and
lower aircraft emissions via the articulation of public demands.

2. Current understanding of the environmental impact of
aviation

Growth in the total volume of air transportation has important

environmental ramiﬁcations associated with climate change and
stratospheric ozone reduction on a global scale. On local to regional
scales, issues such as noise, decreased air quality (related primarily
to ozone production and particulate levels), roadway congestion
(related to airport services), and local water quality are recognized
as important consequences of air transportation. This section fo-
cuses on emissions-related issues and the scientiﬁc uncertainty
in them.

The total mass of emissions from an aircraft is directly related to

the amount of fuel it consumes. Of the exhaust emitted from the en-
gine core, 7–8% is composed of carbon dioxide (CO

) and water vapor

O), with another 0.5% composed of nitrogen oxides (NO

), un-

burned hydrocarbons (HC), carbon monoxide (CO), and sulfur oxides
(SO

). Other trace chemical species include the hydroxy family

(HO

), the extended family of nitrogen compounds (NO

), and car-

bon-based soot particulates. Elemental species such as O, H, and N
are also formed to an extent that is governed by the combustion tem-
perature. The balance (91.5–92.5%) is composed of O

and N

[10]

Aircraft emissions can impact the climate in several ways: (1)

carbon dioxide emissions can alter the radiative balance of the
earth and contribute to global warming; (2) emissions can produce
or destroy radiatively active substances (e.g., NO

, which modiﬁes

concentration); (3) emissions of water vapor and other sub-

stances can trigger the generation of additional clouds (e.g., con-
trails)

[6,10]

. Because the majority of aircraft emissions are

injected into the upper troposphere and lower stratosphere (typi-
cally 9–13 km in altitude), the resulting effects on the global envi-
ronment are unique among all industrial activities. The fraction of
aircraft emissions that is relevant to atmospheric processes ex-
tends beyond the radiative forcing

effects of CO

. The mixture of

exhaust species that is discharged from aircrafts perturbs radiative
forcing 2–3 times more than if the exhaust was CO

alone. In con-

trast, the overall radiative forcing from the sum of all anthropogenic
activities is estimated to be a factor of 1.5 times that of CO

alone

[11]

. Thus the impact of burning fossil fuels at altitude is approxi-

mately double that due to burning the same fuels at ground level.
The enhanced forcing from aircraft compared with ground-based
sources is due to different physical and chemical effects (e.g., con-
trails and ozone formation/destruction) resulting from altered con-
centrations

participating

chemical

species

and

changed

atmospheric conditions

[11]

The Intergovernmental Panel on Climate Change (IPCC) esti-

mates that the radiative forcing by various aircraft emissions for
1992 accounted for 3.5% of the total anthropogenic forcing that oc-
curred in 1992. The IPCC projects that value will increase to an esti-
mated 5% by 2050 for an all-subsonic ﬂeet. Furthermore, different
gases have a range of different lifetimes in the atmosphere

[12]

Associated increases in ozone levels are expected to decrease the
amount of ultraviolet radiation at the surface of the earth. It is
important to note that these estimates are uncertain

[11]

. While

broadly consistent with these IPCC projections, subsequent re-
search reviewed by the Royal Commission on Environmental Pro-
tection (RCEP) in the UK suggests that the IPCC underestimates
the reference value for the impact of aviation on the global climate.
In particular, while the IPCC probably overestimates the impact of
contrails,

aviation-induced cirrus clouds could be a signiﬁcant

contributor to positive radiative forcing

[13]

. No reliable estimate

of the optical properties and radiative forcing exists for contrail cir-
rus. The assumptions in the IPCC estimate of the radiative forcing
by aviation-induced cirrus changes are very uncertain

[14,15]

. Be-

sides radiative forcing by contrails, contrail cirrus, and changed cir-
rus, the climate may also be impacted by other aspects such as
changes in air composition due to reactions on the surface or inside
of the induced cloud particles. Only preliminary estimates of some
aspects are available so far, and these mainly pertain to radiative
forcing by line-shaped contrails

[14]

Marais et al.

