How long should the APU be turned off after the third start attempt?

The correct duration for turning off the Auxiliary Power Unit (APU) after the third start attempt is five minutes. This time is set to ensure adequate cooldown and prevent overheating or damage to the APU. The additional minutes allow for the components to stabilize and mitigate the risk of complications that can arise from repeated start attempts in quick succession.

Understanding the importance of the cooldown period is crucial for maintaining the operational integrity of the APU. If insufficient downtime is allowed, it may lead to premature wear or malfunction of the unit, which is vital for providing electrical power and bleed air in aircraft. Proper adherence to this protocol is critical for both safety and the longevity of the aircraft systems.