What happens if the APU detects a fire on the ground?

• A. In flight shutters down but does not automatically fire the bottle
• B. Ground shuts down the APU and fires the bottle
• C. Ground continues running; bottle is unaffected
• D. In flight shutters down and fires the bottle automatically

Answer: (B)

When an APU fire is detected on the ground, the system is designed to respond immediately by shutting the APU down and automatically discharging the fire suppression bottle. This rapid action stops any fuel or heat sources from feeding the fire and applies extinguishing agent where it’s most needed, reducing the risk to the aircraft and maintenance crew. The other scenarios would either let the APU keep running in a fire condition or apply procedures meant for flight, which don’t provide the same quick, ground-specific protection.