Let’s pick this up right where the technique gets practical — Page 208, Lesson 187

Let’s pick this up right where the technique gets practical. We’ve already got the two methods — the Closing Angle guidelines and the distance-to-go markers — and now I want to show you how to combine them into the recommended technique, because that’s what you’ll actually use in the cockpit. Here’s the idea. You mark up your chart with the Closing Angle guidelines and the distance-to-go markers. Then, when you get a fix — a pinpoint — at a known fraction of the way along your track, you alter heading by a multiple of the Closing Angle. The multiple depends on how far along you are. If your fix is a quarter of the way along track, you alter heading by 4 times the Closing Angle. If it’s a third of the way, alter by 3 times the Closing Angle. If it’s halfway, alter by 2 times the Closing Angle. And so on — the pattern is that the multiplier is the reciprocal of the fraction travelled. A quarter of the way means you’ve got three quarters left, so you need four times the correction to converge. Halfway means half left, so twice the correction. That’s the whole logic of the combination. Now let’s look at what happens when you apply the system a second time, because you won’t always get it right on the first correction. Look at Figure 11.16. You’re flying from A toward B, and you get a pinpoint at C, which is a quarter of the way along. You apply the rule — 4 times the Closing Angle — and you alter heading. Now you’re on a new track, C to B. But you get another pinpoint at D, and D is off this new track. So you apply the system again, but this time with respect to your ‘new’ track C-B. Suppose the 10° lines show that angle DBC — that’s the closing angle with respect to your new desired track C-B — is 2°. And suppose your new pinpoint D is about halfway between C and B. Then your new alteration, to starboard, will be 2 divided by 1, times 2°, which is 4°. The fraction travelled is half, so the multiplier is 2 over 1, and 2 times 2° gives you 4° to starboard. Now here’s the critical point to bear in mind. The closing angle must be calculated from the ‘new’ track, C-B, which will not be drawn on your chart. Your 10° guidelines were drawn with respect to the original track, A-B. So if you use them, you must mentally allow for the first closing angle — the one you already corrected for. And similarly, you must estimate the proportion you have travelled of the ‘new’ distance C-B, not the old distance A-B. That’s the trap — people measure the fraction against the original track and get the multiplier wrong. Let me put that in plain terms. When you re-apply the rule after a second pinpoint, everything is measured relative to the track you are now actually flying, C-B. The guidelines on your chart are still drawn for the original track, so you have to shift your mental reference. And the distance fraction — quarter, third, half — is of the remaining distance, not the total original distance. Now, before we move on, I want to make sure you can actually do the arithmetic, because the questions at the end of this section test exactly that. Let me walk you through the first one. If an aircraft is 3° off required track at a range of 120 NM, how far in nautical miles is it off track? This is the 1 in 60 rule in reverse. The rule says that at 60 NM, 1° of track error equals 1 NM off track. So at 120 NM, which is twice 60, 1° equals 2 NM. Three degrees, then, is 3 times 2, which is 6 NM off track. That’s the direct application. The second question flips it. If an aircraft is 2 miles off required track at a range of 40 NM, what is the track error? Here, at 40 NM, 1 NM off track corresponds to 1.5° — because 60 divided by 40 is 1.5. Two miles off track, then, is 2 times 1.5, which is 3° of track error. The third question is the classic multi-part one. You leave A to fly to B, 95 NM away. After flying 35 NM, you pinpoint your position, and you’re 7 NM right of track. Part (a) asks for the track error. At 35 NM, 1 NM off track is 60 divided by 35, which is about 1.71°. Seven NM off track is 7 times that, which is about 12° of track error. Part (b) asks what alteration of heading is required to fly direct to B. You’ve flown 35 of 95 NM, so you have 60 NM left. The closing angle is the track error times the ratio of distance travelled to distance remaining — 12° times 35 over 60, which is 7°. So you alter heading by 7° to the left, since you’re right of track. Part (c) is the trap question — what is the drift? Drift is the angle between heading and track, and you don’t have enough information to compute it from the data given. The track error and closing angle are about your position relative to the planned track, not about the wind. So the answer is that you cannot determine drift from this information. The fourth question sets up a scenario — flying from Oxford to Cambridge, planned track 074°(M), distance 70 NM, heading 065°(M). After 30 NM, you pinpoint overhead Cranfield, 4 NM left of planned track. That’s the setup for a series of sub-questions, but the key numbers are there: the planned track is 074° magnetic, the heading is 065° magnetic, so there’s an initial drift of 9° left. And you’re 4 NM left of track at 30 NM. Now, these are the book’s practice questions — let’s try them one at a time.