What aspect of the APU is crucial for overall aircraft safety during flight?

The functionality of the electrical system is crucial for overall aircraft safety during flight. The Auxiliary Power Unit (APU) plays a significant role in supplying electrical power to various aircraft systems, particularly when the main engines are not running and during ground operations. A fully operational electrical system provided by the APU ensures that essential aircraft systems, such as avionics, lighting, and communications, remain functional.

Additionally, a reliable electrical supply enhances the safety of the aircraft by enabling effective monitoring of various systems and providing power for onboard systems in the event of engine failure or other emergencies. This capability is vital for maintaining operational readiness and ensuring that the aircraft can be safely managed in flight.

In contrast, the other options do not directly impact flight safety as significantly. For instance, the aesthetic condition of the unit is not relevant to safety, and complete isolation from the aircraft would limit the APU's function, which is intended to integrate with the aircraft systems. Meanwhile, the level of external training of personnel is important for operational effectiveness, but it does not directly pertain to the aircraft's safety in-flight.