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

I want to walk you through the two-drift wind method on the navigation computer. This builds on what we've already covered with three-drift winds, but now we're looking at a practical situation where you only have two drift readings to work with. Let me start with when you'd actually use this. The three-drift wind method is really only necessary for particular applications like bush flying and maritime flying. But for flying on airways in the UK, in an aircraft that's only equipped with the minimum navaids for airways — meaning no RNAV, no GPS, no INS, no FMS — you can get away with just two drift readings. The key requirement is that your two headings need to be at nearly right angles to each other, and certainly in excess of 70 degrees apart. Let me give you a concrete example. Imagine you're flying an airways route from Brecon to Ottringham, going via Wallasey. The planned track from Brecon to Wallasey is 008 degrees magnetic along airway A25. Then from Wallasey to Ottringham, the track is 085 degrees magnetic along airway B1. Here's how the pilot works it in practice. As you're homing into Wallasey, you apply your planned drift to the VOR track. If your drift is wrong, the VOR radial will start to change. So you get back onto the correct centre line, then modify your drift based on how the radial changed. After a short period of trial and error, you establish the correct drift to hold the radial. Then you arrive at Wallasey and turn onto the planned heading to hold the outbound track along B1. Again, if your planned drift wasn't correct, you go through another short series of trials and corrections until you find the heading that holds the VOR radial consistently. Now you have drifts for two headings. At this point, you can use the navigation computer to find the wind point from the cross-cut of those two drifts on their appropriate headings. You're essentially drawing two drift lines and seeing where they intersect — that intersection gives you your wind velocity. Now, there is one problem with a two-drift wind. There's no confirmatory third cut to give you confidence in the other two. With three drifts, if all three lines intersect at the same point, you know your readings are consistent. With only two, you don't have that check. But it should still be possible to assess the drift reasonably accurately when you're flying along a VOR centre line. Now, the excerpt moves into practice questions. These are the book's practice questions — let's try them one at a time.