Lambert’s Conformal Chart - 2 — Page 361, Lesson 316

Let’s pick this up right where the plotting problem gets interesting. We’ve already seen that on a Lambert chart, meridians converge — they’re not parallel. That convergence is exactly what makes plotting a bearing measured in the aircraft different from plotting one measured at the ground station. First, let’s be clear about who measures what. For VDF — that’s VHF Direction Finding, giving you QDMs and QTEs — and for VOR, the bearing is measured at the ground station. Now, with VOR it can look like the aircraft is measuring the bearing, because the instrument is in the cockpit and the display is decoded there. But it isn’t measured in the aircraft. 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 VOR receiver does is de-modulate the encoded signal and display it. It does not actually measure a bearing — unlike 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 will be 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 — you’re plotting it from the ground station. 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’s walk through the example in Figure 22.12. An aircraft measures a bearing of an NDB of 245°(R) — that’s relative, measured from the aircraft’s heading — while on a heading of 025°(T), true. We need to plot the position line on a Lambert chart. Adding the relative bearing of 245° and the true heading of 025° gives the true great circle bearing of the NDB from the aircraft: 270°. So the NDB lies on a true bearing of 270° from the aircraft. Now, if we 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 whole trap. But if we measure the angle of 090° from a line drawn through the NDB that is parallel to the aircraft’s meridian — not parallel to the NDB’s own meridian — then the position line will pass through the aircraft. That’s the key move: you must use the aircraft’s meridian direction, transferred to the NDB, not the NDB’s own meridian. So the rule for ADF and AWR plotting is: take the true great circle bearing from the aircraft, add or subtract 180° to get the reciprocal, and then plot that angle from a line through the NDB that is parallel to the aircraft’s meridian — not the NDB’s meridian. That’s how you make the position line actually pass through the aircraft on a converging chart.