Electronic Flight Information Systems — Page 297, Lesson 350

Let’s pick up with the EHSI—the Electronic Horizontal Situation Indicator—and I want to walk you through the two ILS display modes, because this is where the instrument really earns its keep in the cockpit. We’ll 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 of headings. The ILS source is identified in the lower left corner, and the frequency you’ve tuned is shown in the lower right. So you always know which navaid you’re actually looking at. Now, the course you’ve selected—that’s the localizer course—is shown by the magenta course needle. The tip of that needle points to your selected course, and in this example it’s 150°. So the needle is your reference for where you want to be flying laterally. Localizer deviation is shown by the traditional deviation bar, moving across a scale that’s two dots left and two dots right. Here’s a critical detail: that scale is exponential. What that means is the dots don’t represent a linear distance from the centerline—the sensitivity increases as you get closer to the localizer. Near the center, small angular deviations produce large needle movements; further out, the needle is less sensitive. That’s how the ILS gives you precise guidance as you close in on the runway. On the right-hand side, you have glide slope deviation, shown by a magenta coloured triangle moving up and down the traditional scale. That’s your vertical guidance—keeping you on the correct descent path. In the top left corner, you’ll see DME distance—that’s your slant-range distance to the DME station, in nautical miles. 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°, and the markings either side of the window tell you it’s Magnetic Heading—that’s the current selection. Your current track—that’s the path you’re actually making good over the ground—is shown by the white triangle on the inside edge of the compass rose. Your selected heading—the one you’ve dialled into the autopilot or flight director—is shown by the magenta heading “bug” on the outer scale of the compass rose. In the lower left corner, you get wind speed and direction, and here’s the important part: that wind is orientated to your display selection—whether you’re displaying Heading or Track, and whether that’s Magnetic or True. So the wind arrow always relates to the reference you’ve chosen. And one notable limitation in Full ILS Mode: weather radar displays are not available. You don’t get radar returns in this mode. Now let’s move to Expanded ILS. This is the same ILS guidance, but with a different presentation. Instead of a full compass rose, you get about 90° of compass rose—a sector view, expanded for better readability. The ILS source is still in the lower left, and the frequency in the lower right. At the bottom of the display, the white triangle is the aircraft symbol—that’s you, the aeroplane, drawn on the screen. Your selected course—which here is described as selected track—is again shown by the magenta course needle, tip pointing to 150°. Now, a practical point: 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 course knob. Localizer deviation is the same traditional deviation bar on a two dot left, two dot right scale, and glide slope deviation is on the right in the traditional fashion. DME distance is again in the top left corner. Current heading is in the window and at the lubber line at the top of the compass rose—here it’s 130° Magnetic, as indicated by the markings either side of the window. Current track is shown differently in this mode: it’s the white line running from the tip of the aircraft symbol to the inside edge of the compass rose. So you can see your actual path relative to the aircraft. Selected heading is the magenta heading “bug” on the outer scale, and wind speed and direction are in the lower left corner, again orientated to your display selection—Heading or Track, Magnetic or True. Now here’s the big difference from Full ILS: in Expanded ILS, weather radar displays are available. When you select the radar “on”, you also get range arcs visible. The weather radar is shown in three colours: green, yellow, and red—green being the least turbulence, red being the worst. And if TURBULENCE MODE is available, that’s shown in magenta, marking 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—so the display is showing you a 10-nautical-mile half-scale range. So the key contrast between the two modes: Full ILS gives you the complete 360° picture but no weather radar; Expanded ILS gives you a 90° sector, the aircraft symbol, track as a line from the aircraft, and the ability to overlay weather radar with its colour-coded turbulence. Both give you the same ILS localizer and glide slope guidance, the same DME, heading, track, selected heading, and wind information—just presented differently for different phases of flight.