Which of the following is a listed APU shutdown cause?

• A. Overspeed
• B. DC Signal Lost
• C. Door Closed
• D. Fire

Answer: (D)

The event the question is testing is the safety-driven triggers for shutting down an APU. A fire detected in the APU compartment is a direct, automatic shutdown cause because the fire detection system is designed to cut off fuel and stop the turbine immediately to prevent flame spread and to allow firefighting teams to work safely. This immediate shutdown protects both the aircraft’s electrical power system and the surrounding structure from fire damage.

Other conditions like overspeed, loss of DC signal, or a door status describe faults or operational states, but they aren’t the specific listed automatic shutdown trigger for the APU in the standard safety checklist. Fire directly represents the intentional protective action built into the APU shutdown logic.