What initial action should be taken if a fire is detected in the APU?

In the case of a fire detected in the Auxiliary Power Unit (APU), the correct course of action is to immediately notify the pilot. This step is crucial as the pilot needs to be aware of the emergency to take appropriate action. The APU plays a significant role in the aircraft's systems, and the pilot must assess the situation and coordinate the emergency response effectively.

Notifying the pilot allows them to initiate the emergency shutdown of the APU, which is necessary to prevent the escalation of the fire and to ensure the safety of the aircraft and crew. Additionally, the pilot can make informed decisions regarding the evacuation of the crew and passengers if needed or prepare for any emergency landing procedures.

While activating an emergency shutdown could be an important subsequent action, it relies on the pilot being informed first. Other actions, such as using a water hose to cool or moving the aircraft immediately, could potentially exacerbate the situation without first ensuring that the control center (the cockpit) is aware of the fire, which is why notifying the pilot is the initial priority.