In the case of an APU fire, when should the emergency shutdown switch be activated?

The activation of the emergency shutdown switch in the event of an APU fire is critical for safety and effective response to the situation. The correct choice indicates that this switch should only be activated if the internal fire extinguisher fails to operate. This understanding is based on the operational protocols that prioritize the immediate response to a fire hazard while still initially relying on automatic fire suppression systems designed to control onboard fires.

If the fire starts, the primary step is to assess whether the internal fire extinguishing system can effectively handle the situation. Activating the emergency shutdown switch prematurely might not allow the onboard safety systems to function properly, which could be critical in containing the fire without additional manual intervention.

The point about trying to restart the APU is also pivotal since it reflects that one does not initiate action that could exacerbate the situation until it is clear that all safety protocols and systems are ineffective. Each step in the emergency protocol is designed to ensure that actions taken will prioritize the safety of the crew and equipment while following manufacturer guidelines.