[16]

estimate that in the short-term, the most sig-

niﬁcant impact on surface temperature results from aviation-in-
duced cirrus formation, although the level of conﬁdence in
estimating the radiative forcing due to aviation-induced cirrus is
quite low

[17]

. The second most signiﬁcant impact on surface tem-

perature is that due to ozone changes. A parametric study by Sau-
sen and Schumann

[6]

shows that aviation-induced O

changes

have a greater effect on global temperature and sea level than do
CO

emissions from aviation. Thus, the temperature change result-

ing from non-CO

effects dominates that due to CO

during the

years immediately following the year of emissions. The impact of
CO

dominates in the long-term

[16]

Note also that comparing and addressing the climate impact of

aviation solely on the basis of radiative forcing at one point in time
may lead to less effective policies, since the full future impacts of
effects are not taken into account. Simple radiative forcing esti-
mates can produce misleading comparisons of the relative impor-
tance of short-lived and long-lived effects because different
emissions species have different time-scales. The conclusions of
previous studies

[18,19]

emphasize that reducing the uncertainties

behind the net climate effects of jet engine emissions should be a
priority in order to improve policy-making. As one possible way
to reduce the uncertainties, Boucher and Haywood

[20]

suggest a

thorough analysis and comparison to observation of the different
model estimates.

Forcing is a measure of the change in the Earth’s radiative balance associated with

atmospheric changes. Positive forcing indicates a net warming tendency relative to
pre-industrial times.

Saturation occurs when the moist, high-temperature air in the jet exhaust

condenses into particles in the atmosphere when it mixes with the ambient cold air.
The result is a condensation trail or contrail.

190

J.J. Lee / Energy Conversion and Management 51 (2010) 189–196

To summarize, one consistent and clear message from the cur-

rent studies is that there are many uncertainties about both the
physical processes and economic impacts of climate change. Mar-
ais et al.

[16]

ﬁnd that the climate sensitivity, the radiative forcing

of different short-lived effects, and the choice of emissions scenario
have the most signiﬁcant inﬂuence on estimating the effects of cli-
mate change. To improve policy decisions, it is most important to
articulate the uncertainty in climate sensitivity and short-lived ef-
fects (in particular contrails and aviation-induced cirrus clouds),
and to consider ranges of plausible emissions scenarios

[21]

. More

certain values for climate sensitivity and the radiative forcing val-
ues for short-lived effects may become available as the current
understanding of the physical processes involved in climate change
improves.

3. Progress in aircraft energy efﬁciency

3.1. Measurement and trends in aircraft energy efﬁciency

An aircraft is composed of systems that convert fuel energy to

mechanical energy in order to perform work—the movement of
people and cargo. An aircraft engine converts a ﬂow of chemical
energy contained in aviation fuel and the air drawn into the engine
into power (thrust multiplied by ﬂight speed). Overall engine efﬁ-
ciency is deﬁned by the ratio of power to total fuel energy ﬂow
rate. Only one-fourth to one-third of fuel energy is used to over-
come drag and thus propel the aircraft. The remaining energy is ex-
pelled as waste heat in the engine exhaust

[22]

When judging the efﬁciency of an aircraft system, it is more rel-

evant to consider work in terms of passengers or payload carried
per unit distance. Energy intensity (E

) is an appropriate measure

when comparing efﬁciency and environmental impact to other
modes

[23]

. E

consists of two components; energy use, E

, and

load factor,

, as described by Eq.

(1)

. Energy use is energy con-

sumed by the aircraft per seat per unit distance traversed, and is
determined by aircraft technology parameters including engine
efﬁciency. E

observed in actual aircraft operations reﬂects opera-

tional inefﬁciencies, such as ground delays and airborne holding.
The ﬂeet average E

is of interest because it is the ﬂeet fuel efﬁ-

ciency that determines the total energy use

[5]

. Load factor is a

measure of how efﬁciently aircraft seats are ﬁlled and aircraft kilo-
meters are utilized to generate revenue. Increasing the load factor
leads to improved fuel consumption on a passenger-kilometer
basis.

RPK

ASK

RPK
ASK

ð1Þ

where MJ is the mega joules of fuel energy and ASK is the available
seat-kilometers.

Fig. 1

shows historical trends in E

for the US large commercial

ﬂeet. Lee et al.

[9]

suggest that 57% of the reductions in energy

intensity during the period 1959–1995 were due to improvements
in engine efﬁciency, 22% resulted from increases in aerodynamic
efﬁciency, 17% were due to more efﬁcient use of aircraft capacity,
and 4% resulted from other changes, such as increased aircraft size.
Year-to-year variations in E

for each aircraft type due to different

operating conditions, such as load factor, ﬂight speed, altitude, and
routing controlled by different operators, can be ±30%, as repre-
sented by the vertical extent of the data symbols.

Individual aircraft E

are based on 1991–1998 operational data

with the exception of the B707 and B727, which are based on avail-
able operational data prior to 1991. Fleet averages were calculated
using a revenue passenger-kilometer (RPK) weighting. Data was
not available for the entire US ﬂeet average during 1990 and
1991. Source:

[9]

Note that reductions in E

do not always directly imply lower

environmental impact. For example, the prevalence of contrails is
enhanced by greater engine efﬁciency

[24]

. NO

emissions also be-

come increasingly difﬁcult to limit as engine temperatures and
pressures increase—a common method for improving engine
efﬁciency

[25]

. These conﬂicting inﬂuences make it difﬁcult to

translate between the expected changes in overall system perfor-
mance and the impact on air quality.

