If the recommended octane is not available, which fuel should be substituted?

• A. Automotive gasoline
• B. The next higher octane aviation gas
• C. Jet fuel
• D. The next lower octane aviation gas

Answer: (B)

Octane rating shows a fuel’s ability to resist detonation under the engine’s compression and heat. If the exact recommended octane isn’t available, you should use the next higher octane aviation gasoline. This keeps the anti-knock protection high enough to avoid pre-ignition or knocking, which can cause power loss or engine damage. Automotive gasoline isn’t approved for aircraft engines because it may contain ethanol and other additives that aren’t compatible with aviation systems; jet fuel is for turbine engines and cannot run a piston engine; using a lower octane would reduce resistance to detonation and is unsafe. So the best substitute is the next higher octane aviation gasoline.