Aviation fuels are petroleum-based fuels, or petroleum and synthetic fuel blends, used to power aircraft.
They have more stringent requirements than fuels used for ground use, such as heating and road transport, and contain additives to enhance or maintain properties important to fuel performance or handling.
They are kerosene-based (JP-8 and Jet A-1) for gas turbine-powered aircraft. Piston-engined aircraft use gasoline and those with diesel engines may use jet fuel (kerosene).
By 2012 all aircraft operated by the U.S. Air Force had been certified to use a 50-50 blend of kerosene and synthetic fuel derived from coal or natural gas as a way of stabilizing the cost of fuel.
Specific energy is an important criterion in selecting fuel for an aircraft. The much higher energy storage capability of hydrocarbon fuels compared to batteries has so far prevented electric aircraft from using electric batteries as the main propulsion energy store from becoming viable for even small personal aircraft.
As aviation moves into the renewables era, hydrogen-powered aircraft might enter common use. These are zero-emission for CO2 but not for NO x. Cryogenic hydrogen can be used as a liquid at temperatures below 20 K.
Gaseous hydrogen involves pressurized tanks at 250–350 bar. 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. An alternative to conventional aviation fuel available in the near term is aviation biofuel, sometimes termed sustainable aviation fuel (SAF).
Sustainable Aviation Fuel
SAF is a so-called drop-in fuel, which means that it can be blended with fossil jet fuel and that the blended fuel requires no special infrastructure or equipment changes.
Once it is blended, our fuel is fully certified (ASTM D1655/ DEFSTAN 91-91) and has the same characteristics, and meets the same specifications as fossil jet fuel.
Since the first commercial flight operated by KLM in 2011, more than 150,000 flights were powered by SAF. To date, SkyNRG has supplied SAF that’s produced from waste oils with the so-called HEFA technology.
The United States of America is the top country by the output of jet fuel in the world. As of December 2021, the output of jet fuel in the United States of America was 1,493.7 thousand barrels per day. The top 5 countries also include India, the Republic of Korea, Japan, and the Netherlands.
How is SAF produced?
The SAF production process initially requires the production of feedstock that can be of four types: oil, sugar, starch, and lignocellulosic.
The latter primary stands for the components of vegetal biomass, such as wood and wood residues. There are several sources for obtaining the respective feedstocks, such as agriculture, forestry, organic residues, other waste materials, among others.
After sourcing the feedstocks, they undergo a pre-treatment process intending to achieve the required quality to feed the conversion process, which will finally produce the SAF.
To date, there is no SAF conversion process that produces the complete range of substances of conventional jet fuel, such as aromatics that are needed to avoid fuel leakages.
Therefore, to ensure fuel compatibility with the aircraft engine and fueling infrastructure, the drop-in share or blending ratio of SAF with conventional fuels is limited to 50%.
At present, the international certification association ASTM has approved eight conversion processes. ASTM is responsible for fuel certification and security of commercial aviation fuels.
These processes are specified in the ‘Standard Specification for Aviation Turbine Fuel Containing Synthesised Hydrocarbons’, ASTM D7566. This specification covers in eight annexes each of the individual existing conversion processes.
Aviation Fuel Types for General Use
There are two main types of aviation fuel used in general aviation: jet fuel and AVGAS. Jet fuel is a refined kerosene-based, clear, or straw-colored liquid that is primarily used to power turbine engines, such as turboprop and jet engines.
There are several types of jet fuel, with the main ones being:
- Jet A is primarily used in the United States. This fuel is developed to be heavier with a higher flash point and freezing point than standard kerosene.
- Jet A1 is the most used jet fuel worldwide. Jet A1 has a lower freezing point (-47° C) than Jet A (-40° C) so it is especially suitable for international travel through varying climates. This type of fuel also contains static dissipater additives that decrease static charges that form during movement. Despite the differences between Jet A and Jet A1, flight operators use both fuels interchangeably.
- Jet B is the most common alternative to jet fuel and AVGAS, primarily used in civil aviation. Jet B has a uniquely low freezing point of -76° C, making it useful in extremely cold areas.
- AVGAS (short for “aviation gasoline”) is used by traditional propellor aircraft and small piston-engine airplanes. Tasks that use AVGAS-based aircraft are typically on a smaller scale and include crop-dusting, private flying, and flight training.
- AVGAS notably is still produced tetraethyl lead (TEL), which is a toxic additive used to prevent engine knocking. However, there are currently research studies and experiments aimed to reduce and eliminate the usage of TEL. G100UL, the first unleaded AVGAS (produced by General Electric), was FAA approved in mid-2021!
Sustainable Aviation Fuel (SAF) is a clean substitute for fossil jet fuels. Rather than being refined from petroleum, SAF is produced from sustainable resources such as waste oils from a biological origin, Agri residues, or non-fossil CO2.
Aviation fuels consist of blends of over two thousand chemicals, primarily hydrocarbons (paraffin, olefins, naphthenes, and aromatics), additives such as antioxidants, 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 is 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.