Aircraft Turn Coordinator: Definition, Operation, Use, Errors, Checks

Mounted at 30 degrees to the longitudinal axis, the turn-coordinator is a single gyroscopic instrument in the basic six-pack that fuses rate-of-turn indication with slip/skid sensing. It supplies the pilot with an immediate visual cue of both the direction and the coordination of a heading change, but it does not measure actual bank angle or roll rate.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What is a turn coordinator in an aircraft?

A turn coordinator is an aircraft flight instrument that displays the rate of turn and roll information, offers insight into the aircraft's turn rate and balance, and determines coordination of a turn. The turn coordinator is a gyroscopic instrument and an advancement of the turn and slip indicator. It displays a miniature airplane symbol that rolls right or left depending on bank angle and rate of turn and the pointer deflects until it aligns with marks for a standard-rate turn. Below the instrument, an inclinometer uses a black ball suspended in liquid. That ball rolls right, left, or remains in the middle of the tube to indicate slip or skid: the ball deflects right to indicate skid and left to indicate slip. During taxi, the ball deflects outward from turn, but in flight its position warns the pilot of uncoordinated flight. Thus, the turn coordinator does not directly measure yaw but senses roll and rate of turn while the inclinometer supplies skid/slip information, helping the pilot keep the ball centered for coordinated control.

What are the components of a turn coordinator?

There are two components in a turn coordinator: a rate-of-turn indicator and an inclinometer, inside one housing. The rate-of-turn section is a gyro instrument whose gyro is mounted in a single gimbal tilted thirty degrees upward relative to the longitudinal axis of the aircraft. This tilt lets the gyro sense both roll and yaw, so the visible needle, designed like a small airplane symbol, rolls right or left to show the direction and approximate speed of the turn.

Below the miniature airplane sits the inclinometer, a curved glass tube filled with liquid and a black ball that is free to roll right, left, or remain in the middle. The black ball indicates slip or skid condition. During taxi the ball deflects outward from the turn while in flight, centering the ball with coordinated rudder pressure removes the slip or skid. Before engine start a red flag is visible; engine start activates gyroscopes and the flag disappears, confirming that the turn coordinator is ready for use.

Is the turn coordinator vacuum powered?

The turn coordinator is not usually vacuum powered but works on electrical power. The vacuum pump creates suction that pulls air through filters and the instruments themselves, keeping the gyros stabilized in space. If the vacuum pump quits, you still have an electrically-powered turn coordinator. This electrical design makes the turn coordinator the only instrument that is electrically driven, while attitude indicator and heading indicator are usually vacuum-driven.

How does a turn coordinator work?

Inside the instrument, a gyroscope mounted sideways at 30 degree spins; the gyroscope exhibits rigidity in space, so when the airplane rolls or yaws the instrument measures the torque forces that arise as the gyroscope tries to resist the rotational motion. Because torque produces a reaction 90 degrees ahead of the spin, the airplane symbol rolls right or left depending on bank angle and rate of turn.

Below the miniature aircraft, an inclinometer is a black ball in liquid. The ball measures relative strength of gravity and inertia while the spring restrains rotation plane and keeps the display steady. When the nose points inward, slip occurs and the ball indicates slip toward the inside. When the nose points outward, skid occurs and the ball indicates skid toward the outside. In a perfectly coordinated turn, banking does not apply torque to the axis and the ball stays centered. The instrument's flag shows the instrument is in a non-operational status.

Why is the turn coordinator canted?

The turn coordinator is canted 30 degrees so that its gyro, mounted at a 30-degree angle upward from the longitudinal axis of the aircraft, senses the very first roll rate. Because the gimbal axis is canted 30 degrees while the spin axis of the gyro wheel is not canted, the instrument reacts to both roll and yaw, making it more responsive to the start of a turn than the older turn-and-slip indicator that does not provide roll information. This tilt means it responds immediately when the pilot starts to turn. The gyroscope creates rigidity in space, and when a force is applied to the spinning gyro the resultant force is 90 degrees in the direction of rotation, so the canted gyro can display the initial roll before the aircraft has actually yawed.

How does precession work in a turn coordinator?

Because precession is the force that results in an error, the case and spring are engineered so that the amount of precessional tilt is exactly proportional to the yawing force produced by the turn rate. The off-center gyro causes slow drift of the instrument, but the restrained motion is instead displayed by the needle rotating left or rotating right, alerting the pilot to the direction and rate of the turn.

How does a turn coordinator sense movements?

The turn coordinator senses movements by measuring tilt. The instrument begins with a gyro tilted 30 degrees from the aircraft's longitudinal axis. By tilting the gyro, the same mechanism gains sensitivity to both roll rate and yaw rate. When the aircraft starts to yaw or roll, a banking turn causes force to be translated to the gyro. Force applied to the spinning gyro makes the gyroscope resist rotational motion and in resisting, it generates measurable torque. The instrument measures these torque forces and converts them into movement of the miniature airplane symbol on the dial. Thus, the small airplane symbol rolls left or right in proportion to the sensed roll rate, and the tilt allows the same gyro to sense the accompanying yaw rate, giving the pilot immediate feedback about the turning motion.

Parallel to the gyroscopic channel, an inclinometer-a black ball suspended in liquid-reacts to the uncoordinated unit of the turn. The liquid allows the black ball to remain in the middle of the tube when the rudder and bank are balanced. Any misalignment lets the ball roll left or right, revealing slip or skid. The combined indications of the tilting gyro and the drifting ball let the pilot detect and correct both the rate of turn and the quality of the turn.

How to use a turn coordinator?

