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Fuel economy in aircraft

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Fuel burning in the engines produces water vapor which condenses in contrails behind a Douglas DC-8.

Fuel economy in aircraft is a measure of how much fuel an aircraft, or a fleet of aircraft of mixed types, needs to operate in relation to a service provided (i.e. number of passengers or ton of freight) and the distance between points of travel. It can be expressed in several ways, for example by the litres of fuel consumed per passenger per kilometer.

Flight efficiency theory

Diagram showing the balance of forces on an aircraft
Forces on an aircraft

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

graph of drag forces
Drag forces by velocity

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; however, a higher cruise-speed allows more revenue passenger miles flown per day.

Jet engine efficiency increases with velocity because the speed difference between the flight and the exhaust is lower. However, above the drag divergence Mach number, the aerodynamic drag on the airframe overwhelms this effect because supersonic shockwaves begin to form, greatly increasing drag and needing supercritical airfoil designs for transonic flight.

For supersonic flight, drag increases at Mach 1.0 but decreases again after the transition. With a specifically designed aircraft, such as the (in development) Aerion AS2, the Mach 1.1 range at 3,700 nmi is 70% of the maximum range of 5,300 nmi at Mach 0.95, but increases to 4,750 nmi at Mach 1.4 for 90% before falling again.[1]

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 air show. Their installation adds 200 kilograms (440 lb) but offers a 3.5% fuel burn reduction on flights over 2,800 km (1,500 nmi).[2]

Weight

bar chart of aircraft weight
Aircraft weight at takeoff

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.[3]

The 84-ton maximum payload of a modern Airbus A380 represents 14.6% of its 575-ton maximum take-off weight. There remains considerable room for future improvements in weight reduction. The weight of an aircraft can be reduced by using light-weight materials such as titanium, carbon fiber and other composite plastics. Expensive materials may be used if the reduction of mass justifies the price of materials through improved fuel efficiency over the aircraft's lifetime. Any improvements in fuel efficiency reduce the amount of fuel an aircraft must carry, which reduces the take-off weight of the aircraft and provides further gains in fuel efficiency. For example, the Airbus A380 design includes multiple light-weight materials. The Boeing 787 Dreamliner was the first major commercial airplane to have a composite fuselage, composite wings, and use composites in most other airframe components.[4]

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 the tipping point at 3,000 nautical miles (5,600 km). It is more fuel-efficient to make a non-stop flight below this distance and to make a stop when covering a longer total distance.[5]

Boeing 777-224 fuel burn per distance against range

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.[6] For these reasons the world's longest commercial flights were cancelled c. 2013. An example is Singapore Airlines' former New York to Singapore flight, which could carry only 100 passengers (all business class) on the 10,300-mile (16,600 km) flight. According to an industry analyst, "It [was] pretty much a fuel tanker in the air."[7]

Propulsive efficiency

Propulsive efficiency comparison for various gas-turbine engine configurations

To get thrust, an aircraft engine is either a shaft engine – piston engine or turboprop, with its efficiency measured by its brake-specific fuel consumption – coupled with a propeller having its own propulsive efficiency; or a jet engine with its efficiency measured by its thrust-specific fuel consumption.

Turboprops have an optimum speed below 460 miles per hour (740 km/h).[8] This is less than jets used by major airlines today, but there is reduced drag at the lower speed.

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.[9] Propfans are a more fuel-efficient technology than jet engines or turboprops. NASA has conducted an Advanced Turboprop Project (ATP), where they researched a variable-pitch propfan that produced less noise and achieved high speeds.

Operations

Refuelling an A320 airliner with biofuel

Speed

At constant propulsive efficiency, the maximum range speed is when the ratio between velocity and drag is minimal,[10] 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. This means an aircraft may be more efficient at higher altitude. With increasing altitude, air pressure and temperature both decrease, causing the maximum power or thrust of aircraft engines to reduce. In a piston engine, this trend towards decline in maximum power can be mitigated by installation of a turbocharger. Decreasing air temperature with altitude increases thermal efficiency.[citation needed]

Airlines

A Boeing 787 from Norwegian Air Shuttle

Fuel economy in air transport comes from aircraft fuel efficiency combined with airline efficiency: seating configuration, passenger load factor and air cargo. For instance, over the transatlantic route, the most-active intercontinental market, the average fuel consumption in 2014 was 32 pax-km per L (3.13 litres per 100 kilometres [75 mpg‑US] per passenger). The most fuel-efficient airline was Norwegian Air Shuttle with 40 pax-km/L (2.5 litres per 100 kilometres [94 mpg‑US] per passenger), thanks to its fuel-efficient Boeing 787-8, a high 86% passenger load factor and a high density of 1.18 seat/m2 due to a low 11% premium seating. On the other side, the least efficient was British Airways at 27 pax-km/L (3.7 litres per 100 kilometres [64 mpg‑US] per passenger), using fuel-inefficient Boeing 747-400s with a low density of 0.79 seat/m2 due to a high 24% premium seating, in spite of a high 83% load factor.[11]

