What happens if the APU detects a fire in flight?

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

Answer: (B)

When fire protection detects an APU fire in flight, the immediate priority is to isolate the APU and stop fuel and bleed air to the unit. This is done by shutting the APU down via the fire handle, which cuts off the energy source feeding the fire and helps prevent it from growing.

The extinguishing bottle, however, is not discharged automatically in flight. Releasing the extinguishing agent typically requires crew action, after the APU is shut down and the situation is assessed. This design avoids inadvertent discharge that could affect other aircraft systems or equipment, and it ensures the crew confirms the fire condition before deploying the suppressant.

So, in flight the system shuts the APU down to stop the fire, but does not automatically fire the bottle; the crew would initiate discharge if further action is needed.