The Magnetic Compass: Variation, Deviation, and the Turning and Acceleration Errors (ANDS/UNOS)

The magnetic compass is the oldest instrument in the cockpit and, on paper, the simplest — a magnet floating in fluid, pointing north. But it's also one of the most misunderstood, because the compass lies to you in predictable ways during turns and speed changes. Understanding the magnetic compass means understanding two static errors (variation and deviation) that you correct during planning, plus a set of dynamic errors (acceleration and turning errors) that affect the compass in flight and can fool a pilot into a dangerous heading mistake, especially in instrument conditions.

This post covers the magnetic compass in practical depth: variation and deviation (the planning errors), then the in-flight errors that trip up pilots — acceleration error (ANDS), northerly turning error (UNOS), magnetic dip, and oscillation — plus how to use the compass correctly.

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How the Magnetic Compass Works

Before the errors, understand the instrument:

The basic mechanism:

• A magnetized needle (or card) floats in a fluid-filled housing

• The magnet aligns with the Earth's magnetic field

• The compass card is marked with headings

• The pilot reads the heading against a lubber line

The fluid:

• Dampens oscillation

• Lubricates the pivot

• Allows smooth movement

• (Bubbles indicate a problem — fluid leak)

The Earth's magnetic field:

• The compass aligns with magnetic field lines

• These lines run from magnetic south to magnetic north

• The field has both horizontal and vertical components

• The vertical component causes "dip" (a key source of errors)

The fundamental simplicity and complexity:

• Simple: a magnet pointing north

• Complex: the field isn't aligned with true north (variation), the aircraft interferes (deviation), and the field's vertical component causes dynamic errors (dip-induced errors)

Static Error 1: Magnetic Variation

Variation (also called declination) is the difference between true north and magnetic north.

The two norths:

• True north: The geographic North Pole (Earth's rotational axis)

• Magnetic north: The magnetic pole (in northern Canada, and moving)

The variation angle:

• The angular difference at your location

• East variation: magnetic north is east of true north

• West variation: magnetic north is west of true north

Why variation exists:

• The magnetic pole isn't at the geographic pole

• They're geographically separated

• The angle between them varies by location

• The magnetic pole also moves over time (currently moving toward Siberia)

Reading variation on charts:

• Isogonic lines (dashed magenta) show variation

• Labeled with amount and direction (e.g., "10°W," "6°E")

• The agonic line: where variation is zero (runs through the central U.S.)

• Interpolate between lines

Applying variation:

• "East is least (subtract), West is best (add)"

• Converts true heading to magnetic heading

• East variation: true − variation = magnetic

• West variation: true + variation = magnetic

Variation is location-based:

• Changes as you fly across the country

• Predictable and charted

• Large-scale (the Earth's field)

• Not affected by the aircraft

Static Error 2: Magnetic Deviation

Deviation is compass error caused by the aircraft's own magnetic influences.

The source:

• The aircraft contains metal, wiring, radios, electronics

• These create local magnetic fields

• They distort the compass reading

• Specific to each aircraft

Why deviation varies:

• Different on different headings

• Changes when equipment is added/modified

• Affected by electrical loads

• Unique to each aircraft

The compass swing:

• A maintenance procedure to measure deviation

• The aircraft is aligned to known magnetic headings

• The compass error is recorded for each direction

• Results go on the compass correction card

The compass correction card:

• Mounted near the compass

• Shows "for (magnetic) steer (compass)" values

• Example: "For 090° steer 092°"

• Required to be in the aircraft

Applying deviation:

• Use the card to convert magnetic to compass heading

• Small corrections (usually a few degrees)

• Accounts for the aircraft's specific errors

Deviation is aircraft-based:

• Specific to the individual aircraft

• Localized (the aircraft's own fields)

• Small-scale

• Determined by compass swing

Variation vs. Deviation: The Key Distinction

Feature	Variation	Deviation
Source	Earth's magnetic field	Aircraft's magnetic fields
Scale	Large (geographic)	Small (local)
Cause	Magnetic vs. true north	Onboard interference
Where found	Isogonic lines on charts	Compass correction card
Changes with	Location	Heading, equipment
Predictable	Yes (charted)	Yes (card)

The memory aid:

• Variation: The Earth varies (geographic, large-scale)

• Deviation: The aircraft deviates (local, aircraft-specific)

In the heading sequence:

• True → (variation) → Magnetic → (deviation) → Compass

• Variation converts true to magnetic

• Deviation converts magnetic to compass

The Dynamic Errors: Why the Compass Lies in Flight

Beyond the static errors, the magnetic compass has dynamic errors that occur during flight. These are caused by magnetic dip.

