Airplanes use two types of turn indicators—the turn and- slip indicator and the turn coordinator. Because of the way the gyro is mounted, the turn-and-slip indicator only shows the rate of turn in degrees per second. Because the gyro on the turn coordinator is set at an angle, or canted, it can initially also show roll rate.
Once the roll stabilizes, it indicates rate of turn. Both instruments indicate turn direction and quality (coordination), and also serve as a backup source of bank information in the event an attitude indicator fails. Coordination is achieved by referring to the inclinometer, which consists of a liquid-filled curved tube with a ball inside.
TURN-AND-SLIP INDICATOR
The gyro in the turn-and-slip indicator rotates in the vertical plane, corresponding to the airplane's longitudinal axis. A single gimbal limits the planes in which the gyro can tilt, and a spring tries to return it to center. Because of precession, a yawing force causes the gyro to tilt left or right as viewed from the pilot seat.
The turn-and-slip indicator uses a pointer, called the turn needle, to show the direction and rate of turn.
TURN COORDINATOR
The gimbal in the turn coordinator is canted; therefore, its gyro can sense both rate of roll and rate of turn.
Since turn coordinators are more prevalent in training airplanes, this discussion concentrates on that instrument. When rolling into or out of a turn, the miniature airplane banks in the direction the airplane is rolled. A rapid roll rate causes the miniature airplane to bank more steeply than a slow roll rate.
The turn coordinator can be used to establish and maintain a standard-rate-turn by aligning the wing of the miniature airplane with the turn index. The turn coordinator indicates only the rate and direction of turn; it does not display a specific angle of bank.
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