What is the required action if the APU does not start after two attempts?

When the Auxiliary Power Unit (APU) does not start after two attempts, the recommended action is to wait for 20 minutes before making another start attempt. This waiting period allows the APU's starting system to cool down and reset, which is critical for preventing potential damage from repeated starting attempts in quick succession.

If the APU is trying to start and fails immediately, it may indicate an underlying issue that needs to be diagnosed, and continuously attempting to start it could exacerbate the problem or lead to component failure. Allowing time between attempts helps ensure that any residual heat dissipates and lowers the risk of overheating. This approach also aligns with standard operating procedures designed to maintain the safety and integrity of aircraft systems.

In contrast, initiating a third start immediately could lead to further complications, checking fuel levels is a precaution but does not address the necessity of cooling down the system, and evacuating the aircraft is an extreme measure not warranted by a simple APU failure under normal circumstances.