The attitude indicator (AI) shows the aircraft's attitude relative to the natural horizon — the main reference for instrument flight. Indication: pitch and bank.
Indication — what the AI shows
The attitude indicator shows pitch and bank attitude:
- Pitch: angle between aircraft longitudinal axis and horizon (nose-up / nose-down).
- Bank: roll attitude (left/right banked).
What the AI does NOT show
The rate of climb or descent cannot be read directly from the attitude indicator — reason: the AI shows only pitch attitude, not actual vertical speed. An aircraft can fly nose-up and still descend (e.g. in a stall), or be horizontal and climb (in strong updraft).
→ Read vertical speed from the VSI, not the AI.
Rotational axis — vertical to earth surface
The rotational axis of the attitude indicator is vertical to the surface of the earth:
- The gyro rotates about a vertical axis relative to earth.
- A pendulous mechanism (erection system) keeps the axis pointing down (to earth centre) regardless of aircraft motion.
- The gyro stays "vertical" and the horizon bar parallel to the real horizon.
Construction
| Element | Function |
|---|---|
| Vertical-axis gyro | Maintains vertical relative to earth (pendulum erection aligns with gravity) |
| Horizon bar | Moves relative to the miniature aircraft |
| Miniature aircraft | Fixed to cockpit; does not move |
| Bank index (top) | Shows bank (0°, 10°, 20°, 30°, 60° marks) |
| Pitch scale | Shows pitch in degrees |
| Gimbal mounting | Allows free rotation in all three axes, maintains vertical orientation |
Drive
- Vacuum (classic) — RPM typically 12 000–24 000.
- Electric (more common in modern avionics).
- AHRS (glass cockpit) — no mechanics, MEMS sensors.
Vacuum AI in engine failure — unreliable
A vacuum-driven attitude indicator becomes unreliable in an engine-failure scenario because:
- The vacuum pump is engine-driven — engine failure cuts vacuum supply.
- The gyro spins down and tilts under bearing friction and gravity — indication drifts from true attitude.
- After a few minutes the AI is completely unusable.
→ On engine failure the pilot switches to alternative attitude references: magnetic compass + altimeter + VSI + turn coordinator (often electric, unaffected by engine failure).
Gimbal mounting — errors in prolonged turns
The attitude indicator has a gimbal mounting allowing free rotation in all three axes. In prolonged turns, however, indication errors (gimbal error) are possible:
- During a turn centrifugal force acts on the pendulum mechanism → it displaces the erection mechanism from true vertical.
- Result: after a long turn the AI shows a wrong attitude, often a falsely under- or over-banked position.
- After returning to straight flight the AI re-erects itself (erection system), but this takes seconds to minutes.
→ The pilot is aware of this error — after steep or long turns (e.g. 60° bank for 2 min) the AI may show wrongly for a while.
Other classic vacuum AI errors
| Error | Cause | Effect |
|---|---|---|
| Acceleration error | Pendulum displaced by longitudinal accel | Accelerating (take-off roll) — AI shows apparent climb + slight right bank. Decelerating — apparent descent + left bank. |
| Spin-up time | Gyro needs time to spin up | Wait 5-10 min after start before AI is fully reliable |
ADI — Attitude Director Indicator (combined display)
An Attitude Director Indicator (ADI) is a combined display that integrates the information from the attitude indicator AND the flight director ("an ADI combines information provided by the attitude indicator and the flight director").
- Standard AI: only pitch + bank.
- ADI: pitch + bank + flight director command bars (steering recommendation from autopilot or flight computer).
- Pilot follows the director bars for automated approach or autopilot engagement.
→ ADI is typical for IFR aircraft with autopilot; in modern glass cockpits the ADI is part of the PFD.
AHRS AI
Modern glass-cockpit AIs use MEMS accelerometers and gyros with software filtering. Pros: no mechanics, ready immediately, no acceleration or erection errors of noticeable size. Con: requires electrical power (battery backup needed).