Gyroscopic Instruments: Attitude, Heading and Turn
Attitude indicator
A vertical gyro with two degrees of freedom (horizontal spin axis and vertical gimbal) drives a horizon bar display. It uses an erection mechanism (pendulous vanes or torque motors) that slowly aligns the gyro with the true vertical. Errors include acceleration error (during prolonged acceleration the horizon bar shows a climb, then a descent on deceleration) and turning error (during a prolonged turn the instrument indicates a turn in the opposite direction after rollout, and a small bank error during the turn).
Heading indicator
A horizontal-axis gyro with a vertical gimbal and a compass card. It has rigidity in space, so it drifts relative to the Earth as the aircraft moves over the curved surface, and it suffers real drift, apparent drift and random drift. It must be realigned with the magnetic compass in straight, level, unaccelerated flight. A slaved (gyromagnetic) compass uses a flux valve to correct automatically.
Turn coordinator and turn needle
A rate gyro with a single horizontal gimbal senses yaw rate. The turn coordinator also cants the gyro to sense roll rate, so it indicates rate of turn and quality of turn; the turn needle senses yaw only. Both are rate instruments, not bank instruments. Calibration is at 3°/s (rate 1, 2 minutes for 360°).
Worked rule of thumb
Rate 1 turn: bank angle ≈ airspeed (TAS) ÷ 10 + 7. At 120 kt, about 19°. A standard rate turn is 3°/s, so 30° of heading change takes 10 seconds. Use this to plan timed turns and to check the turn indicator.
Question traps
Do not confuse the turn coordinator (roll + yaw) with the turn needle (yaw only). Remember that the attitude indicator's acceleration error is a false climb indication during acceleration, not a false bank.