Aviation fuel


Aviation fuels are either derived from petroleum or are blends of petroleum and synthetic fuels, and are used to power aircraft. These fuels have more stringent requirements than those used for ground-based applications, such as heating or road transportation. They also contain additives designed to enhance or preserve specific properties that are important for performance and handling. Most aviation fuels are kerosene-based—such as JP-8 and Jet A-1—and are used in gas turbine-powered aircraft. Piston-engined aircraft typically use leaded gasoline, while those equipped with diesel engines may use jet fuel. As of 2012, all U.S. Air Force aircraft had been certified to operate on a 50-50 blend of kerosene and synthetic fuel derived from coal or natural gas, as part of an initiative to stabilize fuel costs.

Types of aviation fuel

Conventional aviation fuels

Jet fuel

Jet fuel is a clear to straw-colored fuel, based on either an unleaded kerosene, or a naphtha–kerosene blend. Similar to diesel fuel, it can be used in either compression ignition engines or turbine engines.
Jet-A powers modern commercial airliners and is a mix of extremely refined kerosene which burns at temperatures at or above. Kerosene-based fuel has a much higher flash point than gasoline-based fuel, meaning that it requires significantly higher temperature to ignite. It is a high-quality fuel; if it fails the purity and other quality tests for use on jet aircraft, it is sold to ground-based users with less demanding requirements, such as railroads.

Avgas

Avgas, or aviation spirit, is used by small aircraft, light helicopters and vintage piston-engined aircraft. Its formulation is distinct from the conventional gasoline used in motor vehicles, which is commonly called mogas or autogas in aviation context. Although it comes in many different grades, its octane ratings are generally much higher than those of road motor gasoline.

Emerging aviation fuels

Biofuels

Alternatives to conventional fossil-based aviation fuels, new fuels made via the biomass to liquid method and certain straight vegetable oils can also be used.
Fuels such as sustainable aviation fuel have the advantage that few or no modifications are necessary on the aircraft itself, provided that the fuel characteristics meet specifications for lubricity and density as well as adequately swelling elastomer seals in current aircraft fuel systems. Sustainable aviation fuel and blends of fossil and sustainably-sourced alternative fuels yield lower emissions of particles and greenhouse gases. They are, however, not being used heavily, because they still face political, technological, and economic barriers, such as currently being more expensive than conventionally produced aviation fuels by a wide margin.

Compressed natural gas and liquified natural gas

and liquified natural gas are fuel feedstocks that aircraft may use in the future. Studies have been done on the feasibility of using natural gas and include the "SUGAR Freeze" aircraft under NASA's N+4 Advanced Concept Development program. The Tupolev Tu-155 was an alternative fuel testbed which was fuelled on LNG. The low specific energy of natural gas even in liquid form compared to conventional fuels gives it a distinct disadvantage for flight applications.

Liquid hydrogen

can be used largely free of carbon emissions, if it is produced with power from renewable energy like wind and solar power.
Some development of technology for hydrogen-powered aircraft started after the millennium and gained track since about 2020, but as of 2022 was still far away from outright aircraft product development.
Hydrogen fuel cells do not produce or other emissions. However, hydrogen combustion does produce emissions. Cryogenic hydrogen can be used as a liquid at temperatures below. Gaseous hydrogen involves pressurized tanks at. With materials available in the 2020s, the mass of tanks strong enough to withstand this kind of high pressure will greatly outweigh the hydrogen fuel itself, largely negating the weight to energy advantage of hydrogen fuel over hydrocarbon fuels. Hydrogen has a severe volumetric disadvantage relative to hydrocarbon fuels, but future blended wing body aircraft designs might be able to accommodate this extra volume without greatly expanding the wetted area.
Even if finally practical, the industry timeline for adopting hydrogen is fairly lengthy. Alternatives to conventional aviation fuel available in the near term include aviation biofuel and synthetically created fuel. These fuels are collectively referred to as "Sustainable Aviation Fuel".

Production of aviation fuel

The production of aviation fuel falls into two categories: fuel suitable for turbine engines and fuel suitable for spark-ignition piston engines. There are international specifications for each.
Jet fuel is a gas turbine fuel used in propeller and jet fixed-wing aircraft and helicopters. It has a low viscosity at low temperature, has limited ranges of density and calorific value, burns cleanly, and remains chemically stable when heated to high temperature.
Aviation gasoline, often referred to as avgas or 100-LL, is a highly refined form of gasoline for aircraft, with an emphasis on purity, anti-knock characteristics and minimization of spark plug fouling. Avgas must meet performance guidelines for both the rich mixture condition required for take-off power settings and the leaner mixtures used during cruise to reduce fuel consumption. Aviation fuel can be used as CNG fuel.
Avgas is sold in much lower volume than jet fuel, but to many more individual aircraft operators, whereas jet fuel is sold in high volume to large aircraft operators, such as airlines and militaries.

