Turn indicator

A Turn Indicator is an aircraft flight instrument that shows the rate of turn. It is used by the pilot to maintain control when flying under Instrument flight rules.

Attitude and heading indicators function on the principle ofrigidity, but rate instruments such as the turn-and-slip indicatoroperate on gyroscopic precession. Precession is the characteristicof a gyroscope that causes an applied force to produce amovement, not at the point of application, but at a point 90°from the point of application in the direction of rotation.

Turn-and-Slip Indicator

The first gyroscopic aircraft instrument was the turn indicatorin the needle and ball, or turn-and-bank indicator, whichhas more recently been called a turn-and-slip indicator.

The inclinometer in the instrument is a black glass ball sealedinside a curved glass tube that is partially filled with a liquid,much like compass fluid. This ball measures the relativestrength of the force of gravity and the force of inertia causedby a turn. When the aircraft is flying straight-and-level, thereis no inertia acting on the ball, and it remains in the center ofthe tube between two wires. In a turn made with a bank anglethat is too steep, the force of gravity is greater than the inertiaand the ball rolls down to the inside of the turn. If the turn ismade with too shallow a bank angle, the inertia is greater thangravity and the ball rolls upward to the outside of the turn.

The inclinometer does not indicate the amount of bank, neitheris it limited to an indication of slip; it only indicates therelationship between the angle of bank and the rate of yaw.The turn indicator is a small gyroscope spun either by air or byan electric motor. The gyro is mounted in a single gimbalwith its spin axis parallel to the lateral axis of the aircraftand the axis of the gimbal parallel with the longitudinal axis.

When the aircraft yaws, or rotates about its vertical axis, itproduces a force in the horizontal plane that, due to precession,causes the gyro and its gimbal to rotate about thegimbal axis. It is restrained in this rotation plane by acalibration spring; it rolls over just enough to cause thepointer to deflect until it aligns with one of the doghouse shapedmarks on the dial, when the aircraft is making astandard-rate turn.

The dial of these instruments is marked “2 MIN TURN.”Some turn-and-slip indicators used in faster aircraft like the Concorde aremarked “4 MIN TURN.” In gliders, the calibration spring is usually set for a one minute standard rate turn. A standard-rateturn is being made whenever the needle aligns with adoghouse.

Turn Coordinator

The major limitation of the older turn-and-slip indicator isthat it senses rotation only about the vertical axis of theaircraft. It tells nothing of the rotation around the longitudinalaxis, which in normal flight occurs before the aircraft beginsto turn.

A turn coordinator operates on precession, the same as theturn indicator, but its gimbal frame is angled upward about30° from the longitudinal axis of the aircraft. This allows itto sense both roll and yaw. Some turn coordinator gyros aredual-powered and can be driven by either air or electricity.Rather than using a needle as an indicator, the gimbal movesa dial on which is the rear view of a symbolic aircraft. Thebezel of the instrument is marked to show wings-level flightand bank angles for a standard-rate turn.

The inclinometer, similar to the one in a turn-and-slipindicator, is called a coordination ball, which shows therelationship between the bank angle and the rate of yaw. Theturn is coordinated when the ball is in the center, between themarks. The aircraft is skidding when the ball rolls toward theoutside of the turn and is slipping when it moves toward theinside of the turn.

A turn coordinator does not sense pitch. This is indicated on some instruments by placing the words "NO PITCHINFORMATION" on the dial.

Sources

*Advisory Circular AC 61-23C, "Pilot's Handbook of Aeronautical Knowledge", U.S. Federal Aviation Administration, Revised 1997.
*FAA-H-8083-15 "Instrument Flying Handbook", U.S. Federal Aviation Administration (IFH), (Update 25 Nov 05)