FAA Ground School Practice Test

If the grade of fuel used in an aircraft engine is lower than specified, what is the most likely consequence?

1. Fuel vaporization

2. Detonation

3. Engine overheating

4. Knocking

The correct answer is: Detonation

Using a fuel grade that is lower than specified can lead to detonation, which is a serious condition in aircraft engines. Detonation occurs when the fuel-air mixture ignites prematurely, causing a rapid and uncontrolled combustion process. This can significantly increase the pressures and temperatures within the engine and can cause mechanical damage to engine components. Detonation is particularly concerning in high-compression engines that require a higher octane fuel to operate smoothly. Lower-grade fuel typically has a higher tendency to knock and detonate under these conditions, resulting in potential engine damage such as burnt pistons, cracked cylinder heads, or even catastrophic failure. In contrast, while fuel vaporization, engine overheating, and knocking can occur under various circumstances, they are not as directly linked to the use of a lower-grade fuel. Vaporization relates more to fuel temperature and pressure, while overheating can result from several factors like inadequate cooling or excessive load, and knocking refers to irregular combustion cycles rather than the fuel grade alone. Thus, the primary consequence of using a fuel with a lower grade is the risk of detonation.