In 2016, over the transpacific routes, the average fuel consumption was 31 pax-km per L (3.23 litres per 100 kilometres [73 mpg‑US] per passenger). The most fuel-efficient were Hainan Airlines and ANA with 36 pax-km/L (2.78 litres per 100 kilometres [85 mpg‑US] per passenger) while Qantas was the least efficient at 22 pax-km/L (4.55 L/100 km [51.7 mpg‑US] per passenger). Key drivers for efficiency were the air freight share for 48%, seating density for 24%, aircraft fuel burn for 16% and passenger load factor for 12%.[12]

MSCI ranked Ryanair as the lowest-emissions-intensity airline in its ACWI index with 75 g CO2-e/revenue passenger kilometre in 2014 – below Easyjet at 82 g, the average at 123 g and Lufthansa at 132 g – by using high-density 189-seat Boeing 737-800s. In 2015 Ryanair emitted 8.64 Bn t of CO2 for 545,034 sectors flown: 15.85 t per 776 miles (674 nmi; 1,249 km) average sector (or 5.04 t of fuel: 4.04 kg/km) representing 95 kg per 90.6 million passengers (30.4 kg of fuel: 3.04 L/100 km or 76 g CO2/km).[13]

The Aeroflot Group fuel consumption is 22.9g/ASK, or 2.86 L/100 km (82 mpg‑US) per seat, 3.51 L/100 km (67.0 mpg‑US) per passenger at its 81.5% load factor.[14] In 2016, Cathay Pacific and Cathay Dragon consumed 4,571,000 tonnes of fuel to transport 123,478 million revenue passenger kilometers, or 37 g/RPK, 25% better than in 1998: 4.63 L/100 km (50.8 mpg‑US).[15] From 2010 to 2012, the most fuel-efficient US domestic airline was Alaska Airlines, due partly to its regional affiliate Horizon Air flying turboprops – it would otherwise be second to Spirit Airlines.[16]

Procedures

An Airbus A330 from Thai Airways at Tokyo Narita

Airbus presented the following measures to save fuel, in its example of an A330 flying 2,500 nautical miles (4,600 km) on a route like Bangkok–Tokyo: direct routing saves 190 kg (420 lb) fuel by flying 40 km (25 mi) less; 600 kg (1,300 lb) more fuel is consumed if flying 600 m (2,000 ft) below optimum altitude without vertical flight profile optimization; cruising 0.01 mach above optimum consumes 800 kg (1,800 lb) more fuel; 1,000 kg (2,200 lb) more fuel on board consumes 150 kg (330 lb) more fuel while 100 litres (22 imp gal; 26 US gal) of unused potable water consumes 15 kg (33 lb) more fuel.[17]

Operational procedures can save 35 kg (77 lb) fuel for every 10-minute reduction in use of the Auxiliary power unit (APU), 15 kg (33 lb) with a reduced flap approach and 30 kg (66 lb) with reduced thrust reversal on landing.[17] Maintenance can also save fuel: 100 kg (220 lb) more fuel is consumed without an engine wash schedule; 50 kg (110 lb) with a 5 mm (0.20 in) slat rigging gap, 40 kg (88 lb) with a 10 mm (0.39 in) spoiler rigging gap, and 15 kg (33 lb) with a damaged door seal.[17]

History

Past

The earliest jet airliner, the de Havilland Comet

Modern jet aircraft have twice the fuel efficiency of the earliest jet airliners.[18] Late 1950s piston airliners like the Lockheed L-1049 Super Constellation and DC-7 were 1% to 28% more energy-intensive than 1990s jet airliners which cruise 40 to 80% faster.[19] The early jet airliners were designed at a time when air crew labor costs were higher relative to fuel costs. Despite the high fuel consumption, because fuel was inexpensive in that era the higher speed resulted in favorable economical returns since crew costs and amortization of capital investment in the aircraft could be spread over more seat-miles flown per day.[20] Productivity including speed went from around 150 ASK/MJ*km/h for the 1930s DC-3 to 550 for the L-1049 in the 1950s, and from 200 for the DH-106 Comet 3 to 900 for the 1990s B737-800.[21]

Today's turboprop airliners have better fuel-efficiency than current jet airliners, in part because of their lower cruising speed and propellers that are more efficient than those of the 1950s-era piston-powered airlines.[16]

The Bombardier CSeries 300 is the most fuel efficient among its competitors, the A319neo and 737 MAX7[22]

