The Magnetic Compass and Its Errors

The magnetic compass is simple and needs no power, which makes it a valuable backup, but its readings are distorted whenever the aircraft turns or changes speed. These distortions come from magnetic dip, the compass’s tendency to tilt toward the magnetic pole. Two mnemonics capture the effects. Turning error is UNOS: Undershoot North, Overshoot South. When rolling out onto a northerly heading the compass lags behind, so you must stop the turn early; onto a southerly heading it leads, so you let the turn continue past the target. Acceleration error is ANDS: Accelerate North, Decelerate South. On an east or west heading, accelerating makes the compass indicate a turn toward north and decelerating toward south, even though the heading has not changed. Because these errors are largest near northerly and southerly headings and during speed changes, pilots rely on the heading indicator for steady reference and use the compass to reset it during straight, unaccelerated flight.