Plotting — Page 491, Lesson 489

Let's get straight into the heart of this. We're plotting bearings on a Lambert chart, and the first thing I want you to understand is why this is actually easier than on a Mercator chart. 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 key advantage. Because of that, there is no conversion angle to apply. On a Mercator, you'd have to fiddle with that conversion angle because great circles curve. Here, you don't. But — and this is the catch — there may be a correction for chart convergence to apply. Whether you apply it depends entirely on one thing: where the bearing is actually measured, at the ground station or at the aircraft. Let's split the world into those two cases. First, bearings measured at the ground station. That covers VDF — which gives you QDMs and QTEs — and VOR. Now, here's a subtle point about VOR that trips people up. It may look like the bearing is measured in the aircraft, because the display is in the cockpit and the signal is decoded there. But it is not measured there. 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 demodulate that encoded signal and display it. It does not actually measure a bearing. That's in direct contrast to ADF, which does measure in the aircraft. So for ground-station-measured bearings — VDF or VOR — the plotting process is simple. You correct for variation at the point where it was 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, because of 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. You're plotting it from the ground station, and it is the correct radio wave path. Now the second case, and this is where it gets complicated: bearings measured at the aircraft. That covers ADF, NDB, and AWR — airborne weather radar. The complication is convergence between the meridian where the bearing is measured, which is at the aircraft, and the meridian from where it will be plotted, which is at 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°. So the NDB lies on a true bearing of 270° from the aircraft. Now, here's the trap. If you take the reciprocal — 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. They converge. That's the chart convergence at work. If you plot that 090° from the NDB's meridian, you get what's labelled the FALSE P.L. — the false position line. It misses the aircraft entirely. The correct approach is shown in Figure 28.4(b). You plot the true great circle bearing of 270° from the aircraft's meridian, and that gives you the CORRECT P.L. — the correct position line that actually passes through the aircraft. So the correction for convergence is applied by plotting from the correct meridian — the one at the measurement point, the aircraft — not blindly from the NDB. So the rule of thumb to carry with you: if the bearing was measured at the ground station, plot it from the ground station, and convergence doesn't bite you. If it was measured at the aircraft — ADF, NDB, or AWR — you must account for the convergence between the aircraft's meridian and the NDB's meridian, and plot from the correct meridian to get the true position line.