What is magnetic variation in aviation, and why does it matter?

Magnetic variation, also called magnetic declination, is the angle between true north and magnetic north at a specific place and time. In Aviation & Real-World Flying, it matters because charts, runways, radio aids and cockpit instruments may use different north references, so an uncorrected course can be wrong.

A 10° reference error maintained over 100 NM produces about 17 NM of cross-track error in still air. Variation therefore affects practical navigation, not merely how a direction is written.

What do east, west and local magnetic variation mean?

Easterly or westerly variation tells you which side of true north magnetic north lies on at your position. With east variation, magnetic north is east of true north; with west variation, it is west.

Local variation means the variation applicable to a particular location. It is not a separate type of compass error. The value changes across the Earth and gradually changes with time as the magnetic field moves, so a variation taken from one airport or an old chart may be unsuitable elsewhere.

Aeronautical charts may show variation as a labelled value, a compass rose or isogonic lines joining locations with equal variation. A chart may also give the reference year and annual change. An agonic line marks zero variation, where true north and magnetic north coincide at that time.

“Magnetic variance” is sometimes used informally, but magnetic variation and magnetic declination are the recognised navigation terms. Our explanation of how true and magnetic headings differ in flight simulators covers how these references appear on cockpit displays.

How do you convert true and magnetic directions?

Convert true to magnetic by subtracting east variation or adding west variation; reverse the operation when converting magnetic to true.

If east variation is treated as positive and west as negative, the same rules are M = T - V and T = M + V. Normalise the answer to a valid direction: 355° plus 10° becomes 005°, not 365°.

The mnemonic “east is least, west is best” works only when converting true to magnetic. A mistake we see constantly is using it in the reverse direction, or applying variation twice after avionics have already converted an internally calculated true track for magnetic display.

1. Identify the reference. Look for TRUE, MAG, °T or °M. Near polar regions, a chart may use grid north instead; do not treat a grid direction as magnetic without following that chart's procedure.
2. Find the applicable local variation. Use the value for the relevant position and date. For a long route, one airport's variation may not represent every leg.
3. Convert only if the references differ. Enter a published magnetic course directly into an instrument expecting magnetic input. Do not add another correction simply because variation appears on the chart.
4. Check the direction of conversion. Label the starting and resulting values with T or M before doing the arithmetic.
5. Sense-check the answer. A result on the wrong side of the true direction usually means east and west were reversed.

Does variation change heading, course, track or bearing?

Variation changes the north reference of a direction; it does not change what heading, course, track or bearing means. A heading is where the aircraft points, while track is its path over the ground, so wind can make them different even when both are magnetic.

Variation also does not provide a wind correction. During conventional flight planning, apply wind where required to obtain the appropriate heading, then ensure that every direction is expressed in the reference expected by the chart or instrument.

Are VOR radials true or magnetic?

VOR radials are normally magnetic and are orientated using the station's assigned magnetic declination, not necessarily the latest local variation. That stored value can lag behind changes in the Earth's magnetic field.

A VOR radial extends from the station. An aircraft on the 090 radial is east of the station; its approximate inbound course is the reciprocal, 270° magnetic. Comparing a radial directly with a GPS bearing to the station therefore creates a 180° error before variation is even considered.

Do not recalculate a published radial using a newer local variation. Select and fly the published radial or procedure course as printed unless it is explicitly marked true. A small mismatch between a VOR indication and a GPS-derived magnetic bearing may come from the station declination, different magnetic models or comparison of an inbound bearing with an outbound radial.

Where a charting authority uses true bearings, the chart or procedure should identify that reference. For the flying technique itself, see our guidance on tracking VORs accurately during a VFR flight.

How does magnetic variation affect runway numbers?

Most runway numbers are based on the runway centreline's magnetic direction, rounded to the nearest ten degrees with the final zero omitted. A magnetic alignment of 184° is therefore designated Runway 18, while the reciprocal end is approximately Runway 36.

The number is only a rounded designator, not a precise heading to fly. Several degrees of difference between the runway number and a magnetic heading display can be entirely normal.

As local variation changes, the magnetic direction of a fixed runway can eventually cross a rounding boundary and require renumbering. The pavement has not moved; its relationship to magnetic north has changed. Older simulator scenery may retain the previous designation, which explains many apparent “magnetic runway” errors. Our detailed guide explains the rounding, reciprocal and renumbering rules for runway designations.

Do not infer a wind's north reference from the runway number. Coded weather reports and pilot-facing ATIS or ATC information can use different conventions; we explain when aviation wind directions are true or magnetic separately.

Variation is not compass deviation

Magnetic variation and compass deviation are different corrections caused by different things. Variation comes from the Earth's magnetic field, while deviation is an individual aircraft compass error caused by nearby metal, electrical systems and equipment.

The traditional correction sequence is true direction to magnetic direction using variation, then magnetic direction to compass direction using deviation. A compass correction card does not replace charted variation. Likewise, acceleration, turning and magnetic-dip errors in a direct-reading compass are not magnetic variation.

Why can magnetic variation disagree in a flight simulator?

A simulator can disagree with a chart when its magnetic model, scenery, navigation database and aircraft avionics represent different dates or use separate data sources.

• An older simulator may use a fixed magnetic-field model from an earlier epoch.
• The selected simulation date may affect one magnetic calculation while scenery and navigation data remain fixed.
• Legacy airport scenery may retain a runway number used before a real-world renumbering.
• An add-on aircraft may calculate magnetic directions independently of the simulator.
• A VOR may use its stored station declination while GPS avionics use another magnetic model.
• One display may be configured for true north while another remains magnetic.
• Heading and ground track may be compared without allowing for wind, slip or compass error.

How should you diagnose a true-versus-magnetic mismatch?

The quickest diagnosis is to identify each number's reference and data source before changing any course.

1. Classify the values. Establish whether each number is a heading, track, bearing, course, VOR radial, runway designator or wind direction.
2. Check display modes. Look for true-north settings and T or M labels on the map, GPS, HSI and aircraft instruments.
3. Compare like with like. Use the same location, date, direction and north reference. Do not compare a magnetic heading with a true ground track.
4. Choose the controlling source. For a published procedure or VOR radial, use the printed course as published. For real-world flying, use approved, up-to-date operational charts and aircraft documentation.
5. Isolate the source. If only one add-on aircraft disagrees, suspect its avionics or magnetic model. If every aircraft shows the same airport discrepancy, ageing scenery or simulator-wide data is more likely.
6. Avoid improvised corrections. Do not force two displays to agree by adding local variation until you know that one is true and the other magnetic; otherwise, variation is often applied twice.