What is the first step if an APU FIRE warning is detected?

The correct response is to immediately shut down the APU when an APU FIRE warning is detected. This action is crucial because it helps to quickly stop any further fuel supply to the APU, reducing the risk of a larger fire and allowing for a more controlled response to the situation. The APU is a critical component of the aircraft that provides power and air for starting engines and running systems while on the ground. In the event of a fire warning, prompt action is necessary to mitigate safety risks to personnel and the aircraft itself.

Taking swift action to shut down the APU will significantly lower the chances of a fire spreading or causing additional hazards. Once the APU is secured, further actions such as deploying fire extinguishers or calling for maintenance support can then be safely addressed, but the priority must always be to eliminate the immediate danger first.