Which testing option would verify APU shutdown capabilities if a self-test feature is available?

Which testing option would verify APU shutdown capabilities if a self-test feature is available?

• A. Presence of a fault indication on the APU control system with no other symptoms.
• B. Normal electrical output but high oil temperature.
• C. Abnormal EGT readings while N2 is within range.
• D. Availability of a self-test feature that can be run to verify shutdown/start controls.

Answer: (D)

Verifying shutdown and start control functionality using a self-test is essential because it directly exercises the control path that stops and starts the APU. A self-test designed for this purpose runs the actual shutdown sequence and startup sequence in a controlled, repeatable way, checks the interlocks and safety conditions, and confirms that status signals return to a safe, known state. This provides concrete evidence that the APU can be reliably shut down when commanded and then restarted if needed, which is the primary safety and operational requirement. Observing a fault indication without additional symptoms only flags a potential problem; it doesn’t prove the shutdown path works. Normal electrical output with high oil temperature points to a fault condition but doesn’t test the ability to stop the APU. Abnormal EGT with N2 within range indicates a fault in operating parameters but again does not verify the shutdown control hardware/software response.

Verifying shutdown and start control functionality using a self-test is essential because it directly exercises the control path that stops and starts the APU. A self-test designed for this purpose runs the actual shutdown sequence and startup sequence in a controlled, repeatable way, checks the interlocks and safety conditions, and confirms that status signals return to a safe, known state. This provides concrete evidence that the APU can be reliably shut down when commanded and then restarted if needed, which is the primary safety and operational requirement.

Observing a fault indication without additional symptoms only flags a potential problem; it doesn’t prove the shutdown path works. Normal electrical output with high oil temperature points to a fault condition but doesn’t test the ability to stop the APU. Abnormal EGT with N2 within range indicates a fault in operating parameters but again does not verify the shutdown control hardware/software response.