What is a critical function of the APU during ground operations?

The critical function of the Auxiliary Power Unit (APU) during ground operations is to maintain cabin temperature and air quality. The APU provides necessary air conditioning and environmental control while the aircraft is on the ground, which is essential for the comfort and safety of both the crew and passengers. Keeping the cabin temperature within a suitable range and ensuring proper ventilation are vital to prevent overheating and maintain a breathable atmosphere, especially during long ground waits or when the external weather conditions are extreme.

While the APU has other support roles, such as charging the aircraft's main battery and providing hydraulic pressure for system operations, these functions are secondary to maintaining an acceptable environment inside the aircraft during ground operations, where the APU is often the sole source of environmental control. This makes its role in managing cabin temperature and air quality so crucial, particularly before departure or during maintenance when the engines are not running.