Plotting — Page 478, Lesson 476

Right, let's get into plotting. This is where all the navigation theory starts to become practical, so I want to build this up carefully. First, we need a common language for bearings and tracks, because these terms get used interchangeably in everyday speech but mean very specific things in navigation. Let's define them precisely. QTE is the true bearing of the aircraft from a DF station. So, if a Direction Finding station tells you your QTE, it's giving you the true bearing from the station to you. Think of it as the direction you'd look from the station to find the aircraft, measured as a true bearing. QDM is the track, magnetic, from the aircraft to the DF station. This is the magnetic track you would need to fly to get to the station. So QDM is always the magnetic direction towards the station. QDR is the bearing, magnetic, of the aircraft from the DF station. This is the magnetic bearing from the station to the aircraft. Notice the contrast: QDM is towards the station, QDR is from the station. Now, VORs work on the same principle. A VOR Radial is sometimes referred to as VOR QDR. It's the magnetic bearing of the aircraft from a VOR beacon. So when you're on a radial, you're on a specific magnetic bearing from the beacon. And VOR QDM is the magnetic track from the aircraft to a VOR beacon. So, if you want to fly directly to the VOR, you'd fly the reciprocal of the radial — that's your VOR QDM. Next, two more terms you'll see constantly on charts. Isogonal — this is a line on the chart joining places of equal magnetic variation. So, along an isogonal, the variation is the same everywhere. And Convergency — this is the angle between two selected meridians at a given latitude or latitudes. The meridians converge towards the poles, and convergency is the measure of that angle. Now, let's talk about the symbols you'll use in plotting. These are standard, and I want you to always use them and always record the appropriate time of each occurrence. That time recording is crucial — a fix without a time is nearly useless. We have a position line at 1115 UTC, plotted as a true bearing. That's a single line of position. Then we have a two position line fix at 1121 UTC — that's where two position lines cross, giving you a fix. We also have a radar fix at 1510 UTC. And the symbol for a DF station, VOR, NDB or VDF facility — that's the standard symbol for any of those ground-based navigation aids. Now, the simplest form of plot. It's obtained by plotting the positions of two fixes. Here's the key condition: provided that the aircraft has been flying a single heading during the interval between the fixes, a straight line joining them represents the track. So, if you hold one heading between two fixes, the straight line connecting those fixes is your actual track over the ground. That track, determined between two fixes between which one heading only has been flown, is called a Track Made Good, or TMG. That's the official term. Now, here's where it gets useful. This track made good is easily measured on the chart in the usual way. And the distance measured along the TMG gives you a method of calculating the actual ground speed. Let me give you the example from Figure 28.1. In that figure, the distance flown in 30 minutes is 100 NM. So, the actual ground speed must be 200 kt. Let's check that logic: 100 NM in 30 minutes means 200 NM in 60 minutes, which is 200 knots. That's your ground speed — the actual speed over the ground, derived directly from your TMG and the time between fixes. So, the whole idea here is: two fixes, one heading, straight line equals TMG, measure the distance, divide by the time, and you've got your ground speed. That's the foundation of all practical plotting.