FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesEasy
A technician is selecting fuel for a reciprocating engine placarded for 100LL aviation gasoline. What does the octane (performance) rating of the fuel primarily indicate?
- AThe fuel's resistance to detonation under compression and heat
- BThe maximum RPM the engine can safely produce
- CThe fuel's tendency to vaporize and cause vapor lock
- DThe heat content (BTU value) of the fuel per gallon
Show answer & explanationAnswer & explanation
Correct answer: A. The fuel's resistance to detonation under compression and heat
Octane (or performance) number rates a fuel's ability to resist detonation — the uncontrolled, explosive combustion that occurs when the fuel/air charge self-ignites from heat and pressure. Higher-octane fuels can withstand higher compression and manifold pressures without detonating.
Why the other options are wrong
- B. RPM limits are set by engine design, not fuel rating.
- C. Vapor lock relates to volatility/Reid vapor pressure, not octane rating.
- D. Heat content relates to energy density, not detonation resistance.
Octane (Performance) Rating
A numerical rating of a fuel's resistance to detonation; higher numbers allow higher compression/manifold pressure without knock.
- 100LL = 100 octane, low lead
- Higher octane = greater detonation resistance
- Using lower-than-specified octane fuel can cause detonation
Memory trick: 'Octane Outlasts Overheating' — higher octane resists knock longer