The magnetic compass has four errors every PPL pilot must know:

1. Variation (magnetic declination)

Angle between True North and Magnetic North. Shown on charts as isogonals (lines of equal variation).

In Germany currently about +2° to +5° E (east variation).
Drifts slowly over the years — charts state value and annual change.

Application:

code

True ± Variation = Magnetic

Variation East → Magnetic less ("Variation East, Magnetic least"). Example: True 100°, Var 5°E → Magnetic = 100° − 5° = 095°.
Variation West → Magnetic more. Example: True 100°, Var 5°W → Magnetic = 100° + 5° = 105°.

2. Deviation

Error caused by the aircraft's own magnetic fields (engine, avionics, cables). Tabulated on the compass correction card next to the instrument.

Application:

code

Magnetic ± Deviation = Compass

Deviation East → Compass less ("Deviation East, Compass least").
Deviation West → Compass more ("Deviation West, Compass best" — common mnemonic).

Deviation is typically small (±3°) but heading-dependent → consult the card.

3. Acceleration error

On an East or West heading in the Northern hemisphere:

Accelerating → compass apparently turns toward North.
Decelerating → apparently turns toward South.

Mnemonic: ANDS — Accelerate North, Decelerate South.

Cause: pendulous mounting is displaced by longitudinal acceleration; the vertical Earth field component then produces an apparent heading change.

4. Turning error

In the Northern hemisphere when turning through North:

Compass lags — reads less than the actual heading.
→ Roll out early (e.g. roll out at indicated 030° when desired heading is 360°).

When turning through South:

Compass leads — reads more than the actual heading.
→ Roll out late.

Mnemonic: UNOS — Undershoot North, Overshoot South.

In the Southern hemisphere the rule reverses (OSUN).