There are two main types of aviation fuel. Commonly known as “jet fuel”, aviation turbine fuel is used in dozens of countries and hundreds of locations worldwide. There’s also aviation gasoline, usually known by its shortened name of Avgas, or sometimes known as “aviation spirit” in the UK. This is used in plane engines that run through spark ignition.
Both airline industry bodies and military aviation users issue detailed specifications for their fuels, which have to be followed by aviation fuel suppliers. For a general fuel supplier, it can make sense to adhere to the most demanding standards, to ensure that their fuel is acceptable across a wide range of requirements.
Jet A-1 is the fuel most widely used for jets in many locations. It has to conform to the Aviation Fuel Quality Requirements for Jointly Operated Systems (AFQRJOS). This standard enforces some precise specifications and stringent standards for cleanliness and safety. There are inspections of fuel to ensure that it adheres to these standards. There are also some other standards for jet fuel, often for specific military uses.
Because the standards and testing regime is complex, specialist fuel companies often produce fuel batches for initial testing or carry out testing to specific requirements. New fuels that are being developed need to be researched in this way because they are not yet in production in a refinery.
Like other types of oil-based fuels, jet fuel is made at oil refineries through a refining, distillation and filtration process. All the different types of aviation fuel start off being produced in the same way. During distillation, the raw materials (“feedstocks”) are separated according to their boiling point, into different flows. Then the materials are boiled, producing kerosene as the basis for the jet fuel.
Once they have been distilled, they are then filtered and processed again, this time to get rid of metals, sulphur or acids that may be present in the fuel but could be harmful to the plane’s engine, or could increase unwanted emissions.
After this, the different fuel streams are precisely reblended to give the exact specification for any given fuel. The reblending recipe is what makes the difference between the different grades of jet fuel. Once the fuel blend is ready, additives are used. All aviation fuel suppliers use these to enhance the performance of the fuel and its stability, and bring it closer to the exact specifications required for the different blends.
Avgas, the fuel for non-jet aviation, is not a particularly clean product. Many of the Avgas fuel mixes in use today have barely changed since they were first developed in the 1940s. As with older car engines, there can be “knocking” - technically known as detonation - and so a lead additive has to be used to prevent this. Research is currently underway to develop fuels for spark ignition plane engines that don’t need this toxic additive.