What is aviation fuel?

In a refinery, crude oil is heated and separated by boiling-temperature ranges. Gasoline comes off the top of the column; gas oil and fuel oil from the bottom. Between the two, around 150-290°C, kerosene is collected, and from that cut — after sulphur, acids and unstable compounds are removed through hydrotreatment — aviation fuel is born. A typical barrel yields around 10% jet kerosene, an amount the refinery adjusts according to seasonal demand and its price relative to diesel, with which it competes for the same molecules.

The first jet engines of the 1940s ran on gasoline or improvised blends. When commercial aviation adopted the turbine in the 1950s, kerosene became the standard for several reasons. It is less volatile, so vapours in the tank are harder to ignite and evaporation losses at altitude are lower. It is denser, and in an aircraft limited by volume, every litre of tank capacity stores more energy. It lubricates pumps and injectors. And since the turbine does not suffer from detonation, it does not need the octane rating that makes gasoline more expensive. The result is a product that is safer to handle, cheaper, and carries more energy per litre.

Aviation fuel is not a generic product: each batch is tested against a standard (ASTM D1655 in the United States, DEF STAN 91-091 in the United Kingdom, and the joint AFQRJOS requirements list used by major oil companies). Density, flash point, freezing point, distillation curve, sulphur and aromatics content, acidity, electrical conductivity, thermal stability and water and particle content are all checked. The result travels with a certificate of analysis from the refinery to the airport hydrant, and at every transfer samples are taken and the fuel is filtered again. For an engine that consumes fuel at a rate of a litre per second, minimal contamination can clog injectors or feed microorganisms that grow at the water-kerosene interface.

Aviation gasoline, or avgas, is the fuel used by light piston aircraft and is manufactured separately, with a 100 octane rating and tetraethyl lead. Sustainable aviation fuels are produced through different chemical routes — from oils, waste or electricity and hydrogen — but are designed to meet the same specification as fossil kerosene, so the engine notices no difference.