Can a second start attempt of the APU be made in flight after a hung start without waiting?

A second start attempt of the Auxiliary Power Unit (APU) in flight after a hung start necessitates a cooling period. This procedure is critical because a hung start occurs when the APU fails to reach its normal operating speed and can indicate that fuel has accumulated in the combustion chamber, which may lead to overheating or damage if restarted immediately.

The one-minute cooling period allows time for the APU components to cool down and ensures that any excess fuel vapor is dissipated, reducing the risk of fire or further mechanical failure. This safety protocol is in place to protect the aircraft and its systems, emphasizing the importance of adhering to guidelines established by manufacturers and aviation authorities regarding APU operations.

In contrast, the options that suggest immediate restart, required permission, or manual restart do not account for the critical safety measures necessary following a hung start and diverge from standard operational procedures.