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

I want to walk you through the multi-drift wind technique on the navigation computer. This is a method we use when we have multiple drift readings from different headings, and we want to determine the wind velocity — the wind direction and speed — without needing a separate wind calculation for each leg. Let me set the scene. We start with a known true airspeed — in this example, 120 knots — set under the centre dot on the computer's wind face. We take our first heading, and we plot the drift we observed on that heading. In the example, the first drift is 10°P — that's 10 degrees of port drift, meaning the wind is pushing the aircraft to the left. Now, we rotate the heading scale to the second reading, which is 330°. We make sure we still have 120 knots TAS under the centre dot — that's critical, we don't change the TAS setting. Then we plot the drift on this second heading. In the example, it's 5°P. But here's a key point: we don't draw a full line across the whole wind face this time. We just make a small cross-cut — about one centimetre either side of the drift line. Just a short mark. Then we rotate to the third heading, 090°. Again, 120 knots TAS under the centre dot. We plot the drift on this heading. In the example, it's also 5°P. Now, if all three drift assessments have been made correctly, the third line should lie over or very near the other two. But if they don't all intersect at the same point, the three lines will form a small triangle where they cross. That triangle has a specific name — it's called the cocked hat. The size of that cocked hat tells us something important. It's an indication of the accuracy of our drift assessments. If all three lines coincide at a single point, it's probable that every drift assessment was correct. A small cocked hat means the probable error is small. A larger cocked hat means that probably one or more of the drifts is in error to some extent. Assuming we have no cocked hat, or only a small one — in which case we take the centre of the triangle as our wind point — we have established the wind velocity by taking the intersection of the drift lines. That intersection point is called the wind point. To read off the wind speed and direction, we rotate the wind face so that the wind point is at the 6 o'clock position — straight down from the centre. Then we read the wind direction from the heading scale at the top, and the wind speed from the concentric speed rings. In this example, the wind velocity is 115/21 — that's 115 degrees true direction, and 21 knots wind speed. Let me show you the diagrams that illustrate this process step by step. So to summarise the key points: we take three drift readings on three different headings, all at the same TAS. We plot short drift marks for the second and third headings. The three lines should ideally meet at one point. If they form a triangle — a cocked hat — its size tells us the accuracy of our drift assessments. We take the intersection as the wind point, rotate it to 6 o'clock, and read off the wind direction and speed.