Gyroscopic Instruments and Heading Systems
Domain 4 covers the gyroscope as a sensing element - attitude, rate of turn and heading - and the heading systems built on it. The exam tests the two properties (rigidity and precession), the difference between attitude and rate gyros, drift and its causes, and the turn-performance numbers.
The two properties
Rigidity - a spinning rotor keeps its axis fixed in space through conservation of angular momentum. Precession - a torque applied to the spin axis produces movement at 90° to the applied force in the direction of rotation, at a rate proportional to the torque and inversely proportional to rotor angular momentum.
Instruments
- Attitude indicator - vertical gyro with two gimbals and an erection mechanism (pendulous vanes or mercury switches); shows pitch and roll.
- Turn indicator - a rate gyro with one gimbal and a restraining spring; needle deflection is proportional to rate of turn, not angle of bank. Rate one = 3 °/s = 180 °/min.
- Turn coordinator - the same gyro canted about 30°, adding roll information.
- Directional gyro / HSI - heading reference slaved to a flux valve, with caging and manual or automatic slaving.
- AHRS - ring laser gyros, fibre-optic gyros and MEMS accelerometers, with no spinning mass.
Drift
Real drift comes from gyro imperfection. Apparent drift comes from the earth's rotation and from transporting the gyro over the earth (transport wander). Gimbal error and random drift also degrade accuracy; slaving or periodic caging corrects heading gyros.
Worked example
A rate-one turn at 300 kt TAS. Time for 180° = 180 ÷ 3 = 60 s. Turn radius r = V/ω, with ω = 3 °/s = 0.05236 rad/s and V = 300 kt = 154.3 m/s, so r ≈ 2947 m ≈ 1.6 nm. The rough rule r (nm) ≈ TAS ÷ 200 gives 1.5 nm - close enough for a quick check.
Traps that cost marks
- Claiming the turn needle shows bank angle; it shows rate, and it still deflects in a skid at zero bank.
- Confusing real and apparent drift, or forgetting that transport wander grows with latitude and east/west heading.
- Assuming the standby attitude indicator is slaved; it is usually a free gyro needing caging and erection.
- Using indicated rather than true airspeed in turn-radius calculations.