The Navigation Computer - Calculation of Heading and Wind Finding — Page 144, Lesson 135

I want to walk you through a practical navigation computer problem — finding the heading to fly and the ground speed you'll achieve, given a planned true airspeed, a forecast wind, and a desired track. Let's set up the scenario. You're in an aircraft with a planned TAS — true airspeed — of 146 knots. The forecast wind velocity, which we write as W/V, is 315/20. That means the wind is coming from 315° true at a speed of 20 knots. You need to fly a track of 040°(T) — that's the path you want to make good over the ground, measured clockwise from true north. We start by plotting the wind on the navigation computer in the usual way. For a wind of 315/20, you'd set that up on the wind side of the computer. Then you put the TAS — 146 knots — under the blue circle on the computer's central disc. Next, you rotate the central disc so that your desired track — 040° — is against the 12 o'clock index, which is the mark at the top of the computer. When you do that, the computer shows a drift of 8 degrees to starboard — that is, to the right. So if you fly a heading of 040°, you'll experience 8° of starboard drift, and your actual path over the ground — your track — will be 048°. But you don't want a track of 048°; you want a track of 040°. So you need to adjust. Here's the key idea: if a heading of 040° gives you 8° starboard drift, you can try aiming off 8° to the left — that is, subtract 8° from your heading. Try a heading of 032°. Rotate the disc to put 032° under the 12 o'clock index, and then check whether the drift has changed. Sometimes it changes, sometimes it doesn't. It depends on the geometry of your particular triangle of velocities — that's the triangle formed by your air vector, the wind vector, and the ground vector. In this example, when you set 032° under the index, the drift is still 8° starboard. That means you've solved the problem. With a heading of 032° and 8° starboard drift, you get a track of 040° — exactly what you wanted. So your heading to fly is 032° true. Now there's one final thing: establish your ground speed. On the computer, you can read off the ground speed directly. In this case, the ground speed will be 143 knots. Let me also explain something important about changing drift. Whether the drift changes from your initial attempt at a heading — 040° in our example — to your final heading that gives the right answer — 032° — depends on the geometry of your triangle of velocities. In this example, the drift did not change; it was 8° starboard in both cases. That can happen, and there's a diagram showing a heading change with no drift change. However, in other situations, a heading change with the same wind can give a change of drift. The drift in the adjusted case might be smaller. It just depends on three things: how big the wind vector is compared to the air vector, how large the heading change is, and the initial value of the angle between the wind direction and the heading. So you always need to check the drift after each heading adjustment — you can't assume it stays the same.