What is necessary when operating the APU with the APU compartment door open?

When operating the APU with the APU compartment door open, it is crucial to station an external fire guard with an adequate fire extinguisher. This requirement is grounded in safety protocols designed to mitigate the risk of fire hazards that may arise during APU operation. The APU generates heat and can be a source of potential ignition for any fuel vapors or flammable materials in the vicinity. By having a fire guard present, there is a dedicated individual monitoring for any signs of fire or smoke and ready to act immediately with a fire extinguisher if necessary, enhancing overall safety during the operation.

Other choices, although they might seem plausible, do not adequately cover the safety measures needed in this scenario. For instance, turning off the APU might stop the immediate risk but does not address the potential hazards present while it is operating with the door open. The suggestion of no special precautions needed overlooks the inherent risks of operating equipment in an open environment. Additionally, while having a safety officer may improve overall safety procedures, the critical requirement of a fire guard specifically addresses potential fire risks associated with the APU operation.