How can a fire extinguisher be discharged into the APU compartment if the APU fire extinguisher bottle does not extinguish the fire?

Discharging a fire extinguisher into the APU compartment through the APU compartment push-in-door is the correct approach because this method allows direct access to the area where the fire is occurring. The push-in-door design is specifically intended to facilitate manual intervention and provide firefighters or crew personnel the ability to apply extinguishing agents effectively.

The push-in-door allows for a more controlled application of the extinguishing agent, targeting the source of the fire more accurately. This is critical in aircraft operations where rapid and effective response to fire situations is necessary to ensure safety.

Other methods listed, such as using an access panel or discharging from the cockpit, may not provide the same level of access, control, and effectiveness. Automatic fire systems are designed to respond to particular conditions, but there may be scenarios where manual intervention is required, making the push-in-door a valuable option for direct firefighting efforts.