Magnetic Compasses and Magnetism
Variation, deviation and the memory aid
Variation is the angle between true north and magnetic north, shown by isogonals on the chart; deviation is the error caused by the aircraft's own magnetic fields and is recorded on the deviation card after a compass swing. The memory aid is 'variation east, magnetic least; deviation east, compass least'. As equations: magnetic = true − easterly variation, and compass = magnetic − easterly deviation.
Dip and the errors it causes
Magnetic dip (inclination) is the angle at which the Earth's field dips into the ground. It is steep in New Zealand, roughly 60° south, which is why a direct-reading compass is so sensitive to acceleration and turning. Turning error is greatest on north–south headings and is roughly equal to the dip angle; in the southern hemisphere the compass lags when turning through south, so the aircraft initially appears to turn the wrong way. Acceleration error is greatest on east–west headings and reverses between hemispheres. Heeling error appears in turns and turbulence.
Using the compass
Read it in smooth, level flight; use it to set and cross-check the directional indicator; it remains the standby heading source after electrical failure.
Worked example
True track 045°, variation 18°E, deviation 3°E. Magnetic = 045 − 018 = 027°. Compass = 027 − 003 = 024°. If the deviation card had shown 3°W, the compass heading would have been 030°.
Traps
Applying variation in the wrong direction; treating the deviation card as one number rather than a heading-by-heading table; quoting the northern-hemisphere ANDS rule in New Zealand airspace; and reading the compass during a turn or in turbulence.