Which bottle should be discharged to the APU when performing the fire emergency procedure?

The procedure during a fire emergency requires the discharge of only one specific bottle, typically designated as Bottle 2. This bottle is specifically configured for use in suppressing fires that could potentially affect the Auxiliary Power Unit (APU). Bottle 2 is designed to effectively target the areas most likely to catch fire and mitigate the risk posed by the APU, which can have critical implications for aircraft safety.

Discharging both bottles is unnecessary and could lead to complications, especially if the situation only requires one targeted response. Similarly, discharging only Bottle 1 does not align with the specific operational protocols established for a fire involving the APU. Therefore, focusing the response on Bottle 2 ensures that the most effective measures are taken in addressing the emergency while conserving resources and maintaining equipment integrity.