What should you do if the APU does not shut down after 11 seconds?

When the Auxiliary Power Unit (APU) does not shut down after 11 seconds, pulling the APU and APU Prime circuit breakers is the appropriate action. This step effectively interrupts the electrical supply to the APU, ensuring that it shuts down safely and quickly to prevent potential damage or hazardous conditions. The purpose of this action is to stop the APU's operation immediately, which helps to mitigate any risks associated with its failure to shut down properly.

Neutralizing the power to the APU eliminates ongoing risks such as fire hazards or system malfunctions. It is crucial to act swiftly in such situations to protect both the aircraft and personnel on the ground.

While other options may seem relevant, they involve further action that could delay the necessary response to the malfunction. For instance, calling for troubleshooting assistance might take time and does not address the immediate need to stop the APU, which could lead to further complications. Replacing the APU is a more involved process requiring significant maintenance resources, and signaling the ground crew does not directly resolve the malfunction at hand. Therefore, pulling the APU and APU Prime circuit breakers is the most effective and immediate response.