Electronic Flight Information Systems — Page 297, Lesson 350

Let's look at the two ILS display modes on the EHSI — the Electronic Horizontal Situation Indicator. We've already covered the VOR modes, so now we're moving to the ILS, or Instrument Landing System, displays. There are two of them: Full ILS Mode and Expanded ILS. Let's start with Full ILS Mode. When you select an ILS frequency, the EHSI shows you a full compass rose — that's the complete 360° circle. The ILS source is identified in the lower left corner, and the frequency you've tuned is shown in the lower right. Now, the key part — the course. The selected course for the localizer is shown by the magenta course needle. The tip of that needle points to your selected course, which in this example is 150°. The localizer deviation — how far left or right you are from the localizer centreline — is shown by the traditional deviation bar. That bar moves across a scale that goes two dots left and two dots right. And here's an important detail: this scale is exponential. That means the sensitivity increases as you get closer to the centreline — the dots aren't evenly spaced in terms of actual angular deviation. On the right-hand side, you have the glide slope deviation. That's shown by a magenta coloured triangle moving up and down the traditional scale. That tells you whether you're above or below the glide path. In the top left corner, you'll see the DME distance — that's the Distance Measuring Equipment readout. Now for the heading information. Your current heading is shown in the window at the top, and also by the lubber line at the top of the compass rose. In this example, the heading is 130°. The display is set to Magnetic Heading, as indicated by the markings on either side of the window. Your current track — the actual path over the ground — is shown by the white triangle on the inside edge of the compass rose. Your selected heading — the heading you've dialled in — is shown by the magenta heading "bug" on the outer scale of the compass rose. In the lower left corner, you'll see wind speed and direction. That wind information is orientated to your display selection — so it's referenced to either Heading or Track, and either Magnetic or True, depending on what you've selected. And one important limitation in Full ILS Mode: weather radar displays are not available. You don't get weather radar in this mode. Now let's move to Expanded ILS. This is the second mode. With an ILS frequency selected, the EHSI shows about 90° of compass rose — so instead of the full circle, you get a quarter of it, expanded. Again, the ILS source is in the lower left, and the frequency is in the lower right. At the bottom of the display, you'll see the white triangle — that's the aircraft symbol. That's your position on the display. The selected course — or track — is again shown by the magenta course needle, with the tip pointing to your selected course, 150° in this case. Now, here's a practical point: the course selectors are usually located 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. The localizer deviation is shown the same way — the traditional deviation bar moving across a two dot left and two dot right scale. And the glide slope deviation is on the right, again in the traditional fashion. DME distance is in the top left corner, just like before. Current heading is in the window and by the lubber line at the top. In this case, the heading is 130° Magnetic, as indicated by the markings either side of the window. Current track is shown differently here — it's the white line from the tip of the aircraft symbol to the inside edge of the compass rose. So you can see the track line extending from your aircraft symbol. Selected heading is again the magenta heading "bug" on the outer scale. Wind speed and direction are in the lower left corner, orientated to the display selection — Heading or Track, Magnetic or True. Now, here's a key difference from Full ILS Mode: weather radar displays are available in Expanded ILS. When you select weather radar "on", you also see range arcs — those are the distance rings. The weather radar is shown in three colours: green, yellow, and red. Green is the least turbulence, red is the worst. And if TURBULENCE MODE is available, that's shown as magenta — that indicates the area of greatest activity within the cloud. Finally, the range of the display can be selected on the control panel. In this example, the half scale range is displayed — that's 10 NM, or nautical miles. So to summarise the key contrast: Full ILS gives you the complete 360° picture but no weather radar. Expanded ILS gives you a 90° expanded view with weather radar available, and it shows your track as a line from the aircraft symbol rather than a triangle on the edge. Both give you the same core information — course, localizer deviation, glide slope, DME, heading, track, selected heading, and wind — but the presentation and the radar availability differ.