Module MOD-05 · 7 min · ACS PA.I.G · ACS PA.VIII

The Magnetic Compass and Its Errors

← Flight Instruments and Avionicsdraft — pending CFI review

Why this matters in flight: The magnetic compass is a reliable backup, but it misbehaves during turns and speed changes. Knowing its errors keeps you from chasing a swinging needle and rolling out on the wrong heading.
Magnetic compass errors: ANDS and UNOSA compass rose. Acceleration error: on an east or west heading, accelerating shows a turn toward north and decelerating toward south — remembered as ANDS. Turning error: rolling out of a turn near north, the compass lags so you undershoot; near south it leads so you overshoot — remembered as UNOS. These are Northern-Hemisphere behaviors.NSEWANDS — accelerationAccelerate → shows turn to NorthDecelerate → shows turn to South(on E/W headings)UNOS — turningUndershoot rollout near NorthOvershoot rollout near South(N. Hemisphere)
DRAFT schematic — pending CFI review. Northern-Hemisphere magnetic compass errors. Mnemonics ANDS (acceleration) and UNOS (turning). Not to scale; not an FAA-approved figure.

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.

Key terms

Magnetic dip
The compass’s tendency to point downward toward the magnetic pole, causing turning and acceleration errors.
UNOS
Undershoot North, Overshoot South — the turning error mnemonic.
ANDS
Accelerate North, Decelerate South — the acceleration error mnemonic.

Summary

The magnetic compass suffers turning error (UNOS) and acceleration error (ANDS), both caused by magnetic dip, so pilots reset the heading indicator from the compass only during straight, unaccelerated flight.

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Using the UNOS rule, what happens to the magnetic compass when rolling out onto a northerly heading?

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Sources

Every claim traces to a source — paraphrased knowledge elements pointing at the governing FAA publication; not yet verified against a retrieved source.

  • PHAK Ch. 8 — Pilot’s Handbook of Aeronautical Knowledge unverified

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