Electronic Flight Information System (EFIS) — Page 287, Lesson 290

Let’s start with the EHSI symbology — the Electronic Horizontal Situation Indicator. This is the display that shows you, in plan view, where the aeroplane is relative to your route and your navaids. The B737-800 uses a specific set of symbols, and I’ll walk you through each one, what it’s called, which modes it appears in, and what it’s telling you. First, the Distance Display. It shows either a waypoint distance in nautical miles, or a tuned DME distance. So you’ll see something like “200 NM” or “DME 124.” The distance is to the next FMC waypoint, or to the tuned navaid if you’re using DME. Below 100 NM, you’ll get tenths of a nautical mile displayed. So above 100, whole miles; below 100, one decimal place. Next, the Heading Indicator. This is the box that shows the actual heading — the number under the pointer is a heading. It’s referenced to Magnetic North, or True North if you’ve selected TRU. So the box tells you which reference you’re using, and the number under the pointer is your current heading. Then the ETA Display — Estimated Time of Arrival. This appears in PLAN and MAP modes. It shows the FMC-computed ETA for the active waypoint. So the computer is telling you when you’ll arrive at the waypoint you’re currently navigating to. Now, the compass roses. There are two types. The Expanded Compass Rose appears in PLAN, MAP, VOR, and ILS modes. The compass data comes from the selected IRS — Inertial Reference System. You have 360 degrees available, but only about 70 degrees are displayed at a time. So it’s a partial arc, not a full circle. The Full Compass Rose appears in Full VOR and Full ILS modes. Again, compass data comes from the selected IRS, but here you see the full 360 degrees. Then the Aeroplane Symbol. There are two versions. In the expanded modes — EXP VOR/ILS, MAP, and PLAN — the aeroplane is a triangle, and its position is at the apex of the triangle. So the point of the triangle marks where the aeroplane is. In the Full VOR/ILS mode, the aeroplane symbol is different — its position is at the centre of the symbol. Finally, the Waypoint Active and Downpath symbols. These appear in MAP and PLAN modes. The Active waypoint is the one the aircraft is currently navigating to. The Downpath waypoint is a navigation point that makes up the selected active route — so it’s a point further along your route, not the one you’re heading to right now. One thing to note: the book says detailed knowledge of these symbols is not required for the EASA ATPL exams. But you still need to recognise them and understand what they represent, because you’ll see them on the display in the aircraft. Let me show you what this looks like on the actual display. That’s the EFIS display. And here’s the map mode, which shows you the route and the waypoints. So to summarise: the Distance Display gives you range to the next waypoint or DME, the Heading Indicator shows your heading reference, the ETA Display gives you arrival time, the compass roses show your heading arc, the aeroplane symbol shows your position, and the waypoint symbols show where you’re going and what’s along your route.