Turn coordinator

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Image showing the face of a turn coordinator during a standard rate coordinated right turn.

Graphic of a turn and bank indicator (above, with inclinometer) and a turn coordinator

The turn coordinator is an aircraft instrument which displays to a pilot information about the rate of turn, rate of roll, and the 'quality' or 'coordination' of the turn. The turn coordinator was developed to replace the older turn and bank indicator, which displayed rate and quality of turn but not rate of roll.

The turn coordinator is, like the turn and bank instrument it replaced, a gyroscopic instrument. An internal gyroscope, typically electrically driven, spins at approximately 20,000 rpm. As the aircraft turns, the principle of gyroscopic inertia causes the gyro to tilt. This tilting force works against a spring; thus, a slow rate of turn deflects the gyro slightly while a higher rate of turn deflects it more. The gyroscope's movements are linked to an indicator on the front face of the turn coordinator.

The indicator looks like a little airplane seen from behind: when the airplane is level, the rate of turn is zero; when it is tilted, the amount and direction of tilt show the pilot the rate of turn. The wings of the symbolic airplane line up on white tick marks at the level position to indicate zero rate of turn. There is another set of tick marks below the level pair. When the symbolic aircraft is tilted so as to align with one of the tick marks, the aircraft is said to be turning at standard rate of turn, which is 3 degrees of heading change per second. This is often marked on the face plate of the instrument as '2 minutes', since it takes two minutes to complete a 360° heading change when turning at 3°/s.

The quality of turn is indicated by an inclinometer. This is a glass tube mounted on the face of the instrument, below the symbolic airplane. It is actually a completely separate instrument. The inclinometer consists of a glass tube filled with kerosene, and a steel ball. The tube is curved such that its center is the lowest point, and each end is higher. Normally, the ball will then sit in the center position of the tube, which represents a 'coordinated' turn. This position is marked by two vertical wires on the tube. The ball is said to be 'centered' when it sits perfectly evenly between the two wires.

The ball is used typically to tell the pilot the correct amount of rudder input is being applied, usually during rolls and turns. If the rudder input produces a coordinated turn, the ball will remain centered during a roll maneuver. If the ball deflects into the roll, the rudder input was insufficient, indicating a slip; if it deflects opposite the direction of the roll, the rudder input was excessive, indicating a skid. The old adage "step on the ball" refers to the pilot having to apply rudder in the same direction as the ball is deflected in order to return the aircraft to coordinated flight.

The turn coordinator differs from the older turn and bank indicator in that the turn coordinator has the gyro mounted at a 30-degree tilt. This allows the turn coordinator to respond to roll as well as turn. When the aircraft is rolling, the turn coordinator deflection is proportional to the rate of roll and not the rate of turn. Once the roll has stopped, the turn coordinator deflection will settle back to an amount which indicates the rate of turn. Pilots who are unfamiliar with this principle sometimes have difficulty using the turn coordinator properly, as they may see a roll indication and interpret it as a rate of turn.

The turn coordinator should be used as a performance instrument when the attitude indicator has failed. Called 'partial panel' operations, this can be unnecessarily difficult or even impossible if either (1) the pilot does not understand that the instrument is showing roll rates at some times and turn rates at others, and (2) if the internal dashpot is worn out. In the latter case the instrument is said to be underdamped; in turbulence it will indicate large full-scale deflections to the left and right, all of which are roll rate responses. In this condition it may not be possible for the pilot to maintain control of the aircraft in partial-panel operations in instrument meteorological conditions.