Magnetic dip:

• The Earth's magnetic field has a vertical component

• Near the equator, the field is horizontal

• Near the poles, the field dips downward (more vertical)

• This vertical pull causes the compass card to tilt

• The tilt creates errors during acceleration and turns

Why dip matters:

• The compass magnet wants to align with the full field (including vertical)

• The vertical component pulls one end of the magnet down

• This makes the compass susceptible to errors when the aircraft accelerates or turns

• The errors are predictable

The two main dynamic errors:

1. Acceleration error (ANDS)

2. Turning error (northerly turning error)

These affect the compass in flight and are critical to understand, especially for instrument flying.

Acceleration Error (ANDS)

The compass gives false indications during acceleration and deceleration on east or west headings.

The mnemonic: ANDS

• Accelerate

• North

• Decelerate

• South

What it means:

• On an east or west heading

• Acceleration causes the compass to indicate a turn toward North

• Deceleration causes the compass to indicate a turn toward South

The mechanism:

• During acceleration, inertia acts on the tilted compass card

• The card swings, creating a false northerly indication

• During deceleration, the opposite occurs (false southerly)

• Most pronounced on east/west headings

• Minimal on north/south headings

The practical impact:

• Accelerating on an east heading: compass shows a turn toward north (but you're not turning)

• Decelerating on a west heading: compass shows a turn toward south

• The error is temporary (during the speed change)

• Don't chase it

Where ANDS matters:

• Instrument flying (no visual reference)

• Speed changes during compass-only navigation

• Recognizing the error prevents false corrections

Northerly Turning Error (UNOS)

The compass gives false or lagging indications during turns, most pronounced near north and south headings.

The mnemonic: UNOS

• Undershoot

• North

• Overshoot

• South

What it means:

• When turning to a North heading: the compass lags — roll out early (undershoot the turn)

• When turning to a South heading: the compass leads — roll out late (overshoot the turn)

The mechanism:

• During a turn, the dip causes the compass card to tilt and lag or lead

• Near north, the compass lags behind the actual turn

• Near south, the compass leads ahead of the actual turn

• Most pronounced at north/south, minimal at east/west

  
  

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The "undershoot north, overshoot south":

• Turning to north: the compass shows the turn lagging, so you roll out before reaching the target (undershoot)

• Turning to south: the compass shows the turn leading, so you continue past (overshoot)

The correction amount:

• Related to your latitude

• A common rule: lead/lag by approximately your latitude

• At 30°N latitude, lead/lag by about 30°

• (Plus the standard rollout lead of half the bank angle)

Where UNOS matters:

• Turning to headings using the compass

• Instrument flying without a heading indicator

• Recognizing the error for accurate turns

The combined memory:

• ANDS: acceleration errors (East/West headings)

• UNOS: turning errors (North/South headings)

Compass Oscillation and Other Errors

Additional compass behaviors to understand:

Oscillation:

• The compass swings back and forth in turbulence

• Erratic readings in bumpy air

• Difficult to read precisely

• Average the swings for a reading

Dip error (general):

• The underlying cause of ANDS and UNOS

• The vertical field component

• Greater at higher latitudes

• The source of the dynamic errors

Northerly/Southerly heading reading:

• The compass is most accurate in straight-and-level, constant-speed flight

• Errors appear during acceleration and turns

• Steady flight gives the truest reading

Using the Compass Correctly

Given all these errors, how to use the compass properly:

For straight-and-level flight:

• The compass is accurate when steady

• Constant speed, wings level

• Read the heading directly (with deviation correction)

• The most reliable compass use

For turns:

• Account for UNOS (turning error)

• Undershoot when turning to north

• Overshoot when turning to south

• Or use the heading indicator and reset it to the compass periodically

For speed changes:

