Electronic Flight Information Systems — Page 289, Lesson 348

Let’s pick up with the Expanded VOR display on the EHSI — this is the format you’ll see when you’ve tuned a VOR frequency, and it’s a great way to see how all the navigation information gets painted on one screen. First, the layout. With a VOR frequency selected, the EHSI shows about 90 degrees of compass rose. That’s not the full 360 — just a quarter of the compass, centred on your current heading. The VOR source is shown in the lower left corner, and the frequency you’ve tuned is in the lower right. So you always know which station you’re working with. At the bottom of the display, there’s a white triangle — that’s the aircraft symbol. It represents your aeroplane, fixed at the bottom, with the compass rose moving behind it as you turn. Now, the course information. The selected course — or track — is displayed by the magenta course needle. The tip of that needle points to the selected course value, which in this example is 150 degrees. The course selectors are usually on either side of the autoflight main control panel — one for the Captain and one for the First Officer. So each pilot has their own selector, and they can set their own selected course. Course deviation is shown by the traditional deviation bar. That’s the vertical bar that moves left or right across a scale. The scale here is two dots left and two dots right — so you have a two-dot deflection limit on each side. If the bar is centred, you’re on course; if it moves, you’re off. There’s also a TO/FROM annunciation — that tells you whether the VOR is indicating that you’re flying toward the station or away from it. That’s a critical piece of information for interpreting the deviation correctly. DME distance is displayed in the top left corner. So you get your slant-range distance to the station right there. Now, heading and track. The current heading is shown in the window and by the lubber line at the top of the compass rose. In this example, the heading is 130 degrees. The current selection is Magnetic Heading — you can see that indicated on either side of the track window. So the heading is referenced to magnetic north, not true north. Current track is shown by the white line that runs from the tip of the aircraft symbol out to the compass arc. That’s the path you’re actually making good over the ground, as opposed to the heading you’re pointing. Selected heading is shown by the magenta heading “bug” on the outer scale of the compass rose. That’s the heading you’ve dialled in as your target, and the bug sits on the outer ring so you can see it at a glance. Wind speed and direction are shown in the lower left corner, and they’re orientated to the display selection — meaning they’re referenced to either Heading or Track, and either Magnetic or True, depending on what you’ve selected. So the wind readout matches the reference you’re using for the rest of the display. Now, weather radar. When you select the weather radar “on”, range arcs become visible on the display. The radar shows three colours: green, yellow, and red. Green is the least turbulence, red is the worst. So green is light precipitation, yellow is moderate, red is heavy. If turbulence mode is available, that’s shown as magenta — and magenta indicates the area of greatest activity within the cloud. That’s the most severe turbulence region. The range of the display can be selected on the control panel. In this example, half-scale range is displayed as 10 NM, which means the display is actually selected to 20 NM. So the half-scale range is half of the full selected range — the outer arc of the compass rose represents the furthest range from the aircraft, which is the full 20 NM. So, to tie it together: the outer arc is your maximum range, the half-scale range is the mid-point marker, and the weather radar colours give you a quick read on precipitation intensity and turbulence. That’s the Expanded VOR display — you’ve got your navigation source, course, deviation, TO/FROM, DME, heading, track, selected heading, wind, and weather radar all on one screen.