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

Let's start with the Horizontal Situation Indicator — the HSI. This is the instrument that combines a compass card with the VOR deviation display, so you get heading and navigation information on one dial. The heading shown at the top of the HSI is always the Compass Heading. In practice, that Compass Heading should be very close to your Magnetic Heading — the difference between them is called deviation, and it's the error caused by the aircraft's own magnetic fields. So when you read the heading on the HSI, you're effectively reading your magnetic heading, give or take a small deviation. Now, the arrowhead on the dial is the Required Course. That's set by you, the pilot, using the course selector. It represents the course you want to fly or intercept. The TO/FROM indicator is not set by you — it's decided automatically by the instrument. Here's the rule: if the actual radial the aircraft is on is within 90° of the Set Course, then FROM is shown. If the actual radial is more than 90° from the Set Course, then TO is shown. So the indicator tells you, relative to the course you've set, whether you're flying toward the beacon or away from it. The Course Deviation Bar shows the angular difference between the Required Course and the actual VOR Radial the aircraft is on. That's the needle that tells you how far off the selected course you are. Now, the scale. On a standard display, 1 Dot equals 5° of deviation, and Full Scale Deflection equals 10°. But here's the important part: an HSI may be either a 2 Dot or a 5 Dot display. Full Scale Deflection is always 10°, regardless of which display you have. So on a 2 Dot display, 1 Dot equals 2° — because 10° divided by 2 dots is 5° per dot? No, wait — let me be careful. On a 2 Dot display, 1 Dot equals 2°. On a 5 Dot display, 1 Dot equals 5°. In both cases, Full Scale Deflection is 10°. So the dot scale tells you the sensitivity: a 2 Dot display is more sensitive, because each dot represents a smaller angular deviation. Here's a key point about the HSI that differs from a simple CDI: aircraft heading is taken into consideration in displaying a fly left or fly right indication. Because the instrument includes heading information, it can 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 onto the radial. The HSI isn't just telling you which side the radial is on; it's telling you which way to turn. Let me also clarify the labels you'd see on the display. The Selected or Required Course is what you've set. The Course Deviation Bar might show, for example, 8° Fly Left. The TO Beacon is the one selected, and the FROM Beacon is de-selected. And the aircraft heading is shown as, say, 280° Compass or Magnetic. Now let's move to the in-flight procedures — how you actually use VOR in practice, when you have both a CDI-type deviation indicator and an RMI. First, radial interceptions. Imagine you want to intercept the 280° radial. In the example, the aircraft is shown intercepting it by flying a heading of about 045° Magnetic. You commence the turn shortly before making good the radial, so you don't overshoot it. Then a heading of 090° Magnetic is selected to allow for starboard drift inbound. So the turn is through 45°, taking about 15 seconds. When you arrive at the 277 radial — that's 3° before the 280 radial — the Left/Right indicator should show about 1.5 dots 'fly left', and the RMI needle should point to a QDM of 097°. At that point, you turn onto 090° Magnetic. Now, inbound track-keeping. Having intercepted the inbound radial, you maintain your heading — 090° Magnetic in this example — and watch the Left/Right needle. Suppose the needle shows a progressively increasing displacement to the left. That means the aircraft is moving to the right of the desired inbound track. Your drift allowance is insufficient. A heading of 085° would perhaps be more suitable. So the pilot would probably alter heading 30° port — that's to the left — onto 060° Magnetic, until the needle centres, indicating the aircraft is back on track. Then you try the new heading of 085° Magnetic and again watch the needle. So the technique is: intercept at a steeper angle, then once you're on track, adjust your heading to compensate for drift, and keep monitoring the needle to confirm you're holding the radial. That's the HSI and the in-flight procedures for VOR interception and track-keeping.