Definition
A measure of an aviation fuel's resistance to detonation (uncontrolled, spontaneous combustion) inside the engine cylinders. Higher antiknock value means the fuel can withstand greater heat and pressure before igniting on its own, allowing the engine to run smoothly without damaging knocking.
Plain English
How well a fuel resists exploding on its own inside the engine before the spark plug fires. The higher the antiknock value, the better the fuel handles heat and pressure without causing knocking.
Context Anchor
Seen when studying aviation gasoline grades and choosing the correct fuel for a piston-engine aircraft.
Derivation
From 'anti-' (against) and 'knock' -- the audible knocking or pinging sound an engine makes when fuel detonates at the wrong moment. The term simply names the property of fighting against that knocking.
Why Pilots Care
Selecting fuel with the proper rating prevents engine damage from detonation and supports safe power output.
Intuition Check
Antiknock value does not mean the fuel fixes an engine that is already knocking. It means the fuel has a rated ability to resist knock before it starts.
Example Sentence 1
The aircraft's engine requires 100LL avgas because of its high antiknock value.
Example Sentence 2
Using a lower antiknock value than specified can lead to detonation during high-power climbs.