Why must you wait five seconds before turning off the battery switch after pulling the APU fire handle?

The correct answer highlights the importance of allowing the fuel valves to close after pulling the APU fire handle. When the fire handle is activated, it initiates a series of safety measures designed to minimize the risk of fire and ensure the safety of the aircraft and crew. One critical aspect of this procedure is to shut off the fuel supply to the APU, which is accomplished by closing the fuel valves.

Waiting for five seconds serves a dual purpose. It ensures that the command for the fuel valves to close is given enough time to be executed properly, thereby preventing any residual fuel from continuing to flow to the APU. This is especially crucial in a fire scenario, where any additional fuel could exacerbate the situation and make extinguishing the fire more difficult.

Additionally, this time frame allows for the confirmation that the fuel valves have indeed closed successfully before the battery switch is turned off. Turning off the battery too quickly could impede this process, potentially leading to an incomplete shutdown and increasing the risk of further complications. Therefore, allowing for this interval enhances the overall safety and effectiveness of the emergency procedure.