Magnetic Variation (Declination)
The angular difference between True North and Magnetic North — what it is, how it varies across the Earth, how to apply it, and worked examples for PPL/CPL exam preparation.
What is Magnetic Variation?
Magnetic variation (also called magnetic declination) is the angular difference between True North (the geographic north pole, aligned with Earth's rotational axis) and Magnetic North (the direction a freely suspended compass needle points).
- Caused by the Earth's molten iron outer core generating a magnetic field that is not aligned with the geographic axis
- The magnetic north pole moves — currently tracking from Canada toward Siberia at approximately 50–60 km per year
- Variation values on aeronautical charts are updated periodically; always use current charts
- Variation is annotated on charts with an annual rate of change (e.g. "decreasing 0.2° annually")
- Ranges from 0° along the agonic line to over 30° at high latitudes
Why it matters operationally: Runway headings, VOR radials, compass headings, ATC clearances, and chart bearings are all given in magnetic. Wind direction in forecasts (TAF, METAR) and true courses derived from charts are in true. Confusing the two can cause gross navigation errors.
Isogonic Lines
Isogonic lines are lines drawn on charts connecting locations of equal magnetic variation. They are printed on WACs (World Aeronautical Charts), topographical charts, and IFR en-route charts.
| Line Type | Definition | Variation Value |
|---|---|---|
| Agonic line | Line of zero magnetic variation — magnetic north equals true north | 0° |
| Isogonic line (West) | Magnetic north is west of true north — compass points too far west | e.g. 5°W, 10°W, 20°W |
| Isogonic line (East) | Magnetic north is east of true north — compass points too far east | e.g. 5°E, 10°E, 20°E |
- East of the agonic line (e.g. eastern USA, UK, Europe): variation is westerly
- West of the agonic line (e.g. western USA, Pacific): variation is easterly
- UK variation (2024–2025): approximately 1–2°W, slowly decreasing toward zero
- Eastern USA: approximately 10–15°W
- Western USA: approximately 10–15°E
Conversion Rules — TVMDC
The standard mnemonic for converting between True, Magnetic, and Compass bearings is TVMDC — True, Variation, Magnetic, Deviation, Compass.
The Rule: Going T → C (True to Compass)
- West variation: ADD — "West is Best" (add west variation)
- East variation: SUBTRACT — "East is Least" (subtract east variation)
- Apply deviation the same way after variation
The Rule: Going C → T (Compass to True)
- West variation: SUBTRACT — reverse the direction
- East variation: ADD — reverse the direction
- Memory: "Cadets Very Methodically Tighten Torque" (C→T reverses the mnemonic)
Memory aid: "West is Best, East is Least" — when going from True to Magnetic (T→M), add west variation (makes the number bigger, i.e. "best" = bigger) and subtract east variation (makes it smaller, i.e. "least"). This is the T→C direction of the chain.
Worked Examples
Example 1 — T to M, Westerly Variation
090°T10°W100°MExample 2 — T to M, Easterly Variation
270°T15°E255°MExample 3 — M to T, Westerly Variation
045°M5°W040°TExample 4 — Full TVMDC Chain
182°C−3° → 182 − 3 = 179°M7°E → East = add going C→T186°TCompass Deviation
Deviation is the error introduced by the aircraft's own magnetic fields — permanent magnets, electrical equipment, and ferrous metals — affecting the magnetic compass. Unlike variation (which is a property of the location), deviation depends on the heading the aircraft is flying.
- Measured during a compass swing — calibration procedure conducted on a compass calibration base
- Results recorded on a deviation card posted near the compass in the cockpit
- Maximum permitted deviation: ±10°; if exceeded the compass must be swung or replaced
- Typical format: "FOR 000° STEER 002°" means compass reads 002° when the aircraft is pointing true north after variation applied
Chart Annotation and Usage
Understanding when true vs magnetic bearings are used prevents costly navigation errors:
| Context | Reference Used | Examples |
|---|---|---|
| Aeronautical chart courses | True | TC drawn on map between waypoints |
| Wind direction (met) | True | TAF, METAR wind (e.g. 27010KT = true 270°) |
| Aircraft headings to fly | Magnetic | ATC clearances, heading bugs, HI/DI |
| VOR radials | Magnetic | R090 = magnetic bearing 090° FROM station |
| Runway designations | Magnetic (rounded to nearest 10°) | RWY 27 = magnetic heading ~270° |
| NDB bearings (QDM/QDR) | Magnetic | QDM 135 = magnetic bearing 135° TO station |
| ATIS/AWOS wind | Magnetic | Some airports report in magnetic for pilot convenience |
Exam tip: Surface wind direction in METAR and TAF is given in true degrees (ICAO standard). However, wind direction broadcast on ATIS and given verbally by ATC to pilots is typically given in magnetic degrees to match what the pilot reads on their compass/HSI. Always clarify the reference when solving navigation problems.