What happens if the Fire Detection System detects an APU fire while the APU is running?

When the Fire Detection System detects a fire in the Auxiliary Power Unit (APU) while it is operational, the correct response is for the APU to shut down automatically through the APU Electronic Control Unit (ECU). This action is a critical safety feature designed to mitigate the risk of fire escalation and protect the aircraft and its occupants. The APU ECU is programmed to respond to fire detection signals by initiating an immediate shutdown of the APU, effectively cutting off fuel supply and electrical power to prevent further combustion.

This automated shutdown is essential because it minimizes the danger posed by an APU fire, preventing potential structural damage to the aircraft and ensuring the safety of the crew and passengers. The APU is a vital component that provides electrical and air systems, but in the event of a fire, prioritizing safety through immediate action is crucial.

Other responses, like sending alarm signals to the cockpit alone or notifying emergency services automatically, do not address the immediate safety concerns associated with an active fire and the need to halt operations of the APU. Allowing the APU to continue running for safety checks is not a prudent or safe course of action when a fire has been detected.