Now, here's an important assumption we make for these small corrections — Page 199, Lesson 176

I want to walk you through the opening of Chapter 11, which is all about Navigation Using the 1 in 60 Rule. This is a rapid mental-maths technique we use in the cockpit to correct track errors without reaching for a computer. Let's start with the core problem: Returning to Track. If you find you have strayed from your planned path, it could be for several reasons. You might have failed to fly the flight plan heading accurately enough, or perhaps you 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 reason is that the wind has changed. That's the real-world culprit we're planning for. Now, here's an important assumption we make for these small corrections. For small changes, we assume that a change of track of, say, 10 degrees is achieved by a change of heading of exactly 10 degrees. In reality, as you should recall from the Navigation Computer and the Triangle of Velocities, when you alter heading, the drift angle changes too. But these rapid navigation techniques deliberately assume you will alter your track by the same amount that you alter heading. It's a simplification that works well for small angles. Let's clarify some key terms using Figure 11.1. 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. Now, I want to be very clear about something that trips up beginners. Track Error Angle is NOT drift. Drift is the angle between your Heading and your Track — either the planned track or the TMG. If we add the heading line to Figure 11.1, it becomes clear. Let me give you a concrete example from Figure 11.2. 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° true, your planned track is 090° true. The difference between heading and planned track is your expected drift — in this case, 10° to the left. But if the actual wind is different, your TMG might be, say, 085° true. The difference between your planned track of 090° and your TMG of 085° is 5° — that's your Track Error Angle, not drift. Drift is still the angle between your heading and your TMG, which would be 15° in this case. So keep those separate: Track Error Angle is the angular difference between planned track and TMG; drift is the angular difference between heading and track. That sets the foundation. Next, we'll look at the methods for actually calculating how much to turn to get back onto your planned track.