Plotting — Page 491, Lesson 491

Let’s pick this up right where the plotting problem gets interesting. We’ve already seen that when you measure a bearing of an NDB, you get a relative bearing — that’s the angle measured from the aircraft’s heading, clockwise, to the station. And we’ve seen the first way to plot it: you take your true heading, add the relative bearing, and that gives you the true great circle bearing of the NDB from the aircraft. Then you plot the reciprocal — the bearing from the NDB back to the aircraft — and that’s your position line. Now here’s the subtlety I want you to really lock in. On a Lambert chart, the meridians converge — they’re not parallel. So if you draw the bearing from the NDB using the meridian that actually runs through the NDB, you have to account for that convergence. But there’s a much cleaner way, and it’s the one we always use in practical plotting. Look at Figure 28.4(b). Instead of using the NDB’s own meridian, we draw a line through the NDB that is parallel to the aircraft’s meridian. That line is called a parallel false meridian. When we measure the bearing from that parallel line, the position line passes straight through the aircraft — no convergence correction needed. Chart convergence has been automatically allowed for, simply by drawing that parallel line. That’s the method that is always used in plotting. Now, if you ever do need to plot from the true meridian of the NDB — method (b) — then the bearing to plot is the parallel-meridian bearing minus the chart convergence. Let me show you with the worked example, because the numbers make it concrete. An aircraft is flying on a heading of 330° true, and measures a bearing of 090° relative to an NDB. First, we convert: 330° plus 090° relative gives 060° true — that’s the true great circle bearing of the NDB from the aircraft. To plot the position line, we take the reciprocal, which is 240°. So for method (a), we plot 240° from a line parallel to the aircraft’s meridian drawn through the NDB. That’s Figure 28.5(a). For method (b), plotting from the meridian of the NDB, we take that 240° and add the chart convergence, which is given as 3°. So the bearing to plot from the NDB’s true meridian is 243°. Notice the sign: we add the convergence here, because the meridians converge toward the pole, and the parallel false meridian is offset from the true meridian by that convergence angle. But here’s the practical takeaway, and I want you to remember this for the exam and for the cockpit: in practice, we would not normally use method (b). We do not plot from the true meridian. For practical plotting, we always draw in that parallel false meridian, as in method (a). It’s simpler, and it automatically handles convergence. Now there’s one more critical point in this passage, and it’s about variation — where you apply it. This is a classic trap. When you measure a bearing at the aircraft, it’s the aircraft’s compass that you use to add to the relative bearing to get the true bearing. So if your heading is magnetic, and you need to correct it to true, you always use the variation at the aircraft. But if the bearing is measured at a ground station — like a VOR or VDF — then you use the variation at the ground station. So the rule is: bearings measured at ground station, variation at ground station; bearings measured at aircraft — NDB, ADF, AWR — variation at aircraft. That distinction is exactly what separates a correct plot from a position line that’s off by the variation error. So to tie it all together: we convert relative bearing to true bearing using heading and variation at the aircraft, we plot the reciprocal from a parallel false meridian through the station, and we let that parallel line handle the chart convergence for us. That’s the clean, professional way to plot an ADF bearing on a Lambert chart.