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

I want to walk you through a practical technique called multi-drift winds. This is one of two further uses of the wind face on the Navigation Computer that can be genuinely useful for a pilot operating in a basic environment without sophisticated navigation aids. The other use we'll get to later is finding headwind/tailwind and crosswind components. Let's start with the problem that multi-drift winds solves. Experienced pilots — bush pilots and maritime patrol pilots especially — can often assess their drift with considerable accuracy just by estimating the angle between their heading and the angle a ground object appears to track underneath the aircraft at low altitude. That's a skill that develops with experience. But here's the difficulty: it's very hard to assess ground speed quickly. The only reliable way is timing the rate of DME distance change over a short period, and that only works when you're tracking directly towards or away from the DME — which may not always be the case. You could use a stopwatch over a short precalculated distance, but that requires detailed preflight preparation and isn't usually practicable in the moment. So the alternative is this: you can find a wind velocity — a W/V — using drift only. But it requires you to fly on two, or preferably three, headings that cut at a large angle to each other. That's why it's called a multi-drift wind. The best results come from a three-heading check, with the headings spaced 60° apart. Let me walk you through an example. Suppose you're flying an initial heading of 030° at a TAS of 120 knots, and you decide you need an update of the wind velocity. 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 return to your original heading. You note the drift in each case. Here's the summary: - Heading 030°: drift 10°P — that's 10 degrees port, meaning the wind is pushing you to the left. - Heading 330°: drift 5°P. - Heading 090°: drift 5°P. Now, here's how you plot this on the wind face of the Navigation Computer. 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 your first drift line. Then you rotate the heading to the 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 under the centre dot, and draw the third drift line along the 5°P line. If the three lines do not coincide at a single point, they will form a small triangle where they intersect. That triangle is your indication of how consistent your drift readings were. The centre of that triangle gives you the wind velocity. So to summarise: multi-drift winds let you find the wind using only drift measurements, without needing ground speed. You fly three headings 60° apart, note the drift on each, and plot those drift lines on the wind face of the computer. The intersection — or the centre of the triangle formed by the three lines — gives you the wind.