The Heading That Was Eight Degrees Wrong
You are planning a 62 NM VFR leg from Fenwick (elevation 240 ft) to Holloway. Planned true track is 090°T, cruise TAS 105 kt, forecast wind 350°/20 kt. Working the triangle gives a true heading of 082°T and a ground speed of 108 kt. The chart isogonals read 4°W along the whole route, and the aircraft's deviation card, dated at the last compass swing, shows 2°E for a 090° magnetic heading. You write: 082 minus 4 equals 078°M, then 078 minus 2 equals 076°C, and you set 076 on the compass. After 33 minutes, about 60 NM along track, a fix checked against two ground features puts you roughly 8 NM left of the intended track. The heading indicator has read exactly the number you set since engine start and has never been checked against the compass in flight; you have made two 180° turns for cloud avoidance since departure.
1. Starting from the planned 082°T heading, derive the correct magnetic heading and compass heading. Show every step and state the rule you are applying at each one.
Read the worked answer
Step 1 – True to magnetic. Variation is applied with the rule "variation west, magnetic best": for a westerly variation the magnetic value is numerically larger than the true value.
- Magnetic heading = 082°T + 4° = 086°M
Step 2 – Magnetic to compass. Deviation is applied with the companion rule "deviation east, compass least": for easterly deviation the compass reads less than the magnetic heading, so the number set on the compass is smaller.
- Compass heading = 086°M − 2° = 084°C
So the correct setting is 084, not 076. Note two things that make this a planning error rather than a rounding error:
- Variation was subtracted when a 4°W isogonal requires it to be added. That alone puts the aircraft 8° left of the planned true heading.
- Deviation must be read off the card for the heading you will actually fly. The card entry for 090°M was used to correct an 086°M heading; that is acceptable here only because 4° away from the card point the deviation changes very little.
A quick check that costs nothing: the correct answer (084°C) and the erroneous one (076°C) differ by exactly twice the variation, which is the fingerprint of an east/west sign error.
2. Confirm whether the size of the track error seen at the fix is consistent with your arithmetic, and explain why the heading indicator gave no warning.
Read the worked answer
What heading was actually flown? Setting 076 on the compass, with 2°E deviation, means:
- Magnetic heading flown = 076 + 2 = 078°M
- True heading flown = 078 − 4 = 074°T
That is 8° left of the 082°T that the wind triangle demanded.
Does 8° explain an 8 NM offset in 60 NM? Using the 1-in-60 rule, a 1° error produces 1 NM of offset in 60 NM. Eight degrees over 60 NM therefore gives about 8 NM, which matches the fix almost exactly. The arithmetic and the observed position agree, so the error is systematic and not a bad fix.
Why the DI stayed silent. A heading indicator is a gyro with no magnetic sensing. It reads whatever it was set to on the ground and then drifts slowly through precession; two 180° turns plus 33 minutes of normal flight is easily enough for several degrees. Because you set the DI to the same wrong number you put on the compass, the instrument confirmed your error rather than exposing it. The compass is the master; the DI must be re-synchronised with it in straight and level, unaccelerated flight.
In-flight action: apply the 1-in-60 correction (roughly 8° right plus a closing angle), then verify with a second fix before trusting the new heading.
3. A colleague says: "If I had added the deviation instead of subtracting it, I would have been nearly right, so the method isn't that critical." Evaluate that claim.
Read the worked answer
Run the numbers. Adding 2° of easterly deviation to the (already wrong) 078°M gives 080°C. What would that produce?
- Magnetic heading flown = 080 + 2 = 082°M
- True heading flown = 082 − 4 = 086°T
That is 4° right of the required 082°T, or about 4 NM right of track in 60 NM.
The claim is arithmetically half true and procedurally dangerous. The variation error pushed the aircraft 8° left; the deviation error would have pushed it 4° right; the two partly cancel and leave a 4 NM offset that could easily be dismissed as wind or as a poor fix. Two mistakes were hiding each other.
Consequences:
- It is not repeatable. Fly a reciprocal leg where variation and deviation have opposite signs relative to the track, and the errors add instead of cancelling - 12° rather than 4°.
- It destroys your error budget. The wind triangle already spends most of the available tolerance; you cannot afford to give 4–8° of it to sign confusion.
- The method is the only defence. Apply true→variation→magnetic→deviation→compass as two independent, separately checked steps. "It looked about right on the map" is not evidence that a heading is right; a fix is.