Engine, aerodynamic, and structural efﬁciencies play an impor-

tant role in determining the energy intensity of an aircraft. Engine
efﬁciency in large commercial aircraft, as measured by the cruise
speciﬁc fuel consumption (SFC) of newly-introduced engines, has

Fig. 1. Historical trends in energy intensity of the US large commercial ﬂeets.

Fraction of passengers per available seats.

J.J. Lee / Energy Conversion and Management 51 (2010) 189–196

191

improved by approximately 40% over the period 1959–2000, which
equates to an average improvement of 1.5% annually

[9]

. Most of

this improvement was realized prior to 1970 with the introduction
of high bypass turbofan engines.

However, as bypass ratios

have

increased, engine diameters have also become larger, leading to an
increase in engine weight and aerodynamic drag

[26]

. Other routes

to improving engine efﬁciency include increasing the peak pressure
and temperature within the engine, which is limited by materials
and cooling technology, and improving engine component efﬁcien-
cies. Aerodynamic efﬁciency in large commercial aircraft has in-
creased by approximately 15% historically, averaging 0.4% per year
for the same period. Better wing design and improved propulsion/
airframe integration are enabled by improved computational and
experimental design tools, and have been the primary drivers

[27]

Historical improvements in structural efﬁciency

are less evident.

One reason is that over the 35-year period between the introduction
of the B707 and the B777, large commercial aircraft in service have
been constructed almost exclusively of aluminum and are currently
about 90% metallic by weight. Composites are used for a limited
number of components. Another reason is that improvements in air-
craft structural efﬁciency have been largely traded for other techno-
logical improvements like larger, heavier engines and increased
passenger comfort

[28,26]

Aircraft E

is also improved through better utilization (e.g., load

factor) and greater aircraft capacity (e.g., number of seats). Thus,
the outlook for future reductions in energy use is necessarily based
on the potential for increased technological and operational efﬁ-
ciencies. If technological and operational improvements in aircraft
systems continue to occur at a pace seen historically, a 30–50%
reduction in E

would be possible by 2025

[9]

This is equivalent to a 1.2–2.2% annual change in E

. This projec-

tion is consistent with the fuel efﬁciency goal of the latest B787 air-
plane

[29]

. The airplane is expected to use 20% less fuel than its

contemporary counterpart. The key technologies include light-
weight structures, highly efﬁcient engines, and aerodynamic
improvements to the body and wings. As much as 50% of the pri-
mary structure (including the fuselage and wing) on the B787 will
be made of composite materials. The advanced engines for the new
airplane are expected to contribute as much as 8% of its increased
efﬁciency. According to the Boeing Company

[30]

, it will be possi-

ble to eliminate 1500 aluminum sheets and 40,000–50,000 fasten-
ers by manufacturing a one-piece fuselage section, and to attain
greatly improved aerodynamic and structural efﬁciency.

3.2. Alternatives fuels for reduced energy use and emissions

Beyond the evolution of the current aircraft platform, hydrogen

and ethanol have been proposed as alternative fuels for future low-
emission aircraft

[31]

. Hydrogen-fueled engines generate no CO

emissions at the point of use, may reduce NO

emissions, and

greatly diminish emissions of particulate matter. However, hydro-
gen-fueled engines would replace CO

emissions from aircraft with

a threefold increase in emissions of water vapor. Considering
uncertainties over contrails and cirrus cloud formation, and the
radiative impact of water vapor at higher altitudes, it is not clear
whether the use of hydrogen would actually reduce the contribu-
tion from aircraft to radiative forcing

[13,32]

. In addition, there

are several issues that must be resolved before a new fuel base is

substituted for the existing kerosene infrastructure. The usefulness
of such alternative fuels requires a balanced consideration of many
factors, such as safety, energy density, availability, cost, and indi-
rect impacts through production. Renewable biomass fuels such
as ethanol have much lower energy density than kerosene or even
hydrogen, thus requiring aircraft to carry more fuel. They would
again increase water vapor emissions from aircraft in ﬂight

[5]

Some experts believe nuclear-powered passenger aircraft are the
option to propel future air transport systems. However, this will re-
quire a major research program to help the aviation industry con-
vert from fossil fuels to nuclear energy

[33]

