VHF Omni-directional Range (VOR) — Page 129, Lesson 121

We're now into the operational heart of VOR flying — what actually happens to your instruments as you fly toward, over, and away from the beacon. Let's pick up right where the homing procedure leaves off. After you've intercepted your inbound track and settled the aircraft, you'll be holding a steady heading with the needle reasonably central. Now, visualize the RMI — the Radio Magnetic Indicator — during that homing. Say you've intercepted on a heading of 090°. That heading shows against the heading index at the top of the instrument. The RMI needle itself will be indicating 100°, and that 100° is the QDM — the magnetic bearing you must fly to reach the VOR. So the needle points at the beacon, and the number under the needle tip is the bearing to it. Now we come to station passage — flying directly over the VOR. Overhead the beacon there's what we call a 'cone of confusion', sometimes called a zone of confusion. It's a conical volume of space directly above the transmitter where the VOR signals become unreliable. Its vertical angle is about 60° to 80°, though the ICAO minimum is 40°. Why does this matter? Because inside that cone, the indications become indeterminate — they don't mean anything. And here's the key point: at high level, that cone extends over a considerable area on the ground. For instance, at 30,000 feet, the cone of confusion reaches out to about 4 nautical miles radius. So at altitude, you can be a long way horizontally from the beacon and still be inside the confused zone. What do you actually see during station passage? On the VOR/ILS indicator — that's the instrument with the vertical needle and the TO/FROM flag — the needle swings between hard left and hard right. The OFF flag may appear temporarily, which is the instrument telling you it's lost a reliable signal. And the TO/FROM indicator changes to FROM, because you've now passed the beacon and are flying away from it. On the RMI, the needle fluctuates — it wavers — and then rotates through 180° to indicate the QDM back to the beacon. That makes sense: before passage the needle pointed ahead to the beacon; after passage it points behind you, back to the beacon you've just crossed. One more operational point: at low altitude these station passage indications are rapid — they happen quickly. At high altitude they're slow, because you're moving through that large cone gradually. Now let's look at outbound flight — you've passed the beacon and you're flying away. The aircraft is shown outbound on the 150 radial. Remember, a radial is the magnetic bearing FROM the VOR. So on the 150 radial, you're flying away from the beacon on a track of 150° magnetic. The indications here are ideal. The TO/FROM flag shows 'FROM', confirming you're outbound. The L/R needle — the left/right deviation needle — is centralized, showing you're exactly on the selected track of 150°. And that deviation information is confirmed by the RMI needle, which shows a QDM of 330 back to the beacon. Check that: 150 plus 180 equals 330. The bearing back to the beacon is the reciprocal of your outbound radial. So the RMI needle pointing at 330 confirms you're precisely on the 150 radial. Now, what if those indications change? Suppose the needle starts to deflect, showing a track error developing. The pilot would normally make a firm heading alteration — typically 30° — to regain track. Then, once back on track, you steer a revised outbound heading appropriate to your revised assessment of drift. So the procedure is: detect the error, make a firm correction of about 30°, regain the track, then settle on a new heading that accounts for the drift you've now measured. Let me show you what this looks like on the instruments. — that's the RMI showing the QDM to the VOR during the homing. And shows the course at the top of the dial — the required course to fly to achieve your desired aim. So the whole picture: inbound you fly the QDM to the beacon, you cross the cone of confusion with its characteristic needle swings and flag changes, and outbound you fly a radial, confirming your position with the reciprocal QDM back to the beacon.