The Navigation Computer - Multi-drift Winds and Wind Components — Page 173, Lesson 148

I want to walk you through a really practical technique now — finding the wind without ever measuring your ground speed. This is called the multi-drift wind, and it's one of two further uses of the wind face on the Navigation Computer. The other is working out head/tailwind and crosswind components, which we'll get to shortly. Let me set the scene first. Experienced pilots — bush pilots and maritime patrol pilots especially — can often judge their drift just by looking at the angle between their heading and the way a ground object appears to track underneath them at low altitude. That's a skill built up over years. But here's the problem: judging ground speed quickly is genuinely hard. You can time the rate of DME distance change over a short period, but that only works when you're tracking directly towards or away from the DME, which isn't always the case. The alternative is a stopwatch over a short precalculated distance, but that needs detailed preflight preparation and usually isn't practical. So the multi-drift wind technique lets you find the wind velocity — the W/V — using drift only. The catch is you have to fly on two, or preferably three, headings that cut at a large angle to each other. Now, the best results come from a three-heading check, with the headings at 60° to each other. Let me give you a concrete example. Say you're flying an initial heading of 030° at a TAS of 120 knots, and you decide you need an update of the W/V. You assess your drift on your present heading, then turn 60° left and make another drift assessment, then turn 120° right for a third drift assessment. Then you return to your original heading. Here's the summary you'd note down. On heading 030°, you get a drift of 10°P — that's 10 degrees port, so the wind is pushing you to the left. On heading 330°, you get 5°P. And on heading 090°, you get 5°P. Now here's where the wind face of the Navigation Computer comes in. First, rotate the heading to 030° and make sure the TAS of 120 knots is under the centre dot. Then, using a straight-edge, draw a line along the whole length of the 10°P drift line. That's the first step of the plotting. Let me show you what's happening on the wind face. So you've drawn your first drift line. The next step is to rotate the heading to your second reading of 330°, still keeping 120 knots TAS under the centre dot, and draw in the second drift line along the 5°P line. Then you rotate to the third heading of 090°, again with 120 knots TAS, and draw the third drift line along the 5°P line. Now here's the key idea. If the three lines all coincide at a single point, that point gives you the wind. But if they don't coincide, the three lines will form a small triangle by their intersection — and that triangle is your clue to the accuracy of your drift assessments. The smaller the triangle, the more confident you can be in the wind you derive from it. So the whole technique rests on this: you're using drift readings from three headings that cut at large angles to each other, plotting each drift line on the wind face, and where those lines converge — or the small triangle they form — tells you the wind velocity without ever needing a ground speed measurement.