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 how a Lambert chart handles great circles and rhumb lines, and now we’re moving into the real-world task of plotting radio bearings — and this is where the chart’s convergence really starts to bite. First, I want to separate the two families of radio bearings, because the plotting method depends entirely on where the bearing is measured. For VDF — that’s VHF Direction Finding, giving you QDMs and QTEs — and for VOR, the bearing is measured at the ground station. Now, I know it can look like the VOR bearing is worked out in the aircraft, because that’s where the display is. But it isn’t measured there. The VOR ground station encodes the difference between its reference phase and the bearing phase at the moment of transmission, on that particular bearing. All the aircraft’s VOR receiver does is demodulate that encoded signal and display it. It does not actually measure a bearing — unlike ADF. So for bearings measured at the ground station — VDF or VOR — the plotting 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. It is the correct radio wave path. Now the complicated case: bearings measured at the aircraft. That’s ADF, so NDB bearings, and AWR — airborne weather radar. Here the problem is convergence between the meridian where the bearing is measured — that’s the aircraft — and the meridian from where it will be plotted — that’s the NDB. Let’s walk through the example at 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. So first, we add the relative bearing of 245° to the true heading of 025°. That gives us 270° — the true great circle bearing of the NDB from the aircraft. Now here’s the trap. 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. So the line drawn from the NDB’s meridian at 090° will miss the aircraft entirely. That’s Figure 22.12(a). The fix is in Figure 22.12(b). Instead of measuring 090° from the NDB’s own meridian, we draw a line through the NDB that is parallel to the aircraft’s meridian, and measure the 090° from that. Then the position line passes through the aircraft, as it should. So the rule you want to carry: for a bearing measured at the aircraft, you must plot it from a line parallel to the aircraft’s meridian, drawn through the station — not from the station’s own meridian. That’s the convergence correction that makes the position line come out right on a Lambert chart.