061 General Navigation topic guide
Compass Turning and Acceleration Errors
A magnetic compass card is pendulously suspended below its magnet system so that it stays level as the aircraft banks, but that same pendulous mounting is what makes it misbehave. Away from the magnetic equator the Earth's field has a vertical component, dip, which pulls the suspended magnet down on one side; combined with ordinary manoeuvring forces, that pull produces indication errors that have nothing to do with the aircraft's actual heading.
Two distinct effects follow from this: accelerating or decelerating on an east or west heading makes the compass show a false turn even though the heading has not changed, and turning through north or south makes the compass lag behind or run ahead of the true heading. Both effects reverse between the hemispheres, because dip itself reverses sign at the magnetic equator.
Acceleration errors: ANDS
In the Northern Hemisphere, accelerating on an easterly or westerly heading makes the compass show an apparent turn towards north, and decelerating on the same headings shows an apparent turn towards south. The mnemonic ANDS, Accelerate North Decelerate South, captures it. The effect is at its maximum on headings of east and west and disappears on headings of north and south, the opposite pattern from the turning errors covered next.
Nothing about the aircraft's actual track has changed during a simple speed change; the compass card itself is briefly displaced by the combination of dip and the acceleration force, and it settles back to the correct reading once the speed stabilises. A pilot who chases the false indication during the acceleration ends up turning the aircraft off its real heading to correct an error that was never really there.
Turning errors: UNOS
Turning through north or south produces the opposite style of error. In the Northern Hemisphere the mnemonic is UNOS: Undershoot North, Overshoot South. Rolling out of a turn exactly when the compass reads the desired heading leaves the aircraft short of that heading if the turn passed through north, because the compass lags during that part of the turn, so the pilot must roll out early. The same turn through south makes the compass lead, so the pilot must roll out late, after the compass has already shown the desired heading.
This error is largest on turns through north or south and effectively vanishes on turns through east or west, exactly why acceleration errors and turning errors are taught as a pair: one is at its worst precisely where the other is zero.
The Southern Hemisphere mirror
Below the magnetic equator the vertical component of dip reverses, and so does every rule above. Acceleration on an easterly or westerly heading now shows an apparent turn towards south, and deceleration shows an apparent turn towards north, the exact reverse of ANDS. Turning through south now undershoots and turning through north now overshoots, the exact reverse of UNOS. A pilot who learns only the Northern Hemisphere mnemonics and applies them unchanged south of the magnetic equator corrects every one of these errors in the wrong direction.
Worked example
Worked example: rolling out of a turn through north
Not quite. The correct answer is B.
Correct answer: B
- A. This ignores turning error altogether. Through a turn passing north the compass lags the true heading, so waiting for it to show 020 means the aircraft has already turned past 020 degrees.
- B. Correct. This is the UNOS undershoot-north technique: because the compass lags while turning through north, rolling out early leaves the aircraft settling accurately on 020 degrees once the turning error clears.
- C. This is the overshoot-south technique applied to a turn through north instead, the opposite correction to the one this turn actually needs.
- D. This confuses turning error with acceleration error. Acceleration error is worst on east and west and absent on north and south; turning error is the reverse, worst on north and south and absent on east and west.
Step by step
- Identify which pole the turn passes through: the heading changes from 340 to 020 through 000 (north), not through south.
- Recall the Northern Hemisphere turning-error mnemonic UNOS: turns through north undershoot, meaning the compass lags the true heading during the turn.
- Because the compass keeps indicating a heading behind the true one while turning through north, the pilot must roll level before the compass reaches 020 degrees, not after.
- Rule out the distractors: waiting for exactly 020 (option A) or overshooting further (option C) both fail to anticipate the lag, and claiming no correction is needed (option D) mixes up turning error with the unrelated acceleration error.
Common mistakes
Mixing up ANDS and UNOS
ANDS governs steady-heading acceleration and deceleration; UNOS governs turns through north and south. Applying the acceleration mnemonic to a turning question, or the reverse, gives an answer with the right shape but the wrong reasoning and often the wrong sign.
Forgetting the hemisphere reversal
Every one of these rules flips south of the magnetic equator. A question set in the Southern Hemisphere that is answered with Northern Hemisphere ANDS or UNOS gives a heading correction in exactly the wrong direction.
Chasing the compass during a simple acceleration or deceleration
The apparent turn during a speed change is not a real change of heading. Turning the aircraft to follow the false indication introduces a genuine heading error where none previously existed.
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Last reviewed July 2026