Magnetic and True Bearings: Variation and Deviation
Two corrections, one chain
True direction is measured from true north; magnetic direction from magnetic north; compass direction from the aircraft's own compass. Variation is the angle between true and magnetic north at your position, shown by isogonals on the chart, with the agonic line joining points of zero variation. Deviation is the error of the aircraft compass from magnetic north, caused by the airframe's own fields and recorded on the deviation card after a compass swing.
Rules
- Magnetic bearing = true bearing minus easterly variation (plus westerly). The mnemonic is: variation east, magnetic least; variation west, magnetic best.
- Compass = magnetic minus easterly deviation (plus westerly). Deviation west, compass best.
- Carry the signs explicitly rather than relying on memory when the values are large.
- Over New Zealand variation is roughly 20 degrees east, increasing toward the south and changing slowly with secular variation, so it must be read from the current chart, not remembered.
- A direct-reading compass is also affected by turning and acceleration errors, and by magnetic dip at high latitude, where a gyro or grid reference becomes preferable.
Worked example
A runway has a true bearing of 090. Variation is 12 degrees east, so magnetic = 090 minus 12 = 078. The deviation card shows 3 degrees west on that heading, so compass = 078 plus 3 = 081. To go the other way, from compass 081 to true 090, reverse each step exactly.
Traps
Applying variation to a compass heading instead of a true heading; reversing the east-is-least rule; adding deviation to true rather than to magnetic; and assuming variation from one part of the country applies everywhere on a long leg.