Which instrument is primarily used to maintain coordinated flight?

The turn coordinator is primarily used to maintain coordinated flight as it provides crucial information about the aircraft's rate of turn and coordination. This instrument displays both a rate of turn and the coordination of the turn, helping pilots to understand if the airplane is in a coordinated turn or if it is skidding or slipping.

In a coordinated turn, the aircraft banks appropriately and uses aerodynamic forces to turn efficiently without excessive load on the airframe. The turn coordinator will show when the aircraft is at the correct angle for a smooth turn versus when it may be over-banked or under-banked, which can lead to adverse flight conditions.

The other instruments mentioned serve different primary functions. The attitude indicator reflects the aircraft's pitch and bank relative to the horizon, while the horizontal situation indicator provides pertinent navigational information but does not specifically indicate coordination of the turn. The altitude indicator simply shows the aircraft's altitude above sea level, unrelated to turns or coordination. Thus, the turn coordinator uniquely serves the purpose of facilitating and ensuring coordinated flight.