In what scenario might external power sources be preferable over the APU?

External power sources are preferable over the APU during prolonged ground operations primarily to conserve the operational hours of the APU. The APU is often used to provide electrical power, air conditioning, and pneumatic pressure while the aircraft is on the ground, but it has a limited lifespan measured in running hours. By utilizing an external power source, operators can reduce wear and tear on the APU, extending its service life and decreasing maintenance needs.

This approach not only helps in the operational efficiency of the APU but can also reduce fuel consumption and emissions, contributing to overall operational economy. While using the APU is convenient, external power is a more sustainable choice over an extended period, especially during long waits on the tarmac or when parked.

In contrast, scenarios involving flight operations, maintenance requirements, or emergency situations do not focus on the prolonged use of resources in the same way. During flight operations, the APU is not in use; maintenance requires the APU to be shut down, and in emergencies, reliance on the APU may not be optimal when immediate power needs cannot be met through external sources.