What is the sequence of events for an APU fire while on the ground?

The sequence of events for an Auxiliary Power Unit (APU) fire on the ground typically begins with the APU automatically detecting the fire and responding within a short timeframe. The automatic activation of the squib, which is a device that initiates the discharge of the fire extinguishing agent, occurs within about five seconds of the detected fire. This quick response is crucial to mitigate the risks associated with APU fires, as it helps to contain the situation before it escalates.

This sequence ensures that there is a prompt reaction to potential danger, minimizing harm to the aircraft and enhancing safety for those on board. In comparison, although manual squib activation is an option within some protocols, it is not the primary or most effective response to an APU fire on the ground, as the automatic system is designed to act swiftly without the need for manual intervention. The other events, such as the alarm sounding and emergency power deployment, may occur as part of overall safety protocols or subsequent actions but are not the critical initial response to an APU fire.