Quick answer: The magnetic compass tells the truth only in straight, level, unaccelerated flight. Everywhere else it lies in predictable ways, and nearly all of those lies trace back to a single cause — magnetic dip. Two mnemonics cover the errors that matter in flight: UNOS (Undershoot North, Overshoot South) for turns, and ANDS (Accelerate North, Decelerate South) for speed changes on easterly and westerly headings. Both apply in the Northern Hemisphere and reverse in the Southern.
Most pilots learn these as exam answers and promptly forget them, which is fine right up until the day the vacuum pump quits, the heading indicator topples, and the compass becomes the only heading reference in the aircraft. That is the moment these errors stop being trivia.
Start With the Only Rule That Really Matters
Before the mnemonics, internalise this: the compass is accurate only when the aircraft is straight, level, and unaccelerated. If you are banking, climbing, descending, speeding up, or slowing down, the reading is suspect.
This is why the technique for using a compass is not really about correcting the errors at all. It is about waiting — establishing stable flight, letting the card settle, and only then believing it.
The Six Errors, and Which Ones Matter in Flight
A common memory aid is VD-MONA: Variation, Deviation, Magnetic dip, Oscillation, Northerly turning error, and Acceleration error. They are not equally important, and lumping them together is part of why students find this topic muddy.
| Error | When it bites | Do you correct in flight? |
|---|---|---|
| Variation | Flight planning | No — handled on the chart |
| Deviation | Every heading | No — handled by the correction card |
| Magnetic dip | Underlies the rest | No — it is the cause, not a symptom |
| Oscillation | Turbulence | No — average the swing |
| Turning error (UNOS) | Rolling out of turns | Yes |
| Acceleration error (ANDS) | Speed changes on E/W | Yes |
Only the last two require you to actively do something with the controls. The first two are bookkeeping, and the middle two are physics you simply accommodate.
Variation and Deviation: The Two That Aren’t Really Flight Errors
Variation
True north and magnetic north are not in the same place, and the angular difference between them is variation. It changes with geography — lines of equal variation (isogonic lines) are printed on sectional charts, and a line of zero variation (the agonic line) runs where the two norths happen to align.
Variation is not a compass fault. The instrument is correctly pointing at magnetic north; it is simply that your chart’s true north is somewhere else. You resolve it during planning by converting between true and magnetic, not by doing anything in the cockpit.
Deviation
Deviation is error caused by your own aircraft — its metal structure, wiring, radios, and electrical equipment all generate small magnetic fields that tug the compass card.
Unlike variation, deviation is unique to the individual airframe, which is why every aircraft carries a compass correction card near the instrument showing the actual heading to fly for a given desired heading. It is also why deviation can change when equipment changes: install a new radio, and the card should be recalibrated by a technician during a compass swing.
A practical habit worth building: notice what your electrical load does. Turning on landing lights, pitot heat, or an anti-collision light can shift the card slightly, and the correction card is typically produced with radios on.
Magnetic Dip: The Cause of Everything Else
This is the concept that makes the rest make sense, and it is the one most often skipped.
The Earth’s magnetic field does not run neatly parallel to the surface. It emerges from the planet at an angle, running roughly horizontal near the equator and plunging increasingly steeply toward the magnetic poles. Because the compass magnets try to align with that field, they are pulled not just around but also downward. That downward pull is magnetic dip.
Compass designers fight it by mounting the card on a pivot, weighting it slightly, and suspending it in fluid — which damps the motion but cannot eliminate the vertical pull. The residual dip is what produces both turning error and acceleration error.
Two consequences follow directly, and they are worth remembering because they explain the mnemonics rather than just asserting them:
- Dip effects grow with latitude. They are negligible near the equator, where the field is nearly horizontal, and pronounced at high latitudes. A pilot in Alaska deals with far more compass error than one in southern Florida.
- Dip effects reverse across the equator. UNOS and ANDS are Northern Hemisphere rules. In the Southern Hemisphere the field dips the other way and the errors invert.
Turning Error: UNOS
When you bank, the compass card banks too — and the dip-induced vertical pull now has a sideways component that swings the card. The result is that the compass leads or lags the aircraft’s actual heading depending on which way you are turning and where you are pointed.
The behaviour, in the Northern Hemisphere:
- Turning through north, the compass lags — it initially shows a turn in the opposite direction, then catches up sluggishly.
- Turning through south, the compass leads — it races ahead of the aircraft’s actual heading.
- On east and west, turning error is at its minimum.
Hence the mnemonic: UNOS — Undershoot North, Overshoot South.
The practical rule
Because the compass lags near north, you must roll out early — before it reads the heading you want. Because it leads near south, you must roll out late — after it has already sailed past.
