The fuel economy in aircraft is the measure of the transport energy efficiency of aircraft.
Fuel efficiency is increased with better aerodynamics and by reducing weight, and with improved engine brake-specific fuel consumption and propulsive efficiency or thrust-specific fuel consumption.
Endurance and range can be maximized with the optimum airspeed, and economy is better at optimum altitudes, usually higher. An airline efficiency depends on its fleet fuel burn, seating density, air cargo and passenger load factor, while operational procedures like maintenance and routing can save fuel.
Average fuel burn of new aircraft fell 45% from 1968 to 2014, a compounded annual reduction 1.3% with a variable reduction rate. In 2018, CO2 emissions totalled 747 million tonnes for passenger transport, for 8.5 trillion revenue passenger kilometers (RPK), giving an average of 88 grams CO2 per RPK; this represents g of fuel per kilometer, or a fuel consumption per passenger, on average. The worst-performing flights are short trips of from 500 to 1500 kilometers because the fuel used for takeoff is relatively large compared to the amount expended in the cruise segment, and because less fuel-efficient regional jets are typically used on shorter flights.
New technology can reduce engine fuel consumption, like higher pressure and bypass ratios, geared turbofans, open rotors, hybrid electric or fully electric propulsion; and airframe efficiency with retrofits, better materials and systems and advanced aerodynamics.
Flight efficiency theory
A powered aircraft counters its weight through aerodynamic lift and counters its aerodynamic drag with thrust. The aircraft's maximum range is determined by the level of efficiency with which thrust can be applied to overcome the aerodynamic drag.
Aerodynamics
A subfield of fluid dynamics, aerodynamics studies the physics of a body moving through the air. As lift and drag are functions of air speed, their relationships are major determinants of an aircraft's design efficiency.
Aircraft efficiency is augmented by maximizing lift-to-drag ratio, which is attained by minimizing parasitic drag, and lift-generated induced drag, the two components of aerodynamic drag. As parasitic drag increases and induced drag decreases with speed, there is an optimum speed where the sum of both is minimal; this is the best glide ratio. For powered aircraft, the optimum glide ratio has to be balanced with thrust efficiency.
Parasitic drag is constituted by form drag and skin-friction drag, and grows with the square of the speed in the drag equation. The form drag is minimized by having the smallest frontal area and by streamlining the aircraft for a low drag coefficient, while skin friction is proportional to the body's surface area, and can be reduced by maximizing laminar flow.
Induced drag can be reduced by decreasing the size of the airframe, fuel and payload weight, and by increasing the wing aspect ratio or by using wingtip devices at the cost of increased structure weight.
Design speed By increasing efficiency, a lower cruise-speed augments the range and reduces the environmental impact of aviation. According to a research project completed in 2024 and focusing on short to medium range passenger aircraft, design for subsonic instead of transonic speed (about 15% less speed) with turboprop instead of turbofan propulsion would save 21% of fuel compared to an aircraft of conventional design speed and similar characteristics in terms of size, range and expected general technology improvements. Another analysis from 2014 compared the Airbus 320 from 2009 with a hypothetical turboprop successor flying at a 33% lower Mach number, concluding that the slower aircraft would have 36% less fuel consumption. Both state that the decrease of fuel costs enabled by lower design speed would overcompensate the increase of time-related costs resp. the decrease in revenue passenger miles flown per day. In other words, subsonic turboprop aircraft would be more profitable than transonic turbofan aircraft even at current energy prices without additional costs related to climate action like emission fees, aviation fuel taxation or higher prices for sustainable aviation fuels compared to fossile kerosene.
For supersonic flight, drag increases at Mach 1.0 but decreases again after the transition. With a specifically designed aircraft, such as the (discontinued) Aerion AS2, the Mach 1.1 range at 3,700 nmi is % of the maximum range of 5,300 nmi at Mach 0.95, but increases to 4,750 nmi at Mach 1.4 for % before falling again.
Wingtip devices Wingtip devices increase the effective wing aspect ratio, lowering lift-induced drag caused by wingtip vortices and improving the lift-to-drag ratio without increasing the wingspan. (Wingspan is limited by the available width in the ICAO Aerodrome Reference Code.) Airbus installed wingtip fences on its planes since the A310-300 in 1985, and Sharklet blended-winglets for the A320 were launched during the November 2009 Dubai Airshow. They add but offer a 3.5% fuel burn reduction on flights over .
On average, among large commercial jets, Boeing 737-800s benefit the most from winglets. They average a 6.69% increase in efficiency but depending on the route have a fuel savings distribution spanning from 4.6% to 10.5%. Airbus A319s see the most consistent fuel and emissions savings from winglets. Airbus A321s average a 4.8% improvement in fuel consumption, but have the widest swing based on routes and individual aircraft, recognizing anywhere from 0.2% improvement to 10.75%.
Weight
As the weight indirectly generates lift-induced drag, its minimization leads to better aircraft efficiency. For a given payload, a lighter airframe generates a lower drag. Minimizing weight can be achieved through the airframe's configuration, materials science and construction methods. To obtain a longer range, a larger fuel fraction of the maximum takeoff weight is needed, adversely affecting efficiency.
