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

I want to walk you through a practical application of the Navigation Computer's wind face that goes beyond the basic triangle-of-velocities exercise. In the previous chapter, we covered how, if you know your heading, true airspeed (TAS), and a forecast wind velocity (W/V), you can use the wind face to calculate the resulting track and ground speed. That's the fundamental operation, but it's only one of several. Let me lay out the three most common operations the wind face supports. I'll read them from the table, then we'll talk through each. First operation: finding track and ground speed. You start with heading, TAS, and W/V. When do you use this? When you are being held on a mandatory heading — for example, because air traffic control has given you a radar vector that takes you off your planned route. Second operation: finding heading and ground speed. Here you start with track, TAS, and W/V. This is used during flight planning, or when you need to re-calculate a heading in flight because you have a new track to fly or a new wind has been encountered. Third operation: finding the wind velocity (W/V). For this you start with heading, TAS, track, and ground speed. This is wind finding — you're working backwards from what the aircraft actually did to determine the wind that was acting on it. The found wind will be more recent and more local than a forecast W/V, which is valuable for updating your navigation. Now, the first operation — finding track from heading — might seem like the simplest and least practical. You might think it's only used to explain the triangle of velocities concept. But I want to show you a real-world scenario where it becomes essential. Imagine you are flying along a pre-planned track, say airway R123 between Brookman's Park and Clacton. You've already used your Navigation Computer to calculate the heading needed to maintain that track — we haven't covered that calculation yet, but we will shortly. You still have the forecast W/V marked up on your computer. Then, because of a conflict with other traffic, ATC gives you a mandatory radar heading of 030°T. This takes you temporarily off the airway at time 1003. You make a note of your VOR/DME position at that moment. After a while, the conflict is resolved. At time 1009, ATC tells you, "You are cleared own navigation to Clacton." But at that exact moment, your VOR receiver unlocks — you lose your VOR signal. You have no area navigation equipment. So how are you going to find the required track to Clacton? This is where the first operation becomes critical. You have been flying on a known mandatory heading of 030°T for six minutes. You know your TAS and you still have the forecast W/V marked on your computer. By using the wind face to find track from heading, you can determine the actual track you've been making good while on that radar heading. You also know your position at time 1003 from your VOR/DME fix. From that fix, you can plot the track you've actually flown for six minutes, and that gives you a new, known position at time 1009 — even without the VOR. From that position, you can then work out the track required to Clacton. So the seemingly simple "find track from heading" operation has a very practical use: it lets you reconstruct your position when your navigation aids fail, as long as you have a known starting point and a mandatory heading to work from.