FAA Powerplant Written Practice Test

What is the purpose of a cool-off period before shutting down a turbine engine?

1. To reduce fuel consumption

2. To allow the turbine wheel to oil before the case contracts

3. To check engine parameters

4. To prevent engine flooding

The correct answer is: To allow the turbine wheel to oil before the case contracts

The purpose of a cool-off period before shutting down a turbine engine is primarily to allow the turbine wheel to cool gradually, which helps to prevent thermal shock as the metal components contract. When a turbine engine operates, it generates a substantial amount of heat, particularly in the turbine section. If the engine is shut down immediately after operation, the rapid cooling can lead to significant differences in temperature between the turbine wheel and the surrounding engine case, causing stresses that could potentially warp or damage components. By allowing for a cool-off period, the temperature differential between the turbine and the case becomes less drastic. This gradual cooling helps maintain the integrity of engine components, ensuring that the metal parts contract uniformly and reduces the risk of thermal fatigue or cracking. Hence, this preventive measure is critical for extending the lifespan and maintaining the performance of the turbine engine over time.