Airplane fuel is made from refined petroleum, primarily a blend of hydrocarbons with 9 to 16 carbon atoms per molecule. It’s essentially a highly refined form of kerosene, sitting between gasoline and diesel on the refining spectrum. The exact recipe depends on whether the aircraft uses jet engines or piston engines, but the vast majority of commercial aviation runs on a fuel called Jet A or Jet A-1.
The Core Chemistry of Jet Fuel
Jet fuel is not a single chemical compound. It’s a mixture of hundreds of different hydrocarbon molecules, each with slightly different properties. These molecules fall into a few major families, and the balance between them determines how the fuel performs.
The two dominant groups are paraffins (straight and branched chains of carbon and hydrogen) and naphthenes (ring-shaped carbon structures). Together, these make up over 70% of jet fuel by weight, typically landing between 73% and 82% depending on the crude oil source and refining process. Paraffins alone can range from about 33% to 59% of the total, while naphthenes fill in most of the rest. These components are the workhorses of the fuel: they burn cleanly, release a lot of energy, and remain liquid across a wide temperature range.
Aromatic hydrocarbons, a family of molecules built around stable ring structures, make up the remaining portion. Aromatics are capped at 25% of the total because they produce more soot when burned and contribute to engine deposits. In practice, most jet fuel samples contain between 18% and 27% aromatics. A tiny fraction, less than 1%, consists of olefins, which are reactive molecules generally kept to a minimum because they can form gums and residues.
Why Kerosene Instead of Gasoline
Jet engines need fuel that won’t evaporate too easily at high altitude, won’t freeze at temperatures around minus 40 to minus 50 degrees Celsius, and packs as much energy as possible per kilogram. Kerosene-type fuels hit this sweet spot. They boil between 145°C and 300°C, which is higher than gasoline (making them safer and less prone to vapor ignition) but lower than diesel (keeping them liquid in extreme cold).
The energy density of jet fuel is about 43.5 megajoules per kilogram. That’s roughly 15% more energy per kilogram than gasoline, which matters enormously when every extra pound of fuel means less cargo or fewer passengers. Weight is everything in aviation, and kerosene delivers one of the best energy-to-weight ratios of any liquid fuel.
Additives That Keep Engines Running
Raw kerosene alone isn’t enough to meet aviation standards. A range of additives are blended in at very low concentrations to protect both the fuel and the engine during storage, transport, and flight. These include oxidation inhibitors that prevent the fuel from degrading over time, corrosion inhibitors that protect metal fuel tanks and pipelines, and anti-icing additives that stop ice crystals from forming in the fuel at high altitude, where temperatures can plunge well below freezing.
Other common additions include static dissipaters, which prevent dangerous static electricity buildup during fueling, metal deactivators that neutralize trace metals picked up during refining, thermal stability additives that help the fuel withstand the heat near engine components, and biocide additives that kill microorganisms. Bacteria and fungi can actually grow in the water that collects at the bottom of fuel tanks, and left unchecked, they form sludge that clogs filters.
Avgas: Fuel for Smaller Aircraft
Not all airplanes run on jet fuel. Smaller propeller-driven aircraft with piston engines (similar in concept to car engines) use aviation gasoline, or avgas. Almost all avgas sold in the United States today is a grade called 100LL, where “100” refers to its octane rating and “LL” stands for low lead.
Avgas is a complex mixture of hydrocarbons, much like car gasoline but formulated for higher performance and greater reliability. The key difference is that 100LL still contains tetraethyl lead, an organic lead compound added in small quantities to boost the octane rating and prevent engine knock. Lead was phased out of car gasoline decades ago, but piston aircraft engines were designed around it and many still require it to operate safely. This makes avgas the largest remaining source of lead emissions in the United States, and the FAA has been working to certify unleaded alternatives.
Military Jet Fuels
Military aircraft use fuels designated JP-5 and JP-8 rather than commercial Jet A-1, though the base chemistry is nearly identical: the same C9 to C16 hydrocarbon range, the same dominance of paraffins and naphthenes, and the same aromatic content cap of 25%. The differences come down to additives and specifications. Military fuels include additional corrosion inhibitors, thermal stability packages, and other performance additives tailored to the demands of combat aircraft, which operate under more extreme conditions than commercial planes. JP-5, used primarily by the Navy, has a higher flash point for safer storage on aircraft carriers.
Sustainable Aviation Fuel
A growing share of aviation fuel is being produced from non-petroleum sources. Sustainable aviation fuel, or SAF, is chemically similar enough to conventional jet fuel that it can be blended directly into existing supplies without modifying engines or airport infrastructure.
The feedstocks are surprisingly varied. The most commercially mature pathway uses fats, oils, and greases: think used cooking oil, waste animal fats, and plant oils from crops like camelina and jatropha. These triglyceride feedstocks are processed with hydrogen to break apart the long fatty acid chains and rearrange them into the same types of paraffinic hydrocarbons found in petroleum-based jet fuel. Fuels made this way can currently be blended up to 50% with conventional jet fuel.
Other production methods use woody biomass (forest waste, agricultural residue, energy crops) or even the food and yard waste portion of municipal solid waste. These are converted into fuel through a process called Fischer-Tropsch synthesis, which breaks the raw material into simple gases and then rebuilds them into liquid hydrocarbons. A third pathway ferments sugars from cellulosic biomass using engineered microbes that convert the sugars directly into hydrocarbons, though this route is currently approved at only a 10% blend ratio.
Regardless of the source, the end product has to meet the same chemical and performance specifications as petroleum jet fuel. SAF doesn’t change what airplane fuel is made of at the molecular level. It changes where those molecules come from.