Attitude indicator reading & errors
The attitude indicator (AI) shows a picture of aircraft attitude: miniature aircraft against horizon bar matches real aircraft against real horizon (PHAK ch.8). Reacts instantaneously, even to the smallest attitude change (TP 1102).
Parts and controls:
| Part | Behaviour | Cite |
|---|---|---|
| Horizon bar | Fixed to gyro, which spins in a horizontal plane. Stays horizontal as aircraft pitches or banks; aircraft rotates around the gyro | PHAK ch.8 |
| Miniature aircraft | Normally adjusted so wings overlap the horizon bar in straight-and-level cruise | PHAK ch.8 |
| Adjustment knob | Moves the miniature aircraft up or down, not the gyro, to suit pilot's line of vision. Set for level-flight indication at normal cruise | PHAK ch.8; TP 1102 |
| Banking scale | Bank angle shown by the miniature aircraft against the bar and by the pointer against the banking scale at the top | TP 1102 |
Indication type (PHAK glossary; TP 1102):
- Direct: miniature aircraft against horizon bar.
- Indirect: any other instrument showing pitch or bank, such as the VSI.
- Advantage: pitch and bank in one glance (TP 1102).
- Taxiing: AI bar is always steady (TP 1102).
Reading the AI (TP 1102 unless noted):
- Attitude always referenced to the horizon: earth's natural horizon and, by inference, the AI horizon bar.
| Pitch | Miniature aircraft | Airspeed |
|---|---|---|
| Up | Rises above bar | Falls |
| Down | Drops below bar | Rises |
- Banking scale: read for the DEGREE of bank only (PHAK ch.8).
- Miniature aircraft position against horizon bar: read for the DIRECTION of bank (PHAK ch.8).
- Example: nose-down left turn shows miniature aircraft below and banked left of the horizon bar.
Errors and erection: AIs are free from most errors (IFH ch.5).
| Condition | Effect | Cite |
|---|---|---|
| Rapid acceleration | Slow erection system can give a slight nose-up indication | IFH ch.5 |
| Rapid deceleration | Slight nose-down indication | IFH ch.5 |
| 180 degree turn | Small bank and pitch error can follow | IFH ch.5 |
| Return to straight-and-level | These small errors self-correct within a minute or so | IFH ch.5 |
| Engine start | Gyro not erect; a gravity-actuated self-erecting mechanism precesses it to vertical | IFH ch.5 |
Trap: horizon bar is fixed to the gyro; the adjustment knob only moves the miniature aircraft to line up with it.
References
- PHAK ch.8 (FAA-H-8083-25), Attitude Indicator and glossary
- TP 1102 Ex.2, Instrument Flying
- IFH ch.5 (FAA-H-8083-15), Attitude Indicators: erection and errors