Electronic Flight Information Systems — Page 304, Lesson 360

Let's look at the EHSI symbology table. This is the heart of understanding what you see on an Electronic Horizontal Situation Indicator. Each row describes one symbol, which modes it appears in, and what it tells you. First, the Vertical Pointer and Deviation Scale. This is marked with an M, meaning it's a magenta symbol. It shows vertical deviation from your selected vertical profile, but only in MAP mode and only during descent. The scale shows plus or minus 400 feet of deviation. So if you're descending on a computed profile, this pointer tells you how far above or below that profile you are, up to 400 feet either way. Next, the Glide Slope Pointer and Deviation Scale, also magenta. This one operates in ILS mode. It displays the glide slope position and your deviation from it. This is your vertical guidance on an ILS approach. Then we have the Drift Angle Pointer, marked W for white. It appears in Full VOR and Full ILS modes. It displays the difference between the FMC track angle and the IRS heading. In plain terms, it shows you the angle between where the flight computer wants you to go and where the inertial system says the nose is pointing — that's your drift. The Wind Speed and Direction indicator, white, shows in MAP, VOR, and ILS modes. It gives wind speed in knots and wind direction referenced to the map display orientation and the compass reference. The Offset Path and Identifier, magenta, appears in MAP and PLAN modes. After you select it on the FMC CDU, it presents a dot-dash line parallel to and offset from the active route. This is for flying a parallel offset from your planned track. The North Pointer, green, appears in PLAN mode. It indicates the map background is oriented and referenced to true north. Then the Altitude Profile Point and Identifier, green, in MAP mode. This represents an FMC-calculated point labelled on the flight plan path. The labels are "T/C" for top of climb, "T/D" for top of descent, and "S/C" for step climb. Finally, the Weather Radar Returns and Mapping Radar Returns, both shown in green, amber, and red, appear in MAP, VOR, and ILS modes. These multicoloured returns are presented when either "WXR ON" switch is pushed. The most intense regions display in red, lesser intensity in amber, and the lowest intensity in green. That figure shows example displays of these modes. Then the chapter closes with practice questions referencing Annex A, which shows various EHSI displays. Those are the book's practice questions — let's try them one at a time.