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

I want to walk you through a technique called balancing the drift on the navigation computer. This is how we find the true heading we need to steer and the ground speed we'll actually make good, given a forecast wind and a desired track. Let me set up the situation we're working with. Our true airspeed, or TAS, is 76 knots. The desired track — the path we want to follow over the ground — is 230°. And the forecast wind velocity, or W/V, is 110/40 — that means the wind is coming from 110° true at 40 knots. Here's how we start. First, we plot the wind velocity of 110/40 on the computer in the usual way — you'll have already learned that step. That places a wind point on the disc. Next, we put the blue centre circle — that's the central dot on the slide — onto the TAS point of 76 knots. So we slide the disc so that 76 knots on the inner scale lines up under that centre dot. Now we bring the desired track of 230° up to the top of the computer, against the index mark. At this point, we look at where the wind point is relative to the centre line. The drift under the wind point reads 20° starboard — meaning the wind is pushing us to the right, so we need to correct into wind. We rotate the disc 20° clockwise to bring the desired track of 230° against the 20° mark on the DRIFT STBD scale — that's the drift starboard scale. But now, when we look at the wind point again, the drift has increased to 25° starboard. That's because we've moved the disc, and the relative wind angle has changed. So we rotate the disc a further 5° clockwise, bringing 230° against 25° on the DRIFT STBD scale. We check the drift again — it's now 27° starboard. So we rotate another 2° clockwise, bringing track 230° against 27° on the DRIFT STBD scale. Now we check the wind point once more — the drift is still 27° starboard. That means we've balanced it: the drift reading is no longer changing as we rotate. We also check that 76 knots is still exactly under the centre dot — the blue circle — because if it's slipped, our TAS setting is wrong and we need to reset. Once everything is stable, we can read off our answers. The true heading we need to steer is read against the TRUE HEADING index — that's 203°. And the ground speed is read from under the wind point — that's 87 knots. So in summary: with a TAS of 76 knots, a desired track of 230°, and a forecast wind of 110/40, we end up steering a true heading of 203° and making a ground speed of 87 knots. The iterative process of rotating the disc until the drift stops changing is what we call balancing the drift.