What happens if the elapsed time of 30 seconds to ON SPEED is exceeded during APU start?

During the start process of an Auxiliary Power Unit (APU), it is critical to adhere to specific time limits for various phases to ensure proper operation and prevent potential damage. The elapsed time of 30 seconds to ON SPEED is a critical threshold during the APU start sequence. If this time limit is exceeded, the safest course of action is to terminate the start immediately.

Exceeding the 30-second mark can indicate a problem in the starting sequence, such as insufficient fuel flow, ignition issues, or mechanical failures, which could lead to an unsuccessful start or even a hazardous situation. Therefore, terminating the start process helps prevent further complications and protects both the APU and the aircraft systems.

In summary, if the APU fails to reach ON SPEED within the specified time, the protocol is to immediately stop the start process to maintain safety standards and equipment integrity.