Drift and wind correction: flying the planned track

Heading, track and course: three words, three things

Navigation goes wrong when these get mixed up, so it is worth being exact about them.

• Heading is where the nose points, read off the compass or heading indicator. It is what you steer.
• Track is the path over the ground. The required track is the line on the chart; the track made good is where you have actually been.
• Course is the American word for the intended path, so an FAA textbook’s “true course” is the UK student’s “true track”. The ideas are identical.

In still air the heading and the track are the same. In any crosswind they are not, and the aeroplane moves over the ground slightly sideways, crabbing, with the nose pointed a little into wind. That is not a mistake to be fixed. It is the whole point.

What drift is, and why it never goes away on its own

The aeroplane sits in a moving mass of air. Whatever heading you fly, the whole block of air, with you in it, is carried downwind. The angle between the heading and the track that results is the drift angle: wind from the left drifts you right, wind from the right drifts you left.

Drift is an angle, so the distance off track grows steadily with distance flown. In the trainer’s leg, 36 nautical miles (NM) from Wizard Farm to Ridge bridge on a track of 060°, a 25-knot wind from 330° sits square on the left at a true airspeed (TAS) of 105 knots. Steer 060 and ignore it, and the drift is about 13°: a mile off after four miles, six miles off after 24, and eight and a half miles off abeam the destination. Nothing was flown badly. The wind was simply left out.

Try it: Open the wind and drift trainer and pick “Ignore the wind”, then “Allow for it”: the same wind, with one thing changed

The wind triangle

The wind triangle is the picture behind every heading you work out. It adds two movements together to get a third:

• The air vector: heading and TAS, where the aeroplane goes through the air.
• The wind vector: where the air itself goes, and how fast. Winds are reported by the direction they blow from, so the arrow points the other way.
• The ground vector: track and groundspeed, the result of the other two.

Before a flight you know the track you want, the TAS and the forecast wind, and you solve for the heading and groundspeed. The angle between heading and track in the solved triangle is the wind correction angle (WCA): the allowance you turn into wind so the drift carries you along the line instead of off it.

On the ground you solve the triangle on a flight computer, or a navigation app does it for you; the guide to the wind side of the E6B walks through it step by step. In the air you need something quicker.

Wind correction angle in your head: maximum drift and the clock code

The mental method has two parts. First, work out maximum drift, the drift you would get with the wind at 90° to the track:

Second, scale it by how far the wind is off the track, using the clock code. Read the angle as minutes on a clock face and take that fraction of an hour:

• 15° off the track: a quarter of maximum drift.
• 30° off: a half.
• 45° off: three quarters.
• 60° or more off: all of it.

On the trainer’s leg the wind is 90° off, so the whole 14° applies. Wind from the left means turn left, and 060 − 14 gives a heading of 046, the heading the “Allow for it” scenario flies.

Try one where the wind is not square on. Track 060°, wind 030° at 20 knots, TAS 100 knots. Maximum drift is 60 × 20 ÷ 100 = 12°. The wind is 30° off the track, so half: 6°, from the left, so steer 054. The exact answer is a little under 6°, which is as close as anyone can steer.

Headwind and tailwind component

The same clock gives the part of the wind along the track, which sets your groundspeed and so your timings. This time apply it to how far the wind is from being straight across (90° minus its angle off the nose or tail). In the last example the wind is 30° off the nose, so 60° from straight across: the clock says all 20 knots is headwind, for a groundspeed of about 80 knots. The true figure is nearer 17 knots, so the clock errs on the cautious side. With the wind square on, as on the trainer’s leg, there is no headwind at all, but the crab still costs a few knots: about 102 over the ground from 105 through the air.

Set your own wind below and see how close the clock code gets to the exact answer.

The 1 in 60 rule: working out how far off you are

Forecast winds are never quite right, so however carefully you plan you will find yourself off track. The 1 in 60 rule says that one degree of angle opens up a gap of about one mile for every sixty miles flown. Turn that round, and two distances you can estimate from the chart give you an angle:

• Track error = 60 × distance off track ÷ distance gone. This is how far your track made good differs from the one you wanted.
• Closing angle = 60 × distance off track ÷ distance still to go. This is the extra turn that brings you back to the line exactly at the destination.

Track error plus closing angle, turned towards the line, takes you straight to the destination. The rule rests on a small-angle approximation and gets less accurate as the angles grow, which is one more reason to check your position often.

Getting back on track: a worked example

Take the “Notice, and double the track error” scenario. Same leg, same wind, no allowance made at the start. At 18 NM along, a feature on the ground puts you 4.3 NM right of track.

1. Track error: 60 × 4.3 ÷ 18 is about 14°. You have been drifting about 14° right of where you meant to go.
2. Double it and turn towards the track: 29° left, onto 031. Turning only 14° would stop the gap growing but never close it.
3. Hold that heading. The gap closes at the rate it opened, so you rejoin the track after roughly the same distance again: in the trainer, at about 34 NM.
4. Back on the line, take half of the correction off (turn 14° right, onto 046) and hold the rest. That remaining 14° is the drift allowance you should have had from the start.

Doubling needs no idea of how far there is still to go, which is why it is so widely taught for use in the air. It only works if you spot the error before halfway: any later and you would rejoin the line beyond the destination, so use track error plus closing angle instead. Here the fix happened at the halfway point, so the closing angle is also 60 × 4.3 ÷ 18, about 14°, and both methods give the same 29° turn.

Fix earlier and the numbers shrink: in “Fix early, turn small” the error is spotted at 9 NM, just over 2 NM off. Track error is again about 14° and the closing angle, with 27 NM to go, about 5°, so a turn of about 19° left takes you straight to Ridge bridge with no second turn. The rule is an approximation, so you arrive within half a mile rather than exactly overhead, which is as close as anyone needs.

Try it: Pick “Notice, and double the track error” in the wind and drift trainer, then switch the correction to “Closing angle” or “Track error only” and compare

What the wind and drift trainer shows

The trainer draws the 36 NM leg from Wizard Farm to Ridge bridge with the planned track as a straight line and the aeroplane’s real path as a coloured line beside it, plus a dashed circle around the destination showing roughly how far off you could still expect to see it. As it flies, the readouts give heading, track made good, drift, distance off track and distance gone. Once you are off the line it works the 1 in 60 sums for you: track error, closing angle and the turn to regain.

The five lessons use the same 330/25 wind at 105 knots so that only one thing changes each time: “Ignore the wind”, “Allow for it”, “Notice, and double the track error”, “Fix early, turn small” and “The half-correction”. You can move the wind direction, wind speed and TAS while it flies, set where the error is noticed (6 to 30 NM) and choose the correction method. Free play hands you the heading to steer, with a flight computer beside it for working the heading out first. Tracks in the trainer are true; on a real flight log you apply variation to get the magnetic heading you actually steer.