Definition
The standardized classifications of aviation gasoline (avgas) based on octane rating, which indicates the fuel's resistance to detonation under engine pressure and temperature. Each grade is identified by a number and a dye color so pilots and line crews can verify the correct fuel is being used. Common piston-aircraft grades include 80 (red), 100 (green), and 100LL (low lead, blue), with 100LL being the most widely available today.
Plain English
Different types of aviation gasoline are sorted into grades. Each grade has a number that shows how much pressure the fuel can take inside the engine before it burns the wrong way, and each grade is dyed a specific color so you can tell them apart by sight.
Context Anchor
Seen in the aircraft handbook, fuel filler placards, refueling procedures, and powerplant discussions about approved fuel.
Derivation
Grade' comes from the Latin gradus, meaning 'a step' or 'a level on a scale.' In fuel terms, it refers to a step on the octane scale -- a higher grade means a higher rating against detonation.
Why Pilots Care
Selecting the wrong grade risks engine damage from detonation or loss of power.
Intuition Check
Do not read grade as meaning simply “better” or “higher quality.” In aviation, a fuel grade means a specific approved fuel category that must match the engine’s requirements.
Example Sentence 1
Before fueling, the pilot checked the placard near the filler cap and confirmed the truck was dispensing 100LL, the correct fuel grade for the aircraft.
Example Sentence 2
The aircraft's operating handbook lists the approved fuel grades for safe operation.