Types of Avgas for Piston Aircraft
Learn the main avgas types, what octane ratings and fuel colors mean, and why pilots must verify approved aircraft fuel.
Avgas is aviation gasoline used in piston-engine aircraft. It is not the same thing as jet fuel, and it is not something pilots should treat casually. The wrong fuel can damage an engine and create a serious in-flight emergency.
For student pilots, the practical goal is simple: know what fuel your aircraft requires, verify the fuel during preflight, and never guess.
What the Numbers Mean
Avgas names usually include an octane rating. Octane describes the fuel's resistance to detonation, which is abnormal combustion inside the engine.
Higher-compression engines generally need fuel that can resist detonation better. Using fuel with too low an octane rating can lead to engine damage.
The aircraft's approved fuel types are listed in the Pilot's Operating Handbook, aircraft flight manual, placards, and engine documentation. Those references matter more than what happens to be available at the pump.
Fuel Colors
Avgas is dyed so pilots can visually identify fuel type during preflight. When you sump the tanks, you are not only checking for water and contamination. You are also confirming that the fuel looks like the correct type.
Color alone is not a complete fuel-quality inspection, but it is an important clue. If the fuel color does not match what you expect, stop and investigate before flight.
100LL
100LL is the common blue aviation gasoline used by many piston aircraft. The "100" refers to the octane rating, and "LL" means low lead compared with older high-lead aviation fuels.
Low lead does not mean no lead. 100LL still contains lead additives. That lead helps prevent detonation and protects certain engine components, but it also creates environmental and maintenance concerns.
Because 100LL is widely available, many student pilots first learn fuel handling with this fuel.
Unleaded Avgas
Unleaded aviation gasoline options exist for some aircraft and engines. Examples discussed in aviation training include fuels such as UL91, UL94, and newer 100-octane unleaded options.
The key word is approved. Do not assume an aircraft can use a fuel because another similar aircraft can. Some unleaded fuels require specific authorization, supplemental type certificate approval, or manufacturer guidance.
Unleaded avgas approval depends on the aircraft, engine, and applicable authorization, which is why this topic deserves official-source review during aircraft ownership or dispatch planning.
Older Avgas Types
Older fuel grades such as avgas 80, avgas 100, and avgas 115 appear in training history and older aircraft discussions. Some were identified by different colors and lead content.
Many of these fuels are rare or largely replaced in normal general aviation operations. They still matter because older aircraft manuals, placards, or maintenance conversations may refer to them.
If you fly a vintage or unusual aircraft, fuel approval deserves extra care.
Avgas vs. Jet Fuel
Avgas is for spark-ignition piston engines. Jet fuel is for turbine engines and is kerosene-based.
Misfueling is dangerous. Putting jet fuel in a piston aircraft can lead to engine failure. Putting avgas in a turbine aircraft can also create serious problems depending on the engine and operation.
Always verify fuel grade, truck markings, nozzle type, fuel order, receipt, and the fuel sample when appropriate. Be present for fueling when practical.
If the Preferred Fuel Is Not Available
Do not solve a fuel availability problem from memory. Check the aircraft documents and approved fuel placards. Some engines may allow certain higher-octane fuels, while lower-octane fuel may be prohibited because of detonation risk.
If you are away from home and unsure, call maintenance, the aircraft owner, or an instructor before fueling. It is better to delay the flight than to depart with an unapproved fuel choice.
Fuel availability is also part of cross-country planning. Before relying on an airport as a fuel stop, confirm that the needed grade is available and that the facility will be open when you arrive.
Preflight Fuel Check
During preflight, sump each required drain point using the aircraft checklist. Look for water, sediment, unusual smell, wrong color, and anything that does not look normal.
Water often appears as a separate layer or bead because it is heavier than fuel. Contaminants may appear as particles. Wrong fuel may look or smell different from what you expect.
If something is questionable, keep sampling, ask for help, and do not fly until the issue is resolved.
Student-Pilot Rule
Use only fuel approved for that aircraft and engine. If the correct fuel is not available, do not substitute casually.
Fuel decisions are not a place for shortcuts. The right habit is to verify, sample, confirm, and ask when unsure.
Related Reading
Official References
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