Approach idle is selected inflight with _____ or _____ or when landing gear is down.

The correct answer indicates that approach idle may be selected inflight with CONF 3 or FULL configurations, or when the landing gear is down. This is tied to the aircraft's aerodynamics and operational procedures.

When in CONF 3 or with the landing gear deployed, the aircraft is configured to provide maximum lift and drag, which supports flight stability during the approach phase. Selecting approach idle under these conditions ensures the engines can respond adequately to the pilot's commands, allowing for a controlled descent and approach to landing.

Moreover, the use of approach idle during high lift configurations (like CONF 3) is essential for managing the energy state of the aircraft, as it allows for smooth transitions in power settings while maintaining the desired altitude and airspeed during approach.

The other configurations, such as CONF 1 or CONF 2, do not support as much aerodynamic drag and lift as CONF 3 or FULL, making them less suitable for maintaining approach stability. Therefore, the selection criteria hinge on the aerodynamic performance related to the specific configurations during the landing phases.