Gyroscopic flight instruments
Some instruments run off a fast-spinning gyro, which behaves two ways. Rigidity in space: it keeps pointing where it was set, so it can show attitude or heading. Precession: it reacts to a push 90° further around the wheel, so it can sense a turn.
| Instrument | Principle | Usual power | Reset to compass? |
|---|---|---|---|
| Attitude indicator (AI) | rigidity | engine-driven vacuum (suction) or pressure (some electric) | no |
| Heading indicator / DG (HI) | rigidity | engine-driven vacuum | yes, ~every 15 min |
| Turn coordinator | precession | electric | no |
Why the HI needs resetting: it can't find north on its own and slowly precesses off its heading (friction, earth's rotation), so reset it to match the magnetic compass (in straight, level, unaccelerated flight) about every 15 minutes (TP 1102).
Why the turn coordinator is electric: it is a backup. A vacuum failure kills the AI and HI together, so the turn instrument runs off the electrical system and keeps working (POH/AFM).
Trap: don't swap the principle (rigidity is AI/HI, precession is the turn coordinator) or the power source (vacuum for AI/HI, electric for the turn coordinator). Only the HI is reset to the compass; the compass is the reference, not a gyro (TP 1102).
Note: precession and the 15-minute HI reset are in TP 1102; "rigidity in space" and the vacuum-vs-electric power split are standard gyro theory / aircraft-specific (POH/AFM), not named in TP 1102.
References
- TP 1102
- POH/AFM