VHF Omni-directional Range (VOR) — Page 129, Lesson 115

Let’s start with the RMI — the Radio Magnetic Indicator — because that’s the first display we’re looking at for VOR. I want you to picture the instrument face: there’s a rotating compass card, and on top of it a single needle. That needle has an arrowhead at one end and a tail at the other. Here’s the key relationship. The arrowhead always points TO the beacon, and it shows you the QDM — that’s the magnetic bearing you’d fly to reach the VOR. The tail of the needle points in the exact opposite direction, and that shows the QDR — the radial you’re on, which is the bearing from the beacon. So on the example we have: QDM 070° at the arrowhead, QDR 250° at the tail, and the aircraft heading is 030° magnetic. Notice 070 and 250 are 180° apart — they’re reciprocals. Now, the heading displayed on the RMI is always Compass Heading, which should be very close to Magnetic Heading — the difference between them is just deviation, the compass error. For exam purposes, remember this: the VOR QDR/QDM shown is the radial the aircraft is actually on. And if the arrowhead and tail don’t agree — a “bent needle” situation — the arrowhead is the correct reading. Here’s a really important point about how this instrument behaves. The aircraft’s heading does not affect the readings. The radial you’re on is a fact of your position, not your direction of travel. But — and this is the subtle part — if your heading is correct, then both the Relative Bearing and the Radial will be correct. If your heading is in error, the Radial stays correct, but the Relative Bearing will be wrong. Why? Because relative bearing is measured from your heading — it’s the angle from the nose to the beacon. So a heading error shifts that angle. The radial, though, is purely positional, so it’s unaffected. And the great thing is, no action is required by the pilot — the RMI does all this automatically. Let me give you the picture from the figure: the aircraft is on heading 030°(M), the VOR beacon is off to the right, and the radial from the VOR is 250°(M) — that’s the QDR. The arrowhead points to the beacon, showing QDM 070°. Magnetic North is at the top of the card. Now, why do large aircraft prefer the RMI despite some disadvantages? Because it gives you a continuous indication of the QDM to a VOR, and the reciprocal — the radial — at the tail. You get magnetic heading on the same instrument, which is a real asset when homing to a VOR or maintaining a track outbound. The approximate relative bearing of the beacon is immediately apparent — you get a “plan view” of the local navigation situation, which is most useful in a holding procedure. Because the pointer continuously shows the VOR bearing, you can easily assess the rate of crossing radials during an interception. With a two-needle RMI, you can display the bearings of two beacons at once — that’s particularly useful on an airway, using one beacon ahead or astern for track-keeping and a second beacon off the airway for reporting abeam. And finally, ADF bearings can also be displayed on an RMI. Now let’s move to the second display — the CDI, the Course Deviation Indicator. This is the instrument with the vertical needle and the little TO/FROM flag. The course shown at the top or centre of the dial is the Required Course — the course you’ve selected to fly to achieve your desired aim. Here’s how the TO/FROM indicator works. The instrument decides it, not you. If the actual radial you’re on is within 90° of the Set Course, then FROM is shown. If the actual radial is more than 90° from the Set Course, then TO is shown. So the flag tells you whether the radial you’re on is generally toward or away from the beacon relative to your selected course. The Course Deviation Bar shows the angular difference between the Required Course and the actual VOR Radial you’re on. And here are the numbers you need to memorise: 1 dot = 2°, and Full Scale Deflection = 10°. The edge of the inner circle is the first dot, so that’s 2°. So if the bar is sitting one dot off centre, you’re 2° off the radial; if it’s at full deflection, you’re 10° off. Just like the RMI, the aircraft’s heading does not affect the readings. The pilot sets the Required Course using the OBS knob — that’s the Omni-Bearing Selector. In the figure, the selected course is TO the beacon, the Course Deviation Bar shows 6° fly left, and FROM is de-selected. So to summarise the contrast between the two instruments: the RMI gives you a continuous, automatic picture — QDM at the arrowhead, QDR at the tail, heading on the card, no pilot action needed. The CDI requires you to set a course with the OBS, and then it tells you the angular deviation from that course, with the 2°-per-dot scale and the 90° TO/FROM logic. Both are immune to heading, but they answer different questions.