VHF Omni-directional Range (VOR) — Page 129, Lesson 118

Let's pick this up with the Horizontal Situation Indicator — the HSI — because this is where the VOR information gets presented to you in the cockpit in a very specific way. First, the heading. The heading displayed on the HSI will always be Compass Heading, which should be very close — within deviation — to Magnetic Heading. So the compass card you see is your actual magnetic heading, not a track or a course. Now, the arrowhead. That's the Required Course, and it's set by you, the pilot. You rotate it to the course you want to fly. Next, the TO/FROM indicator. This is decided by the instrument, not by you. Here's the rule: if the actual radial the aircraft is on is within 90° of the Set Course, then FROM will be shown. If the actual radial is more than 90° from the Set Course, then TO will be shown. So the instrument compares where you actually are against the course you've set, and decides which side of the beacon you're on. Then we have the Course Deviation Bar. This shows the angular difference between the Required Course and the actual VOR Radial the aircraft is on. So it's telling you, in degrees, how far off the radial you are. Now, the scaling. On this instrument, 1 Dot equals 5°, and Full Scale Deflection equals 10°. But here's the catch — an HSI may be either a 2 Dot or a 5 Dot display. Full Scale Deflection will always be 10°. So if you have a 2 Dot display, 1 dot equals 2°. If you have a 5 Dot display, 1 dot equals 5°. Same full-scale deflection, different dot resolution. You need to know which one you're flying. Here's the key difference from a simple CDI. The HSI takes aircraft heading into consideration when displaying a fly left or fly right indication. Because the instrument includes heading, it's able to determine the best direction to turn to achieve the required radial. So it's possible to be right of the radial but still be given a turn right indication — because turning right is the shorter, more efficient way to get back on the radial. The HSI is thinking about the geometry, not just left and right. Let me show you the instrument layout. Here you see the heading displayed, which will always be Compass Heading, very close to Magnetic Heading. And here's the arrowhead showing the Required Course. The course shown at the top and centre of the dial is the Required Course to fly to achieve the desired aim. And in this figure you can see the Selected or Required Course, the Course Deviation Bar showing 8° fly left, the TO beacon selected, the FROM beacon de-selected, and the aircraft heading of 280° Compass or Magnetic. Now let's move to the in-flight procedures, because this is where it all comes together. We have an aircraft equipped with both a CDI-type deviation indicator and an RMI, and we're going to look at radial interceptions and inbound track-keeping. First, radial interception. In the example, the aircraft is intercepting the 280° radial by flying a heading of about 045°(M). The pilot commences the turn shortly before making good the radial, so as not to overshoot it. A heading of 090°(M) is selected to allow for starboard drift inbound. So the turn is through 45°, taking about 15 seconds. Arrival at the 277 radial should be announced by the Left/Right indicator showing about 1.5 dots 'fly left', and the RMI needle pointing a QDM of 097°. At that point, the pilot would turn onto 090°(M). Now, inbound track-keeping. Having intercepted the inbound radial, the pilot maintains his heading — 090°(M) in the example — and watches the Left/Right needle. Suppose the needle shows a progressively increasing displacement left. That means the aircraft is moving to the right of the desired inbound track. The drift allowance is insufficient, and a heading of 085° would perhaps be more suitable. So the pilot would probably alter heading 30° port, onto 060°(M), until the needle centres, indicating the aircraft is back on track. Then he tries the new heading of 085°(M) and again watches the needle. So you see the technique: intercept with a deliberate turn, watch the needle for the arrival cue, then maintain heading and correct for drift with a defined intercept angle, not a guess. That's the professional way to fly a VOR radial.