Deviation
Deviation is the angular difference between magnetic north (as indicated by the magnetic compass) and compass north (as indicated by the compass in the aircraft due to the magnetic influence of the aircraft’s own metallic structure and electrical equipment). Deviation varies with heading and is corrected by a compass deviation card mounted near the compass.
Deviation vs. Variation
- Magnetic variation: The angular difference between true north and magnetic north at a given geographic location; caused by the earth’s magnetic field; shown on charts as isogonic lines
- Magnetic deviation: The additional error caused by the aircraft’s own magnetic fields; corrected by the compass deviation card specific to each individual aircraft
Compass Compensation
Aircraft compasses contain small adjustable magnets that are set during compass swinging (a calibration procedure) to minimize deviation. Residual deviation after compensation is recorded on the compass correction card. The pilot adds or subtracts the published deviation for each heading to get the correct magnetic heading to steer.
TVMDC
The conversion sequence: True (heading/course) → apply Variation (+ W, − E) → Magnetic → apply Deviation (from card) → Compass. The reverse for converting compass to true.