What must be available when starting an APU without external power?

When starting an APU without external power, it is critical to have a fire truck standing by as a safety precaution. The APU can produce various exhausts and flammable materials during its start-up phase, which inherently poses a risk of fire. Having a fire truck readily available ensures that swift action can be taken in case of an emergency, such as a fire outbreak, which could occur due to the nature of the equipment and its operation.

This consideration reflects standard safety protocols in aircraft operations, ensuring that any potential hazards during the ignition and operation of the APU are mitigated appropriately. The presence of emergency equipment, like a fire truck, is a vital aspect of risk management in aviation ground operations.