Energy content

The net energy content for aviation fuels depends on their composition. Some typical values are:
  • BP Avgas 80,, density at is .
  • Kerosene type BP Jet A-1,, density at 15 °C is .
  • Kerosene type BP Jet TS-1,, density at 15 °C is .

    Density

In performance calculations, airliner manufacturers use a density of jet fuel around.
Specific cases are:
Aviation fuels consist of blends of over two thousand chemicals, primarily hydrocarbons, additives such as antioxidants and metal deactivators, biocides, static reducers, icing inhibitors, corrosion inhibitors, and impurities. Principal components include n-heptane and isooctane. Like other fuels, aviation fuel for spark-ignited piston engines are described by their octane rating.
Alcohol, alcohol mixtures, and other alternative fuels may be used experimentally, but alcohol is not permitted in any certified aviation fuel specification. In Brazil, the Embraer Ipanema EMB-202A is a version of the Ipanema agricultural aircraft with a modified Lycoming IO-540-K1J5 engine so as to be able to run on ethanol. Other aircraft engines that were modified to run on 100% ethanol were several other types of Lycoming engines and certain Rotax engines.

Tax

The Convention on International Civil Aviation exempts air fuels already loaded onto an aircraft on landing from import taxes. Bi-lateral air services agreements govern the tax exemption of aviation fuels. In the course of an EU initiative, many of these agreements have been modified to allow taxation. A motion for a European Parliament resolution on a European Strategy for Low-emission Mobility has stated that "the possibilities for harmonised international measures for kerosene taxation for aviation" needs to be explored.
A worry is that a local aviation fuel tax would cause increased tankering, where airlines carry extra fuel from low tax jurisdictions. This extra weight increases fuel burn, thus a local fuel tax could potentially increase overall fuel consumption. To avoid increased tankering, a worldwide aviation fuel tax has been proposed. Australia and the United States oppose a worldwide aviation fuel tax, but a number of other countries have expressed interest.
During a debate in the UK Parliament, the foregone tax income due to the exemption of tax on aviation fuel was estimated at £10 billion annually.
The planned inclusion of international aviation into the European Union Emission Trading Scheme in 2014 has been called an "illegal tax" by countries including the US and China, which cite the Chicago Convention.

Certification

Fuels have to conform to a specification in order to be approved for use in type certificated aircraft. The American Society for Testing and Materials developed specifications for automobile gasoline as well as aviation gasoline. These specifications are ASTM D910 and ASTM D6227 for aviation gasoline and ASTM D439 or ASTM D4814 for automobile gasoline.

In use

Aviation fuel generally arrives at the airport via pipeline systems, such as the CEPS. It is then pumped over and dispensed from a tanker or bowser. The fuel is then driven up to parked aircraft and helicopters. Some airports have pumps similar to filling stations to which aircraft must taxi. Some airports have permanent piping to parking areas for large aircraft.
Aviation fuel is transferred to an aircraft via one of two methods: overwing or underwing.

Overwing

Overwing fueling is used on smaller planes, helicopters, and all piston-engine aircraft. Overwing fueling is similar to car fueling — one or more fuel ports are opened and fuel is pumped in with a conventional pump.

Underwing

Underwing fueling, also called single-point refueling or pressure refueling where not dependent on gravity, is used on larger aircraft and for jet fuel exclusively.
For pressure refueling, a high-pressure hose is attached and fuel is pumped in at and a maximum of for most commercial aircraft. Pressure for military aircraft, especially fighters, ranges up to. Air being displaced in the tanks is usually vented overboard through a single vent on the aircraft. Because there is only one attachment point, fuel distribution between tanks is either automated or controlled from a control panel, either at the fueling point or in the cockpit. An early use of pressure refueling was on the de Havilland Comet and Sud Aviation Caravelle. Larger aircraft allow for two or more attachment points; however, this is still referred to as single-point refueling, as either attachment point can refuel all of the tanks. Multiple attachments allow for a faster flowrate.