How many minutes of cooling must elapse between APU start attempts when the initial attempts fail?

The correct answer indicates that there should be a cooling period of 1 minute between APU start attempts if the initial attempts fail. This brief cooling time is important to prevent potential overheating or damage to the APU system due to repeated attempts in quick succession. By allowing a short interval for the APU to cool down, it helps ensure that the system can reset and potentially function properly on subsequent attempts.

In aviation procedures, these cooling periods are often outlined in the operational manuals to maintain the integrity of the APU and to enhance safety during operation. A longer cooling period would not be necessary given the APU's design and operational parameters, which allow for this relatively quick turnaround. Therefore, adhering to the 1-minute cooling period is a practical and efficient practice when troubleshooting APU start failure.