A pilot in Toronto, planning a flight to Ottawa, opens an enroute chart and reads off a track of 078° T. The flight plan, the FMS, and the ATC clearance all want different answers to the same physical question — which way is the nose pointing? The compass on the panel will show something close to 090° C. Twelve degrees of confusion that aren't a mistake — they are two well-defined corrections in sequence.
The first correction is variation: true north and magnetic north aren't in the same place, and the angle between them depends on where in the world you're standing. The second is deviation: even after you've corrected for variation, the compass mounted in your aeroplane has its own steel and electrical fields nudging the needle by a few more degrees, differently for every heading.
True, magnetic, and compass — three different norths
True north
True north is the direction of the geographic North Pole — the axis the Earth spins on. It is fixed, defined by geometry, and it never moves. Every meridian on a chart points at it. True heading (TH) is the angle from true north to the aeroplane's nose, measured clockwise.
Magnetic north
Magnetic north is the direction in which the magnetic compass needle points. It is the direction of the Earth's magnetic North Pole, a point in the Arctic Ocean north of Canada that moves a few tens of kilometres each year. — Oxford ATPL General Navigation, Ch. 5
In plain terms: a freely-suspended magnet points at magnetic north, not true north. The angle between the two — measured at your present position — is variation. East variation means magnetic north is to the east of true north; west variation, the other way.
Variation is a property of location, not of the aeroplane. It is printed on every aeronautical chart as isogonal lines (lines of equal variation) or sometimes a single chart-wide value. Pilots in Mumbai see roughly 0° variation; pilots in Vancouver see 16° E; pilots in Toronto see 12° W; pilots in Reykjavik see 11° W. The value is the same whether you're flying east, west, or sitting on the ground.
Compass north
The deviation of a compass is defined as the angle between the compass north and the magnetic north, caused by ferrous components in the aircraft and electrical equipment. Deviation changes with heading. — Oxford ATPL Instruments, Ch. 1
In plain terms: every aircraft has its own metal and wiring, and the compass card sits in the middle of those magnetic fields. Where the compass says north is depends on which way the aircraft is pointing. Deviation is small (usually within ±5°) and is measured for the specific airframe by swinging the compass during maintenance. The result is a deviation card mounted next to the compass, listing the correction for each heading (typically every 30°).
The conversion cascade
Going from a chart bearing to a compass reading is a two-step correction, always in the same order:
Variation = True − Magnetic
Deviation = Magnetic − Compass
Each correction is just the difference between the two headings on either side of it (with East as positive and West as negative). To go in the chart-to-compass direction, rearrange and chain the two steps — this is exactly what the TVMDC mnemonic spells out:
T − V = M − D = C
True − Variation = Magnetic − Deviation = Compass
Read left to right. Each letter of TVMDC is the next item in the chain: start at True, subtract Variation to get Magnetic, then subtract Deviation to get Compass. The minus signs are how it works when East is positive — flip them to plus signs (or use the four-line memory aid below) when applying westerly values.
The mnemonic — and it is on every ATPL exam — is TVMDC ("True Virgins Make Dull Companions"), where each letter is the next term in the chain.
For the signs, every pilot learns the four-line rule:
Variation East, Magnetic Least (Magnetic < True) Variation West, Magnetic Best (Magnetic > True) Deviation East, Compass Least (Compass < Magnetic) Deviation West, Compass Best (Compass > Magnetic)
"Least" means the next heading is smaller than the previous one; "Best" (biggest) means larger than. Apply the rule once for variation (T → M), then again for deviation (M → C). So:
- Variation 5° E → MH = TH − 5°
- Variation 5° W → MH = TH + 5°
- Deviation 2° E → CH = MH − 2°
- Deviation 2° W → CH = MH + 2°
If you treat E as positive and W as negative, both rules collapse to "subtract the signed value":
MH = TH − Variation
CH = MH − Deviation
Same direction the other way (chart-from-compass) — just add instead.
A worked example — Toronto departure
You are airborne off Toronto City (CYTZ) heading for Ottawa. The chart track is 078° T. Local variation is 12° W. Your deviation card, opposite "090 heading", reads 2° E.
Step 1 — True to magnetic.
MH = TH − Variation
= 078 − (−12) (West is negative)
= 078 + 12
= 090° M
The pilot rule "Variation West, Magnetic Best" applied: westerly variation made magnetic heading bigger than true.
Step 2 — Magnetic to compass.
You'll be flying near 090° M, so use the deviation card row for "090" — which says 2° E.
CH = MH − Deviation
= 090 − (+2) (East is positive)
= 088° C
The pilot rule "Deviation East, Compass Least" applied: easterly deviation made compass heading smaller than magnetic.
Step 3 — Putting it together.
The chart-line of 078° T turned into the compass-card reading of 088° C. The same physical flight path. Three different numbers for the same nose direction.
If wind correction adds another 5° of right drift to keep the track on 078, the heading you fly increases by 5° in each frame: 083° T → 095° M → 093° C. The wind correction goes on top of the variation/deviation cascade — never mixed into it.
Common mistakes
- Adding when you should subtract. Almost every exam wrong-answer is a sign error. Memorise either the four-line rule (Variation East, Magnetic Least / Variation West, Magnetic Best / Deviation East, Compass Least / Deviation West, Compass Best) or signed = E positive, W negative; always subtract. Pick one and never deviate from it.
- Picking the wrong row of the deviation card. Deviation is per heading, not per location. If you're flying 090, you read the 090 row — not whichever value happened to be calibrated when the compass was swung. The cell value applies to the nearest listed heading, usually rounded to the closest 30°.
- Applying variation to wind correction angle. Variation converts headings between frames of north. Wind correction converts a track into a heading in any frame. They commute (you can do them in either order) but they are not the same correction.
- Forgetting that variation drifts. The magnetic North Pole moves ~50 km a year. Charts list a date alongside each variation value along with an annual change (e.g., "12°W 2025, decreasing 4'/yr"). In high-latitude theatres a chart that's a decade out of date can be a degree or more wrong.
Why it matters
Almost every ATPL navigation question that gives you a track or a heading specifies which frame it's in (T, M, or C). Mixing them is the most common single source of marks lost in the General Navigation paper. Operationally, the FMS, GPS, and ATC use magnetic headings in most jurisdictions; the chart prints true tracks; and the wet compass on the panel is the only purely compass instrument left. You routinely have to translate between three of these in a single pre-flight, and a missed sign costs ten degrees of track every leg.