Which instrument helps to maintain a constant heading during a turn?

The Turn Coordinator is designed specifically to assist pilots in maintaining a constant heading during turns. It provides visual indications of the rate of turn and the coordination of the aircraft during the maneuver. The instrument displays a miniature airplane symbol that reflects the aircraft's turn rate; when the airplane symbol aligns with the reference lines on the instrument, it indicates that the turn is properly coordinated. This is crucial because a properly coordinated turn helps in ensuring the aircraft maintains balanced flight and doesn't enter into a skidding or uncoordinated condition.

The Heading Indicator, while useful for displaying the aircraft's current heading, does not provide information about the rate of turn or the coordination of the turn. The Vertical Speed Indicator shows how quickly an aircraft is climbing or descending, which is unrelated to turns. The Airspeed Indicator monitors the speed of the aircraft but does not assist in maintaining heading during turns. Therefore, the Turn Coordinator is the most relevant instrument for maintaining a constant heading during a turn.