The Navigation Computer - Multi-drift Winds and Wind Components — Page 173, Lesson 150

We're now into the multi-drift wind technique on the navigation computer. This is the method you use when you're in flight, you don't have a forecast wind, and you want to find the actual wind velocity — the W/V — by observing your drift on three different headings. Let me set the scene. You're flying at a constant true airspeed — in our example, 120 knots TAS. You've already taken three drift readings on three different headings: the first one gave you 10°P, the second, on a heading of 330°, gave you 5°P, and the third, on 090°, gave you 5°P again. The "P" stands for port, meaning the wind is pushing your track to the left of your heading. Now, the technique on the computer. You start with the first drift reading — 10°P — and you draw a line along the wind face at that drift angle. That's your first drift line, as in Figure 9.1. Then you rotate the heading ring to your second heading, 330°. Make sure you still have 120 knots TAS under the centre dot — that's critical, the TAS must stay fixed under the centre dot for every reading. Plot the drift on this heading, which is 5°P. But this time, you don't need to draw the line all the way across the wind face. You just make a small cross-cut — about 1 centimetre either side of the centre dot. That's Figure 9.2. Then rotate to your third heading, 090°. Again, keep 120 knots TAS under the centre dot. Plot the drift — 5°P again — and again just make a small cross-cut. Now here's the key logic. If your drift assessments have been correct, the third line should lie over, or very near, the other two. That's Figure 9.3. But what if they don't coincide? Then the three lines form a triangle by their intersection. That triangle has a specific name — it's called the "cocked hat". And the size of that triangle is your indication of the accuracy of your drift assessment. If the lines coincide, it's probable all your drift assessments were correct. A small cocked hat means the probable error is small. A larger cocked hat means probably one or more of your drifts is in error to some extent. So, assuming you have no cocked hat, or only a small one — in which case you take the centre of the triangle — you've established the W/V by taking the intersection of the drift lines. The three lines intersect at what we call the wind point. Now, to read off the wind speed and direction: you rotate the wind point round to the 6 o'clock position — that's the position on the computer where you read the wind velocity. In our example, that gives you 115/21 — meaning the wind direction is 115° and the wind speed is 21 knots. That's Figure 9.4. So the whole sequence: draw the first full drift line, then two small cross-cuts on the second and third headings, all at constant TAS under the centre dot. The intersection — or the centre of the cocked hat — is your wind point, and rotating that to the 6 o'clock position gives you the W/V.