Presently, it appears that a blend of kerosene and synthetic fuel

will be possible for use in existing and near-term aircraft. Synthetic
jet fuels are very similar in performance to conventional jet fuel,
but have almost zero sulfur and aromatics while producing lower
particulate exhaust emissions. Future mid-term aircraft may use
a blend of bio-fuels and synthetic fuels in ultra-efﬁcient airplane
designs. The major challenges of using pure bio-fuels in a commer-
cial aircraft are that they may freeze at normal cruising tempera-
tures and exhibit thermal instability at high temperatures. In
addition, present bio-fuels are incapable of supplying a large per-
centage of fuel without displacing food production. Future long-
term engines and aircraft in the 50-plus year horizon may be spe-
ciﬁcally designed to use alternative fuels with low to zero carbon
content, such as liquid hydrogen or liquid methane. To use liquid,
cryogenic fuels in aircraft engines, modiﬁcations are necessary to
the combustor and fuel system components

[34]

. Schumann

[35]

points out that the use of cryogen fuels instead of kerosene in-
creases the frequency of contrail formation for constant air trafﬁc.
This is because the threshold temperature increases with the
hydrogen content of the fuel, and engines driven by liquid hydro-
gen emit 2.5 times more water vapor mass than kerosene-burning
engines for the same energy content

[35]

. The climate impact,

however, may not be much different because of a cleaner exhaust
with fewer particles available to form ice particles

[36,37]

While reducing energy intensity tends to reduce overall emis-

sions, there are factors inherent to air transportation that can act
to counter the potential beneﬁts. Reductions in emissions are
hindered by the relatively long lifespan and large capital and oper-
ating cost of individual aircraft, and the inherent lag in the adop-
tion of new technologies throughout the aviation ﬂeet as a result

[9]

. Perhaps most importantly, the cost of change is uncertain. Air-

lines are willing to pay higher acquisition costs if they can save in
direct operating costs, mainly through lower fuel and maintenance
costs during the lifetime of aircraft. However, it is unclear whether
future technologies can be delivered at an acceptable price. If the
price is too high, airlines may choose not to pay more for energy
saving technologies, in which case further improvements in energy
use for the aviation sector may be limited.

3.3. Operational measures to reduce aircraft energy use and emissions

Operational measures are cost-effective ways to lower the en-

ergy intensity of aviation and mitigate subsequent climate effects.
First, mitigating effects due to jet engine-induced contrails could
signiﬁcantly reduce the total climate impact of aviation. Myhre
and Stordal

[38]

suggest that shifting the peak trafﬁc periods to-

wards sunrise and sunset could reduce the contrail impact because
the amount of solar radiation blocked by the contrails is higher at
these times and acts to cancel the warming effect. An alternative
method would be to reduce contrail production by restricting
cruise altitudes. The formation of contrails and cirrus clouds can
be reduced by reducing ﬂights in air masses that are supersatu-
rated with ice. Sausen et al.

[39]

ﬁnd that changes in the cruise alti-

tude of aircraft could eliminate the contrail contribution, but
constraints on cruise altitudes would prevent some aircraft from

The dominant mode of propulsion for commercial aircraft today. A turbofan

engine derives its thrust primarily by passing air through a large fan system driven by
the engine core.

The ratio of the air passed through the fan system to that passed through the

engine core.

The ratio of aircraft operating empty weight to maximum take-off weight. This is

a measure of the weight of the aircraft structure relative to the weight it can carry
(the structure itself + payload + fuel).

192

J.J. Lee / Energy Conversion and Management 51 (2010) 189–196

operating at their maximum speed and efﬁciency. As a result, the
implications for total fuel burn, and hence the radiative impact of
increased CO

, must be considered. Williams et al.

[40]

study these

trade-offs using an air space simulation model, as applied to Euro-
pean airspace. Based on a one-day Western European trafﬁc sam-
ple, calculations suggest that annual mean CO

emissions would

increase by only 4% if cruise altitudes were restricted to prevent
contrail formation. The change in ﬂight time depends on aircraft
type and route, but average changes are less than 1 min

[40]

. The

cruise altitude change strategy will work immediately if the pilot
has access to an accurate device that measures relative humidity,
and if there is enough ﬂexibility in the selection of ﬂight levels

[24,35]

Currently, technological and operational measures are available

for the aviation sector to reduce the environmental impact of jet
engine emissions. The most important factor is the ethical and
political will to mitigate the climate impact of air trafﬁc

[24]

. To

become truly environmentally responsible, the aviation industry
needs to make progress on all aspects of regulatory, technological,
and operational fronts

[41]

. To this end, the next section explores a

strategy to induce environmental innovations for the aviation
sector.

