Gyro limits & heading indicator reset
Gyro instruments ride on a gimbal mount; its design sets how far pitch or bank can go before readings go bad (PHAK ch.8).
| Instrument | Mount | Limits | Past limit |
|---|---|---|---|
| Attitude indicator (AI) | Double gimbal: free in pitch and bank | Bank 100 to 110 degrees, pitch 60 to 70 degrees (typical, varies by make and model) | Tumbles ("spills"): leaves plane of rotation, erroneous readings until reset |
| Heading indicator (HI) | Double gimbal: free in pitch and bank | Some light-aircraft HIs: about 55 degrees pitch and bank | Tumbles: erroneous readings until reset |
| Turn coordinator | Built so it can't tumble | None | None |
Recovering a tumble: spilled HI is reset with the caging knob (PHAK ch.8).
Routine HI reset (different from a tumble recovery): the HI can't find north, so it slowly precesses (friction, earth's rotation). Corrects only ordinary precession drift.
- Reset to magnetic compass in straight, level, unaccelerated flight.
- About every 15 minutes (TP 1102).
- Earth's rotation alone can drift it up to 15° an hour (PHAK ch.8).
- Unlike the compass, HI stays accurate through turns, acceleration, deceleration, so it is kept and reset periodically, not read directly (TP 1102).
HI numbers (TP 1102):
| Motion | Direction | HI numbers |
|---|---|---|
| Turning | Left | Decrease ("left for less") |
| Turning | Right | Increase |
| Taxiing | Straight | Steady |
| Taxiing | Left | Decreasing |
| Taxiing | Right | Increasing |
Trap: turn coordinator has no bank limit (cannot tumble); AI and HI tumble past their make's limits. Only the HI is reset to the compass; the compass is the reference, not a gyro (TP 1102).
References
- PHAK ch.8 (FAA-H-8083-25)
- TP 1102
- IFH ch.5 (FAA-H-8083-15)