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

Right, let's get stuck into the navigation computer. We're going to solve a classic problem: finding the heading to fly and the ground speed, given a wind. Here's the scenario. You're in an aircraft with a planned TAS of 146 knots. TAS is True Air Speed — the actual speed of the aircraft through the air. The forecast wind is 315/20, meaning the wind is coming from 315° True at a speed of 20 knots. You need to fly a track of 040°(T). Track is the path over the ground you intend to make good. The question is: what heading do you steer, and what will your ground speed be? We start by plotting the wind on the computer in the usual way. We set the wind up as a vector: 315/20. Then we put the TAS, 146 knots, under the blue circle, and rotate the central disc to put the desired track, 040°, against the 12 o'clock index. Now, when we do that, the computer shows us a drift of 8 degrees starboard. Starboard means to the right. So what this is telling us is: if we fly a heading of 040°, we will experience 8° starboard drift and therefore fly a track of 048°. Heading is where the nose points; track is where you actually go over the ground. The wind pushes you 8° to the right. But we don't want a track of 048°. We want a track of 040°. So we need to aim off to the left to compensate. If a heading of 040° gives 8° starboard drift, we try aiming off 8° to the left — a heading of 032°. We rotate the disc to put 032° under the 12 o'clock index and check whether the drift has changed. And here's the key point: sometimes it changes, sometimes it doesn't. It depends on the geometry of your particular triangle of velocities. The triangle of velocities is the vector triangle formed by the air vector (TAS and heading), the wind vector, and the ground vector (track and ground speed). In this example, the drift is still 8° starboard. That means we've solved it. With a heading of 032° and 8° starboard drift, we get a track of 040° — exactly what we wanted. The drift didn't change when we changed heading, so the correction worked perfectly. Now the final step: establish the ground speed. Ground speed is your speed over the ground, the resultant of TAS and wind. Reading it off the computer, we see the ground speed will be 143 knots. Now, let's talk 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 our example, the drift did not change; it was 8° starboard in both cases. There are cases where a heading change gives no change of drift, as shown in Figure 8.7. But in other cases, as in Figure 8.8, the heading change with the same wind does give a change of drift — the drift in the right-hand diagram is obviously smaller. So what determines whether drift changes? 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 wind and heading. That's the geometry of the triangle of velocities at work. So the practical takeaway: you aim off by the initial drift, re-check, and if the drift has changed, you iterate until the track comes out right. That's the core of wind finding on the navigation computer.