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

The purpose of a cool-off period before shutting down a turbine engine is primarily to cool the turbine wheel before the case contracts. During operation, the turbine wheel reaches very high temperatures due to the combustion process, and the materials expand as a result. If the engine is shut down immediately without allowing for a gradual cooling period, the sudden temperature change can cause thermal stress on the turbine components, particularly between the hot turbine wheel and the cooler engine casing.

By implementing a cool-off period, the engine can stabilize at a lower temperature gradually, which minimizes the risk of damage, such as cracking or warping, during the transition from high temperature to ambient temperature. This practice contributes to the overall longevity and reliability of the engine, ensuring that it maintains its structural integrity over time.

While the other options may seem relevant, they do not address the primary concern of thermal stress and the physical properties of materials involved in turbine engines during the shutdown process.