How to use the E6B wind side

What the wind side of an E6B actually does

The E6B (and its UK cousins, the CRP-type navigation computers) solves the wind triangle: the course you want over the ground, your true airspeed (TAS) and the wind make three sides, and the computer finds the heading to steer and the groundspeed you will make. It does this with no arithmetic at all, just three parts that move.

• The frame, with the TRUE INDEX at the top. It never moves.
• The transparent disc, with a compass ring round its edge and a small hole at its centre called the grommet. The disc turns, and anything you pencil on it turns with it.
• The slide underneath, printed with speed arcs and drift lines. It moves up and down behind the disc.

The order you move them in is the whole method. Get the order right and the answer simply appears; get it wrong and every number that follows is wrong with it.

How to use the E6B wind side, step by step

This is the “wind up” method, the usual one in American E6B teaching and the one the trainer on this site uses (many UK courses teach “wind down” instead; see below). It needs four numbers before you touch the computer: the true course measured off the chart, the true airspeed, and the forecast wind direction and speed.

1. Set the slide to start. Put the grommet on a round speed near the middle of the window, such as 100 knots. This does not affect the answer; it only leaves room above the grommet for the wind mark.
2. Wind direction under the true index. Turn the disc until the direction the wind is blowing from sits under the TRUE INDEX.
3. Mark the wind. Count up the centre line from the grommet by the wind speed and make a pencil dot. It is on the disc, so from now on it turns with the disc.
4. True course under the true index. Turn the disc until the course you want to make good is at the top. Watch the dot swing round: it ends up on the side the wind is coming from.
5. Slide until the dot is on the true airspeed. Leave the disc alone and move only the slide, until the dot lies on the arc for your TAS.
6. Read the answer. The arc under the grommet is the groundspeed. Count the drift lines from the centre line out to the dot: that is the wind correction angle. Dot to the left, steer left of the course; dot to the right, steer right.

A worked example: 060° true, 105 knots, wind 300/25

This is the first problem in the trainer, “Out to the coast”. The line on the chart measures 060° true, the aeroplane makes 105 knots TAS at the cruising level, and the forecast wind there is 300° at 25 knots.

The wind is from behind and to the left, so expect a heading a little left of 060 and a groundspeed above 105. Now work it:

• Grommet on 100. Turn the disc to 300 under the true index.
• Pencil a dot 25 knots up the centre line from the grommet.
• Turn the disc to 060. The dot swings round to below and left of the grommet: behind you, from the left.
• Slide until the dot sits on the 105 arc. The grommet now reads about 115 knots, and the dot lies about 12° left of the centre line.
• Heading: 060 − 12 = 048° true. Groundspeed: 115 knots.

The exact answer is 048° and 115 knots, and a well-set computer gets within a degree or two of it. The return leg on 240° in the same wind (“The leg back” in the trainer) swaps everything round: 12° right, heading 252° true, and a groundspeed of only about 90 knots. One wind, two very different legs.

Try it: Open the E6B trainer, pick “Out to the coast” and press Show me how to watch the disc and slide move

True course, true wind, then magnetic heading

Everything on the wind face is in degrees true: the course measured from the chart against a meridian, the forecast wind, and so the heading that comes out. Only once you have the true heading do you apply variation to get the magnetic heading you will fly.

The old rhyme does the job: variation west, magnetic best (add it); variation east, magnetic least (take it off). With 2° west, the 048° true from the example becomes 050° magnetic. Variation across the UK is now only a degree or two, which makes it easy to forget and easy to apply the wrong way; in much of the United States it is ten degrees or more, and the wrong way round costs twice that. Compass deviation, from the card in the aeroplane, comes last.

The trainer’s flight log does this for a three-leg route: one turn of the wind face per leg, the variation applied, and the groundspeed turned into leg times and fuel. For why the correction is the size it is, and a mental method to check the computer, see the wind correction angle guide.

Wind up or wind down?

There are two ways to use the wind face. The method above marks the dot up from the grommet, then slides so the dot sits on the TAS and the grommet shows the groundspeed. It is exact in one pass.

The other, “wind down”, puts the TAS under the grommet from the start and marks the wind below the centre, the way the wind blows rather than where it comes from. It is the method many UK schools and ATPL theory courses teach on their navigation computers. It gets the same answer by a different route, usually with a second small adjustment: you apply the drift to find a heading, read the drift again, and repeat until it stops changing. Both are legitimate; the trouble starts when a student learns one from a book and the other from an instructor and ends up with half of each. Follow the method your instructor uses and do not switch mid-sum.

Common E6B wind side mistakes

• Using a magnetic wind. Forecast winds aloft, such as the UK’s spot wind charts, and the coded METAR and TAF are in degrees true. The surface wind you hear from ATC or on the ATIS (automatic terminal information service) is magnetic, so that it lines up with the runway numbers. Only true winds go on the wind face.
• Putting the direction the wind blows towards under the index. A wind of 300 blows from 300. Set 300, not 120.
• Setting a magnetic course. Measure the course against a meridian on the chart, not the nearest compass rose, and keep it true until the very end.
• Using indicated airspeed. Find the TAS first on the other side of the computer (the trainer’s back face does this from pressure altitude and temperature, with the rough rule of adding 2% per thousand feet beside it).
• Moving the disc during the slide. In step five only the slide moves. Nudge the disc and the course is no longer under the index.
• Applying the drift the wrong way. You steer into wind. Dot on the left, the wind is from the left, so the heading is less than the course.

Try it: Then fly it: open the wind and drift trainer, press Open the E6B to work the leg, and see whether your heading holds the track