Certified Flight Instructor - Flight Instructor Airplane Practice Exam

What may occur if the grade of fuel used in an aircraft engine is lower than specified?

1. Detonation.

2. Lower cylinder head temperatures.

3. Decrease in power which could overstress internal components.

4. Improved efficiency.

The correct answer is: Detonation.

Using a fuel grade that is lower than specified can lead to detonation in an aircraft engine. Detonation occurs when the fuel-air mixture in the combustion chamber ignites prematurely, which can be caused by the fuel's octane rating being insufficient to handle the compression ratio and operating temperatures of the engine. This early ignition can lead to a rapid and violent release of energy, resulting in a knocking or pinging sound. Detonation can cause significant damage to engine components, including the pistons, cylinder heads, and valves, due to the increased pressures and temperatures associated with this abnormal combustion process. The potential for engine failure is a serious concern, which is why it is critical to adhere strictly to the fuel specifications outlined by the aircraft manufacturer. In contrast, using the appropriate fuel grade helps maintain optimal combustion characteristics, promotes better engine performance, and reduces the likelihood of destructive phenomena like detonation.