The Navigation Computer - Calculation of Heading and Wind Finding — Page 144, Lesson 137

Let’s pick this up right where the procedure gets interesting — the balancing act. We’ve already plotted the wind vector, so now we’re going to use the navigation computer to find the true heading and ground speed for a given desired track, and we’re going to do it by balancing the drift. Here’s the situation we’re working with. Our true airspeed, TAS, is 76 knots. Our desired track is 230°. And the forecast wind velocity, W/V, is 110/40 — that means the wind is coming from 110° true at 40 knots. We start in the usual way. We plot the W/V of 110/40 on the computer — that puts the wind point on the disc. Then we put the blue centre circle onto the TAS point of 76 knots. That sets our airspeed on the slide. Now we bring the desired track of 230° up to the top, against the index. At this stage, we’re not yet flying a heading — we’re just looking at the track we want to make good. Now here’s the key observation. Look under the wind point. The drift is 20° starboard. That means the wind is pushing us to the right of our track by 20°. So to counteract that, we rotate the disc 20° clockwise. That brings the desired track of 230° against the 20° mark on the DRIFT STBD scale — the starboard drift scale. But here’s the subtlety. When we rotate the disc, the wind point moves relative to the track, so the drift changes. Let’s check it again. Now the drift under the wind point has increased to 25° starboard. So we rotate the disc a further 5° clockwise, bringing 230° against 25° on the DRIFT STBD scale. Check the drift again. It’s now 27° starboard. So we rotate the disc a further 2° clockwise, bringing track 230° against 27° on the DRIFT STBD scale. Now check the wind point once more. The drift is still 27° starboard — it’s stabilised. That’s the balance. The drift we’ve set on the scale matches the drift the wind is actually producing. That’s why this is called balancing the drift. One more check before we read off the answers. Make sure that 76 is still exactly under the centre dot — the blue circle. If the slide has shifted, the whole solution is off, so we confirm the TAS is still set correctly. Now we can read off the results. Against the TRUE HEADING index, we read the true heading — that’s 203°. And from under the wind point, we read the ground speed — that’s 87 knots. So the full solution: to make good a track of 230° with a TAS of 76 knots in a wind of 110/40, we fly a true heading of 203°, and we’ll make good a ground speed of 87 knots. The whole point of the balancing procedure is that the drift we set on the scale must equal the drift the wind actually produces — and we iterate, rotating the disc a little at a time, until those two agree. Once they do, the heading and ground speed we read off are correct.