What is the cooling period required after three aborted APU start attempts?

The correct cooling period required after three aborted APU (Auxiliary Power Unit) start attempts is 15 minutes. This cooling period is essential to prevent overheating and potential damage to the APU due to repeated unsuccessful attempts within a short duration. After three failed attempts, the recommended wait time allows the system components to cool down sufficiently before attempting another start, thereby reducing the risk of mechanical failure or increased wear on the components. This protocol ensures that the APU operates within its specified limits and maintains reliability for subsequent operations.

Understanding the importance of respecting these cooldown periods is crucial for maintaining the integrity and longevity of the APU, as well as ensuring overall operational safety.