What is the primary function of the APU and main battery?

The primary function of the Auxiliary Power Unit (APU) and main battery is to provide emergency power for essential systems when the main engines are not operating. In the event of a power failure, the APU or main battery can supply power to critical systems, allowing the crew to maintain control of the aircraft and ensure safety. This emergency power can sustain operations needed for a minimum of 30 minutes, which is vital to manage emergency situations effectively.

This capability makes the APU and main battery essential for situations where the regular power sources are unavailable, ensuring that the aircraft's essential systems remain operational during critical times, such as an emergency landing or during pre-flight checks while on the ground. The other options do not accurately reflect the primary purpose of the APU and battery; while they support various aircraft functions, their critical role in providing emergency power is the most significant aspect.