What triggers the automatic start of a hydraulic pump when the switch is set to AUTO?

The automatic start of a hydraulic pump when the switch is set to AUTO is triggered when the flaps are out of the zero-degree detent. This condition indicates that the aircraft is in a configuration requiring additional hydraulic pressure to support various flight control systems and mechanisms, particularly when extending the flaps for takeoff or landing.

In this scenario, having the flaps extended necessitates increased hydraulic system performance to ensure reliable operation of the systems that depend on hydraulic pressure, such as the landing gear and braking systems. Therefore, the design of the hydraulic system includes an automatic activation feature that responds to flap position to ensure that adequate hydraulic power is available during crucial phases of flight.

While the other choices involve critical functions of the aircraft, they do not directly influence the hydraulic pump's operation under the specified condition pertaining to the flap position.