Plotting — Page 486, Lesson 479

Right, let's get into the plotting chapter. We're going to look at how you actually put these radio bearings down on a chart, and the first thing I want to establish is the difference between where a bearing is measured and where it's used. Look at this worked example. We have a ground station, and it measures the true bearing of the aircraft from the station. That's the QTE, and here it's 140°(T). Now, the station has a variation of 10°W. To convert that true bearing into a magnetic bearing, we apply the variation. With 10°W, we add it to the true value, giving us 150°(M). That magnetic bearing of the aircraft from the ground station is the QDR. Now, the reciprocal of that QDR — the magnetic track of the ground station from the aircraft — is the QDM, and that's 330°(M). So you can see the relationship: QTE is true, QDR is magnetic from the station, QDM is the reciprocal, magnetic from the aircraft. Now, here's the key point. Whether you see a QDM or a QDR on your indicator depends on which one the operator selects in the aircraft. But the crucial thing is where the measurement happens. With VDF — that's VHF Direction Finding — it's obvious the ground station does the measuring. But with VOR, even though the bearing is received by the aircraft, it's still determined at the ground station. Because the bearings are measured at the ground station, we use the ground station's variation to convert them to magnetic. That's a subtle but important distinction. So now, how do we plot these? QDMs and QDRs are magnetic bearings. If the chart has a magnetic north arrow, we can plot them directly from that magnetic datum. Otherwise, we have to apply variation and plot them as QTEs — true bearings of the aircraft from the ground station. Let's look at the three ways to plot a QDM or QDR. First, with a QDM, we apply the reciprocal to convert it to a radial. Then, method one: align your protractor with the magnetic north arrow on the chart. The problem is those arrows are very short — usually about 1½ centimetres — so it's difficult to align the protractor accurately. Method two is better. Find the variation from the isogonals — those are the lines of equal magnetic variation on the chart. Then use your protractor to plot that variation at the VOR, using the nearest meridian as true north. Now you've constructed your own magnetic north arrow, but a much more accurate one. You can plot the magnetic radial directly without arithmetically applying variation. Method three is the best if the VOR is on an airway, or better still, defines several airways. Then the airway centre lines are defined by printed radials on the chart. Take Shannon, for example — that's SHA at 5243.3N 00853.1W on the E(LO)1 chart. It has radials of 052(M), 071(M), 080(M), 115(M), and 173(M). 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. Finally, let's talk about bearings measured by the aircraft. There are two sources: ADF — that's Automatic Direction Finding — and Airborne Weather Radar, or AWR. ADF is the more common and familiar method. But you can use AWR in the MAP mode. In that case, the normal pencil beam becomes a cosecant² beam, which gives better performance as a map-painting radar. You can then obtain ranges and bearings from suitable ground returns, particularly headlands and islands. So to summarise the whole picture: the source of the bearing determines whose variation you use. Ground-station measurements — VOR and VDF — use the ground station's variation. Aircraft measurements — ADF and AWR — use the aircraft's. And the plotting method depends on what's available on your chart: a magnetic arrow, isogonals, or printed radials.