Plotting — Page 491, Lesson 489

Let's pick this up right where the plotting gets interesting. We're on a Lambert chart now, and I want to walk you through why plotting bearings here is actually simpler than on a Mercator, and where the one remaining trap hides. On a Lambert chart, great circles — and remember, radio waves travel along great circles — are drawn as straight or near-straight lines. That's the big win. Because of that, there is no conversion angle to apply. On a Mercator you'd have to fiddle with that correction; here you don't. But there may be a correction for chart convergence to apply. And whether you need it depends entirely on one thing: where the bearing was measured — at the ground station or at the aircraft. Let's sort out who measures what. The bearing measurement process for VDF — that's VHF direction finding, giving you QDMs and QTEs — and for VOR, takes place at the ground station. Now, with VOR it can look like the aircraft is doing the measuring, because the bearing is displayed in the cockpit and decoded there. But it has not been measured in the aircraft. Here's the key: the difference between the VOR reference phase and the bearing phase is encoded at the moment of transmission, on that particular bearing, at the ground station. All the aircraft's VOR receiver does is de-modulate that encoded signal and display it. It does not actually measure a bearing. That's in contrast to ADF, which does measure in the aircraft. So for bearings measured at the ground station — VDF or VOR — the plotting process is simple. You correct for variation at the point where it has been added, which is the ground station, and you plot the true bearing from the ground station. That gives you the correct great circle track at the ground station. Now, the straight-line bearing will change direction with respect to north as it crosses meridians and encounters convergence, so it won't be the same great circle track direction at the aircraft. But that doesn't matter, because you're not plotting it from the aircraft. It is the correct radio wave path. Now the complicated case: bearings measured at the aircraft — that's ADF with an NDB, and AWR, airborne weather radar. The complication is convergence between the meridian where the bearing is measured, which is the aircraft, and the meridian from where it will be plotted, which is the NDB. Let me walk you through the worked example in Figure 28.4. An aircraft measures a bearing of an NDB of 245°(R) — that's relative — while on a heading of 025°(T), true. We need to plot the position line on a Lambert chart. First step: add the relative bearing of 245° to the true heading of 025°. That gives you the true great circle bearing of the NDB from the aircraft, which is 270°. Now here's the trap. If you take the reciprocal of that, 090°, and plot it from the meridian of the NDB, the position line will not pass through the aircraft position. Why? Because the meridian through the aircraft is not parallel to the meridian through the NDB. That's chart convergence in action. That's the false position line — Figure 28.4(a) shows it missing the aircraft. The correct approach, shown in Figure 28.4(b), accounts for that convergence so the position line actually passes through the aircraft. So the rule of thumb: when the bearing is measured at the aircraft, you can't just plot the reciprocal from the station's meridian — you must correct for the convergence between the two meridians. That's the whole distinction in a nutshell: ground-station-measured bearings plot straight from the station with just a variation correction; aircraft-measured bearings need the convergence correction because the meridians aren't parallel.