The Navigation Computer - Multi-drift Winds and Wind Components — Page 173, Lesson 152

We're moving into the two-drift wind technique now. This is the practical, everyday version of what you've just learned with three drifts. Let me set the scene. The three-drift wind is the gold standard, but it's only really necessary for special operations—bush flying and maritime flying, where you're out over featureless terrain or open water with no navaids to confirm your position. But here's the key point: if you're flying on airways in the UK, in an aircraft that only has the minimum navaids for airways flying—so no RNAV, no GPS, no INS, no FMS—then you can get away with just two drift readings. The critical constraint is the angle between your two headings. They need to be at nearly right angles to each other, and certainly in excess of 70 degrees. That's the geometric requirement that makes the wind solution reliable. If the headings are too close together, the drift lines will cross at a very shallow angle, and your wind point becomes ill-defined—a small error in drift becomes a large error in wind. Let me walk you through the practical scenario. Look at Figure 9.5: an airways route from Brecon to Ottringham, via Wallasey. The planned track from Brecon to Wallasey is 008° magnetic, along airway A25. Then from Wallasey to Ottringham, the track is 085° magnetic, along airway B1. Now, the pilot homes into Wallasey by applying his planned drift to the VOR track. Here's the feedback loop: if his drift is wrong, the VOR radial will change. He gets back onto the correct centre line, then modifies his drift depending on how the radial changed. After a short period of trial and error, he establishes the correct drift to hold the radial. That's his first drift reading. He arrives at Wallasey, turns onto the planned heading to hold the outbound track along B1. Again, if the planned drift wasn't correct, there's another short series of trials and corrections until he finds the heading that holds the VOR radial consistently. That's his second drift reading. So now he has drifts for two headings. He can use the Navigation Computer to find the wind point from the cross-cut of the drifts on those two headings. The technique is exactly what you did with three drifts—plot each drift line on the computer, and where they cross is your wind point. Now, the honest limitation: the only problem with a two-drift wind is that there's no confirmatory third cut to give confidence in the other two. With three drifts, if all three lines coincide, you know your readings are consistent. With two, you have no such check. But—and this is the practical saving grace—it should be possible to assess the drift reasonably accurately when flying along a VOR centre line, because the radial gives you a precise reference to hold. Now let's work through the practice questions. These are the book's practice questions—let's try them one at a time.