When the APU MASTER switch is selected OFF, how long may the APU continue to run to cool before shutting down?

When the APU MASTER switch is selected to the OFF position, the APU continues to run for a specified period to cool down properly. The correct answer is based on the operational procedures designed to ensure that the APU does not overheat and that all systems are safely powered down. Allowing the APU to cool for two minutes helps to dissipate residual heat from the engine components and prevent damage that could occur from an abrupt shutdown. This cooling down period is essential for maintaining the overall health of the APU and ensuring its reliability for future operations.

In contrast, shorter cooldown periods might not effectively reduce temperature buildup, increasing the risk of mechanical failures, while longer periods could lead to unnecessary delays in operations. Therefore, the two-minute cooling time strikes a balance between efficiency and safety.