We have a planned track, a heading, and a track made good — the TMG — Page 199, Lesson 178

Let’s start with the picture in front of us, because it sets up the whole chapter. We have a planned track, a heading, and a track made good — the TMG. The planned track here is 090°(T), and the heading is 100°(T). So there’s a 10° difference between the heading and the planned track. That 10° is what we call the expected drift — the drift we predicted in the flight plan, shown in red on the diagram. Now, the track made good is the path the aircraft actually follows over the ground. The difference between the planned track and the TMG is the track error angle — that’s shown in green. And the difference between the heading and the TMG is the actual drift experienced — shown in blue. Here’s the key point I want you to lock in: the track error angle is the difference between the expected drift and the actual drift. But the track error angle is not drift itself. Drift is the angular difference between heading and track. Track error angle is the angular difference between your planned track and your actual track. They’re related, but they’re not the same thing. That distinction matters, because the techniques we’re about to use treat them differently. Now, there are three basic techniques for getting back to track. The first is the Double Track Error Angle method. The second is the Track Error Angle and Closing Angle method. The third is the Combined Track Error Angle and Closing Angle Single Calculation — and that last one is simply a variation on the second, a quicker way of getting the same answer. From these three, the Oxford preferred technique is called New Track Reference, and we’ll build up to that. Let’s work through the Double Track Error Angle method with a concrete example. You’re planning to fly a track of 090°(T). You’ve done your flight plan and calculated a heading to fly. You fly that heading accurately — we assume you hold it perfectly. But after 30 miles along track, you get a pinpoint — a visual fix — and it puts you 4 miles left of track. First, calculate the track error angle. Four miles off in 30 miles along is 8°. That’s the 1 in 60 rule at work: 4 miles off over 30 miles along gives you 8° of track error. If you do nothing, you’ll continue to diverge from track at the same rate — the aircraft will keep drifting further left. So the first thing to do is stop that trend. You’re diverging by 8° to the left, so you turn 8° right. That puts you on a heading that parallels the planned track — you’re no longer diverging, but you’re still 4 miles left of where you want to be. That’s the first step of the Double Track Error Angle method: stop the divergence by turning through the track error angle. Now, the name of the method tells you the next step — you’re going to double that track error angle. But before we go there, make sure you’ve got the picture: 8° left of track after 30 miles, turn 8° right to parallel, and now you’re tracking parallel to your planned track but displaced 4 miles left. The next move is to close that 4-mile gap, and that’s where the doubling comes in. Let’s continue with that.