4. Inducing environmental innovations in aircraft systems

Experts say that jet engines are already energy efﬁcient, and

technology to signiﬁcantly reduce carbon dioxide from engines is
not as far along as it is for land-based pollution sources

[8]

Improvements in aviation energy efﬁciency have been difﬁcult
and slow, while most recent research has targeted only the reduc-
tion of NO

and persistent contrails

[42]

. Given the technological,

operational, and economic constraints on controlling increases in
aircraft energy consumption and the impact of emissions, this sec-
tion examines the question, ‘‘Can we accelerate improvements in
energy efﬁciency in aircraft technologies and operations, and
achieve sustainable industry growth?” By comparing the historical
trend of aircraft noise reduction to that of jet efﬁciency improve-
ment, this paper examines the process of inducing environmental
innovations for the aviation industry.

For the aviation industry, the word sustainable means increased

mobility for people around the world, proﬁtable industry growth,
protection of the environment, and continued improvements to
safety and security. Regarding protection of the environment, it
is important to stabilize, reduce, or even eliminate conventional
greenhouse gas emissions from aircraft engines. To do so, the avi-
ation sector must consider not only the technological solutions,
operational solutions, and economic costs, but also where aviation
stands in relation to society. Currently, the public demand to re-
duce the contribution of aircraft emissions to global climate
change is not strong. In the cases of aircraft noise and automobile
emissions, a clear demonstration of health damages followed by
strong public pressure to reduce the environmental nuisances have
led to dramatic improvements in both technologies and the way
both systems operate

[2]

. On the contrary, public awareness about

the effects of aircraft engine emissions on global warming is rela-
tively low today. Also, there are large scientiﬁc uncertainties about
the potential climate change effects of jet engine emissions that are
discharged at altitude

[11]

In this regard, it is important to continue the advancement of

atmospheric science regarding the environmental effects of jet en-
gine emissions, and to raise general public awareness about the
impact of aviation on local and global air quality. Such efforts,
along with modeling and assessments of various options to reduce
emissions, will be important steps toward sustainable air trans-
port. To clarify this point and help formulate a strategy to induce

environmental innovations in aircraft technologies and operations,
this section examines the case of aircraft noise reduction. Based on
the case study, it is possible to understand the drivers of techno-
logical and operational change, and the evolution of effective gov-
ernment regulations to guide policy toward sustainable air
transport.

4.1. Drivers of aircraft noise reduction: strong social pressure

Historically, strong public demand supported by scientiﬁc evi-

dence of health damage caused by aircraft noise and subsequent
government regulation to limit the operation of noisy aircraft have
led to large reductions in noise around airports. This section ﬁrst
reviews the metrics used to judge the magnitude and scope of avi-
ation noise effects.

The US Federal Aviation Administration (FAA) employs day-

night noise level (DNL) to determine the compatibility of airport-
local land uses with aircraft noise levels. At 55 dB DNL (indoors
or outdoors), a community will generally perceive aviation noise
as no more important than various other environmental factors,
and about 3% of the population will be highly annoyed. At 65 dB
DNL, 12% of the population may be highly annoyed, and the com-
munity will generally consider aviation noise to be an important
and adverse aspect of the environment

[43]

. The historical evolu-

tion of noise exposure in these zones and future projections devel-
oped by the FAA show reductions of over 80% in the population
areas affected by commercial aviation noise. The greatest reduc-
tions have resulted primarily from two factors. The ﬁrst is low-
noise aircraft operations enabled by advances in aircraft communi-
cation, navigation, and surveillance, and air trafﬁc management
(CNS/ATM) technology. The second is the phase-out of loud aircraft
through regulatory action and enabled by the availability of im-
proved engine technology (e.g., increased bypass ratio)

[2]

The key drivers of this drastic noise reduction were accumula-

tion of knowledge and diffusion of information about the health
damage caused by aircraft noise, and subsequent government reg-
ulations responding to the public demand for relief from jet aircraft
noise. It was in the 1960s that people gained scientiﬁc knowledge
that noise produces a variety of adverse physiological and psycho-
logical effects. Common among these are speech interference and
sleep disturbance, which may result in reduced productivity for a
variety of tasks associated with learning and work. Deﬁnitive evi-
dence of other non-auditory health effects as a direct consequence
of aviation noise is not available

[43]

, but some studies suggest

such connections. Rogen and Olin

[44]

show that the high levels

of aircraft noise prevalent near major commercial airports increase
blood pressure and contribute to hearing loss. Some research also
indicates that aircraft noise contributes to heart diseases, immune
deﬁciencies, neurodermatitis, asthma, and other stress related dis-
eases

[44,45]

