Let’s pick this up right where the RMI is doing its clever work — Page 124, Lesson 113

Let’s pick this up right where the RMI is doing its clever work. I want to walk you through what happens when that needle points at the VOR, and why the numbers on the card mean what they mean. First, the key idea: the RMI needle shows you the QDM to the VOR — that’s the magnetic bearing from you to the station. But here’s the subtle part. The equipment measures a phase difference from the VOR signal, and from that it derives a QDM. Then it subtracts your magnetic heading to get a relative bearing, which is what actually swings the needle around the card. Let me give you the worked example, because it ties it all together. Suppose your aircraft heading is 040°(M) — that’s 040 degrees magnetic. The measured phase difference is 270°. From that phase difference, the equipment derives a QDM of 090°. It then subtracts your heading of 040° to get a relative bearing of 050°. That relative bearing positions the RMI needle 50° clockwise from the heading index at the top of the card. Now watch what happens on the display. The heading index reads 040°, and the needle indicates 040° plus 050°, which equals 090° — and that is the correct QDM to the VOR, based on the magnetic meridian at the beacon. That’s the crucial contrast I want you to hold onto: the QDM shown by the RMI is referenced to the magnetic variation at the VOR station, not at your aircraft. Compare that with an ADF bearing displayed on an RMI — there, the magnetic bearing shown is based on the magnetic variation at the aircraft. Two different reference points, two different instruments, and you need to know which one you’re looking at. Now, if you want to convert that QDM into a True bearing for plotting on a chart, you have to apply the variation at the VOR station. That’s the correction you add or subtract to go from magnetic to true. There’s one more elegant feature of the RMI presentation worth mentioning. The arrowhead of the needle shows the QDM — the bearing to the beacon. But the needle is a full-diameter pointer, so the tail end of it indicates the reciprocal of the QDM. And the reciprocal of the QDM is exactly the radial on which your aircraft is positioned — that’s the bearing from the station. So with a single glance, the RMI displays both the bearing TO and the bearing FROM the station. That’s a genuinely useful piece of situational awareness. Now let’s be honest about the trade-offs, because the RMI isn’t free. Compared to the OBS-type deviation indicator, the RMI has some real disadvantages. It is a more complex instrument — it requires additional hardware, including a remote-reading magnetic compass and the appropriate power supplies. All of that extra equipment makes it heavier, it occupies more space, and it is more costly. So you’re paying a price in weight, space, and money for that beautiful TO/FROM presentation. So the picture you should carry away is this: the RMI needle gives you QDM referenced to the VOR’s magnetic meridian, the tail gives you the radial you’re on, and the whole instrument costs you in complexity, weight, space, and money compared to the simpler OBS indicator.