The Navigation Computer - Calculation of Heading and Wind Finding — Page 144, Lesson 130

Let’s pick this up right where the wind face left off. In the last chapter, we used the wind face to find track and ground speed from a known heading, TAS, and forecast wind. Now I want to show you that the wind face is far more versatile than that — it’s a tool with several distinct jobs, and the book tabulates the three most common ones. Here’s the first row of that table. To find track, you start from heading, TAS, and W/V — that’s the wind velocity, the forecast wind. And you use this when you are held on a mandatory heading. That’s the situation we just described: ATC tells you to fly a specific heading, and you want to know what track that heading will actually produce over the ground. The second row flips it. To find heading, you start from track, TAS, and W/V. This is the one you use in flight planning, or when you’re re-calculating a heading in flight because you’ve been given a new track or a new wind. This is the operation we haven’t covered yet — calculating the heading you need to fly to maintain a desired track. That’s coming shortly. The third row is the one I want to focus on now. To find the W/V — the wind velocity — you start from heading, TAS, track, and ground speed. And this is called wind finding. The key point here is that the wind you find this way is more recent and more local than a forecast W/V. A forecast wind is a prediction made hours ago for a broad area; the wind you derive from your actual flight is what the air is really doing right where you are. Now, you might have thought that finding track from heading — the first operation — is just a teaching exercise, the simplest way to explain the Triangle of Velocities, with no real-world use. The book explicitly tells you that’s not true. Let me walk you through the practical scenario it gives. Suppose you’re on a pre-planned track — say, airway R123 between Brookman’s Park and Clacton. You’ve already used your Navigation Computer to calculate the desired heading to maintain that track, and you still have the forecast W/V marked up on the computer. Then, because of a conflict of tracks with other traffic, ATC gives you a mandatory radar heading of 030°T — that’s 030 degrees true. This takes you temporarily off the airway at time 1003. You make a note of your VOR/DME position at that moment. That’s Figure 8.1 — you can see the aircraft being vectored off the airway on that mandatory radar heading. Now, after a while, the conflict is resolved. At 1009, ATC informs you: “You are cleared own navigation to Clacton.” And at just that moment, the VOR unlocks — you lose your VOR/DME position information. You have no area navigation equipment. So the question is: how are you going to find the required track to Clacton? Here’s the beauty of it. While you were on that mandatory radar heading of 030°T, you knew your heading, you knew your TAS, and you had a forecast W/V marked up. So you could use the wind face to find your track and ground speed during that radar vector. And you noted your VOR/DME position at 1003. So you know exactly where you were when you left the airway, and you know the track and ground speed you made good while on the radar heading. That means you can plot where you are at 1009 — even without the VOR — because you know how far you’ve travelled along that track in those six minutes. And that’s the practical application of finding track from heading: it lets you maintain a precise position plot while ATC has you on a mandatory heading, so that when you’re cleared back to own navigation — even if your VOR fails — you still know where you are and can work out the track you need to fly to get to Clacton. That’s why the simplest operation on the wind face is genuinely useful in the real world.