Definition
A specialized gasoline fuel formulated for use in piston-engine aircraft. AVGAS is refined to tighter standards than automotive gasoline, with a specified octane rating, a controlled volatility range, and additives that allow it to perform reliably across the wide temperature, pressure, and altitude conditions encountered in flight. It is identified by grade (most commonly 100LL, meaning 100-octane Low Lead) and is dyed a specific color so pilots can verify the correct fuel during preflight.
Plain English
A special kind of gasoline made for small piston-engine airplanes. It is not the same as the gasoline used in cars. Each grade is dyed a specific color so the pilot can check the fuel is the right type before flight.
Context Anchor
You will see AVGAS in fuel system discussions, aircraft manuals, fuel placards, and during preflight fuel checks and fueling decisions.
Derivation
A blend of 'aviation' and 'gasoline,' shortened to AVGAS. The term came into use to clearly distinguish aircraft piston fuel from automotive gasoline and from jet fuel (which is a kerosene-type fuel, not gasoline at all).
Why Pilots Care
Using the wrong fuel type can cause detonation, engine damage, or complete failure in flight.
Intuition Check
Do not assume “gasoline” means the same fuel used in cars. In this context, AVGAS means aircraft-approved gasoline for engines designed to use it.
Example Sentence 1
Before departure, the pilot checked the fuel sample and confirmed the blue color of 100LL AVGAS.
Example Sentence 2
The checklist required verifying the aircraft placard called for aviation gasoline rather than jet fuel.