The standard rule of thumb ties the amount to your latitude:
- Rolling out on north: begin the rollout when the compass indicates approximately your latitude short of north. At 30°N, start rolling out around 030° when turning right to north, or 330° when turning left.
- Rolling out on south: continue past south by approximately your latitude. At 30°N, roll out around 150° or 210° depending on turn direction.
- Rolling out on east or west: essentially no correction needed.
Treat the latitude figure as an approximation, not a computation. The point is the direction and rough magnitude of the correction, not precision to the degree.
Acceleration Error: ANDS
The second dip-driven error appears when airspeed changes. The compass card is slightly weighted, so acceleration and deceleration make it tilt — and a tilted card, under a downward magnetic pull, rotates.
In the Northern Hemisphere, on an easterly or westerly heading:
- Accelerating makes the compass indicate a turn toward north.
- Decelerating makes it indicate a turn toward south.
Hence ANDS — Accelerate North, Decelerate South.
Two details students routinely miss. First, this error is greatest on east and west, and essentially absent on north and south — the exact opposite of turning error, which is why the two mnemonics complement rather than conflict. Second, the aircraft is not actually turning at all; only the indication moves.
The correction is simply to disregard the compass during speed changes and read it once the airspeed is stable. Level-off after a climb is the classic trap: you lower the nose, the aircraft accelerates, and the compass swings convincingly toward north while your heading has not changed by a degree.
Oscillation
The least interesting error, and the easiest to handle. In turbulence the card swings back and forth around the correct heading. The fluid damps most of it, but rough air will still produce noticeable movement.
There is nothing to correct. Read the average of the swing rather than chasing individual indications, and do not attempt to fly a precise heading off a compass that is oscillating through twenty degrees.
Putting It Together: Flying a Compass Turn
When the heading indicator is unavailable, the compass becomes your primary heading reference — a scenario worth practising deliberately rather than meeting for the first time on a bad day.
- Use timed turns where possible. A standard-rate turn covers 3° per second, so 30 seconds gives 90°. This sidesteps compass errors entirely and is generally more accurate than chasing the card.
- If you must use the compass, roll out and wait. Establish straight, level, unaccelerated flight, give the card several seconds to settle, then read it.
- Apply UNOS if rolling out near north or south. Early for north, late for south, by roughly your latitude.
- Ignore it during speed or pitch changes. Re-read once stable.
- Make small corrections. Chasing a compass produces oscillation of both the card and the aircraft.
If your aircraft has a functioning turn coordinator and clock, timed turns paired with an occasional settled compass check will hold a heading far better than continuous reference to the compass alone.
The Bottom Line
The magnetic compass is the most reliable instrument in most light aircraft — it needs no vacuum, no electrical power, and no gyro — and also the most misread. Its quirks are not random. They come almost entirely from magnetic dip, they scale with latitude, and they show up in two specific situations: rolling out of turns, and changing speed.
Remember that it is honest only in steady flight, keep UNOS and ANDS straight, and it will do exactly what you need on the day everything else has quit.
Frequently Asked Questions
What does UNOS mean in aviation? UNOS stands for Undershoot North, Overshoot South. It describes magnetic compass turning error in the Northern Hemisphere: when rolling out on a northerly heading you must roll out early because the compass lags, and when rolling out on a southerly heading you must roll out late because the compass leads. The correction is roughly equal to your latitude.
What does ANDS mean in aviation? ANDS stands for Accelerate North, Decelerate South. In the Northern Hemisphere, on easterly or westerly headings, accelerating causes the magnetic compass to indicate a turn toward north and decelerating causes it to indicate a turn toward south, even though the aircraft’s heading has not changed.
What causes magnetic compass errors? Nearly all in-flight compass errors trace to magnetic dip — the Earth’s magnetic field runs into the ground at an angle rather than parallel to the surface, pulling the compass card downward as well as around. That vertical pull produces both turning error and acceleration error, and both grow with latitude.
What is the difference between variation and deviation? Variation is the angular difference between true north and magnetic north, which varies by geographic location and is shown on charts by isogonic lines. Deviation is error caused by magnetic fields within your own aircraft, such as wiring and avionics, and is documented on a compass correction card specific to that airframe.
When is the magnetic compass accurate? Only in straight, level, unaccelerated flight. During turns, climbs, descents, or airspeed changes the indication is unreliable. The practical technique is to establish stable flight, allow the card several seconds to settle, and only then read it.
Do compass errors work the same in the Southern Hemisphere? No. UNOS and ANDS are Northern Hemisphere rules. Because magnetic dip acts in the opposite direction south of the equator, both turning and acceleration errors reverse. The errors are also smallest near the equator and largest at high latitudes.
This article is general educational information and is not flight instruction. Aircraft equipment and procedures vary — follow your POH, your compass correction card, and guidance from your flight instructor.