Jet aircraft efficiency is improving: between 1960 and 2000 there was a 55% overall fuel-efficiency gain (as compared to the Boeing 707).[19] Most of the improvements in efficiency were gained in the first decade when jet craft first came into widespread commercial use. Between 1971 and 1998 the fleet-average annual improvement per available seat-kilometre was estimated at 2.4%. Nonetheless, from the 1960s to the present, total fleet emissions of the greenhouse gas CO2 have outpaced efficiency gains, due to growth in the air-travel market.[23]

Concorde, a supersonic transport, managed about 17 passenger-miles to the Imperial gallon, which is 16.7 L/100 km per passenger; similar to a business jet, but much worse than a subsonic turbofan aircraft. Airbus states a fuel rate consumption of their A380 at less than 3 L/100 km per passenger (78 passenger-miles per US gallon);[24] however this is for an unspecified number and configuration of seats. The maximum configuration is 850 economy passengers, but with a typical 525-seat multi-class configuration the fuel consumption is "comparable to that of a B747-400ER and even about 15% worse than a B777-300ER on a passenger-mile basis."[25]

Example values

The aviation fuel density used is 6.7 lb/USgal or 0.8 kg/l.

Commuter flights

For flights of 300 nmi (560 km):

Model First flight Seats Fuel burn Fuel per seat
ATR 42-500 1995 48 1.26 kg/km (4.5 lb/mi) 3.15 L/100 km (75 mpg‑US)[26]
ATR 72-500 1997 70 1.42 kg/km (5.0 lb/mi) 2.53 L/100 km (93 mpg‑US)[26]
Beechcraft 1900D (226 nm) 1982 19 1.00 kg/km (3.56 lb/mi) 6.57 L/100 km (35.8 mpg‑US)[27]
Bombardier Dash 8 Q400 1998 78 2.16 kg/km (7.7 lb/mi) 3.46 L/100 km (68.0 mpg‑US)[28]
Dornier 228 1981 19 0.94 kg/km (3.3 lb/mi) 6.22 L/100 km (37.8 mpg‑US)[29]
Dornier 328 1991 32 1.22 kg/km (4.3 lb/mi) 4.76 L/100 km (49.4 mpg‑US)[30]
Embraer Brasilia 1983 30 0.92 kg/km (3.3 lb/mi) 3.82 L/100 km (61.6 mpg‑US)[31]
Pilatus PC-12 (500 nm) 1991 9 0.41 kg/km (1.5 lb/mi) 5.66 L/100 km (41.6 mpg‑US)[32]
Saab 340 1983 32 1.1 kg/km (3.9 lb/mi) 4.29 L/100 km (54.8 mpg‑US)[33]
Saab 2000 1992 50 1.75 kg/km (6.2 lb/mi) 4.39 L/100 km (53.6 mpg‑US)[34]
Xian MA700 2019 78 1.69 kg/km (6.0 lb/mi) 2.71 L/100 km (87 mpg‑US)[35]

Regional flights

For flights of 500–660 nmi (930–1,220 km)

