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

Let's pick this up right where the aircraft has just intercepted the inbound track. After you've made that interception, you're settled on a heading of 090° against the heading index, and your RMI needle is indicating 100° — that's the required QDM to the VOR. Now, the key point here is that you may still need further small heading alterations before the aircraft is truly settled on a good inbound heading, with the needle reasonably steady in the central position. So don't expect perfection on the first turn — homing is an iterative process. Now let's talk about what happens when you actually fly over the station — this is called station passage. Overhead a VOR, there's a region called the cone of confusion, sometimes called the zone of confusion. It has a vertical angle of about 60° to 80°, though the ICAO minimum is 40°. What this means is that directly above the beacon, the signals become indeterminate — the equipment can't give you reliable directional information. At high level, this cone extends over a considerable area. For instance, at 30,000 feet, it can reach out to about 4 nautical miles in radius. So the higher you are, the bigger the area of confusion. Now, how does this show up on your instruments? On the VOR/ILS indicator, the needle swings between hard left and hard right — it's erratic. The OFF flag may appear temporarily, and the TO/FROM indicator changes to FROM. On the RMI, the needle fluctuates and then rotates through 180° to indicate the QDM back to the beacon. So the RMI needle physically swings around to point back the way you came. The timing of these indications depends on your altitude: at low altitude, station passage is rapid — it happens quickly; at high altitude, it's slow — the indications linger because the cone is wider. After you pass the station, you're now on the outbound flight. Let's look at the ideal case. The aircraft is shown outbound on the 150 radial — that's the radial you're flying away from the station on. The indications are ideal: the TO/FROM flag shows 'FROM', and the left/right needle is centralized, showing you're exactly on the selected track of 150°. The deviation indicator's information is confirmed by the RMI needle, which shows a QDM of 330 back to the beacon. Notice the relationship: if you're outbound on the 150 radial, the bearing back to the station is 330 — that's 180° different, which makes sense. Now, what if those indications change? If you see a track error developing — the needle starts to deflect, or the RMI shows a different QDM — the pilot would normally make a firm heading alteration, typically 30°, to regain track. Then 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 drift. Let me make sure you've got the key relationships clear. The RMI needle always points to the QDM — the magnetic bearing to the station. When you're inbound, that's the heading you want to fly. When you're outbound, it points back behind you. The TO/FROM flag tells you which side of the station you're on. And the cone of confusion is that indeterminate zone overhead — the higher you are, the wider it is, and the longer the indications take to settle.