Why should turbine engines normally be operated at idle for a period of time before shutdown?

Operating turbine engines at idle for a period of time before shutdown is important primarily because it allows the turbine case and components to cool down gradually. When a turbine engine operates at high power settings, components, including the turbine blades and case, can reach extremely high temperatures. If the engine is shut down immediately from a high power setting, the rapid cooling can cause the turbine case to contract considerably faster than the turbine blades.

This mismatch in cooling rates can lead to a situation where the turbine blades become physically constrained by the shrinking turbine case, potentially causing them to seize. By allowing the engine to operate at idle for a while, the temperatures can stabilize, enabling more uniform cooling of both the case and the blades. This practice helps maintain the integrity of the components and minimizes the risk of mechanical failure due to thermal stress, ultimately enhancing the durability and reliability of the engine over time.