Model First flight Seats Sector Fuel burn Fuel efficiency per seat
Airbus A319neo 2015 144 600 nmi (1,100 km) 3.37 kg/km (11.94 lb/mi) 2.92 L/100 km (80.6 mpg‑US)[36]
Airbus A319neo 2015 124 660 nmi (1,220 km) 2.82 kg/km (10 lb/mi) 2.82 L/100 km (83.5 mpg‑US)[37]
Airbus A320neo 2015 154 660 nmi (1,220 km) 2.79 kg/km (9.9 lb/mi) 2.25 L/100 km (104.7 mpg‑US)[37]
Airbus A321neo 2015 192 660 nmi (1,220 km) 3.30 kg/km (11.7 lb/mi) 2.19 L/100 km (107.4 mpg‑US)[37]
Boeing 737-300 1984 126 507 nmi (939 km) 3.49 kg/km (12.4 lb/mi) 3.46 L/100 km (68 mpg‑US)[38]
Boeing 737-600 1998 110 500 nmi (930 km) 3.16 kg/km (11.2 lb/mi) 3.59 L/100 km (65.5 mpg‑US)[39]
Boeing 737-700 1997 126 500 nmi (930 km) 3.21 kg/km (11.4 lb/mi) 3.19 L/100 km (74 mpg‑US)[39]
Boeing 737 MAX 7 2017 128 660 nmi (1,220 km) 2.85 kg/km (10.1 lb/mi) 2.77 L/100 km (84.8 mpg‑US)[37]
Boeing 737 MAX 7 2017 144 600 nmi (1,100 km) 3.39 kg/km (12.01 lb/mi) 2.93 L/100 km (80.2 mpg‑US)[36]
Boeing 737-800 1997 162 500 nmi (930 km) 3.59 kg/km (12.7 lb/mi) 2.77 L/100 km (85 mpg‑US)[39]
Boeing 737 MAX 8 2017 166 660 nmi (1,220 km) 3.04 kg/km (10.8 lb/mi) 2.28 L/100 km (103.2 mpg‑US)[37]
Boeing 737-900ER 2006 180 500 nmi (930 km) 3.83 kg/km (13.6 lb/mi) 2.66 L/100 km (88 mpg‑US)[39]
Boeing 737 MAX 9 2017 180 660 nmi (1,220 km) 3.30 kg/km (11.7 lb/mi) 2.3 L/100 km (103 mpg‑US)[37]
Boeing 757-200 1982 200 500 nmi (930 km) 4.68 kg/km (16.61 lb/mi) 2.91 L/100 km (80.7 mpg‑US)[40]
Boeing 757-300 1998 243 500 nmi (930 km) 5.19 kg/km (18.41 lb/mi) 2.66 L/100 km (88.4 mpg‑US)[40]
Bombardier CRJ1000 2009 100 500 nmi (930 km) 2.66 kg/km (9.4 lb/mi) 3.33 L/100 km (71 mpg‑US) [41]
Airbus A220 100 2013 115 600 nmi (1,100 km) 2.8 kg/km (10.1 lb/mi) 3.07 L/100 km (76.7 mpg‑US)[36]
Airbus A220 300 2015 140 600 nmi (1,100 km) 3.10 kg/km (11.01 lb/mi) 2.75 L/100 km (85.6 mpg‑US)[36]
Airbus A220-100 2013 125 500 nmi (930 km) 2.57 kg/km (9.1 lb/mi) 2.57 L/100 km (92 mpg‑US)[42]
Airbus A220-300 2015 160 500 nmi (930 km) 2.85 kg/km (10.11 lb/mi) 2.23 L/100 km (105 mpg‑US)[43]
Bombardier Dash 8 Q400 1998 82 600 nmi (1,100 km) 1.83 kg/km (6.5 lb/mi) 2.79 L/100 km (84 mpg‑US)[44]
Dornier 328 1991 31 600 nmi (1,100 km) 1.08 kg/km (3.8 lb/mi) 4.35 L/100 km (54.1 mpg‑US)[45]
Embraer E-Jet E2-175 2020 88 600 nmi (1,100 km) 2.44 kg/km (8.64 lb/mi) 3.44 L/100 km (68.3 mpg‑US)[36]
Embraer E-Jet E2-190 2018 106 600 nmi (1,100 km) 2.83 kg/km (10.04 lb/mi) 3.32 L/100 km (70.8 mpg‑US)[36]
Embraer E-Jet E2-195 2019 132 600 nmi (1,100 km) 3.07 kg/km (10.91 lb/mi) 2.9 L/100 km (81 mpg‑US)[36]
Embraer E-Jet-190 2002 98 500 nmi (930 km) 2.98 kg/km (10.6 lb/mi) 3.81 L/100 km (61.7 mpg‑US)[46]
Saab 340 1983 31 500 nmi (930 km) 0.95 kg/km (3.4 lb/mi) 3.83 L/100 km (61.4 mpg‑US)[33]
Saab 2000 1992 50 500 nmi (930 km) 1.54 kg/km (5.5 lb/mi) 3.85 L/100 km (61.1 mpg‑US)[34]
Sukhoi SSJ100 2008 98 500 nmi (930 km) 2.81 kg/km (10.0 lb/mi) 3.59 L/100 km (65.5 mpg‑US)[46]
Xian MA700 2019 78 650 nmi (1,200 km) 1.56 kg/km (5.5 lb/mi) 2.5 L/100 km (94 mpg‑US)[35]

Short-haul flights

For flights of 1,000 nmi (1,900 km):