• Account for ANDS (acceleration error)

• Don't correct during the acceleration/deceleration on east/west headings

• Wait for steady flight to read

The heading indicator relationship:

• The heading indicator (HI/DG) is easier to use (no dip errors)

• But it drifts and must be reset to the compass

• Reset the HI to the compass during steady, straight-and-level flight

• Use the HI for turns, the compass to reset it

The practical workflow:

• Set the heading indicator to the compass (in steady flight)

• Fly using the heading indicator

• Periodically reset the HI to the compass

• Use the compass directly if the HI fails

The Compass as a Backup

In modern aircraft, the compass is often a backup:

Primary heading sources:

• Heading indicator (gyroscopic)

• HSI (horizontal situation indicator)

• Glass cockpit heading displays

• These are easier to use (no dynamic errors)

The compass backup role:

• Required equipment (FAR 91.205)

• Backup if other systems fail

• Independent of electrical/vacuum systems

• Always available

When the compass is essential:

• Heading indicator failure

• Electrical failure (glass cockpit)

• Vacuum failure (gyroscopic instruments)

• The compass keeps working

The skill:

• Know how to use the compass despite its errors

• Account for ANDS and UNOS

• Use it as a reliable backup

• Required competency

Common Misconceptions

• "Variation and deviation are the same."

  • No — variation is the Earth's (true vs. magnetic north); deviation is the aircraft's (onboard interference).

• "The compass is always accurate."

  • The compass is accurate in steady flight but has dynamic errors (ANDS, UNOS) during acceleration and turns.

• "ANDS and UNOS are the same error."

  • No — ANDS is acceleration error (east/west headings); UNOS is turning error (north/south headings).

• "Acceleration error happens on all headings."

  • ANDS is most pronounced on east/west headings, minimal on north/south.

• "Turning error happens on all headings."

  • UNOS is most pronounced on north/south headings, minimal on east/west.

• "The heading indicator has the same errors."

  • No — the gyroscopic heading indicator doesn't have dip errors, but it drifts and must be reset to the compass.

On the Written Test and Checkride

Compass errors appear consistently on tests. The most commonly tested topics:

• Variation vs. deviation (sources and differences)

• "East is least, West is best"

• Acceleration error (ANDS)

• Northerly turning error (UNOS)

• Magnetic dip as the cause

• Using the compass correctly

Quick Reference

Variation:

• True north vs. magnetic north

• Earth's magnetic field (geographic)

• Isogonic lines on charts

• "East is least, West is best"

Deviation:

• Aircraft's own magnetic interference

• Compass correction card

• Compass swing to determine

• Specific to each aircraft

Variation vs. Deviation:

	Variation	Deviation
Source	Earth	Aircraft
Scale	Large	Small
Found	Charts	Correction card

Acceleration Error (ANDS):

• Accelerate North, Decelerate South

• On east/west headings

• Acceleration: false turn toward north

• Deceleration: false turn toward south

Turning Error (UNOS):

• Undershoot North, Overshoot South

• On north/south headings

• Turning to north: compass lags (undershoot/roll out early)

• Turning to south: compass leads (overshoot/roll out late)

• Lead/lag ≈ latitude

Magnetic Dip:

• Vertical component of Earth's field

• Causes ANDS and UNOS

• Greater at higher latitudes

The Memory Pattern:

• ANDS: acceleration (East/West)

• UNOS: turning (North/South)

Using the Compass:

• Accurate in steady, level, constant-speed flight

• Account for ANDS during speed changes

• Account for UNOS during turns

• Reset heading indicator to compass in steady flight

Compass as Backup:

• Required equipment (FAR 91.205)

• Independent of electrical/vacuum

• Works when other systems fail

Heading Sequence:

• True → (variation) → Magnetic → (deviation) → Compass

Key Principle:

Variation (Earth) and deviation (aircraft) are static errors corrected in planning. ANDS (acceleration, E/W) and UNOS (turning, N/S) are dynamic errors caused by magnetic dip — know them to use the compass correctly in flight.

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Author: Nathan Hodell

CFI, CFII, MEI, ATP, Creator and CEO

Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.