Information about the health damage caused by aircraft noise in

the vicinity of airports spread quickly via media and environmental
groups. The diffusion of such information to the public resulted in
civil lawsuits totaling billions of dollars in such cities as New York,
Chicago, Los Angeles, and Washington, DC. Newspapers, journals,
and reports highlighted the potential danger of aircraft noise to hu-
man health. In 1975, the US Environmental Protection Agency
(EPA) reported ‘‘An estimated 16 million Americans are now sub-
jected to a wide range of aircraft noise. Such noise can interfere
with the normal use of homes and yards and poses a particularly
serious problem for such institutions as schools and hospitals”

[46]

The accumulation of knowledge and diffusion of information

about the health damage from aircraft noise caused the US govern-
ment to initiate the Noise Control Act of 1972 (42 USC 4901–4918).
Under the NCA, engine-nacelle combinations must be quieted, and

J.J. Lee / Energy Conversion and Management 51 (2010) 189–196

193

therefore all civil, subsonic airplanes powered by turbojet engines
must comply with the noise level requirements of FAR 36

[46]

. The

federal noise regulations in FAR 36 deﬁne aircraft according to four
classes, and set a phase-out schedule based on the weight and
number of engines, and resulting noise level under various operat-
ing conditions. Around 20% of the current ﬂeet already achieves a
noise target 14 dB better than the current (Chapter 3) standards.
The majority of aircraft designed in recent years are already quiet
enough to meet the impending (Stage 4) standards

[47]

The substantial reductions in the noise of individual aircraft are

largely due to improvements in aircraft technology. Aircraft noise
arises from engines and from the movement of turbulent air over
the airframe. To date, noise reduction has focused mainly on reduc-
ing engine noise, and it has become increasingly important to
tackle noise from the airframe, which may be more challenging
to reduce. To address continued noise concerns, the FAA has
adopted a ‘balanced approach’—a combination of operational
changes, land-use planning, abatement (e.g., insulation programs)
and technological improvements (e.g., increasingly stringent noise
standards)

[48]

In summary, the social demand to reduce aircraft noise was

well supported by scientiﬁc evidence and the diffusion of informa-
tion about the detrimental effects to human health. It then
triggered government regulations, which have driven the techno-
logical and operational innovations to reduce the impact of noise
in the vicinity of airports.

4.2. Drivers of aircraft emissions reduction: fuel cost is main reason to
improve fuel efﬁciency

Historically, fuel cost has been the main driver for improve-

ments to aircraft fuel efﬁciency. Fuel efﬁciency gain was strongest
during the 1970s when oil prices were highest

[9,26]

. When oil

prices soar, airlines actively adopt advanced aircraft with greatly
improved fuel economy. In order to achieve continued improve-
ments in aircraft fuel efﬁciency, as well as reductions in jet engine
emissions that adversely impact global climate and local air qual-
ity, there should be a stronger social pressure for the aviation sec-
tor. Currently, the social demand for low-emission aircraft is not
strong enough because the general public is not well aware of
the effects of aviation emissions on the global climate

[49,50]

. At

the same time, the effects of aircraft engine emissions on the global
atmosphere are not well understood scientiﬁcally.

To address the potential impact of aviation emissions on cli-

mate change, international dialogues have taken place. The Inter-
national Civil Aviation Organization (ICAO) was ﬁrst created at
the 1944 Chicago Convention as the UN specialized agency with
the authority to develop standards and recommended practices
regarding all aspects of aviation, including certiﬁcation standards
for emissions and noise. Since 1977, the ICAO has promulgated
international emissions and noise standards for aircraft through
its Committee on Aviation Environmental Protection (CAEP)

[4]

The ICAO CAEP is primarily responsible for monitoring the avi-

ation sector’s emissions and noise reduction efforts, and seeking
further options to mitigate the impacts of aviation on noise in local
communities, local air quality, and the global atmosphere. Over the
years, CAEP has set aircraft engine certiﬁcation standards and
phase-outs of noisy aircraft. In a recent meeting (CAEP/6), increas-
ing the stringency of NO

emissions standards was one of the is-

sues under consideration

[51]

. In a broader perspective of

climate change, the UN framework convention on climate change
seeks to stabilize atmospheric greenhouse gases from all sources
and sectors, but it does not speciﬁcally refer to aviation.

