If the APU electric fuel pump fails while the APU is running, what will happen?

When the APU electric fuel pump fails while the APU is running, the system is designed to have a fuel supply mechanism that allows the APU to continue operating normally for a certain period. The APU typically relies on both an electric fuel pump and a mechanical fuel pump to supply fuel. If the electric fuel pump fails but the APU is already running, the mechanical pump can continue to draw fuel from the tank, ensuring the APU does not shut down immediately.

However, if the APU is subsequently shut down, the failure of the electric fuel pump means it would not be able to restart the APU because it relies on that electric pump for starting. Therefore, while it will continue to run with the existing fuel supply, once it is shut off, it cannot restart without the electric fuel pump functioning. This understanding illustrates the redundancy built into the APU systems, allowing for continued operation even if one part fails, but highlighting the importance of the electric pump for restart capabilities.