VHF Omni-directional Range (VOR) — Page 124, Lesson 112

Let's pick up with the VOR deviation indicator, because there are a few important details I want you to have before we move on. If the instrument you're using also has an ILS glide path needle, that needle will be inoperative, centralized, and flagged 'OFF' when you're displaying VOR information. In other words, when you're flying on VOR, that glide path needle is dead — it sits centered, and the flag tells you it's not giving you valid data. Conversely, when you're displaying ILS information, the OBS is inoperative, and the TO/FROM indication is meaningless. So the OBS knob and the TO/FROM flag only have meaning in VOR mode; in ILS mode, you ignore them entirely. Now, let's talk about the Radio Magnetic Indicator, or RMI. This is an alternative way of presenting VOR bearing information, and it's described at length in the ADF notes. Briefly, the RMI has a remote-reading compass repeater card. That card indicates the aircraft's magnetic heading against a fixed heading index at the top of the instrument. So the card rotates under a fixed lubber line at the top, and that tells you your magnetic heading. A pointer on the instrument indicates on the compass card the aircraft's QDM to the beacon. QDM is the magnetic bearing from the station to the aircraft — the bearing you'd fly to get to the beacon. Two needles are common, so you can display two bearings simultaneously, one from each of two different beacons. Now, here's a critical point for professional licence students. Before display on the RMI, VOR information must be processed differently from ADF information. Why? Because the aircraft receives a magnetic bearing from the VOR, dispensed in the form of a phase difference. The ADF equipment, by contrast, gives you a direct indication of relative bearing. So the raw data is fundamentally different. Let me explain that. The VOR QDM is derived from the measured phase difference between the reference and variable signals. That phase difference is what encodes the magnetic bearing. To display that on the RMI, the VOR QDM must be converted to a relative bearing. This is achieved by means of a differential synchro, which automatically subtracts the aircraft's magnetic heading from the VOR QDM. So the differential synchro takes the QDM, subtracts your heading, and the result is a relative bearing — the bearing of the beacon relative to your nose. That relative bearing then positions the RMI needle. But here's the clever part: the point of the needle indicates the original QDM to the VOR, because the magnetic heading that was subtracted is, in effect, re-applied by the compass card. The card rotates with your heading, so the needle's tip ends up pointing at the true magnetic bearing to the station, even though the processing inside used a relative bearing. That's the whole point of the RMI — you get a direct read of QDM without doing mental arithmetic. Let me show you the geometry of that with a diagram.