What happens if the APU fire extinguishing indicator light goes out?

• A. APU automatically shuts down and releases extinguishing agent after approximately 11 seconds
• B. Extinguishing agent is released immediately on detection
• C. APU continues operation until manual shutdown
• D. No automatic action occurs

Answer: (A)

When APU fire protection is armed, detection triggers the extinguishing sequence. There’s a built-in delay of about 11 seconds after fire detection before the extinguishing agent is released, and the APU is automatically shut down to stop the fire from spreading. The indicator light going out signals that the extinguishing bottle has discharged and the system has taken automatic action. So, the correct behavior is automatic shutdown with agent release after roughly 11 seconds. The other options aren’t correct because the agent isn’t released instantly, the APU doesn’t keep running after a detected fire, and there is automatic action in response to fire.