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

Let's start with the RMI — the Radio Magnetic Indicator. This is the instrument that combines a compass card with a bearing pointer, and it's the first display we use for VOR work. Here's the key idea: the arrowhead of the pointer always shows you the QDM — that's the magnetic bearing you'd fly TO the beacon. The tail of the needle shows the reciprocal, which is the QDR — the radial you're on, the bearing FROM the beacon. So on Figure 8.12, we have a beacon on the 270 radial, giving a QDM of 090, with a phase difference of 270 degrees. The heading index at the top shows your magnetic heading. Now, why do large aircraft prefer the RMI over other displays? Let me give you the advantages. First, it gives you a continuous indication of the QDM to a VOR, and the reciprocal — the radial — at the tail of the pointer. Second, magnetic heading is displayed on the same instrument, which is a real asset when homing to a VOR or maintaining a track outbound. Third, the approximate relative bearing of a beacon is immediately apparent — you get a plan view of the local navigation situation, which is most useful when flying a holding procedure. Fourth, because the pointer automatically gives a continuous indication of the VOR bearing, you can easily assess the rate of crossing radials during an interception. Fifth, with a two-needle RMI, you can display the bearings of two beacons simultaneously — particularly useful when flying along 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 look at the details of reading the RMI, as in Figure 8.13. The arrowhead gives you the QDM — say 070 degrees. The tail of the needle gives you the QDR — say 250 degrees. And the aircraft heading is shown at the top, say 030 degrees magnetic. Now, an important point: the heading displayed will always be Compass Heading, which should be very close — within deviation — to Magnetic Heading. Here's a critical rule for examination purposes: the VOR QDR/QDM displayed will be the radial that the aircraft is actually on. If the arrowhead and the tail do not agree, due to a bent needle, then the arrowhead will be the correct reading. The arrowhead will always point TO the beacon — that's the QDM. Now, a crucial point about how this instrument behaves: the aircraft's heading will not affect the readings. If your heading is correct, then both the Relative Bearing and the Radial will be correct. But if your heading is in error, then the Radial will still be correct, but the Relative Bearing will be wrong. And no action is required by the pilot — the RMI is a passive display; it just shows you the information. Let me clarify that with Figure 8.14. We have an aircraft heading of 030 degrees magnetic, on the 250 degree radial from the VOR — that's the QDR. The VOR beacon is at the centre, and Magnetic North is at the top. The pointer shows you where the beacon is relative to your heading. Now let's move to the CDI — the Course Deviation Indicator. This is a different display, and it works on a different principle. The course shown at the top or centre of the dial is the Required Course to fly to achieve the desired aim. You set this course yourself, using the OBS knob — that's the Omni-Bearing Selector. Here's how the TO/FROM indicator works. It will be decided by the instrument, not by you. If the actual radial the aircraft is on is within 90 degrees of the Set Course, then FROM will be shown. If the actual radial is more than 90 degrees from the Set Course, then TO will be shown. So the instrument decides based on the angular relationship between your actual radial and the course you've set. The Course Deviation Bar shows the angular difference between the Required Course and the actual VOR Radial the aircraft is on. And here are the calibration numbers you need to remember: 1 Dot equals 2 degrees, and Full Scale Deflection equals 10 degrees. The edge of the inner circle is the first dot — that's 2 degrees. Again, the aircraft's heading will not affect the readings. The pilot sets the Required Course using the OBS knob, and the instrument does the rest. Let me put this together with Figure 8.15. We have a TO Beacon selected, a Course Deviation Bar showing 6 degrees fly left, and a FROM Beacon de-selected. The Selected or Required Course is shown at the top of the dial. So the key contrast between the two instruments: the RMI gives you a continuous, passive display of QDM and QDR without any pilot action, while the CDI requires you to set a Required Course with the OBS knob, and then it shows you the deviation from that course in dots, where each dot is 2 degrees and full scale is 10 degrees.