What should you do if the APU still does not shutdown after ensuring the battery switch is on?

If the APU does not shutdown after confirming that the battery switch is on, the recommended action is to open door 13L and check the APU Prime circuit breaker. This is a key troubleshooting step because the circuit breaker is designed to protect the APU system from electrical faults. By inspecting the APU Prime circuit breaker, you can determine if it has tripped due to an overload or fault condition.

If the circuit breaker has tripped, it may require resetting to allow the APU to function properly again. This approach emphasizes a systematic response to potential issues with the APU, allowing for safe handling of the situation without resorting to more drastic measures. It is important to follow protocol in this scenario, as addressing the circuit breaker directly pertains to the operational health of the APU system.

In contrast, other options might not directly address the root cause. Calling for emergency service could be necessary in extreme situations but is not the first step in standard troubleshooting. Waiting for the APU to reset itself does not address the underlying issue and could lead to unnecessary delays or complications. Disconnecting power from the APU may lead to additional safety concerns or loss of functionality but does not solve the immediate problem of the APU not shutting down properly.