Model First flight Seats Fuel Burn Fuel efficiency per seat
Airbus A319 1995 124 2.93 kg/km (10.4 lb/mi) 2.95 L/100 km (80 mpg‑US)[47]
Airbus A319Neo 2015 136 2.4 kg/km (8.6 lb/mi) 1.93 L/100 km (122 mpg‑US)[22]
Airbus A320 1987 150 3.13 kg/km (11.1 lb/mi) 2.61 L/100 km (90 mpg‑US)[47]
Airbus A321-200 1996 180 3.61 kg/km (12.8 lb/mi) 2.5 L/100 km (94 mpg‑US)[47]
Airbus A330-200 1997 293 5.6 kg/km (19.8 lb/mi) 2.37 L/100 km (99 mpg‑US)[47]
Boeing 737-600 1998 110 2.77 kg/km (9.8 lb/mi) 3.15 L/100 km (75 mpg‑US)[39]
Boeing 737-700 1997 126 2.82 kg/km (10.0 lb/mi) 2.79 L/100 km (84 mpg‑US)[39]
Boeing 737-700 1997 128 2.8 kg/km (9.9 lb/mi) 2.71 L/100 km (87 mpg‑US)[47]
Boeing 737 MAX-7 2017 140 2.51 kg/km (8.91 lb/mi) 1.94 L/100 km (121 mpg‑US)[22]
Boeing 737-800 1997 162 3.17 kg/km (11.2 lb/mi) 2.44 L/100 km (96 mpg‑US)[39]
Boeing 737-800 1997 160 3.45 kg/km (12.23 lb/mi) 2.68 L/100 km (88 mpg‑US)[47]
Boeing 737-800W 1997 162 3.18 kg/km (11.3 lb/mi) 2.45 L/100 km (96 mpg‑US)[48]
Boeing 737 MAX-8 2017 162 2.71 kg/km (9.6 lb/mi) 2.04 L/100 km (115 mpg‑US)[48]
Boeing 737-900ER 2006 180 3.42 kg/km (12.1 lb/mi) 2.38 L/100 km (99 mpg‑US)[39]
Boeing 737-900ERW 2006 180 3.42 kg/km (12.1 lb/mi) 2.37 L/100 km (99 mpg‑US)[48]
Boeing 737 MAX-9 2017 180 2.91 kg/km (10.3 lb/mi) 2.02 L/100 km (116 mpg‑US)[48]
Boeing 757-200 1982 190 4.60 kg/km (16.33 lb/mi) 3.02 L/100 km (78 mpg‑US)[47]
Boeing 757-200 1982 200 4.16 kg/km (14.76 lb/mi) 2.59 L/100 km (90.8 mpg‑US)[40]
Boeing 757-300 1998 243 4.68 kg/km (16.62 lb/mi) 2.4 L/100 km (98 mpg‑US)[40]
Airbus A220-100 2013 125 2.28 kg/km (8.1 lb/mi) 2.28 L/100 km (103 mpg‑US)[42]
Airbus A220-300 2015 160 2.56 kg/km (9.08 lb/mi) 2.00 L/100 km (118 mpg‑US)[43]
Airbus A220-300 2015 135 2.30 kg/km (8.17 lb/mi) 1.85 L/100 km (127 mpg‑US)[22]
Quest Kodiak 2004 9 0.71 kg/km (2.52 lb/mi) 6.28 L/100 km (37.5 mpg‑US)[49]

Medium-haul flights

For flights of 1,750–3,400 nmi (3,240–6,300 km). The larger end of this range includes transatlantic flights (e.g. New York JFK – London-Heathrow is 3,000 nmi).[50]

