First, the core situation — Page 199, Lesson 176

I want to walk you through the start of Chapter 11, which is called "Navigation Using the 1 in 60 Rule." This is a set of rapid, practical navigation techniques you'll use in the air, so let's build it up properly. First, the core situation. If you find you have strayed from track, it could be for a few reasons. You might have failed to fly the flight plan heading accurately enough, or you might have made too shallow a turn from the last track when you were over the initial turning point. But for en route navigation, we assume the most likely culprit is that the wind has changed. Now, there's a key assumption here that underpins these rapid techniques. We assume that, for small changes, a change of track of, say, 10 degrees is achieved by a change of heading of exactly 10 degrees. Now, in reality, as you recall from the Navigation Computer and the Triangle of Velocities, when you alter heading, the drift changes too. But these rapid navigation techniques deliberately assume you will alter your track by the same amount that you alter heading. So it's a simplification, and you need to remember that. Let's nail down the terminology, because this is where beginners often trip up. The track you intended to fly is called the "planned track." The track you actually make over the ground — presumably because the actual wind is different from the forecast — is called the "track made good," or TMG for short. The difference between them is called the "Track Error Angle." And here's the critical point I want to hammer home: Track Error Angle is NOT drift. This is a very common mistake among beginners. Drift is the angle between Heading and Track — and that track could be either the planned track or the TMG. So drift is about your heading versus where you're actually going. Track Error Angle, on the other hand, is purely the angular difference between where you planned to go and where you actually went over the ground. Let me make this concrete with the example from the text. Suppose that, with the forecast wind on the day, you needed to fly a true heading of 100°(T) to give you a planned track of 090°(T). So your heading is 100 degrees true, and that's supposed to produce a track of 090 degrees true. The difference there — 10 degrees — that's your expected drift. Now, if the actual wind differs from forecast, your TMG might be something else, say 082 degrees. The angle between your planned track of 090 and your TMG of 082 — that 8 degrees — that's your Track Error Angle. So you can see: drift is heading-to-track, Track Error Angle is planned-track-to-TMG. Two different things. Let me show you this visually. In Figure 11.1, we have the planned track and the track made good drawn as two lines diverging from your starting point. The angle between them is the Track Error Angle. Then in Figure 11.2, we add in the heading line, and now you can see the expected drift between heading and planned track, and the actual drift between heading and TMG, and the Track Error Angle sitting between the two tracks. That diagram really clarifies why Track Error Angle is not drift. So, to summarize where we are: we've established why you might be off track — most likely wind change — we've established the assumption that heading changes equal track changes for small angles, and we've precisely defined planned track, track made good, and Track Error Angle, and distinguished Track Error Angle from drift. That's the foundation. Next we'll move into the actual methods for getting back on track, starting with the Double Track Angle Error Method.