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

I want to walk you through the symbology of the Electronic Horizontal Situation Indicator — the EHSI — as it appears on the Boeing 737-800. This is the display that shows you your navigation picture: where you are, where you're heading, and where you're going. Let me start with the first symbol in the table. It's called the Distance Display, and it's applicable in ALL modes. This is the readout that shows you either a distance in nautical miles to the next waypoint, or the DME distance to a tuned navaid. So you'll see something like "200 NM" or "DME 124." The key detail here is that below 100 nautical miles, tenths of a nautical mile will be displayed — so you get finer resolution as you get closer. Next we have the Heading Indicator, also applicable in ALL modes. This is the box that indicates your actual heading. The number under the pointer is a heading, and it's referenced to either Magnetic North, shown as "M," or True North, shown as "TRU." So when you see that heading box, you know the number inside is your current heading relative to one of those two references. Then there's the ETA Display, which is applicable in PLAN and MAP modes only. ETA stands for Estimated Time of Arrival. This shows the FMC-computed ETA for the active waypoint — the FMC being the Flight Management Computer. So it's telling you when you expect to arrive at the waypoint you're currently navigating toward. Now we come to the compass displays, and here we have a distinction between two types. First, the Expanded Compass Rose, applicable in PLAN, MAP, VOR, and ILS modes. The compass data is provided by the selected IRS — that's the Inertial Reference System. The full 360 degrees are available, but only approximately 70 degrees are displayed at any one time. That's why it's called "expanded" — you're zoomed in on a portion of the compass. The second type is the Full Compass Rose, applicable in Full VOR and Full ILS modes. Again, compass data comes from the selected IRS, but here you see the complete 360-degree rose. Now, the aeroplane symbol. This appears in two different forms depending on the mode. In the EXP VOR/ILS, MAP, and PLAN modes, the aeroplane symbol is a triangle, and the aeroplane's position is at the apex of that triangle — the point of the triangle. In the Full VOR/ILS mode, the aeroplane symbol represents the aircraft at the centre of the symbol instead. Finally, we have the Waypoint Active and Downpath symbols, applicable in MAP and PLAN modes. The Active waypoint represents the waypoint the aircraft is currently navigating to. The Downpath symbol represents a navigation point that makes up the selected active route — so it's a point further along your route that you haven't reached yet. One thing I want to stress: the table says detailed knowledge of these symbols is not required for the EASA ATPL examinations. So you need to recognise them and understand what they represent, but you don't need to memorise every nuance. Let me show you what this looks like in practice. That's the EFIS display itself. And here's the map mode, which shows you how these symbols come together in a real navigation picture. So to tie it all together: the EHSI gives you a horizontal picture of your navigation. The distance display tells you how far to the next point. The heading indicator tells you which way you're pointing. The ETA tells you when you'll get there. The compass rose gives you the directional context. And the aeroplane symbol plus the waypoint symbols show you where you are relative to your route. Each symbol has its own applicable modes, and knowing which mode you're in tells you which symbols you should expect to see.