Let's pick this up right where the needle sits on the RMI — Page 124, Lesson 113

Let's pick this up right where the needle sits on the RMI. We've just worked out how the instrument derives a QDM by subtracting the aircraft's magnetic heading from the measured phase difference. Now I want to walk you through the crucial detail that makes the RMI so clever, and then we'll compare it with the older OBS-style indicator. First, the key point about that QDM you see on the RMI needle. The instrument subtracts your magnetic heading to get a relative bearing, but then the compass repeater card effectively re-applies that heading. So the needle ends up showing you the original QDM to the VOR — the magnetic bearing from your position to the beacon, based on the magnetic meridian at the beacon itself. That's the whole trick: the subtraction is undone by the rotating card, so what you read is the true bearing to the station, not a relative bearing. Now, here's a subtle but important distinction. If you want to plot that QDM as a True bearing on a chart, you must apply the magnetic variation at the VOR station — not at your aircraft. That's because the QDM is referenced to the beacon's magnetic meridian. I'll come back to this contrast in a moment, because it's exactly where the RMI differs from an ADF. Let me give you a concrete example, and I'll reference Figure 8.12 as we go. Suppose your aircraft heading is 040° magnetic, and the measured phase difference is 270°. The equipment takes that phase difference and 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 — so if you look at the instrument, the needle sits 50° to the right of the top of the card. Now, if there were a difference in magnetic variation between your position and the VOR station, that derived relative bearing of 050° would carry a corresponding error. But — and this is the elegant part — the QDM indicated by the needle would still be correct. Why? Because the compass repeater card is aligned to your heading, and the whole display is referenced to the beacon's meridian. The error in the relative bearing is exactly cancelled by the card's own reference. So the needle reads correctly regardless of variation differences. Continuing the example: the RMI heading index reads 040°, and the needle indicates 040° plus 050°, which equals 090°. That 090° is the correct QDM to the VOR, based on the magnetic meridian at the beacon. Now compare that with an ADF bearing displayed on an RMI. In the ADF case, the magnetic bearing indicated is based on the magnetic variation at the aircraft — not at the beacon. So you have two different reference meridians at play: the VOR QDM is referenced to the station, the ADF bearing is referenced to your own position. That's a critical operational distinction you'll need to remember when you're mixing these instruments. Now, one really useful aspect of the RMI presentation deserves mention. The arrowhead of the needle shows the QDM of the beacon — the bearing TO the station. Consequently, the tail end of this full-diameter pointer indicates the reciprocal of the QDM, which is the radial on which the aircraft is positioned — the bearing FROM the station. So with a single glance, you see both the bearing TO and the bearing FROM the station clearly displayed. That's a huge advantage in situational awareness: you know where you're heading to reach the beacon, and you know which radial you're on. Finally, let's make a worthwhile comparison between the RMI and the OBS-type deviation indicator. The RMI has certain disadvantages. It's a more complex instrument requiring additional hardware, including a remote-reading magnetic compass and the appropriate power supplies. Because of that extra hardware, it is heavier, occupies more space, and is more costly. So while the RMI gives you that beautiful simultaneous TO/FROM presentation, you pay for it in weight, space, and money — and that's a real trade-off you'll weigh when you're designing or equipping an aircraft. So to tie it all together: the RMI shows you a QDM referenced to the beacon's meridian, the tail shows your radial, and the whole thing works because the compass card re-applies the heading that was subtracted. The cost is complexity, weight, and expense compared to the simpler OBS indicator.