Model First flight Seats Sector Fuel burn Fuel per seat
Airbus A320 1987 150 2,151 nmi (3,984 km) 2.91 kg/km (10.3 lb/mi) 2.43 L/100 km (97 mpg‑US)[51]
Airbus A321NeoLR 2016 154 3,400 nmi (6,300 km) 2.99 kg/km (10.6 lb/mi) 2.43 L/100 km (97 mpg‑US)[52]
Airbus A330-200 1997 241 3,000 nmi (5,600 km) 6 kg/km (21 lb/mi) 3.11 L/100 km (76 mpg‑US)[53]
Airbus A330-300 1992 262 3,000 nmi (5,600 km) 6.25 kg/km (22.2 lb/mi) 2.98 L/100 km (79 mpg‑US)[53]
Airbus A330neo-900 2016 310 3,350 nmi (6,200 km) 6 kg/km (21 lb/mi) 2.42 L/100 km (97 mpg‑US)[54]
Airbus A340-300 1992 262 3,000 nmi (5,600 km) 6.81 kg/km (24.2 lb/mi) 3.25 L/100 km (72 mpg‑US)[53]
Boeing 737 MAX-8 2017 168 3,400 nmi (6,300 km) 2.86 kg/km (10.1 lb/mi) 2.13 L/100 km (110 mpg‑US)[55]
Boeing 737 MAX-9 2017 144 3,400 nmi (6,300 km) 2.91 kg/km (10.3 lb/mi) 2.53 L/100 km (93 mpg‑US)[52]
Boeing 747-400 1988 416 2,151 nmi (3,984 km) 10.77 kg/km (38.2 lb/mi) 3.24 L/100 km (73 mpg‑US)[51]
Boeing 747-8 2011 467 3,000 nmi (5,600 km) 9.9 kg/km (35 lb/mi) 2.65 L/100 km (89 mpg‑US)[56]
Boeing 757-200W 1981 158 3,400 nmi (6,300 km) 3.79 kg/km (13.4 lb/mi) 3 L/100 km (78 mpg‑US)[52]
Boeing 767-200ER 1984 181 3,000 nmi (5,600 km) 4.83 kg/km (17.1 lb/mi) 3.34 L/100 km (70 mpg‑US)[57]
Boeing 767-200ER 1984 193 3,400 nmi (6,300 km) 5.01 kg/km (17.8 lb/mi) 3.25 L/100 km (72 mpg‑US)[52]
Boeing 767-200ER 1984 224 3,000 nmi (5,600 km) 4.93 kg/km (17.5 lb/mi) 2.75 L/100 km (86 mpg‑US)[57]
Boeing 767-300ER 1988 218 2,151 nmi (3,984 km) 5.38 kg/km (19.1 lb/mi) 3.09 L/100 km (76 mpg‑US)[51]
Boeing 767-300ER 1988 218 3,000 nmi (5,600 km) 5.39 kg/km (19.1 lb/mi) 3.09 L/100 km (76 mpg‑US)[57]
Boeing 767-300ER 1988 269 3,000 nmi (5,600 km) 5.51 kg/km (19.5 lb/mi) 2.56 L/100 km (92 mpg‑US)[57]
Boeing 767-400ER 1999 245 3,000 nmi (5,600 km) 5.78 kg/km (20.5 lb/mi) 2.95 L/100 km (80 mpg‑US)[57]
Boeing 767-400ER 1999 304 3,000 nmi (5,600 km) 5.93 kg/km (21.0 lb/mi) 2.44 L/100 km (96 mpg‑US)[57]
Boeing 767-400ER 1999 304 3,265 nmi (6,047 km) 5.92 kg/km (21 lb/mi) 2.43 L/100 km (96.9 mpg‑US)[38]
Boeing 777-200 1994 305 3,000 nmi (5,600 km) 6.83 kg/km (24.2 lb/mi) 2.8 L/100 km (84 mpg‑US)[58]
Boeing 777-200ER 1996 301 3,000 nmi (5,600 km) 6.96 kg/km (24.7 lb/mi) 2.89 L/100 km (81 mpg‑US)[53]
Boeing 777-300 1997 368 3,000 nmi (5,600 km) 7.88 kg/km (28.0 lb/mi) 2.68 L/100 km (88 mpg‑US)[58]
Boeing 787-8 2009 291 3,400 nmi (6,300 km) 5.26 kg/km (18.7 lb/mi) 2.26 L/100 km (104 mpg‑US)[55]
Boeing 787-8 2009 238 3,400 nmi (6,300 km) 5.11 kg/km (18.1 lb/mi) 2.68 L/100 km (88 mpg‑US)[52]
Boeing 787-9 2013 304 3,350 nmi (6,200 km) 5.77 kg/km (20.5 lb/mi) 2.37 L/100 km (99 mpg‑US)[54]
Irkut MC-21 2017 163 1,750 nmi (3,240 km) 3.04 kg/km (10.8 lb/mi) 2.33 L/100 km (101 mpg‑US)[59]

Long-haul flights

For flights of 4,650–7,200 nmi (8,610–13,330 km). This includes transpacific flights (e.g. Hong Kong international – San Francisco International is 6,000 nmi).[60]

