Gyroscopic instruments use two fundamental physical properties of rotating masses:
1. Rigidity in space
A spinning gyro maintains its plane of rotation in inertial space, independent of housing motion. This enables:
- Attitude indicator (vertical-axis gyro) shows absolute attitude relative to the horizon.
- Directional gyro (horizontal-axis gyro) shows a reference direction.
→ A rotating gyro in space tends to maintain its position in space.
2. Precession — 90° response
When a force is applied to the rim of a spinning gyro, the reaction acts 90° offset in the direction of rotation of the gyro — not at the point of force. So:
- Pushing a spinning disc at one point makes the gyro respond with an evasive motion at 90° to the applied force.
- This property is gyroscopic precession, key to understanding all gyro errors.
Examples:
- Bearing friction braking the gyro causes drift in the indication.
- Turn indicators use precisely this principle to create their reading.
Gyroscopic instruments
Turn indicator and artificial horizon (attitude indicator) are the main so-called gyroscopic instruments in the cockpit. The directional gyro is also a gyro instrument.
Gyro drives
| Drive | Advantage | Use |
|---|---|---|
| Vacuum pump (engine-driven) | Works without bus; simple | Classic analog AI, DG (Cessna, Piper) |
| Electric | Higher RPM, faster spin-up | Turn coordinator (often electric), modern AI |
| AHRS (MEMS, no mechanical gyro) | Solid state, no mechanics | Glass cockpit (G1000, Aspen, Dynon) |
Suction gauge — availability of pneumatic gyros
The suction gauge indicates the operability of pneumatically driven gyros. If the suction value is not within the green arc (typically 4.5–5.5 inHg), the pneumatically driven gyroscopic instrument is NOT reliable and must not be used for attitude or heading reference.
→ At low suction (e.g. vacuum-pump failure) the attitude indicator and directional gyro fail — the pilot must fall back on alternatives (magnetic compass, electric turn coordinator).