Plotting — Page 486, Lesson 482

Right, let's get into the plotting chapter. We're dealing with radio bearings now, and the first thing I want you to understand is why we have to do any conversion at all. When you measure a bearing to a ground transmitter, like an NDB, the equipment measures it relative to the aircraft's own fore-and-aft axis. That's the line from the tail to the nose. So the raw number you get is not measured from north; it's measured from where the nose is pointing. To make that number useful for plotting on a chart, we have to convert it into a true direction, and we do that using the aircraft's compass. Now, here's a critical point. The compass is affected by the local variation at the aircraft's position. Not at the transmitter, but at the aircraft. So when we do this conversion, we use what's called AIRCRAFT variation. That's the variation at your position, where the measurement is being taken. Let's talk about the two main instruments you'll see. First, the ADF, which is the Automatic Direction Finder. It measures the bearing of a ground transmitter from the aircraft. The bearing it shows is normally either a relative bearing or a magnetic bearing, depending on the presentation. Let's define those two precisely. A Relative Bearing is measured not from true or magnetic north, but from the direction in which the aircraft is pointing. Let me give you the example from the book. Say your aircraft is heading 030° true, and there's an NDB due east of you. Due east is 090° true. The bearing of that NDB from the aircraft, measured from your heading, is 060° relative. That's the angle between your nose and the transmitter. This type of bearing appears on an instrument called an RBI, which stands for Relative Bearing Indicator. Here's the key relationship you can see in Figure 28.2. If you add the relative bearing and the true heading together, you get the true bearing. So in that example, 030° true heading plus 060° relative gives you 090° true. That's the true bearing to the NDB. The second type is a Magnetic Bearing. This one is measured from the direction of magnetic north, and it appears on an instrument called an RMI, the Radio Magnetic Indicator. To change a magnetic bearing into a true bearing, all you need to do is apply the variation at the aircraft's position, where the measuring is being done. Now, there's a specific term you need to know: AWR. An AWR bearing is a relative bearing from the aircraft. The process is exactly as I described. You combine it with the true heading of the aircraft to get a true bearing TO the response. Then, and this is important, you reciprocate it, meaning you add or subtract 180°, to get the true bearing to plot FROM the response. Because on your chart, you plot the line from the transmitter or radar response outwards, not towards it. So let's put this into a proper conversion procedure. This works for AWR bearings, which are always relative, and for ADF bearings when they're presented on an RBI. Here are the steps. Step one: Apply AIRCRAFT variation to the magnetic heading to get true heading. Remember, aircraft variation, because that's where you are. Step two: Add the relative bearing to the true heading to get the true bearing TO the NDB or radar response. If your answer comes to more than 360° true, you subtract 360. Step three: Take the reciprocal to get the true bearing to plot FROM the NDB or radar response. Let's work the example from the book together. Your RBI reading is 207. Your aircraft heading is 315° magnetic. The NDB variation is 7° West. The aircraft variation is 9° West. And the question is, what is the true bearing to plot from the NDB? First, we get the true heading. We take the magnetic heading of 315° and apply the aircraft variation of 9° West. Since it's West, we subtract. 315 minus 9 gives us a true heading of 306° true. Next, we add the relative bearing. 306° true plus the relative bearing of 207° gives us 513° true. That's more than 360, so we subtract 360, giving us 153° true. That's the true bearing TO the NDB. Finally, we take the reciprocal to plot FROM the NDB. 153 plus 180 gives us 333° true. That's the bearing you plot on your chart from the NDB. Notice we never used the NDB variation of 7° West in this calculation. That's because the measurement is done at the aircraft, so only the aircraft variation matters here. That's a classic trap, so keep it in mind.