Magnetism and Compasses: Terrestrial Field, Variation, Deviation and Compass Systems
The terrestrial magnetic field
The Earth behaves like a tilted dipole. The magnetic poles drift and do not coincide with the geographic poles. Field elements you must know: variation (angle between true and magnetic meridians), dip/inclination (angle of the field to the horizontal, zero at the magnetic equator, 90° at the magnetic poles), and horizontal intensity, which drives the direct-reading compass. The compass is most reliable where horizontal intensity is strong (low latitudes) and unreliable near the magnetic poles, where dip is steep and the directive force weak.
Variation and deviation
Variation is printed on charts as isogonals and changes slowly with time (annual change). Deviation is caused by the aircraft's hard and soft iron fields and by electrical equipment; it is unique to each installation and heading, and is recorded on a deviation card. Compass swinging determines the card. A common exam trap is applying the annual change in the wrong direction, or treating deviation as a property of position rather than of the aircraft and heading.
Compass types
A direct-reading (wet) compass is simple but suffers from turning and acceleration errors. In the northern hemisphere, acceleration causes a false turn indication toward north during acceleration and toward south during deceleration. Turning errors are largest on north/south headings. A remote-reading compass (gyromagnetic or flux valve system) uses a gyro stabilised by flux-valve signals and provides a stable heading output, often with a slaved gyro and a heading reference for the flight director. A standby magnetic compass remains independent.
Worked example
True track 090°, variation 8°W, deviation +3° (on this heading). Magnetic = true − west variation = 090° − 8° = 082°M. Compass = magnetic − deviation = 082° − 3° = 079°C. Always state the reference (T/M/C) with every heading.
Rule of thumb
"True virgins make dull company": True → Variation → Magnetic → Deviation → Compass, adding westerly variation and subtracting easterly when going true to magnetic.