Magnetic compass errors

A compass only reads true in straight, level, unaccelerated flight. The cause is magnetic dip (standard compass theory, not named in TP 1102/TC AIM): the earth's field pulls the compass card down toward the nearer pole, and that downward tug fakes a turn whenever you bank or change speed. Errors are worst near the poles, where the dip is steepest.

Turning error (Northern Hemisphere), UNOS = Undershoot North, Overshoot South:

• Turning onto north the compass lags: roll out early (undershoot).
• Turning onto south the compass leads: roll out late (overshoot).

Acceleration error on E/W headings, ANDS = Accelerate North, Decelerate South:

• Accelerate and the reading swings toward north; decelerate and it swings toward south.
• Wings stay level, so it is an error, not a real turn.

Deviation vs variation:

• Variation = angle between true and magnetic north (the earth's field), applied from the chart.
• Deviation = the aircraft's own magnetism and electrics (the angular difference between magnetic and compass headings, AC 100-001), corrected by the compass correction card (aircraft-specific card, not in TP 1102/TC AIM).

Trap: UNOS is the turning rule (N/S roll-outs), ANDS is the acceleration rule (E/W). Swap the direction (lead vs lag, north vs south) and you double the error instead of removing it (TP 1102).

Practice