Electronic Flight Information Systems — Page 304, Lesson 360

Let's look at the EHSI — the Electronic Horizontal Situation Indicator — and the different symbols and readouts you'll see on it. I want to walk you through each one, because each has a specific job and a specific mode it works in. First, the Vertical Pointer and Deviation Scale, marked with an M for Map mode. This displays vertical deviation from your selected vertical profile, but only during descent, and only in MAP mode. The scale shows plus or minus 400 feet of deviation. So if you're descending on a computed path, this pointer tells you how far above or below that path you are, up to 400 feet either way. Next, the Glide Slope Pointer and Deviation Scale, marked with a W for ILS mode. This one displays the glide slope position and deviation when you're flying an ILS approach. It's the same idea as the vertical pointer, but for the ILS glide slope instead of a computed vertical profile. Then we have the Drift Angle Pointer, marked with a W, and it appears in Full VOR and Full ILS modes. This displays the difference between the FMC track angle and the IRS heading. In plain terms, it shows you how much the wind is pushing you sideways — the drift angle — by comparing where the flight management computer thinks you're tracking versus where the inertial reference system says the nose is pointing. The Wind Speed and Direction indicator, marked with a W, works in MAP, VOR, and ILS modes. It gives you wind speed in knots, and wind direction referenced to the map display orientation and the compass reference. So you see both how fast the wind is blowing and which way it's coming from relative to your display. Now the Offset Path and Identifier, marked with an M, works in MAP and PLAN modes. It presents a dot-dash line that runs parallel to, and offset from, the active route, after you've selected that offset on the FMC CDU — the Control Display Unit. This is useful when you want to fly parallel to your route, say to avoid weather. The North Pointer, marked with a G, appears in PLAN mode. It indicates that the map background is oriented and referenced to true north. So in PLAN mode, your display is always north-up. Next, the Altitude Profile Point and Identifier, marked with a G, works in MAP mode. It represents an FMC-calculated point, and it's labelled on the flight plan path. You'll see "T/C" for top of climb, "T/D" for top of descent, and "S/C" for step climb. These are the points where your computed vertical profile changes. Finally, the Weather Radar Returns and Mapping Radar Returns, marked with G, A, and R, work in MAP, VOR, and ILS modes. These are multicoloured returns that appear when either "WXR ON" switch is pushed. The most intense regions show in red, lesser intensity in amber, and the lowest intensity in green. So you get a colour-coded picture of weather intensity right on your display. That figure shows example displays of these modes. And then the chapter moves into practice questions — those are the book's practice questions, so let's try them one at a time.