If the APU fire does not extinguish after spraying the intake, what should be done next?

Spraying the exhaust with an extinguishing agent is the appropriate action to take if the APU fire does not extinguish after applying it to the intake. This is because the APU's fire may not be limited to just the intake area; it can also involve the exhaust section where flames or hot gases could escape. By targeting the exhaust, you address the potential for fire spread and further combustion in a critical area of the APU.

In firefighting protocols, it's common to evaluate and address multiple locations where a fire may sustain or re-ignite. Although the intake has been treated, if the fire persists, the exhaust system should also be dealt with to ensure thorough fire suppression.

Other responses might focus on different aspects of emergency procedures, but they do not directly engage with the immediate need to extinguish flames from all possible sources. Disconnecting the battery and turning off all electrical systems may help in preventing further ignition but would not directly combat the fire. Signaling the pilot for emergency protocols may be a step taken in a coordinated response, but the priority remains on extinguishing the fire as effectively as possible.