Let’s pick this up right where the deviation bar comes in — Page 124, Lesson 109

Let’s pick this up right where the deviation bar comes in. I want you to picture the VOR indicator in front of you. The left/right displacement of that bar is the whole story of how you fly a VOR radial, and the key idea is this: the bar’s position depends only on the angular position of the beacon relative to the bearing you’ve selected on the Omni-bearing Selector, the OBS. It is completely independent of the way the aircraft is pointing. So for a given position and a given bearing selection, your heading does not affect the display at all. That’s the first thing to lock in — the deviation bar is a function of where you are and what you’ve dialled in, not which way the nose is pointing. Now look at the geometry. If you’re at position 1 or position 3 in Figure 8.10, you get a Fly Right indication. If you’re exactly on the selected bearing, whether you’re going to or from the station, the bar sits central. At positions 2 and 4 you get a Fly Left indication — the bar is to the left of centre. But here’s the subtle trap: the aircraft at position 4 actually has to turn right to reduce its displacement from the selected line. So the bar’s sense is wrong for that aircraft. That’s generally undesirable, and it’s exactly why we have a rule to keep the sense correct. The rule is tied to the automatic To/From flag. When you’re inbound to a VOR, you select the inbound track on the OBS so that a ‘TO’ indication appears. When you’re outbound, you select the outbound track so that a ‘FROM’ indication is seen. And to keep the bar sense correct when flying a track to or from the station, your heading should be about the same as the track selected on the OBS, plus or minus any drift allowance. That drift allowance is the key — you’re matching your heading to the track, not the other way around. Now, the To/From flag itself. It shows ‘TO’ when your QDM is within about 80° of the bearing selected. QDM is your magnetic bearing to the station. It shows ‘FROM’ when your QDR is within about 80° of the bearing selected — QDR being your magnetic bearing from the station. That leaves two sectors, each about 20° wide, where you get an indeterminate TO/FROM indication. So the flag isn’t a simple yes/no — it’s a sector-based logic with a dead zone. And remember, all six indications you’d see in Figure 8.11 are completely independent of the aircraft’s heading. They depend only on your bearing from the beacon and the bearing you’ve selected on the OBS. So don’t ever think the flag is telling you which way you’re pointing — it’s telling you where you are relative to the selected radial. Finally, the failure case. If the VOR transmissions are faulty, or the aircraft is out of range, or the airborne power supply is inadequate, an ‘OFF’ flag appears. That’s your cue that the information is no longer reliable — you treat the display as invalid until the flag clears. So the whole picture: deviation bar for left/right, To/From flag for sense, and OFF flag for failure. Each one is independent of heading, and each one depends on your position and your OBS selection. That’s the core of flying a VOR radial.