The directional gyro (DG; heading indicator HI) shows steering direction — more stable than the magnetic compass, immune to compass errors (acceleration, turning, dip).
Function
- Horizontal-axis gyro with high rigidity.
- The DG does not measure direction — it is set to the magnetic compass in straight flight and then maintains that reference.
- Display typically a compass rose with a lubber line.
Rotational axis — parallel to earth surface
The rotational axis of the directional gyroscope is parallel to the surface of the Earth:
- Unlike the AI (vertical to earth), the DG axis is horizontal.
- This arrangement allows measurement of rotations about the vertical axis (yaw / heading) via precession.
Drift — two sources
| Source | Mechanism | Magnitude |
|---|---|---|
| Mechanical drift (real drift) | Bearing friction causes precession → slow indication shift | ~3°/15 min |
| Apparent drift | Earth rotates beneath the inertially fixed gyro → DG appears to rotate relative to earth | up to ~15°/h at equator, 0 at pole |
Causes of apparent drift
Earth rotation and aircraft movements are the causes for apparent drift of a directional gyroscope:
- Earth rotation: earth rotates 360° in 24 hours → an inertially stable gyro appears to rotate relative to earth.
- Aircraft movement: flying east/west changes geographic position → the effective "earth rotation" under the gyro varies.
"Apparent drift" — definition
"Apparent drift" means a deviation of the directional gyroscope indication relative to the magnetic compass — it is not a real drift (the gyro is space-stable) but arises from earth rotation and aircraft position relative to earth surface.
Reliability limit — 60° pitch and bank
Standard directional gyroscopes operate reliably only up to 60° of pitch and bank attitudes as a basic guideline:
- Beyond 60° the gimbal mechanism reaches its limit.
- The gyro can "tumble" (gimbal lock) or lose attitude.
- After regaining normal attitude the DG must be manually re-synchronised with the magnetic compass.
Gimbal error
The directional gyroscope is affected by gimbal error — indication errors arise from gimbal-mechanism limits and interaction with accelerations / turns. Symptoms:
- After steep or prolonged turns the DG may show incorrectly.
- Correction: re-synchronise with magnetic compass after returning to straight flight.
DG setting — against the magnetic compass
The directional gyroscope is set to the compass indication. Procedure:
- Stable straight flight without acceleration.
- Read the magnetic compass.
- Carefully turn the DG knob until DG matches compass.
- Repeat every ~15 minutes.
Spin-up — ready 3 minutes after engine start
The vacuum-driven directional gyro is ready for operation about 3 minutes after engine start:
- The vacuum pump needs time to spin the gyro up.
- Before this spin-up time the indication is unreliable.
- → Before take-off: wait for DG spin-up and initial sync with compass.
Erection
The DG has an erection system to keep the rotation axis horizontal — otherwise gravity would create precession-induced drift.
Horizontal Situation Indicator (HSI)
The Horizontal Situation Indicator (HSI) is a crucial aviation instrument that combines a heading indicator (directional gyro) with navigation displays (VHF receiver — VOR / ILS) into a single, user-friendly instrument:
- Heading display: compass rose from the directional gyro.
- Course Deviation Indicator (CDI): left/right deviation from the selected VOR radial or ILS localiser.
- Glide slope (ILS): vertical deviation.
- TO/FROM: whether the VOR is ahead of or behind the aircraft.
→ HSI is standard in IFR cockpits and reduces pilot workload by combining heading and course tracking on one display.
Glass cockpit / AHRS
Modern AHRS systems use a magnetometer in addition to gyro-rate measurement — the indication remains synchronised with the magnetic field automatically, no manual adjustment needed.