Electronic Flight Information Systems — Page 289, Lesson 348

Let’s pick this up right where the display starts talking to you. I want to walk you through the Expanded VOR mode on the EHSI — that’s the Electronic Horizontal Situation Indicator, the main navigation display in an EFIS, the Electronic Flight Information System. When you tune a VOR frequency, the EHSI switches into what we call Expanded VOR. The word “expanded” is key: instead of a full 360° compass rose, the display shows you about 90° of the compass rose. That’s the expanded view — you’re zoomed in on the sector ahead of you, which makes the navigation information much easier to read during en-route flying. Now, let’s build the picture from the bottom up. At the bottom of the display, you’ll see a white triangle. That is the aircraft symbol — it represents your aeroplane, fixed at the bottom, with the nose pointing up the display. Everything else on the screen moves relative to that symbol. In the lower left corner of the display, you’ll see the VOR source — that tells you which VOR you’re tuned to. In the lower right corner, you’ll see the frequency itself. So source on the left, frequency on the right. Now, the magenta course needle. This is the selected course, or track. The tip of that magenta needle points to the course you have selected — in the example, 150°. So if you’ve dialled in 150°, the needle tip sits on the 150° mark of the compass rose. The course selectors themselves are usually mounted 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 course selector. Course deviation is shown by the traditional deviation bar. This is the classic VOR needle — it moves across a scale that reads two dots left and two dots right. So if you’re off course, that bar deflects left or right, and the dots tell you how far off you are. There’s also a TO/FROM annunciation. That tells you whether the VOR radial you’re on is taking you toward the station or away from it. It’s a simple flag — TO or FROM — and it’s essential for knowing which side of the station you’re on. DME distance is displayed in the top left corner. DME is Distance Measuring Equipment — it gives you the slant range to the VOR/DME station, in nautical miles. Now, heading. The current heading is shown in the window and by the lubber line at the top of the compass rose. In the example, that’s 130°. The lubber line is the fixed reference mark at the top of the compass — it’s the index against which the rotating compass rose moves. So when the rose rotates, the number under the lubber line is your current heading. The current selection is Magnetic Heading — and you’ll see that indicated on either side of the track window. So the display is telling you that the heading you’re reading is magnetic, not true. Now, current track. This is shown by a white line that runs from the tip of the aircraft symbol out to the compass arc. Track is the actual path you’re making good over the ground — the direction you’re actually moving, as opposed to the direction you’re pointing. That white line shows you that. Selected heading is shown by the magenta heading “bug” on the outer scale of the compass rose. That’s a small magenta marker you can set to a desired heading — it’s your heading reference, the bug you fly to. Wind speed and direction are shown in the lower left corner. And here’s an important detail: the wind is orientated to the display selection — that is, whether you’re displaying Heading or Track, and whether that’s Magnetic or True. So the wind direction you read is referenced to whatever mode you’ve selected on the display. Now, weather radar. The EHSI can display weather radar returns. When you select the radar “on”, range arcs become visible — those are the curved lines that show you distance from the aircraft. The weather 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 — and the more intense the return, the more severe the weather. If the radar has a turbulence mode available, that is shown in magenta. Magenta indicates the area of greatest activity within the cloud — that’s where the turbulence is most severe, even if the precipitation colour might not look that intense. The range of the display can be selected on the control panel. In the example, the half-scale range is displayed as 10 NM — so the display is selected to 20 NM. That means the full range of the display is 20 nautical miles, and the half-scale arc sits at 10. The outer arc of the compass rose is the furthest range from the aircraft — that’s your 20 NM limit. So, to tie it all together: the Expanded VOR mode gives you a 90° window of the compass, your aircraft symbol fixed at the bottom, your selected course in magenta, your deviation on the two-dot scale, your heading under the lubber line, your track as the white line, your selected heading as the magenta bug, and your weather picture in green, yellow, red, and magenta if turbulence mode is on. That’s the full Expanded VOR picture.