Turn and balance indicator
In aviation, the turn and balance indicator (T/B) and the turn coordinator (T/C) variant are essentially two aircraft flight instruments in one device. They each act as a rate of turn indicator that displays the rate the aircraft heading is changing and a balance indicator or slip indicator that displays the slip or skid of the turn.
The turn and balance indicator is often referred to under various names interchangeably, such as the turn and slip indicator or the turn and bank indicator. The turn coordinator, on the other hand, is usually referred to only as such. They each operate by using their own separate working mechanism, as described in the operation section.
Simply put, both the turn and balance indicator (T/B) and the turn coordinator (T/C) use a gyro-driven system. The T/B uses a needle and a ball, while the T/C uses a rolling aircraft depiction and a ball. Although the turn and balance indicator is sometimes called the turn and bank indicator, the instrument does not give the aircraft's true bank angle. In fact, neither the T/B nor the T/C actually give true bank angle information. True bank angle is calculated using the aircraft's speed and rate of turn.
The turn indicator is a gyroscopic instrument that works on the principle of precession. The gyro is mounted in a gimbal. The gyro's rotational axis is in-line with the lateral (pitch) axis of the aircraft, while the gimbal has limited freedom around the longitudinal (roll) axis of the aircraft.
As the aircraft yaws, a torque force is applied to the gyro around the vertical axis, due to aircraft yaw, which causes gyro precession around the roll axis. The gyro spins on an axis that is 90 degrees relative to the direction of the applied yaw torque force. The gyro and gimbal rotate (around the roll axis) with limited freedom against a calibrated spring. The torque force against the spring reaches an equilibrium and the angle that the gimbal and gyro become positioned is directly connected to the display needle, thereby indicating the rate of turn.
The display contains hash marks where the needle may align during a turn. When the needle is lined up with these hash marks, the aircraft is performing a "standard rate turn." The standard rate for most airplanes is three degrees per second, known as "rate one." This translates to two minutes per 360 degrees of turn (a complete circle). This is marked as "2 min. turn" on the display. A standard rate turn may vary for each individual aircraft. For example, a turn and slip indicator used in a very fast aircraft is likely to contain a four minute turn indicator, displayed as "4 min. turn." The supersonic Concorde jet aircraft is one example of an aircraft with a 4 min. turn indicator. On the other side of the spectrum, the much slower glider aircraft are usually set to one minute turns. The hash marks are sometimes called "dog houses," because of their distinct shape on various makes of turn indicators. Under instrument flight rules, using these figures allows a pilot to perform "timed turns" in order to conform with the required air traffic patterns. For a change of heading of 90 degrees, a turn lasting 30 seconds would be required to perform a "rate one" turn.
Balance information of the aircraft is often obtained by an inclinometer, which is recognized as the "ball in a tube." An inclinometer contains a ball sealed inside a curved glass tube, which also contains a liquid to act as a damping medium. Historically, the balance indicator in early aircraft was merely a pendulum with a dashpot for damping. The ball gives an indication of whether the aircraft is slipping, skidding or in balanced flight. The ball's movement is caused by the force of gravity and the aircraft's precession forces. When the ball is centered in the middle of the tube, the aircraft is said to be in balanced flight. If the ball is on the inside (wing down side) of a turn, the aircraft is slipping. And finally, when the ball is on the outside (wing up side) of the turn, the aircraft is skidding.
A simple alternative to the balance indicator is a yaw string, which allows the pilot to simply view the string's movements as rudimentary indication of aircraft balance.
The turn coordinator (T/C) is a further development of the turn and balance (T/B) indicator with the major difference being the display and the axis upon which the gimbal is mounted. The display is that of a miniature airplane as seen from behind. This looks similar to that of an attitude indicator. "NO PITCH INFORMATION" is usually written on the instrument to avoid confusion regarding the aircraft's pitch, which can be obtained from the artificial horizon instrument.
In contrast to the T/B, the T/C's gimbal is pitched up 30 degrees from the lateral axis. This causes the instrument to respond to roll as well as yaw. This allows the instrument to display a balance change more quickly as it will react to the change in roll before the aircraft has even begun to yaw. Although this instrument reacts to changes in the aircraft's roll, it does not display the roll attitude.
The turn coordinator should be used as a performance instrument when the attitude indicator has failed. This is called "partial panel" operations. It can be unnecessarily difficult or even impossible if the pilot does not understand that the instrument is showing roll rates as well as turn rates. The usefulness is also impaired if the internal dashpot is worn out. In the latter case, the instrument is said to be underdamped and in turbulence will indicate large full-scale deflections to the left and right, all of which are actually 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 indicator design.
Slipping and skidding within a turn is sometimes referred to as a sloppy turn, due to the perceptive discomfort it can cause to the pilot and passengers. When the aircraft is in a balanced turn (ball is centered), passengers experience gravity directly in line with their seat (force perpendicular to seat). With a well balanced turn, passengers may not even realize the aircraft is turning unless they are viewing objects outside the aircraft. While aircraft slipping and skidding are often undesired in a usual turn that maintains altitude, slipping of the aircraft can be used for practical purposes. Intentionally putting an aircraft into a slip is used as a forward slip and a sideslip. These slips are performed by applying opposite inputs of the aileron and rudder controls. A forward slip allows a pilot to quickly drop altitude without gaining unnecessary speed, while a sideslip is one method utilized to perform a crosswind landing.
- 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)
- Operation demonstration of a Turn & slip indicator. The indicator cover is removed and gyro is displayed as it operates.
- "How Aircraft Instruments Work." Popular Science, March 1944, pp. 117.