The continued efforts of international organizations including

ICAO have brought greater awareness of aviation-related energy
and environmental issues among stakeholders in the air transport

industry. This is changing the way that current aircraft operate and
new aircraft are developed. European airports and airlines are par-
ticularly concerned with the increasing environmental impacts of
jet engine emissions. For example, Zurich airport has imposed an
emissions surcharge to its landing fee based on engine certiﬁcation
information. An aircraft engine is classiﬁed within one of ﬁve
groups subject to emissions charges at landing. This Zurich emis-
sion charge intends to provide an incentive to airlines to ﬂy their
aircraft with the lowest NO

emissions into Zurich, and to acceler-

ate the use of the best available technology (see report of the FESG
on the analysis of local air quality charges and other work in

[52]

Virgin Airways has earmarked all of the proﬁts to ﬁnancing re-
search and development of alternative fuels. In early 2008, Virgin
Airways conducted a demonstration ﬂight of its Boeing 747 jets
using bio-fuel—the ﬁrst airborne test of a renewable fuel by a com-
mercial jet

[53]

Airlines in North America are also more conscious than ever of

the environmental impact of jet engine emissions. According to the
Air Transport Association of America, many of its member carriers
have already adopted operational practices such as continuous
descent approaches, which have the potential to signiﬁcantly re-
duce noise, fuel burn, and emissions on every landing

[54]

. For

example, FedEx pledges to cut aircraft emissions by 3.7% and in-
crease vehicle fuel efﬁciency by up to 13.7% over a 3 year period

[55]

. Some non-proﬁt organizations are also working to circulate

information on the environmental impacts of jet engine emissions,
and deliver public opinions to policy makers. For example, Sustain-
able Travel International is an advocacy group for eco-friendly tra-
vel. The company is actively involved with educating air carriers
and travelers on environmental issues and suggesting ways the
aviation sector can offset its emissions discharges

[56]

In summary, this section discussed that the primary motivation

to improve aircraft fuel economy has been to lower fuel cost, and
that aircraft manufactures and airlines are increasingly more con-
scious of global climate change due to jet engine emissions. How-
ever, scientiﬁc knowledge and public awareness about the impacts
of aviation emissions on the global atmosphere are still low. This is
the key difference from the case of aircraft noise reduction, where
the scientiﬁc evidence and strong public demand have induced a
large decline in aircraft noise levels. To examine this difference
quantitatively, the next section presents a short data analysis on
the social factors that drive environmental innovation in aircraft
systems.

4.3. Accelerating the improvement of aircraft energy efﬁciency

There is some empirical evidence that knowledge accumulation

and information diffusion were stronger for the reduction of avia-
tion noise than for emissions. Lee

[57]

measures the level of knowl-

edge accumulation by the number of research papers, and that of
information diffusion by the number of newspaper articles that
contain ‘aviation (or airport) noise’ and ‘aviation (or aircraft) emis-
sion’ as keywords. ‘Health’ and ‘environment’ were also included
in the keyword search of research papers and newspaper articles.
While the number of research papers on both aviation noise and
emissions is generally increasing, there are about 50% more papers
that deal with aviation noise than there are papers that examine
aviation emissions. This indicates that knowledge accumulation
has been signiﬁcantly greater for topics pertaining to aviation noise
than for those pertaining to aviation emissions

[57]

In terms of information diffusion, the newspaper coverage of

both aviation noise and emissions is growing, but the newspaper
articles that feature aviation noise are much greater in number than
those that feature aviation emissions. While an in-depth empirical
analysis requires examination of a broader range of data, current
results show that knowledge accumulation and information

194

J.J. Lee / Energy Conversion and Management 51 (2010) 189–196

diffusion have been stronger for ﬁelds pertaining to aviation noise

[57]

To expedite environmentally conscious innovations for low-

emission aircraft, increased amounts of knowledge and informa-
tion should ﬂow between aviation ﬁrms and other societal constit-
uents, such as citizens, governments, and civilian organizations.
Knowledge accumulation is important in two aspects. One is that
it provides a credible basis for the existence of the environmental
problem. The other important aspect of knowledge is to provide
the scientiﬁc capability to solve environmental problems. It is nec-
essary that basic scientiﬁc knowledge be accumulated in order for
ﬁrms to perform further research and develop new products

[58]

The major role of information diffusion is to better inform the gen-
eral public about the importance of environmental conservation
through media including television, radio, newspapers, magazines,
bulletins, and ﬁlms. Events such as environmental week and school
education programs are also good methods of information diffu-
sion

[58]

For knowledge accumulation, the impacts of jet engine emis-

sions on global climate change must be better understood scientif-
ically. For this purpose, the US FAA, NASA, and Transport Canada
are jointly sponsoring research through the Partnership for Air
Transportation Noise and Emissions Reduction (PARTNER) Center
of Excellence to reduce uncertainties associated with the impact
of non-CO

(e.g., contrails, nitrogen oxides, and particulates that in-

duce cirrus clouds) aviation emissions on the climate. The goal of
this research is to reduce uncertainties to levels that enable appro-
priate action (see workshop on the impacts of aviation on climate
change in

