🟢 In 60 seconds
The magnetic compass reads correctly only in unaccelerated, level flight. In turns and during acceleration, systematic errors occur — the pilot must know them to interpret the compass when manoeuvring.

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).

🎯 Exam essentials

Northerly/southerly turning error (NH): turning toward north the compass lags (under-reads), turning toward south it leads (over-reads)
ANDS rule (acceleration on east/west heading, NH): Accelerate → North shown; Decelerate → South shown
Inclination (dip): 0° at magnetic equator, 90° at magnetic poles
Variation (declination): angle between true and magnetic north, varies by location (isogonals on chart)
Deviation: aircraft's own magnetism, heading-dependent → compass deviation card