Total Leg Length = 78 — Page 208, Lesson 185

Let’s pick up with the practical side of the 1-in-60 rule. We’ve already worked through the theory of track error angle and closing angle, and now I want to show you a real example where the leg isn’t a nice round number. Suppose your total leg length is 78 nautical miles. You’ve flown 30 NM along that leg, so you have 48 NM to go. In the air, you won’t normally bother working out distance gone — we mark our maps with distance-to-go, not distance from the start. So you know instantly that 48 NM remains. Now, if you’re 4 NM off track with 48 NM to go, that’s a ratio of 4 in 48. Reduce that: 4 in 48 is the same as 1 in 12. And 1 in 12, scaled to 60, is 5 in 60 — so your closing angle is 5 degrees. Here’s the neat trick. Take the proportion of your distance along track to the total leg length and invert it. In this case, you’ve gone 30 out of 78, so the fraction is 30 over 78. Invert that to get 78 over 30. Now multiply your closing angle by that inverted fraction. So 5 degrees times 78 over 30 gives you 13 degrees. That 13 degrees is the total angle you need to turn to head directly for your destination — all in one calculation. This is not an approximation. There’s a geometrical proof that this method gives exactly the same answer as separately calculating the track error angle and then the closing angle and adding them together. I deliberately chose numbers that aren’t neat halves or thirds to show that the arithmetic works even for awkward results like 13 degrees. In practice, most of the time you’ll just estimate. If you’re roughly one-third of the way along your leg, you know you need to alter heading by about three times your closing angle. No calculator needed. Now, the material I’m about to cover is not examined in the EASA ATPL written exam, but it is used in your actual flying training. In the air, you don’t want to be doing 1-in-60 calculations on the fly. Instead, during preflight preparation, mark up your track on the map with 5-degree and 10-degree guidelines, exactly as shown in Figure 11.12. With those guidelines drawn, you can rapidly assess both the track error angle and the closing angle from any visual fix you get on that leg. Then you simply add the track error angle to the closing angle to know the total heading change needed to head for the destination turning point. Practically speaking, drawing both 5-degree and 10-degree lines at every point can clutter the chart too much. The recommended teaching is to draw just one guideline at each end of the track, at 10 degrees, as shown in Figure 11.13. Similarly, you don’t need to use a ruler in the air to measure distances along and across track. Instead, mark up your map with distance-to-go using 10 NM markers. Do not mark distance gone — that’s of no use to you. What you need is how many miles remain, without having to subtract in your head. Figure 11.14 shows a track of length 44 NM, with markers at 10, 20, 30, and 40 NM to go. Any time you get a visual fix, you can instantly see how far you have to go. You also have a useful 10 NM scale to help assess distances off track if needed. The recommended technique is a combination of these two methods: mark up the closing angle guidelines and the distance-to-go markers together. That way, you can read off your heading correction directly from the map without any mental arithmetic.