The deadweight of the airframe and fuel is non-payload that must be lifted to altitude and kept aloft, contributing to fuel consumption. A reduction in airframe weight enables the use of smaller, lighter engines. The weight savings in both allow for a lighter fuel load for a given range and payload. A rule-of-thumb is that a reduction in fuel consumption of about 0.75% results from each 1% reduction in weight.
The payload fraction of modern twin-aisle aircraft is 18.4% to 20.8% of their maximum take-off weight, while single-aisle airliners are between 24.9% and 27.7%. An aircraft weight can be reduced with lightweight materials such as titanium, carbon fiber and other composite plastics if the expense can be recouped over the aircraft's lifetime. Fuel efficiency gains reduce the fuel carried, reducing the take-off weight for a positive feedback. For example, the Airbus A350 design includes a majority of lightweight composite materials. The Boeing 787 Dreamliner was the first airliner with a mostly composite airframe.
Flight distance For long-haul flights, the airplane needs to carry additional fuel, leading to higher fuel consumption. Above a certain distance it becomes more fuel-efficient to make a halfway stop to refuel, despite the energy losses in descent and climb. For example, a Boeing 777-300 reaches that point at . It is more fuel-efficient to make a non-stop flight at less than this distance and to make a stop when covering a greater total distance.
Very long non-stop passenger flights suffer from the weight penalty of the extra fuel required, which means limiting the number of available seats to compensate. For such flights, the critical fiscal factor is the quantity of fuel burnt per seat-nautical mile.Park Y., O'Kelly M.E. (2014). Fuel burn rates of commercial passenger aircraft: variations by seat configuration and stage distance. Jrnl. Transp. Geog., 41:pp.137-147. For these reasons, the world's longest commercial flights were cancelled . An example is Singapore Airlines' former New York to Singapore flight, which could carry only 100 passengers (all business class) on the flight. According to an industry analyst, "It [was] pretty much a fuel tanker in the air."Why the Longest Nonstop Flights Are Ending, Bloomberg Bus., 31 October 2013. Singapore Airlines Flights 21 and 22 were re-launched in 2018 with more seats in an A350-900ULR.
In the late 2000s/early 2010s, rising fuel prices coupled with the 2008 financial crisis and the Great Recession caused the cancellation of many ultra-long haul, non-stop flights. This included the services provided by Singapore Airlines from Singapore to both Newark and Los Angeles that was ended in late 2013. But as fuel prices decreased and more fuel-efficient aircraft have come into service, many ultra-long-haul routes were reinstated or newly scheduled (see Longest flights).
Propulsive efficiency
The efficiency can be defined as the amount of energy imparted to the plane per unit of energy in the fuel. The rate at which energy is imparted equals thrust multiplied by airspeed.
To get thrust, an aircraft engine is either a shaft engine – piston engine or turboprop, with its efficiency inversely proportional to its brake-specific fuel consumption – coupled with a propeller having its own propulsive efficiency; or a jet engine with its efficiency given by its airspeed divided by the thrust-specific fuel consumption and the specific energy of the fuel.
Turboprops have an optimum speed below . This is less than jets used by major airlines today, but propeller planes are much more efficient. The Bombardier Dash 8 Q400 turboprop is used for this reason as a regional airliner.
Jet fuel cost and emissions reduction have renewed interest in the propfan concept for jetliners with an emphasis on engine/airframe efficiency that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans. Propfans bridge the gap between turboprops, losing efficiency beyond Mach 0.5–0.6, and high-bypass turbofans, more efficient beyond Mach 0.8. NASA has conducted an Advanced Turboprop Project (ATP), where they researched a variable-pitch propfan that produced less noise and achieved high speeds.
Operations
In Europe in 2017, the average airline fuel consumption per passenger was , 24% less than in 2005, but as the traffic grew by 60% to 1,643 billion passenger kilometers, CO2 emissions were up by 16% to 163 million tonnes for g/km CO2 per passenger.
In 2018, the US airlines had a fuel consumption of per revenue passenger for domestic flights, or g of fuel per km, generating g CO2 / RPK of emissions.
Seating classes In 2013, the World Bank evaluated the business class carbon footprint as 3.04 times higher than economy class in wide-body aircraft, and first class 9.28 times higher, due to premium seating taking more space, lower weight factors, and larger baggage allowances (assuming Load Factors of 80% for Economy Class, 60% for Business Class, and 40% for First Class).
Speed At constant propulsive efficiency, the maximum range speed is when the ratio between velocity and drag is minimal, while maximum endurance is attained at the best lift-to-drag ratio.
Altitude Air density decreases with altitude, thus lowering drag, assuming the aircraft maintains a constant equivalent airspeed. However, air pressure and temperature both decrease with altitude, causing the maximum power or thrust of aircraft engines to reduce. To minimize fuel consumption, an aircraft should cruise close to the maximum altitude at which it can generate sufficient lift to maintain its altitude. As the aircraft's weight decreases throughout the flight, due to fuel burn, its optimum cruising altitude increases.
In a piston engine, the decrease in pressure at higher altitudes can be mitigated by the installation of a turbocharger.