3.gyroscopic-instruments-detail. Gyroscopic Flight Instruments
Three of the six primary flight instruments rely on gyroscopic principles: the attitude indicator (AI), the heading indicator (HI), and the turn coordinator (TC) (or older turn-and-slip indicator). Understanding how these instruments are powered, what they sense, and how they fail is essential for instrument flight, because most partial-panel emergencies involve loss of one of these gyros.
Gyroscopic Principles
A gyroscope is any rapidly spinning mass. Two properties give the gyro its usefulness as a flight instrument:
- Rigidity in space. A spinning rotor tends to remain in a fixed orientation relative to inertial space unless acted upon by an outside force. The attitude indicator and heading indicator exploit this property — the rotor stays put while the airplane (and the instrument case) moves around it.
- Precession. When a force is applied to the rim of a spinning rotor, the resulting reaction occurs 90° later in the direction of rotation. The turn coordinator uses precession deliberately to measure rate of turn. Precession also causes unwanted errors: friction, worn bearings, and acceleration cause the AI and HI to drift.
The higher the rotor's mass, the greater its diameter, and the higher its rpm, the more rigidity it has and the more accurate the instrument is.
Power Sources
Gyros are spun up either by a vacuum/pressure system or electrically. A typical light single uses an engine-driven vacuum pump that draws air across vanes inside the AI and HI cases, spinning the rotors at roughly 10,000–18,000 rpm. The turn coordinator is usually electric so the pilot retains turn information if vacuum fails (and vice versa). Many modern aircraft reverse this — electric AI and HI with a vacuum or pneumatic backup, or all-electric glass panels with battery backup.
Monitor the suction gauge (typically 4.5–5.5 inches Hg) and any low-vacuum or low-voltage warning lights before and during flight. A failing vacuum pump may not stop the gyros immediately; rotors coast down slowly, producing subtle, insidious errors before the failure flag appears.
Attitude Indicator
The AI displays pitch and bank against a horizon line. Its gyro spins in the horizontal plane, with its spin axis vertical, and is erected by pendulous vanes (vacuum) or an automatic erection mechanism (electric). Errors include:
- Acceleration error. During takeoff acceleration the AI may show a slight nose-up, climbing-turn-to-the-right tendency; deceleration shows the opposite.
- Turning error. After a 180° turn at standard rate, a small bank and pitch error may appear; both are typically self-correcting within a minute of straight-and-level flight.
- Tumbling. Older AIs were limited to about 100° of pitch and 110° of bank before they tumbled. Modern AIs are non-tumbling.
Always allow at least 5 minutes after engine start for the gyro to spin up and erect before taxi, and recheck just before takeoff.
Heading Indicator
The HI is a directional gyro slaved either to nothing (manual) or to a remote magnetic flux detector (slaved gyro / HSI). The gyro's spin axis is horizontal. Because a free gyro has no north-seeking tendency, an unslaved HI must be set to the magnetic compass during straight-and-level, unaccelerated flight, and rechecked at least every 15 minutes. Real-rate precession drift of about 3° per 15 minutes is normal; greater drift suggests gyro wear.
Turn Coordinator and Turn-and-Slip Indicator
The turn coordinator uses a canted gyro (about 30° from horizontal) so it senses both roll rate and yaw rate; the older turn-and-slip senses yaw only. Precession of the gyro deflects the miniature airplane (or needle) proportional to turn rate. Alignment with the doghouse marks indicates a standard-rate turn of 3°/sec, completing a 360° turn in 2 minutes. The inclinometer (ball) is not gyroscopic — it is a simple pendulum-type instrument showing the quality (coordination) of the turn. Step on the ball to coordinate.
Failure Recognition
Vacuum failure typically produces:
- A drop in suction below 4.5 in Hg.
- Slow, subtle drift of the AI and HI in the same direction at the same time (a strong clue, since these two instruments rarely agree on their errors otherwise).
- The TC continues to function (if electric) and disagrees with the AI.
Cross-check, identify, and verify with pitot-static instruments and the magnetic compass. Cover the failed instrument(s) and fly partial panel.
Preflight and Inflight Checks
- Before taxi: suction gauge in the green; AI erect within 5 minutes; HI set to compass.
- During taxi turns: TC needle deflects in the direction of turn, ball swings opposite (free); AI remains level; HI changes appropriately.
- Before takeoff: re-set HI to compass; verify AI shows level pitch and bank.