To use a turn coordinator, watch the miniature aeroplane: when its wings tilt left the instrument senses a left yaw-rate and if it tilts right, it senses a right yaw-rate. The marks on the glass are calibrated for a standard-rate turn - one that changes heading 3 degrees per second. Once airborne, roll into the bank until the wing tip touches the index which gives a two-minute 360 degrees turn. Beside the aeroplane is a curved race containing a black ball. The ball obeys gravity and centrifugal force: if it rests off-center, the airplane is skidding or slipping. A centered ball and a wing tip on the index together mean a balanced, standard-rate turn. As the aircraft begins to taxi, the ball leans slightly in the direction of each taxi turn. If it does, the gyro is functional and the race is unobstructed.

The turn coordinator became my main indicator for bank steering. I remember rehearsing standard-rate performance and citing the device's rate-of-turn.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What is the rate of turn on a turn coordinator?

The standard rate of turn calibrated to the instrument is 3 degrees/second. This output is called a rate-one or standard rate turn and causes the aircraft to complete a 360 degree turn in precisely two minutes. The instrument displays the angular velocity around the vertical axis; when the miniature aircraft wing aligns with the tick, the pilot is achieving the calibrated 3 degrees/second standard rate.

What does the turn coordinator indicate in a spin?

In a spin, the turn coordinator indicates flight level and direction. On the ground, with the engine running and the gyroscope spinning clockwise as viewed from the right, the miniature airplane of the turn coordinator stands straight up and down to signal that the aircraft is straight and level. The ball rests centered in the inclinometer until a brisk taxi turn or sharp pedal input displaces it, whereupon the ball indicates slip or skid exactly as it does in flight.

In an upright spin, the turn coordinator correctly indicates direction of spin because the turn needle always acts as a yaw indicator: the airplane symbol rolls toward the spin axis and the needle deflects in the same direction as the yaw, so the pilot sees at a glance which way the nose is rotating. The ball, meanwhile, falls toward the inside of the turn, indicating the aerodynamic slip that accompanies the stalled, yawing motion. Because the gyro responds to yaw rate and not bank angle, the indication is achieved due to the effect of precession on gyro, and the display remains valid throughout the spin until recovery is initiated.

How does a turn coordinator integrate with an autopilot?

Autopilot uses turn coordinator to send electric messages to the flight computer. TC cants the gyroscope axis 30 degrees which allows the indicator to respond more quickly to banking which allows autopilot systems to control the bank rate using this instrument. Rate-based autopilots use turn coordinator for pitch and roll information and these are known as rate-based systems. The single-axis autopilot uses the turn coordinator to keep the wings level while the attitude-based autopilots rely on attitude indicators or AHRS for pitch and bank information. Basic autopilots rely on directional gyro heading bugs for lateral control.

What are the errors of a turn coordinator?

Acceleration on an easterly or westerly heading causes the error to appear as a turn indication toward north while deceleration on the same headings shows toward south. The mnemonic ANDS - Accelerate North, Decelerate South - helps pilots remember this heading-dependent error.

Because the gyro is mounted along the longitudinal axis, the instrument senses yaw but cannot separate yaw from roll. It provides a blended indication and cannot determine true yaw or roll independently. When the gyroscope fails, the instrument indicates incorrectly and no red flag is present, since flags only indicate loss of power, not gyro validity.

True airspeed that is too high for the bank angle displaces the ball. A skid occurs when the aircraft's nose points toward the outside of the turn and the ball moves to the outside, while a slip occurs when the nose points toward the inside and the ball moves to the inside. High pitch attitudes due to the P-factor keep the ball right at the center even in balanced flight.

Main failure modes are electrical failure in the a/c motor or inverter driving the gyro and failure of the internal damping system or bearings. Pilots monitor the instrument during engine start and taxi, noting that any heading misalignment occurs while taxiing because the ball moves outward from the turn during surface operations.

Can a turn coordinator tumble?

No, a turn coordinator cannot tumble. The instrument contains a rate gyro that has no gimbals. Because the gyro is fixed relative to its housing, it cannot be displaced off its rotational axis even in unusual attitudes. Consequently, the turn coordinator is incapable of tumbling, whether the miniature airplane is centered or displaced.

What are the limitations of a turn coordinator?

The limitations of a turn coordinator include that it does not display bank angle, yaw angle, or any direct measure of pitch. Because the gyroscopic unit inside the instrument only exhibits rigidity in space, the device is unable to determine the airplane's exact orientation relative to the horizon. Neither can the small inclinometer, a ball suspended in liquid beneath the dial, resolve yaw or roll; it merely signals slip or skid once the turn is already in progress. Pilots thus rely on different instruments when precise attitude is required. The secondary attitude indicator, utilizing a system independent of the primary, has replaced the turn and slip indicator as the approved backup source of bank information.

How to check a turn coordinator?

While checking the turn coordinator, the pilot confirms that black ball suspended in the liquid correctly rolls right, left, or remains in the middle of the tube depending on the aircraft's slip/skid condition. Absence of flags and free motion confirm instrument readiness. During taxi, make gentle turns: taxi indicates a bank in the direction of the turns, and the pointer deflects until it aligns with the doghouse-type marks on the dial. An absence of flags, correct pointer sense, and centered black ball certify proper function.

How do you calibrate the turn coordinator?

The turn coordinator is calibrated on the assumption that every turn is 100% yaw and zero% pitch. Because of this, the gyroscope is mounted at a fixed 30-35-degree angle and restrained by a calibration spring. During production, the manufacturer rolls the miniature airplane symbol until its wing tip aligns with one of the two white turn-alignment marks, then locks the dial. No further pilot adjustment is permitted. The inclinometer, a ball-and-fluid device located below the instrument face, is leveled independently so that the ball rests centered when the aircraft is in unaccelerated flight. If electrical power is lost the gyro ceases to spin and a red flag appears on the right side above the miniature airplane, signaling that the instrument is in a non-operational status.