Turn indicator

In aviation, the turn and slip indicator (T/S, a.k.a. turn and bank indicator) and the turn coordinator (TC) variant are essentially two aircraft flight instruments in one device. One indicates the rate of turn, or the rate of change in the aircraft's heading, the other part indicates whether the aircraft is in coordinated flight, showing the slip or skid of the turn. The slip indicator is actually an inclinometer that at rest displays the angle of the aircraft's lateral axis with respect to horizontal, and in motion displays this angle as modified by the acceleration of the aircraft.

The turn and slip indicator can be referred to as the turn and bank indicator, although the instrument does not respond directly to bank angle. Neither does the turn coordinator, but it does respond to roll rate, which enables it to respond more quickly to the start of a 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. In the turn coordinator, the gyro is canted 30 degrees from the horizontal so it responds to roll as well as yaw.

The display contains hash marks for the pilot's reference during a turn. When the needle is lined up with a hash mark, the aircraft is performing a "standard rate turn" which is defined as three degrees per second, known in some countries as "rate one". This translates to two minutes per 360 degrees of turn (a complete circle). Indicators are marked as to their sensitivity, with "2 min turn" for those whose hash marks correspond to a standard rate or two-minute turn, and "4 min turn" for those, used in faster aircraft, that show a half standard rate or four-minute turn. The supersonic Concorde jet aircraft and many military jets are examples of aircraft that use 4 min. turn indicators. 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 standard rate or "rate one" turn.

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Wikipedia