Which switch must be set to ON to allow external power in the aircraft?

The choice of the battery switch being set to ON is crucial for allowing external power into the aircraft because it serves as the primary source of electrical power when the aircraft is on the ground. When the battery switch is activated, it connects the aircraft's electrical systems to the external power source, enabling the flow of power from ground support equipment or external power units. This connection is necessary to power the aircraft's electrical systems for maintenance, pre-flight checks, and other ground operations without relying solely on the onboard generators.

Other switches, such as the generator, auxiliary battery, and avionics master, play important roles in the aircraft's power management but are not directly responsible for enabling external power. The generator switch is utilized during flight to manage the generator’s output, the auxiliary battery provides back-up power in certain scenarios, and the avionics master controls the power to the avionics systems specifically. However, none of these can replace the need for the battery switch to be ON for external power access.