Magnetism and Compass Conversion
The Magnetic Field
A magnetic compass aligns with the local magnetic field, not with true north. Variation is the angle between true north and magnetic north at a place, stated east or west; it changes with position and slowly with time. Deviation is the angle between magnetic north and the compass needle caused by aircraft magnetism, and it varies with heading.
The Conversion Chain
True → magnetic (apply variation) → compass (apply deviation). The standard mnemonic is "variation west, magnetic best; variation east, magnetic least": add westerly variation to true to get magnetic, subtract easterly. Deviation is handled the same way using the deviation card for the heading.
Compass Errors
Turning errors (northerly/southerly turning error) and acceleration error affect a magnetic compass during turns and speed changes, largest on north–south headings. A remote-reading gyromagnetic compass avoids these but needs synchronising to magnetic north.
Traps
Questions often give true track and ask for compass heading, requiring both corrections in the correct order, or give a compass reading and ask for true. Sign conventions are the usual failure point. Deviation is aircraft-specific; variation is geographic.
Rule of Thumb
Use CADET: Compass + Deviation = Magnetic; Magnetic + Variation = True (east add, west subtract when going compass to true). Example: compass 030°, deviation 3°E, variation 5°W → magnetic 033°, true 028°.