You draw a line on the chart, measure 090°, and yet 090 is not what you'll fly. Between the two sit three corrections, each with a sign convention the exam loves to test.
The full chain
Starting from the chart and ending at the instrument, in order:
- True track (TT): the line measured on the chart, relative to true north (the meridians).
- True heading (TH): the track corrected for drift caused by wind.
- Magnetic heading (MH): the true heading corrected for magnetic variation.
- Compass heading (CH): the magnetic heading corrected for the deviation specific to the aeroplane.
The classic mnemonic for the sequence is True Virtue Makes Dull Companions: True, Variation, Magnetic, Deviation, Compass. Three corrections, three separate causes: the wind pushes you sideways, the Earth's magnetic field isn't aligned with geographic north, and your own aeroplane's metal disturbs its own compass.
Step 1: drift, the wind carries you off
The aeroplane flies inside a moving mass of air. Point the nose straight at your destination with a crosswind, and you'll arrive somewhere else. To make good the intended track, you must point the nose towards the side the wind is coming from.
The angle between the heading flown and the track actually made good is the drift, corrected for by the wind correction angle (WCA). The sign follows simple logic:
Wind from the left → correct to the left → heading is lower than track. And symmetrically for the right.
A useful rule of thumb: maximum drift, wind at right angles, is roughly 60 × wind speed ÷ true airspeed. With a 20 kt crosswind and 100 kt TAS, that's about 12°. Apply a fraction of that value for a wind that isn't exactly 90° off: half for a wind 30° off the track, three-quarters for 45°.
Wind also changes your groundspeed, hence your flight time and fuel. Work both effects together on the navigation computer, not separately.
Step 2: magnetic variation, north isn't quite north
Magnetic north and true north don't coincide. The difference between them is magnetic variation, printed on aeronautical charts as an isogonal value, E or W. It varies by region and drifts slowly year on year.
The mnemonic for the sign, going from true to magnetic, is East is least, West is best:
MH = TH − Variation (E) and MH = TH + Variation (W)
Watch the direction of travel. That rule only holds true-to-magnetic. Read a compass and want the true heading back out, and the signs flip: east variation is now added, west variation subtracted. This is exactly the reversal that catches pilots out, because the mnemonic was learned in one direction and applied blindly in the other.
Step 3: compass deviation, your instrument lies a little
The magnetic compass sits close to metal and electronics. The result is an error specific to each airframe, the compass deviation, logged on a small compass correction card in the cockpit, typically a few degrees at most.
CH = MH + deviation (deviation read from the card, sign included).
The full calculation, worked through
Track measured on the chart: TT = 090°. Wind from 360° at 20 kt, TAS 100 kt. Variation 2°E. Deviation +1°.
- Wind roughly at right angles to the track, coming from the left. Drift ≈
60 × 20 ÷ 100= 12°, corrected to the left. - TH = 090 − 12 = 078°
- MH = 078 − 2 = 076° (variation east, subtract: east is least)
- CH = 076 + 1 = 077°
| Stage | Value | Correction applied |
|---|---|---|
| True track | 090° | — |
| True heading | 078° | −12° drift (wind from the left) |
| Magnetic heading | 076° | −2° variation (east is least) |
| Compass heading | 077° | +1° deviation |
You fly 077 on the compass to make good a track of 090 on the chart: a 13° gap. Over 60 NM, ignoring that correction alone would put you more than 13 NM off your turning point.
Compass errors in manoeuvring flight
Two classic errors that have nothing to do with the calculation above, and everything to do with the instrument's physics, in the northern hemisphere:
- Acceleration error: accelerating on an easterly or westerly heading, the compass shows a turn towards north; decelerating, towards south. Mnemonic: ANDS (Accelerate North, Decelerate South).
- Turning error: rolling out of a turn through a northerly heading, the compass lags; through a southerly heading, it leads. So you roll out early on a northerly heading and late on a southerly one. Mnemonic: UNOS (Undershoot North, Overshoot South).
This is exactly why heading is flown on the directional gyro, re-aligned against the compass periodically in steady, unaccelerated flight, and not read off the compass directly.
What this changes in flight
- The line on the chart is a starting point, never a heading to fly.
- The forecast wind briefed before departure is rarely the actual wind: after the first turning point, compare track made good against track planned, and correct from what you observe, not from what you calculated.
- A heading held 5° off for 30 minutes puts you several nautical miles off track. Dead reckoning isn't fragile because the arithmetic is wrong — it's fragile because heading discipline slips.