During an APU emergency, which procedure should be prioritized first?

In the event of an APU emergency, the priority is to ensure safety and mitigate risks associated with a potential fire. The correct response would be to pull the applicable APU fire pull handle. This action is critical as it directly addresses the immediate threat of fire by cutting off fuel and electrical power to the APU, thereby reducing the risk of an escalation of the emergency.

Addressing a potential fire hazard is paramount because if the APU is on fire, other systems and personnel are at significant risk. Once the fire handle is pulled, it initiates emergency procedures aimed at extinguishing the fire and safeguarding the aircraft and crew.

Other actions, although important, come after addressing the immediate danger posed by a fire. Checking for physical injuries would be a sensible step, but it is secondary to eliminating the source of potential harm. Powering off all systems and activating backup power supply might be necessary in broader emergency procedures, but the central concern in an APU emergency specifically revolves around the fire risk. Thus, taking immediate action to control and eliminate fire hazards is of utmost importance.