Plotting — Page 486, Lesson 479

Let's start with the QTE, QDR, and QDM. These are the three classic bearing codes you'll see in navigation. QTE is the true bearing of the aircraft measured by the ground station. QDR is the magnetic bearing of the aircraft from the ground station. QDM is the magnetic track of the ground station from the aircraft — in other words, the magnetic bearing you'd steer to fly directly to the station. Look at the example I've got here. We have a true bearing of the aircraft measured by the ground station, a QTE, of 140 degrees true. The ground station has a variation of 10 degrees west. To convert that true bearing into a magnetic bearing, we add the variation — because it's west, we add it — giving us a QDR of 150 degrees magnetic. Then, to get the reciprocal, the bearing from the aircraft back to the station, we add 180 degrees, giving us a QDM of 330 degrees magnetic. So the chain is: QTE plus variation gives QDR, and QDR plus 180 gives QDM. Now, the key point here is where the measurement happens. With VDF — that's VHF Direction Finding — the bearing is measured at the ground station. But here's the subtle part: with VOR, even though the bearing is received by the aircraft, it's actually determined at the ground station too. Because the bearings are measured at the ground station, we use the variation of the ground station to convert them to magnetic direction. That's a crucial distinction — the variation you apply is the one at the station, not at your aircraft. Now, how do we plot these? QDMs and QDRs are magnetic bearings. If the chart has a magnetic north arrow, you can plot them directly from that magnetic datum. Otherwise, you have to apply variation and plot them as QTEs — true bearings of the aircraft from the ground station. Let me walk you through the three ways to plot a QDM or QDR. First, with a QDM, you apply the reciprocal to convert it to a radial — that's the bearing from the station out to you. Then you have three plotting options. The first method is to align your protractor with the magnetic north arrow on the chart. The problem is these arrows are very short — usually about one and a half centimetres — and it's difficult to align the protractor accurately. The second method is better. You find the variation from the isogonals — those are the lines of equal magnetic variation on the chart. Then you use your protractor to plot that variation at the VOR, using the nearest meridian as true north. Now you've created your own magnetic north arrow, but a much more accurate one. You can now plot the magnetic radial directly, without needing to arithmetically apply variation. The third method is the best when the VOR is on an airway — or better still, when it defines several airways. In that case, the airway centre lines will be defined by printed radials on the chart. Take Shannon, for example — that's SHA at 52 degrees 43.3 minutes north, 008 degrees 53.1 minutes west — on the E(LO)1 chart. There are radials of 052 magnetic, 071 magnetic, 080 magnetic, 115 magnetic, and 173 magnetic. You put the centre of your protractor over the VOR symbol, then turn it until the 052 on its scale goes through the 052 radial, the 071 goes through the 071 radial, and so on. You've now aligned the protractor with the same magnetic north that Jeppesen used, and you can plot a magnetic radial directly. Alternatively, you can convert the magnetic radial to a true bearing by applying variation arithmetically, then plot by reference to the nearest meridian. Now, there's another source of bearings — ones measured at the aircraft, not the ground. There are two of these: ADF and Airborne Weather Radar, or AWR. ADF is the more common and familiar method. But it's possible to use AWR in the MAP mode. In that case, the normal pencil beam becomes a cosecant-squared beam — that's a beam shaped to give better performance as a map-painting radar. With that, you can obtain ranges and bearings from suitable ground returns, particularly headlands and islands. So to tie it together: the critical idea is always where the bearing is measured. If it's measured at the ground station, you use the ground station's variation. If it's measured at the aircraft, you use the aircraft's variation. And when you plot, you have those three methods — the short magnetic arrow, the constructed arrow from isogonals, or the printed radials on an airway chart.