When is it permissible for a ground observer to fight a fire related to the APU?

The correct choice indicates that it is permissible for a ground observer to fight a fire related to the APU after the APU start has been terminated. This approach is based on safety protocols and best practices regarding fire management around aircraft systems.

When the APU is operational, various hazards and risks are present due to fuel, electrical systems, and the potential for re-ignition. Attempting to engage with a fire while the APU is still running increases the likelihood of serious injury or exacerbating the fire. Therefore, a critical safety measure is to ensure that the APU is no longer operating before attempting to combat any fire that may arise.

Once the APU has been shut down, the situation becomes less volatile, allowing trained personnel to assess the fire safely and take appropriate actions to extinguish it. Moreover, this protocol aligns with emergency response guidelines that emphasize the importance of securing the source of potential hazards before approaching fire situations.

In contrast, engaging with a fire immediately upon detection or before the APU even starts would expose individuals to greater risk without proper safety measures in place. The option involving the arrival of an external fire truck also emphasizes the importance of professional assistance in managing aircraft-related emergencies.