When does fire warning protection become active during APU operations?

Fire warning protection during APU operations is designed to ensure that any potential fire hazards are monitored and addressed promptly. This system becomes active once AC power is online. The APU's main function includes providing electrical power and pneumatic support for aircraft systems, and it is equipped with various safety features, including fire detection and warning systems.

When AC power is available, the fire warning system can actively monitor for signs of fire or overheating in the vicinity of the APU. This proactive approach is crucial as it allows for early detection of any abnormal conditions that could lead to a fire, thus enhancing the safety of the aircraft and its personnel during ground operations and prior to engine start.

Other phases of APU operation, such as starting the APU or reaching operating temperature, depend on the functionality of the fire protection system being powered and operational. Therefore, having AC power available is a prerequisite for ensuring that fire warning systems can function correctly.