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 key idea here is that the displacement of that bar depends on the angular position of the beacon relative to the bearing you’ve selected on the Omni-bearing Selector — the OBS. And here’s the crucial part: 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 a fundamental point, so let’s make sure it sticks. Now, look at Figure 8.10. It shows four aircraft positions around a station, with a course set of 080°. Aircraft at positions 1 and 3 both get a Fly Right indication — the deviation bar sits to the right of centre. If the aircraft lay exactly on the selected bearing, either to or from the station, the deviation bar would be central. That’s the null position — no displacement. But here’s where it gets interesting. Aircraft at positions 2 and 4 both receive a Fly Left indication — the bar is to the left of centre. Yet note this: the aircraft at position 4 must actually turn to the right to reduce its displacement from the selected line. So the deviation bar’s sense is wrong for that aircraft. That’s generally undesirable, and it’s a classic trap. To keep the deviation bar sense correct when you’re flying a track to or from a VOR station, your heading should be about the same as the track you’ve selected on the OBS — plus or minus any drift allowance. That’s the rule of thumb. Now, because the equipment normally includes an automatic To/From flag, there’s a simple rule to follow to keep that sense correct. When you’re inbound to a VOR, select the inbound track on the OBS so that a ‘TO’ indication appears. When you’re outbound from a VOR, select the outbound track on the OBS so that a ‘FROM’ indication is seen. That’s the operational rule you’ll use every time. Let’s dig into the To/From flag itself. In addition to the Left/Right display, the deviation indicator shows a ‘TO’ or a ‘FROM’ flag depending on two conditions. First, if the aircraft’s QDM is within about 80° of the bearing selected, ‘TO’ appears. QDM is the magnetic bearing from the aircraft to the station — the direction you’d fly to reach it. Second, if the aircraft’s QDR is within about 80° of the bearing selected, ‘FROM’ appears. QDR is the magnetic bearing from the station to the aircraft — the reciprocal. So those two conditions leave two sectors, each about 20° wide, where you get an indeterminate TO/FROM indication. That’s the ambiguity zone. Figure 8.11 depicts the deviation indicator in those various sectors about the VOR beacon. And I want to stress this: the six indications in that figure are completely independent of the aircraft’s heading. They depend only on the aircraft’s bearing from the beacon and on the bearing selected on the OBS. So don’t think of the flag as telling you which way you’re pointing — it’s telling you where you are relative to the selected radial. Finally, there’s a failure condition. 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 warning that the information is no longer reliable. So you’ve got three possible flag states: TO, FROM, and OFF — and each one has a precise meaning tied to the geometry and the equipment health.