7.gyroscopic-instruments. Gyroscopic Flight Instruments
Three of the most common flight instruments — the attitude indicator, heading indicator, and turn indicator — operate on the principles of the gyroscope. A gyroscope is a spinning wheel or disk mounted in gimbals so that its spin axis is free to assume any orientation. Once spinning at high speed, a gyro exhibits two fundamental properties that the flight instruments exploit: rigidity in space and precession.
Rigidity in space means a spinning rotor resists displacement of its spin axis from the direction in which it is pointed. The faster the wheel spins, the greater its mass, and the farther the mass is from the spin axis, the greater the rigidity. Rigidity is what allows the attitude indicator and heading indicator to provide a stable reference against which the airframe's pitch, bank, and yaw can be measured.
Precession is the tilting or turning of a gyro in response to a deflective force. When a force is applied to the rim of a spinning rotor, the resulting reaction occurs not at the point where the force was applied, but 90° ahead in the direction of rotation. Precession is the operating principle behind the turn indicator and is also responsible for many of the small errors found in heading and attitude instruments — particularly during prolonged turns, climbs, and descents.
Sources of Power
Gyros may be driven by a vacuum (suction) system, a pressure system, or electrically. A typical light single-engine airplane uses an engine-driven vacuum pump to spin the attitude and heading gyros, while the turn coordinator is electrically driven so that at least one bank reference remains available if the vacuum system fails.
- Vacuum systems maintain suction between approximately 4.5 to 5.5 in. Hg. A suction gauge allows the pilot to verify proper operation.
- Electrical systems rely on the aircraft's DC bus and are monitored by warning flags on the instrument face.
In glass cockpits, traditional spinning-mass gyros are replaced by solid-state AHRS (Attitude and Heading Reference Systems) using MEMS accelerometers, rate sensors, and magnetometers, but the displayed information is functionally the same.
Attitude Indicator
The attitude indicator (artificial horizon) gives a direct, immediate indication of pitch and bank attitude. Its gyro spin axis is mounted vertically, so the gimbals are free to rotate about the lateral and longitudinal axes. A miniature airplane is fixed to the case and a horizon bar represents the natural horizon. Bank angle is read against an index calibrated in 10°, 20°, 30°, 60°, and 90° marks; pitch is read in degrees of nose-up or nose-down displacement. Modern attitude indicators have minimal precession errors and are reliable through 360° of bank and approximately 100° of pitch.
Heading Indicator
Also called the directional gyro (DG), the heading indicator provides a stable directional reference free from the lead/lag and turning errors of the magnetic compass. Its spin axis is mounted horizontally, parallel to the line of flight. Because the gyro is not magnetically slaved in basic installations, it must be manually set to agree with the magnetic compass during straight-and-level, unaccelerated flight, and rechecked at least every 15 minutes due to precession and apparent drift caused by Earth's rotation.
Turn Indicators
Two instruments use precession to display rate of turn:
- Turn-and-slip indicator: a single needle indicates rate of turn only; the gyro is mounted with its spin axis along the lateral axis.
- Turn coordinator: the gyro is canted approximately 30° upward, so the instrument senses both roll rate and yaw rate. A small symbolic airplane displays bank initially and turn rate once established.
A standard-rate turn is 3° per second — 360° in 2 minutes — indicated when the wing of the symbol or needle aligns with the standard-rate index. The inclinometer (ball) below the symbol shows quality of turn (coordination): a centered ball means coordinated flight; a ball to the inside indicates a slip; to the outside, a skid. The pilot corrects with rudder pressure on the side the ball is displaced toward ("step on the ball").
Common Errors and Checks
- After engine start, verify suction in the green arc and check that gyros spool up without abnormal noise.
- During taxi, the attitude indicator should remain erect within 5°, the heading indicator should track turns smoothly, and the turn coordinator should bank in the direction of taxi turns with the ball swinging to the outside.
- Vacuum failure can cause the attitude and heading indicators to give slowly degrading, misleading information without an obvious flag, which is why scan, cross-check, and partial-panel proficiency are essential.