[52]

In addition, research and development should continue to cre-

ate technological and operational solutions to the environmental
concerns caused by jet engine emissions. Advanced concepts under
investigation range from aircraft and engines that burn half the
amount of fuel used by today’s aircraft to converting propulsion
systems to hydrogen fuel and introducing new energy conversion
technologies (i.e., air-breathing fuel cells), which will eliminate
CO

emissions. Future aircraft will use ultra-light-weight materials

in the airframe and engine with a fraction of the millions of parts
now required. Electromechanical sensors will constantly monitor
emissions and environmental performance

[59]

. However, given

that it takes 20–40 years to go from basic research to ﬂeet average
performance

[60]

, concerns exist regarding the likelihood of meet-

ing the environmental goals due to budget restrictions. According
to a recent report published by the US National Research Council,
while ‘‘the goals of the federal research program are admirable
and focused on the right issues, the schedule for achieving the
goals is unrealistic in view of shrinking research budgets.” The re-
port calls for further federal investment in aircraft engine research
and technology development

[61]

To raise the public awareness level (i.e., information diffusion),

much more active dissemination of and education about the envi-
ronmental effects of aircraft engine emissions are needed

[50]

. For

example, Japan Airlines launched an eco-jet program to raise
awareness of the global environment. The company decorated a
777–200 aircraft with a ‘green’ design to raise people’s awareness
of the issues of climate change and global warming, and to clearly
reiterate the company’s strong commitment to reducing the im-
pact of its business activities on the global environment

[62]

. Some

European environmentalists are pushing for programs that in-
crease awareness of air travel passengers, along with their sense
of responsibility for global environmental protection. The pro-
grams enable passengers to pay a fee to mitigate their share of
the damage from the carbon dioxide emitted during each ﬂight.
Environmental companies then use the money to plant trees,
which remove carbon dioxide from the air. British Airways has
an ‘‘emissions calculator” on its website so that passengers can

determine how much carbon dioxide is emitted due to their ﬂying

[8]

. In the short-run, the environmental performance of an incen-

tive-based program or regulation might be poor, since the most di-
rect way of reducing emission is to reduce the number of ﬂights.
However, experts also ﬁnd that the dynamic effects of such pro-
grams and regulations are clearly better than those of command-
and-control regulations in the long-term

[63]

For the governments of developing countries, core environmen-

tal issues such as the impact of aviation on the global climate may
not rank as highly as more pressing problems, such as caring for
the basic needs of much of the population

[64]

. Nonetheless, the

importance of public awareness programs on environmental pro-
tection should be not be ignored because dealing with the inevita-
ble upsurge in demand while minimizing air pollution is one of the
main challenges facing the world air transportation sector during
the 21st century. All of these activities pertaining to knowledge
accumulation and information diffusion will help construct an
environmentally conscious market for the aviation sector and
eventually give aviation ﬁrms a corporate social responsibility to
adopt environmental performance improvement as part of their
business strategy.

5. Conclusion and recommendations

Aviation energy consumption and emissions are expected to in-

crease and constitute a greater proportion of the total anthropo-
genic climate impact. Air transport growth has outpaced
reductions in energy intensity and will continue to do so through
the foreseeable future, perhaps by an increased margin. Unless
measures are taken to signiﬁcantly alter the dominant historical
rates of change in technology and operations, the impact of avia-
tion emissions on local air quality and climate will continue to
grow.

To expedite technological and operational innovations that re-

duce aircraft energy use and the impact of emissions, we must bet-
ter understand the effect of aviation emissions on the climate, and
public pressure should be higher for faster improvement of energy
efﬁciency in aircraft systems. For this purpose, this paper notes the
importance of knowledge accumulation and information diffusion
about the atmospheric effects of jet engine emissions. Atmospheric
science should advance to lower the large uncertainty in the cli-
mate effects of aviation emissions. Public awareness regarding
the environmental effects of aviation should also advance so that
social pressure can build up for accelerated improvement of energy
efﬁciency in aircraft systems. The case analysis of aircraft noise
reduction reveals that the scientiﬁc evidence of health damage
caused by aircraft noise and strong public pressures led to large
reductions in noise around airports. Likewise, it is necessary for
the aviation sector to build up knowledge and disseminate infor-
mation about the effects of aircraft emissions on the global climate.
Institutionalizing knowledge accumulation and information diffu-
sion among ﬁrms and other societal stakeholders can set a path
for the environmentally conscious market, in which greener air-
craft technologies and operational measures are valued highly
and give a competitive advantage to environmentally innovative
ﬁrms.

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