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

I want to walk you through the Radio Magnetic Indicator, or RMI, and how it presents VOR information. This is a different way of showing you the same bearing data, and it's important you understand how it differs from the conventional VOR indicator. First, let me set the scene. The RMI is an alternative means of presenting VOR bearing information. It's the same instrument you've seen described in your ADF notes. Briefly, it 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. Now, a pointer indicates on the compass card the aircraft's QDM to the beacon. QDM is the magnetic bearing you'd fly to reach the station. Two needles are common, so that two bearings can be simultaneously displayed — you could have one needle on a VOR and another on an ADF, for example. Here's the critical part for you as a professional licence student. 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, and that bearing is 'dispensed' in the form of a phase difference. The ADF equipment, by contrast, gives you a direct indication of relative bearing. Let me unpack that. The VOR transmits two signals — a reference phase and a variable phase. The phase difference between them encodes the magnetic bearing from the station to the aircraft. So the VOR gives you a magnetic bearing directly, not a relative bearing. The ADF, on the other hand, points at the station relative to the aircraft's nose — that's a relative bearing. So the VOR QDM, derived from the measured phase difference between the reference and variable phase signals, is converted to a relative bearing for display on the RMI. This conversion is achieved by means of a 'differential synchro', which automatically subtracts the aircraft's magnetic heading from the VOR QDM. Let me make sure that's clear. The differential synchro takes the magnetic bearing from the VOR, subtracts your magnetic heading, and the result is a relative bearing — the bearing of the station relative to your nose. That relative bearing then positions the RMI needle. But here's the elegant part. The point of the needle, the sharp end, indicates the original QDM to the VOR. Why? Because the magnetic heading that was subtracted is, in effect, re-applied by the compass card. The card is rotating to show your heading, so when the needle is positioned at the relative bearing, the card's rotation effectively adds your heading back. The needle tip ends up pointing at the true magnetic bearing to the station — the QDM — even though the internal processing used a relative bearing. Now, let me also touch on the deviation indicator aspects mentioned at the start. If the instrument has an ILS glide path needle, that needle will be inoperative, centralized, and flagged 'OFF' when the indicator is being used to display VOR information. So you'll see the glide slope needle sitting centred with an OFF flag, telling you it's not active. Conversely, when ILS information is being displayed, the OBS is inoperative and the TO/FROM indication is meaningless. The Omni-Bearing Selector, the OBS, does nothing in ILS mode, and you should ignore the TO/FROM flag entirely. So to tie it together: the RMI gives you a direct, easy-to-read presentation of QDM, but the VOR data has to be converted from a magnetic bearing to a relative bearing internally, using that differential synchro, before the needle can be positioned correctly. And remember, the needle tip still shows you the original QDM because the compass card re-applies your heading.