What is the required cooling period for the APU after it has been shut down?

The required cooling period for the Auxiliary Power Unit (APU) after it has been shut down is 60 seconds. This cooling period is crucial to ensure that the APU components, particularly the turbine and any associated systems, can dissipate heat effectively. Allowing for this cooldown minimizes the risk of potential overheating or damage that could occur if the APU were to be restarted immediately after shutdown. The 60-second wait provides sufficient time for the temperature to stabilize, ensuring reliable operation and extending the lifespan of the APU. Understanding the necessity of this cooling period helps in maintaining the safety and functionality of aircraft systems.