Learning objective: You decide when the heading indicator (directional gyro, DG / Kurskreisel) beats the compass — and when you must realign it, because it is not a magnet.
60 seconds: You are flying straight and unaccelerated; you align the DG with the magnetic compass. Twenty minutes later it has wandered, still in level flight. The gyro is not “magnetically broken”. Remember: axis parallel to the Earth, the reference comes from the compass; drift is normal.
The DG uses rigidity in space. It does not sense variation. You give it a heading; it holds that direction in space — until friction and the Earth lie to it.
Two drifts examiners want split:
| Drift | What happens |
|---|---|
| Real drift | Bearing friction → slow precession |
| Apparent drift | Earth rotates under the space-stable gyro; plus your change of position |
Textbook orders of magnitude (often ~3°/15 min friction, Earth rate depending on latitude) are teaching, not AFM. Reset in unaccelerated level flight, often every few minutes in school rules — interval is type/ops, not a universal AFM minute.
Standard DGs are reliable only to about 60° pitch/bank (teaching): beyond that gimbal lock. Not a compass replacement in the turn and after a loop.
Typical trainer: DG on the same vacuum as the AI. Electrics dead, suction alive → DG can live, electric TC not. Suction gone → AI and DG doubt together.
“The heading gyro reads magnetic north, so it never needs resetting.” That is wrong. People see a compass rose and think magnet. Without a reset the rose is just a pretty circle.
Exam knowledge
- DG axis horizontal / parallel to the Earth. AI axis vertical.
- No magnet of its own — align with the compass, straight and unaccelerated.
- Apparent drift = Earth + movement, while the gyro stays space-stable.
- Real drift = friction. School numbers = teaching, not AFM.
- ~60° pitch/bank limit (teaching) → gimbal lock.
- Often vacuum like the AI — check flag/suction.
Source: typical EASA PPL 020 DG teaching. Drift figures and the 60° limit are school teaching; power and reset AFM.
Check yourself
1. DG spin axis — does it sense magnetic heading itself?
- A) Axis parallel to the Earth (horizontal); no own heading sense; aligned with the magnetic compass
- B) Vertical like the AI; reads the Eatth's field
- C) Along the fuselage; fully replaces the compass
- D) No drift, no reset needed
2. Apparent drift of the DG is?
- A) Bearing friction only
- B) Gimbal lock above 60° only
- C) Compass disagreement while the gyro stays space-stable — Earth rotation and aircraft movement
- D) The ~3 min spin-up after engine start
- A — horizontal axis, compass as the reference. Vertical axis is the AI.
- C — apparent drift ≠ real (friction) drift.