What should the pilot use during an APU emergency situation?

In the context of an APU (Auxiliary Power Unit) emergency situation, the most appropriate response involves utilizing the extinguishing system. The APU fires can pose serious risks, and its malfunction can lead to hazardous conditions. The extinguishing system is designed to suppress and extinguish fires that may arise from APU operations or failures.

Utilizing the extinguishing system in an emergency helps to minimize damage to the aircraft and ensures the safety of both the crew and any personnel in the vicinity. This system may include fire suppression agents that can quickly eliminate flames and control the situation, making it a critical first response.

Other systems or manuals, such as the emergency manual or communications system, while important in broader emergency contexts, do not provide immediate assistance in controlling an APU-related fire. The backup controller is typically used to manage other systems but may not specifically address the emergency posed by an APU incident. Therefore, promptly activating the extinguishing system is paramount for addressing the risks associated with an APU emergency.