What happens if an APU is shut down without allowing for a cooling period?

When an Auxiliary Power Unit (APU) is shut down without allowing for a proper cooling period, it could be damaged by thermal shock. APUs operate at high temperatures, and when they're abruptly shut down, the rapid temperature change can lead to structural integrity issues in engine components. This is primarily because the materials expand when heated; if they cool too quickly, they might contract unevenly, leading to stress fractures or cracks. Allowing a cooling period helps mitigate this risk by ensuring that the APU cools down in a controlled manner, preserving the lifespan and reliability of the unit.

The other outcomes—such as automatic restarting, failure to restart, or causing a fuel leak—are not typical consequences of an improper shutdown in the context of thermal stress and do not directly relate to the risks associated with temperature changes in the engine components.