Model First flight Seats Sector Fuel burn Fuel per seat
Airbus A330-200 1997 241 6,000 nmi (11,000 km) 6.4 kg/km (23 lb/mi) 3.32 L/100 km (71 mpg‑US)[53]
Airbus A330neo-800 2017 248 4,650 nmi (8,610 km) 5.45 kg/km (19.3 lb/mi) 2.75 L/100 km (86 mpg‑US)[61]
Airbus A330neo-900 2017 300 4,650 nmi (8,610 km) 5.94 kg/km (21.1 lb/mi) 2.48 L/100 km (95 mpg‑US)[61]
Airbus A340-300 1992 262 6,000 nmi (11,000 km) 7.32 kg/km (26.0 lb/mi) 3.49 L/100 km (67.4 mpg‑US)[53]
Airbus A350-900 2013 315 4,972 nmi (9,208 km) 6.03 kg/km (21.4 lb/mi) 2.39 L/100 km (98 mpg‑US)[54]
Airbus A350-900 2013 315 6,542 nmi (12,116 km) 7.07 kg/km (25.1 lb/mi) 2.81 L/100 km (84 mpg‑US)[62]
Airbus A380 2005 525 7,200 nmi (13,300 km) 13.78 kg/km (48.9 lb/mi) 3.27 L/100 km (72 mpg‑US)[63]
Airbus A380 2005 544 6,000 nmi (11,000 km) 13.78 kg/km (48.9 lb/mi) 3.16 L/100 km (74 mpg‑US)[64]
Boeing 747-400 1988 416 6,000 nmi (11,000 km) 11.11 kg/km (39.4 lb/mi) 3.34 L/100 km (70 mpg‑US)[65]
Boeing 747-8 2011 467 6,000 nmi (11,000 km) 10.54 kg/km (37.4 lb/mi) 2.82 L/100 km (83 mpg‑US)[56]
Boeing 747-8 2011 405 7,200 nmi (13,300 km) 10.9 kg/km (39 lb/mi) 3.35 L/100 km (70 mpg‑US)[63]
Boeing 777-200ER 1996 301 6,000 nmi (11,000 km) 7.42 kg/km (26.3 lb/mi) 3.08 L/100 km (76 mpg‑US)[53]
Boeing 777-200ER 1996 301 6,000 nmi (11,000 km) 7.44 kg/km (26.4 lb/mi) 3.09 L/100 km (76 mpg‑US)[58]
Boeing 777-200LR 2005 291 4,972 nmi (9,208 km) 7.57 kg/km (26.9 lb/mi) 3.25 L/100 km (72 mpg‑US)[54]
Boeing 777-300ER 2003 365 6,000 nmi (11,000 km) 8.49 kg/km (30.1 lb/mi) 2.91 L/100 km (81 mpg‑US)[58]
Boeing 777-300ER 2003 344 7,200 nmi (13,300 km) 8.58 kg/km (30.4 lb/mi) 3.11 L/100 km (76 mpg‑US)[63]
Boeing 777-9X 2020 395 7,200 nmi (13,300 km) 9.04 kg/km (32.1 lb/mi) 2.85 L/100 km (83 mpg‑US)[63]
Boeing 787-8 2011 243 4,650 nmi (8,610 km) 5.38 kg/km (19.1 lb/mi) 2.77 L/100 km (85 mpg‑US)[61]
Boeing 787-9 2013 294 4,650 nmi (8,610 km) 5.85 kg/km (20.8 lb/mi) 2.49 L/100 km (94 mpg‑US)[61]
Boeing 787-9 2013 304 4,972 nmi (9,208 km) 5.63 kg/km (20.0 lb/mi) 2.31 L/100 km (102 mpg‑US)[54]
Boeing 787-9 2013 291 6,542 nmi (12,116 km) 7.18 kg/km (25.5 lb/mi) 3.08 L/100 km (76 mpg‑US)[62]

For comparison, a Volvo bus 9700 averages 0.41 L/100 km (570 mpg‑US) per seat for 63 seats.[66] In highway travel an average auto has the potential for 1.61 L/100 km (146 mpg‑US)[67] per seat (assuming 4 seats) and for a 5-seat 2014 Toyota Prius, 0.98 L/100 km (240 mpg‑US).[68] While this shows the capabilities of the vehicles, the load factors (percentage of seats occupied) may differ between personal use (commonly just the driver in the car) and societal averages for long-distance auto use, and among those of particular airlines.

General aviation

For private aircraft in general aviation, the current official record (FAI "Efficiency Aircraft") is 37 km/kg fuel or 3.7 L/100 km in a Monnett Sonerai.[69]

Future

Boeing/NASA X-48B blended wing body demonstrator
Boeing blended wing body concept
NASA / Aurora Flight Sciences D8 airliner concept
Boeing Volt truss-braced wing concept

NASA and Boeing flight-tested a 500 lb (230 kg) blended wing body (BWB) X-48B demonstrator from August 2012 to April 2013. This design provides greater fuel efficiency, since the whole craft produces lift, not just the wings.[70] The BWB concept offers advantages in structural, aerodynamic and operating efficiencies over today's more-conventional fuselage-and-wing designs. These features translate into greater range, fuel economy, reliability and life-cycle savings, as well as lower manufacturing costs.[71][72] NASA has created a cruise efficient STOL (CESTOL) concept.

Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) have researched a sharkskin-imitating paint that would reduce drag through a riblet effect.[73] Aviation is a major potential application for new technologies such as aluminium metal foam and nanotechnology.

The International Air Transport Association (IATA) technology roadmap envisions improvements in aircraft configuration and aerodynamics. It projects the following reductions in fuel consumption:[74]

Today's tube-and-wing configuration could remain in use until the 2030s due to drag reductions from active flutter suppression for slender flexible-wings and natural and hybrid laminar flow. Large, ultra high bypass engines will need upswept gull wings or overwing nacelles as Pratt & Whitney continue to develop its geared turbofan to save a projected 10–15% of fuel costs by the mid-2020s. NASA indicates this configuration could gain up to 45% with advanced aerodynamics, structures and geared turbofans, but longer term suggests savings of up to 50% by 2025 and 60% by 2030 with new ultra-efficient configurations and propulsion architectures: hybrid wing body, truss-braced wing, lifting body designs, embedded engines, and boundary-layer ingestion. By 2030 hybrid-electric architectures may be ready for 100 seaters and distributed propulsion with tighter integration of airframe may enable further efficiency and emissions improvements.[75]

