The Navigation Computer - Calculation of Heading and Wind Finding — Page 144, Lesson 133

Right, let's pick this up. We've just worked through the practical example where you're held on a fixed heading and you use the Navigation Computer to find your track and ground speed. Now I want to show you the reverse — and this is the one you'll use most often in real navigation and flight planning. Here's the key point. In the last example, you had a heading and you wanted to find your track. But in planning a trip, you do the opposite. You join up your turning points on a map using a ruler, and you measure the track of the straight line between them. So you already know your track. What you need to find is the heading to fly, because the wind will probably cause some drift. It's the reverse of the problem we solved earlier. So let's look at how you use the Navigation Computer, given a track, to find the heading to fly. Now, before we go further, I want to make sure you understand why this matters. The whole point of the Navigation Computer is that it lets you solve for one of three things: heading, track, or ground speed. In the ATPL examination, you'll be tested on all three of these Navigation Computer operations. You need to be completely familiar with all three, and — this is crucial — you need to be able to distinguish the use of one from the other. So when you're given a heading, you solve for track and ground speed. When you're given a track, you solve for heading and ground speed. And there's a third operation we'll get to, which is wind finding. Let me walk you through the practical example we just did, because it sets up the logic. You have a forecast wind of 315/15 — that's wind from 315 degrees at 15 knots. You should have that marked up on the wind face of your Navigation Computer from when you were flight planning before the flight. You put your TAS — let's say 160 knots — under the centre circle. You set the heading of 030 degrees, and you read off the drift and ground speed, exactly as in the last lesson. This gives you 5 degrees of starboard drift — that's drift to the right — making a track of 035 degrees true, and a ground speed of 157 knots. Now, from the position you noted at 1003, you draw a track line of 035 degrees true on your chart. You've gone 6 minutes — that's one-tenth of an hour — at 157 knots ground speed, so that's 15.7 nautical miles. That is your best estimate of position. This is known as a "dead reckoning" or "DR" position, and it's shown by a specific symbol on the chart. From that DR position, you draw a straight line to Clacton. That is your new desired track. The purpose of that example is to make the point that there are occasions where you need to be able to calculate track from a given heading — not the other, more common, way round. But the most common requirement in navigation and planning is finding the heading to fly. So that's what we're going to focus on now. Let me make sure you've got the distinction clear in your mind. When you're given the heading, the TAS, and the wind velocity, you can find your track and ground speed. That's what Chapter 7 showed, and what the first few pages of this chapter used practically when you're held on a fixed heading. But when you're planning a trip, you know your track from the map, and you need to find the heading to fly, because the wind will cause drift. That's the reverse problem. So the procedure is: you have your track measured from the map, you have your TAS, you have the forecast wind marked on the wind face. You set the track on the computer, and you read off the heading you need to fly to maintain that track, accounting for the drift the wind will cause. Now, I want to show you the actual setup on the Navigation Computer. Let me bring up the figure that shows this. That figure shows the mandatory radar heading off the airway — it's an example of a situation where you need to hold a specific heading. But the principle is the same: you're using the computer to convert between heading and track. So here's the core idea I want you to take away. The Navigation Computer has three operations you must master: finding track and ground speed from a heading; finding heading and ground speed from a track; and finding the wind. In this lesson, we're focusing on the second one — the most common in planning. You measure your track from the map, you know your TAS, you have the forecast wind marked up, and you solve for the heading to fly. Let me just recap the numbers from our example so you can see the relationship. Wind 315/15, TAS 160 knots, heading 030 degrees true. The computer gives you 5 degrees starboard drift, so your track is 035 degrees true, and your ground speed is 157 knots. That's the forward problem. The reverse problem — which is what you'll do in planning — is: you know your track is 035 degrees true, you know your TAS is 160 knots, you have the same wind, and you solve for the heading, which comes out to 030 degrees true. So the drift is the bridge between heading and track. Starboard drift means your track is greater than your heading. Port drift means your track is less than your heading. In our example, 5 degrees starboard drift took us from 030 heading to 035 track. Now, one thing I want to emphasise about the DR position. That's your best estimate of where you are based on your heading, TAS, and the forecast wind — it's not a fix from any navigation aid. It's a calculated position, and it's shown with that specific symbol on the chart. When you draw the line from the DR position to your destination, that gives you your new desired track — the track you actually need to fly to get where you're going. So let me summarise where we are. We've established that the most common navigation problem is finding the heading to fly, given a track. We've seen the forward problem — heading to track — with our worked example. And we've set up the reverse problem — track to heading — which is what we'll solve next. The key distinction is: heading is what you fly, track is where you actually go over the ground, and the wind causes the difference, which is drift. That's the foundation. Now we're ready to actually work through the procedure on the Navigation Computer for finding the heading, given the track.