What is jet fuel like?

What a jet airplane fills up with is closer to the fuel of a kerosene heater than to a car's gasoline. Jet A-1, the dominant specification outside the United States, is a middle distillate cut of petroleum, with hydrocarbon chains of between 8 and 16 carbon atoms. To the eye it's a clear liquid, colorless or slightly straw-colored, denser than gasoline (about 0.78-0.84 kg/L at 15 °C versus 0.72-0.77) and considerably less volatile: its flash point must exceed 38 °C, meaning that at room temperature it doesn't easily form a flammable mixture with air. That low volatility is one of the reasons commercial aviation adopted it instead of gasoline.

At cruise altitude the outside air temperature is around −55 °C, and the fuel inside the wing tanks cools for hours. That's why the specification requires that Jet A-1 not begin forming wax crystals above −47 °C (American Jet A settles for −40 °C). Water is the other enemy: kerosene dissolves small amounts of it and releases it as it cools, so facilities filter and let the product settle, and aircraft without fuel heaters carry an anti-icing additive. The formula usually also includes an antistatic additive, to dissipate the charge generated by fast pumping, and in some cases corrosion inhibitors and lubricity improvers.

Aviation kerosene delivers about 43 MJ per kilogram, a high energy density for a liquid that stays stable across a wide temperature range. The engine atomizes it in the combustion chamber, where it burns continuously; there's no periodic spark or octane rating to worry about, and what matters is the cleanliness of combustion, the sulfur and aromatics content, and the liquid's thermal stability, because before being burned it's used to cool the engine oil. Sustainable aviation fuels (SAF) are manufactured to be chemically equivalent and are now authorized in blends of up to 50% with conventional kerosene, with no changes to the aircraft.

Light aircraft and helicopters with piston engines burn avgas, a high-octane gasoline. The most common grade, 100LL ("low lead"), is dyed blue to avoid confusion and still contains tetraethyl lead, the last significant use of that additive in the world. Regulators and manufacturers are working on lead-free substitutes, and some modern light-aviation engines have been redesigned to run on Jet A-1.