Gyro principles & power
Fast-spinning gyro rotor, two behaviours:
- Rigidity in space (inertia): heavy spinning rotor resists any change of its plane of spin. Gimbals tilt, rotor holds its plane. Holds attitude or heading (PHAK ch.8; glossary "rigidity in space").
- Precession: rotor reacts to a push 90° further around the wheel in the direction of rotation, not at the point pushed. Senses a turn (PHAK ch.8; TP 1102).
- Faster spin: slower precession for the same push (PHAK ch.8).
| Instrument | Principle | What it shows | Usual power | On vacuum failure |
|---|---|---|---|---|
| Attitude indicator (AI) | rigidity | pitch and bank attitude | engine-driven vacuum (suction) or pressure (some electric) | Fails together with HI (PHAK ch.8) |
| Heading indicator / DG (HI) | rigidity | heading | engine-driven vacuum | Fails together with AI (PHAK ch.8) |
| Turn coordinator | precession | rate of turn | electric | Keeps working (backup) (PHAK ch.8) |
Vacuum system (PHAK ch.8):
- AI and HI usually share one vacuum pump.
- Failing pump does not quit outright: gyros slow and lose accuracy first.
- Flagged by a warning light or the vacuum gauge.
Uncoordinated Flight: turn with lateral acceleration (slip or skid, ball leaves centre) (AC 700-031).
Trap: rigidity is the AI and HI; precession is the turn coordinator. Vacuum powers AI and HI, electric powers the turn coordinator; never swap them.
References
- TP 1102
- PHAK ch.8 (FAA-H-8083-25)
- AC 700-031