Aviation fuel is a refined petroleum product made almost entirely of hydrocarbons, molecules built from hydrogen and carbon atoms. The exact composition depends on whether the fuel powers a jet turbine or a piston engine, but both types start as crude oil and are separated through distillation. Here’s what goes into each type and how newer alternatives compare.
Jet Fuel: Kerosene at Its Core
The fuel that powers commercial airliners, military jets, and most turboprop aircraft is a refined form of kerosene. Sold under names like Jet A, Jet A-1, and JP-8, it consists of hydrocarbon molecules with carbon chains ranging from 8 to 16 atoms long. That carbon chain length places it between gasoline (shorter chains, lighter) and diesel (longer chains, heavier).
More than 70% of jet fuel by volume is made up of two hydrocarbon families: paraffins (straight and branched chains) and naphthenes (ring-shaped molecules also called cycloalkanes). These components give the fuel its energy density and stability. Aromatic hydrocarbons, a category that includes compounds like alkylbenzenes and naphthalenes, make up no more than 25% of the total. A tiny fraction, less than 1%, consists of olefins, which are reactive molecules refiners try to minimize because they can form gummy deposits in engines.
This blend isn’t random. Refiners target a specific boiling range of roughly 150 to 300°C during fractional distillation, the process where crude oil is heated in a tower and separated into layers by weight. The kerosene fraction condenses in the middle of the tower, lighter than diesel but heavier than gasoline. After distillation, the fuel goes through additional treatment to remove sulfur, water, and other contaminants before it meets the strict specifications airlines require.
What Happens When Jet Fuel Burns
When jet fuel combusts inside a turbine engine, it reacts with oxygen to produce a surprisingly simple set of exhaust products. About 72% of the combustion output by mass is carbon dioxide, and roughly 27.6% is water vapor. The remaining fraction, less than 1%, includes nitrogen oxides, carbon monoxide, unburned hydrocarbons, and soot particles. Overall, more than 99.5% of commercial engine exhaust by molar content is nitrogen, oxygen, CO₂, and water. That narrow sliver of residual pollutants still matters for air quality and climate, but the chemistry itself is straightforward: hydrogen and carbon atoms in the fuel combine with oxygen from the air.
Aviation Gasoline: The Last Leaded Fuel
Piston-engine aircraft, the kind used for private flying, flight training, and crop dusting, run on aviation gasoline, commonly called avgas. The most widely used grade is 100LL (“low lead”), a high-octane fuel made from a complex mixture of hydrocarbons with carbon chains in the C4 to C12 range. That makes it chemically closer to car gasoline than to jet fuel, but with a critical difference: it still contains tetraethyllead.
Lead was phased out of automobile gasoline decades ago, yet avgas 100LL contains between 0.06% and 0.12% tetraethyllead by weight, with a maximum allowable concentration of 0.56 grams per liter (2.12 grams per gallon). The lead is there to prevent engine knock in the high-compression engines common in general aviation. It accumulates in the human body and is a known health hazard, which is why the FAA has been working to approve unleaded replacements. A lead-free alternative called UL94 exists for lower-compression engines, and newer formulations are in testing for high-performance aircraft.
Sustainable Aviation Fuel
Sustainable aviation fuel, or SAF, is designed to be chemically similar to conventional jet fuel so it can work in existing engines without modification. The difference is the source material. Instead of crude oil, SAF is produced from biological or synthetic feedstocks: used cooking oils, animal fats, agricultural waste, woody biomass (lignocellulosic material), oil-seed crops, and even microalgae.
The most commercially mature production method is called HEFA, which stands for hydroprocessed esters and fatty acids. It takes fats and oils, strips away the oxygen, and reshapes the molecules into the same paraffin-heavy kerosene that refineries produce from petroleum. Another established pathway is Fischer-Tropsch synthesis, which converts carbon-containing feedstocks into liquid hydrocarbons through a series of chemical reactions involving heat and catalysts. Both methods yield fuel that meets the same specifications as petroleum-derived Jet A.
SAF currently makes up a small fraction of total jet fuel supply, limited by feedstock availability and production cost. But because its carbon was recently absorbed from the atmosphere by plants or captured from waste streams rather than pumped from underground, it can reduce lifecycle carbon emissions significantly compared to fossil-derived fuel. Airlines can blend SAF with conventional jet fuel at approved ratios, and several airports already offer it as a standard option.
Why the Composition Matters
The specific chemistry of aviation fuel isn’t just a technical detail. The balance of paraffins, naphthenes, and aromatics determines how much energy each kilogram delivers, how cleanly it burns, how it behaves in extreme cold at cruising altitude, and whether it’s safe to store for long periods. Jet fuel needs to remain liquid at temperatures below minus 40°C, resist forming ice crystals that could block fuel lines, and ignite reliably across a wide range of pressures. Every component in the blend serves a purpose, and refining standards exist to keep each one within tight limits. That precision is what lets a single fuel specification work in everything from a regional turboprop to a wide-body airliner crossing the Pacific.