Research projects such as Boeing's ecoDemonstrator program have sought to identify ways of improving the fuel economy of commercial aircraft operations. The U.S. government has encouraged such research through grant programs, including the FAA's Continuous Lower Energy, Emissions and Noise (CLEEN) program, and NASA's Environmentally Responsible Aviation (ERA) Project.[citation needed]

Multiple concepts are projected to reduce fuel consumption:[76]

  • the Airbus/Rolls-Royce E-Thrust is an hybrid electric with a gas turbine engine and electric ducted fans with energy storage allowing peak power for takeoff and climb while for the descent the engine is shut down and the fans recover energy to recharge the batteries;
  • Empirical Systems Aerospace (ESAero) is developing the 150-seat ECO-150 concept for turboelectric distributed propulsion with two turboshaft engines mounted on the wing and driving generators powering ducted fans embedded in the inboard wing sections, effectively increasing the bypass ratio and propulsive efficiency for 20–30% fuel savings over the Boeing 737 NG, while providing some powered lift;
  • NASA's single-aisle turbo-electric aircraft with an aft boundary layer propulsor (STARC-ABL) is a conventional tube-and-wing 737-sized airliner with an aft-mounted electric fan ingesting the fuselage boundary layer hybrid-electric propulsion, with 5.4 MW of power distributed to three electric motors: the design will be evaluated by Aurora Flight Sciences;[77]
  • The Boeing blended wing body (BWB) with a wide fuselage mated to high-aspect-ratio wings is more aerodynamically efficient because the entire aircraft contributes to the lift and it has less surface area, producing less drag and offering weight savings due to lower wing loading, while noise is shielded by locating the engines on the aft upper surface;
  • Developed with the U.S. Air Force Research Laboratory and refined with NASA, the Lockheed Martin Hybrid Wing Body (HWB) combines a blended forward fuselage and wing with a conventional aft fuselage and T-tail for compatibility with existing infrastructure and airdrop; the engines in overwing nacelles on struts over the trailing edge enable higher-bypass-ratio engines with 5% less drag, provide acoustic shielding and increases lift without a thrust or drag penalty at low speed;
  • Airbus-backed German Bauhaus-Luftfahrt designed the Propulsive Fuselage concept, reducing drag with a fan in the tail ingesting air flowing over the fuselage via an annular (ring-shaped) inlet and re-energizes the wake, driven with a gearbox or as a turbo-electric configuration;
  • Conceived by the Massachusetts Institute of Technology for NASA, Aurora Flight Sciences developed the "double-bubble" D8, a 180-seat aircraft with a wide lifting fuselage, twin-aisle cabin to replace A320 and B737 narrowbodies, and boundary-layer ingestion with engines in the tail driving distortion-tolerant fans for a 49% fuel-burn reduction over the B737NG;
  • The Boeing truss-braced wing (TBW) concept was developed for the NASA-funded Subsonic Ultra Green Aircraft Research program with an aspect ratio of 19.5 compared to 11 for the Boeing 787: the strut relieves some bending moment and a braced wing can be lighter than a cantilevered wing or longer for the same weight, having better lift-to-drag ratio by lowering the induced drag and thinner, facilitating natural laminar flow and reducing wave drag at transonic speeds;
  • Dzyne Technologies reduces the thickness of the blended wing body for a 110–130-seat super-regional, a configuration usually too thick for a narrowbody replacement and better suited for large aircraft, by placing the landing gear outward and storing baggage in the wing roots, enabling 20% fuel savings;
  • the French research agency ONERA designed two concepts for a 180-seat airliner Versitalie Aircraft (NOVA) including turbofans with higher bypass ratios and fan diameter: a gull wing with increased dihedral inboard to accommodate larger geared turbofans under without lengthening the gear and the other with engines embedded in the tail to ingest the low-energy fuselage boundary layer flow and re-energize the wake to reduce drag;[78]
  • with Cranfield University, Rolls-Royce developed the Distributed Open Rotor (DORA) with high-aspect-ratio wing and V-tail to minimize drag, and turbogenerators on the wing driving electric propellers along the inboard leading edge with open rotor high-propulsive efficiency and increasing the effective bypass ratio.

Climate change

Main article: Environmental impact of aviation

The growth of air travel outpaces its fuel-economy improvements and corresponding CO2 emissions, compromising climate sustainability. Although low-cost carriers' higher seat-density increases fuel economy and lowers greenhouse gas emissions per-passenger-kilometer, the lower airfares cause a rebound effect of more flights and larger overall emissions. The tourism industry could shift emphasis to emissions eco-efficiency in CO2 per unit of revenue or profit instead of fuel economy, favoring shorter trips and ground transportation over flying long journeys to reduce greenhouse gas emissions.[79]

See also

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