What happens if the APU fire loop detects a fire?

What happens if the APU fire loop detects a fire?

• A. After 11 seconds, the APU shuts down and the fire bottle discharges; the APU is not certified for unattended use.
• B. The APU automatically continues running until manual shutoff.
• C. Only the fire loop trips, with no shutdown of the APU.
• D. The aircraft automatically selects alternate power sources.

Answer: (A)

When the APU fire loop detects a fire, the protection system is designed to respond immediately by shutting the APU down and deploying the fire extinguishing agent, typically after a short delay of about 11 seconds. This rapid sequence is to suppress the fire quickly while preventing continued operation of a burning APU. The APU isn’t certified for unattended use, so once shutdown is initiated, the crew must manage the situation rather than leaving the APU running. The action is specific to fire suppression and doesn’t automatically switch to alternate power sources in response to the fire loop; power source changes, if needed, are handled by separate electrical-system procedures.

When the APU fire loop detects a fire, the protection system is designed to respond immediately by shutting the APU down and deploying the fire extinguishing agent, typically after a short delay of about 11 seconds. This rapid sequence is to suppress the fire quickly while preventing continued operation of a burning APU. The APU isn’t certified for unattended use, so once shutdown is initiated, the crew must manage the situation rather than leaving the APU running. The action is specific to fire suppression and doesn’t automatically switch to alternate power sources in response to the fire loop; power source changes, if needed, are handled by separate electrical-system procedures.