How long must the APU be allowed to stabilize and cool before shutdown?

The correct choice, which indicates that the APU must be allowed to stabilize and cool for 2 minutes before shutdown, is based on procedural protocols designed to ensure the longevity and safety of the auxiliary power unit. Allowing the APU to stabilize after operation is critical, as it ensures that the engine speed decreases to idle and that temperature levels drop adequately. This practice helps minimize thermal stress and potential damage to the unit, as rapid shutdown can lead to oil and fuel accumulation within the APU, potentially causing internal components to degrade or malfunction.

Typically, operational guidelines emphasize a cool-down period that is long enough to ensure safe temperatures but short enough to maintain operational efficiency. A 2-minute cooldown period strikes a balance, allowing enough time for the system to adjust while preventing excessive delays in readiness for subsequent missions or power requirements.