What is the effect of a defective APU shutoff valve?

• A. APU shuts down immediately when commanded, preventing run-on.
• B. No safety impact; the APU remains easily controllable.
• C. Uncontrolled fuel flow or an inability to terminate APU operation, increasing fire risk or unintended run-on.
• D. The valve failure causes a minor leak with no safety consequence.

Answer: (C)

The fuel supply control for the APU is meant to stop fuel flow when a shutdown is commanded. If that shutoff valve is defective, it can fail to close or it can stick in an open position, so fuel continues to reach the APU even when you try to shut it down. That means the APU may keep running regardless of the shutdown signal, or fuel can keep flowing and pooling, making it impossible to terminate operation cleanly. The result is an increased fire risk and the possibility of unintended run-on, since the APU could stay energized and fueled when you don’t want it to. This is far more dangerous than a simple minor leak or no safety impact, and it contradicts the idea of an immediate, clean shutdown.