What primary instrument is used to control bank angle while turning?

The primary instrument used to control bank angle while turning is the attitude indicator. This instrument directly shows the aircraft's orientation relative to the horizon, displaying whether the wings are level and at what angle the aircraft is banked during a turn.

When executing a turn, the pilot must manage the bank angle to maintain control of the aircraft's flight path and ensure a coordinated turn. The attitude indicator provides real-time feedback, allowing the pilot to maintain the desired bank angle for a smooth turn without exceeding the aircraft's limitations.

While the turn coordinator could also provide information on the quality of a turn and assist in monitoring the rate of turn, it does not provide a direct representation of the aircraft's orientation concerning the horizon. The airspeed indicator and heading indicator do not relate directly to controlling bank angle during a turn, as they serve different functions; airspeed is about the speed of the aircraft, while the heading indicator shows the direction in which the aircraft is pointed.