Learn what aviation fuel is made of, how kerosene-based Jet A and Jet A-1 differ from 100LL avgas, and why they cannot be interchanged.
Aviation fuel is made from carefully refined and blended hydrocarbons, usually derived from crude oil, plus tightly controlled additives. Jet A and Jet A-1 are kerosene-type fuels for turbine engines; avgas is a lighter, more volatile, high-octane gasoline for spark-ignition piston engines. They are not interchangeable.
In Aviation & Real-World Flying, “aviation fuel” is an umbrella term rather than one product. The required grade depends on the engine and aircraft approval, not simply on whether the aircraft has a propeller. Our guide to which fuels different aircraft engines use covers those applications in more detail.
Jet fuel vs avgas: key differences
| Property | Jet A and Jet A-1 | 100LL avgas |
|---|---|---|
| Base fuel | Kerosene-range hydrocarbon blend | Gasoline-range, high-octane hydrocarbon blend |
| Normal application | Turbojets, turbofans, turboprops and approved compression-ignition piston engines | Spark-ignition piston engines approved for the grade |
| Volatility | Lower; evaporates less readily | Higher; designed to vaporise readily in piston-engine induction systems |
| Ignition requirement | Reliable continuous combustion, thermal stability and suitable low-temperature flow | Strong resistance to detonation under spark ignition |
| Lead | No tetraethyl lead | 100LL contains tetraethyl lead; “low lead” does not mean lead-free |
| Typical appearance | Clear to pale straw-coloured | 100LL is dyed blue |
| Cold-weather specification | Jet A has a maximum freezing point of −40°C; Jet A-1 is −47°C | Different volatility and freezing requirements apply to aviation gasoline |
How is aviation fuel made?
- Separate the hydrocarbon ranges. Conventional aviation fuel production begins by distilling crude oil. Lighter gasoline-range material can become part of avgas, while the heavier kerosene-range fraction supplies jet fuel.
- Refine and blend the components. Processing removes contaminants and adjusts properties such as sulphur content, volatility, anti-knock performance, freezing point and thermal stability. The finished fuel is a controlled mixture, not one chemical compound.
- Add approved additives and test the batch. Depending on the fuel specification and supply contract, these may include antioxidants, corrosion inhibitors, static dissipaters or icing inhibitors. Every finished batch must meet its applicable fuel standard.
Some jet fuel can also contain approved synthetic components produced from renewable or recycled feedstocks. Once qualified and blended within the permitted limits, this sustainable aviation fuel is intended to behave as specification-compliant, drop-in turbine fuel. An unapproved biofuel cannot simply be substituted for Jet A or Jet A-1.
What exactly is in jet fuel?
Jet fuel contains a mixture of paraffins, isoparaffins, cycloparaffins and controlled amounts of aromatic hydrocarbons. Refiners balance these components to provide energy, pumpability at altitude, acceptable combustion, seal compatibility and resistance to deposit formation in hot fuel systems.
Jet A and Jet A-1 are chemically similar. Their best-known operational difference is freezing point: Jet A-1 remains within specification at a lower temperature, making it the more common international grade. Both generally have a minimum flash point of 38°C, although that does not make spilled jet fuel safe or difficult to ignite.
Additive content should never be assumed from the name alone. Civil Jet A or Jet A-1 does not automatically contain fuel-system icing inhibitor, while particular military specifications or operator contracts may require a defined additive package.
What is 100LL avgas made from?
Avgas is blended from lighter, highly volatile hydrocarbons selected for vapour formation and resistance to detonation. The familiar 100LL grade also contains tetraethyl lead to achieve its required anti-knock performance, along with blue dye and permitted additives.
The “100” is an aviation anti-knock grade, not directly equivalent to the octane number displayed at a road filling station. “LL” means the grade contains less lead than older high-lead aviation fuels, not that it is unleaded. Lead-free avgas grades exist, but an aircraft may use one only when its engine and airframe approvals permit it.
Avgas is also not ordinary automotive petrol. Differences in volatility, additives, vapour-lock behaviour and quality control explain why mogas is limited to specifically approved aircraft.
Why can’t jet fuel and avgas be interchanged?
Jet fuel and avgas are designed for different combustion systems, fuel pumps and operating temperatures. Putting kerosene-based jet fuel into an avgas engine can reduce volatility and anti-knock performance, leading to rough running, detonation, power loss or engine failure.
Avgas in a turbine engine has a much lower flash point and different lubrication, volatility and thermal characteristics. A small number of aircraft manuals permit restricted emergency use under stated limits, but that exception must never be treated as general compatibility.
A common source of confusion is the propeller. Turboprops use jet fuel, while some modern compression-ignition piston aircraft also burn Jet A or Jet A-1. Our comparison of diesel-type aircraft engines and avgas engines explains why engine design and certification decide the fuel.
Which weighs more, jet fuel or avgas?
Jet fuel is normally denser than avgas, so the same volume generally weighs more. Density changes with fuel composition and temperature, however, so flight planning should use the aircraft’s approved figures or reliable delivery data rather than one universal conversion.
This difference matters when fuel is ordered by volume but performance and loading are calculated by mass. See our explanation of how fuel density affects aircraft weight and balance before converting litres or gallons into kilograms or pounds.
What should you do if the wrong fuel may have been added?
Do not start the engine when misfuelling or contamination is suspected. Fuel colour, odour and a normal-looking sump sample are not reliable proof that the correct grade is aboard.
- Stop the operation. Prevent starting or further fuelling and tell the responsible operator or maintenance personnel.
- Check the approved grade. Compare the aircraft flight manual, fuel placards, uplift record and supplier documentation.
- Preserve evidence. Retain properly handled fuel samples and records for examination under the operator’s procedure.
- Use an approved corrective procedure. Draining, flushing, inspection and return to service must follow the aircraft and engine maintenance requirements. Simply topping up with the correct fuel does not make a contaminated tank acceptable.