What action should be taken if an APU FIRE warning message appears?

When an APU FIRE warning message appears, the immediate action is to shut down the APU. The APU, or Auxiliary Power Unit, is a critical component that provides electrical power and pneumatic pressure while the aircraft is on the ground or during certain phases of flight. A FIRE warning indicates an unsafe condition that could lead to severe damage or fire hazard not only to the APU but also to the aircraft and personnel.

Shutting down the APU promptly can prevent an escalation of the situation, such as an actual fire breaking out. This action is vital for ensuring the safety of the aircraft and those on board. Operating the APU under alarming conditions could result in a catastrophic failure or fire, impacting emergency response measures.

Increasing APU power or performing a diagnostic check could exacerbate the situation, risking further damage to the aircraft or injury. Notifying ground control, while important for situational awareness and coordinating emergency measures, does not address the immediate threat indicated by a FIRE warning. Therefore, shutting down the APU is the correct and necessary action in response to a fire warning.