The
turn coordinator (TC) is a flight instrument which displays to a
pilotAn aviator is a person who flies aircraft for pleasure or as a profession. The first recorded use of the term was in 1887 as a variation of the French 'aviation', from the latin 'avis', coined 1863 by G. de la Landelle in "Aviation ou Navigation Aérienne"...
information about the rate of
yaw (turn), rollYaw, pitch, and roll, also known as Tait–Bryan angles, named after Peter Guthrie Tait and George Bryan, are a specific kind of Euler angles very often used in aerospace applications to define the relative orientation of a vehicle...
, and the "quality" or "coordination" of the turn. The turn coordinator was developed to replace the older
turn and bank indicatorIn aviation, the turn and bank indicator shows the rate of turn and the coordination of the turn. The rate of turn is indicated from a rate gyroscopically and the coordination of the turn is shown by either a pendulum or a heavy ball mounted in a curved sealed glass tube...
, which displayed rate and quality of turn but not rate of yaw.
History
The initial reason for the development of the TC was to create a single instrument that could be used by an autopilot to control the roll axis of an airplane. Due to the "newer" look the same instrument without autopilot sensing became popular in small general aviation airplanes but rarely if ever appeared in jets or large airplanes.
Use
The indicator looks like a little airplane seen from behind: when the airplane is level, the rate of yaw plus the rate of roll is zero; when it is tilted, the amount and direction of tilt show the pilot the rate of turn plus the rate of roll. 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 (and the airplane is in a constant bank), 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 ball is used typically to tell the pilot the correct amount of
rudderA rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft,or other conveyance that moves through a fluid . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
input is being applied, usually during rolls and turns and when engine power is higher or lower than a cruise setting. 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
slipA slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow. In other words, for a conventional aircraft, the nose will not be pointing directly into the relative wind .A slip is also a piloting maneuver where the pilot...
; if it deflects opposite the direction of the roll, the rudder input was excessive, indicating a
skidIn flying, a skid is a specific type of slip.It often means a turn where the sideways movement of the aircraft is outwards from the centre of the turn...
. The 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.
A turn coordinator does not sense pitch. This is indicated on some instruments by placing the words
"No pitch information" on the dial.
Operation
The turn coordinator is, like the turn and bank instrument it replaced, a gyroscopic instrument. An internal gyroscope, typically electrically driven (although some turn coordinator gyros are
dual-powered and can be driven by either vacuum or electricity), spins at approximately 20,000 rpm with the spin axis perpendicular to the longitudinal axis of the airplane and the free axis tilted up 30° from it. As the aircraft rotates about the yaw or roll axis, the principle of gyroscopic inertia causes the gyro to "resist" the change in its rotational axis about the free axis. This resisting force works against a spring; thus, a slow rate of turn deflects the gyro slightly while a higher rate of roll or yaw deflects it more. The
gimbal250px|thumb|right|Illustration of a simple two-axis gimbal set.A gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of two gimbals, one mounted on the other with pivot axes orthogonal, may be used to allow an object mounted on the innermost gimbal to...
's movements are linked to the indicator dial on which is the rear view of a symbolic aircraft.
The quality of turn is indicated by an
coordination ball, which works on the same principle as an
inclinometerAn inclinometer or clinometer is an instrument for measuring angles of slope , elevation or inclination of an object with respect to gravity...
. 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.
Variations
The turn coordinator differs from the older
turn and bank indicatorIn aviation, the turn and bank indicator shows the rate of turn and the coordination of the turn. The rate of turn is indicated from a rate gyroscopically and the coordination of the turn is shown by either a pendulum or a heavy ball mounted in a curved sealed glass tube...
in that the turn coordinator has the gyro mounted at a 30° tilt. This allows the turn coordinator to respond to roll as well as turn. The TC indication represents a sum of the roll rate and the yaw rate so it responds more quickly at the beginning and end of a turn than a turn and bank indicator. 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 indicatorAn attitude indicator , also known as gyro horizon or artificial horizon, is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to earth. It indicates pitch and bank or roll and is a primary instrument for flight in instrument meteorological conditions...
has failed. Called "partial panel" operations, this can be unnecessarily difficult or even impossible if either the pilot does not understand that the instrument is showing roll rates at some times and turn rates at others, or the internal
dashpotA dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring...
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. For this and other reasons many highly experienced pilots prefer the "older"
turn and bank indicatorIn aviation, the turn and bank indicator shows the rate of turn and the coordination of the turn. The rate of turn is indicated from a rate gyroscopically and the coordination of the turn is shown by either a pendulum or a heavy ball mounted in a curved sealed glass tube...
design.