Electronic Flight Information Systems — Page 297, Lesson 358

I want to walk you through the symbology of the Electronic Flight Information System — specifically the EHSI, the Electronic Horizontal Situation Indicator. This is the display that replaces the old mechanical compass rose and gives you a moving map of your world. What I have here is essentially the legend, the dictionary of every symbol you'll see on that screen, and I want to teach you to read it the way a professional reads it — instantly, and without hesitation. Let's start with the first symbol on the list: the Distance Display. You'll see it labelled with the letter W, which stands for Window — meaning it's a readout that appears in a window on the display. It's applicable in the PLAN and MAP modes, or in the VOR and ILS modes. The distance is shown either to the next FMC waypoint, in nautical miles, or to the tuned navaid — that's a navigational aid like a VOR or DME — and again it's in nautical miles. So when you see "200 NM" or "DME 124", that's the distance to your next programmed point or to the radio beacon you've tuned. The display can show either the FMC distance or the DME distance, depending on what's selected. Next is the Heading symbol, marked with the letter M for Marker. This is the heading readout. The number under the pointer is a heading, and the box around it indicates your actual heading. Now here's a critical detail for professional operation: this heading is referenced to magnetic north between 60° South and 73° North latitude. Above those latitudes — beyond 73° North or below 60° South — the reference switches to true north. That's a real operational limitation you need to know, because in polar operations the magnetic reference becomes unreliable. Then we have the ETA Display, marked both M and W — Marker and Window. This shows the Estimated Time of Arrival at the active waypoint, and it's calculated based on your current ground speed. Not airspeed — ground speed. That's the speed over the Earth's surface, and it's what actually determines when you'll arrive. The Selected Heading Marker is next, marked M. This indicates the heading you've set in the MCP — the Mode Control Panel, which is where the autopilot and flight director get their commands. A dotted line extends from the marker to the aeroplane symbol, so you can track where that selected heading is even when it's off-screen. That dotted line appears in all modes except PLAN mode. Now let's talk about the compass roses, because there are two of them. The Expanded Compass Rose, marked W, is used in PLAN, MAP, VOR, and ILS modes. The compass data comes from the selected IRS — that's the Inertial Reference System. The full 360° is available, but only approximately 70° is displayed at any one time. That's the "expanded" concept — you see a magnified slice of the compass, which gives you better resolution for tracking. The Full Compass Rose, also marked W, is used in Full VOR and Full ILS modes. Here, the compass data also comes from the selected IRS, but you see the entire 360° circle. So the difference is: expanded shows you a 70° slice, full shows you everything. Let me pause here and point you to the figures, because these will make it concrete. shows the expanded compass rose with a VOR frequency selected — you can see about 90° of the rose there. shows the full compass rose with an ILS frequency selected. And shows the expanded version again, but with the ILS source. Take a moment to look at those — the difference between expanded and full will jump out at you. Now, moving on. The Present Track Line and Range Scale — this is a big one. It's marked with the number 80, and it's applicable in MAP, VOR, and ILS modes. This line predicts the ground track that will result from your present heading and the winds. So it's not just where you're pointing — it's where the wind is actually pushing you. The displayed range mark is one-half the actual selected range. So if you've selected a 40 NM range, the mark on the display represents 20 NM. That's a scale detail you need to internalize. The Aeroplane Symbol, marked W, appears in MAP, VOR, and ILS modes. It represents the aeroplane, and its position is at the apex of the triangle — the point of the triangle is where you are. But in Full VOR and Full ILS modes, the aeroplane symbol is different: it's at the centre of the symbol, not the apex. So the same symbol, two different reference points, depending on the mode. Next, the Waypoint symbols. The Active waypoint, marked M, represents the waypoint the aircraft is currently navigating to. The Inactive waypoint, marked W, represents a navigation point that makes up the selected active route — it's on your route, but it's not the one you're flying to right now. You'll see names like "AMBOY" on the display — that's an example of a waypoint identifier. The Altitude Range Arc, marked G for Graphic, is applicable in MAP mode. When this arc intersects with the track line, it predicts the point where the reference altitude will be reached. So if you're climbing or descending to a target altitude, this arc shows you where along your track you'll hit that altitude. The Trend Vector is next, and this is a beautiful piece of predictive display. It predicts the aeroplane's directional trend at the end of 30, 60, and 90 second intervals. It's based on bank angle and ground speed. Now here's the detail: the number of segments displayed depends on your selected range. When the selected range is greater than 30 NM, you see three segments. On the 20 NM scale, you see two segments. And on the 10 NM scale, you see one segment. So the trend vector adapts to how zoomed in you are. Finally, let's look at the Route displays. The Active Route, marked M, is displayed with continuous lines between waypoints. The Active Route Modifications, marked W, are displayed with short dashes between waypoints. When you make a change in the FMC — the Flight Management Computer — and you activate that change, the short dashes are replaced by a continuous line. So the dashes tell you "this is a proposed change, not yet active." The Inactive Route, marked C for Colour, is displayed with long dashes between waypoints. So you have three line styles: continuous for active, short dashes for pending modifications, long dashes for inactive routes. That's your visual language for route status. And I want to point you to , which shows the MAP display — the mode used normally in flight. It works in conjunction with the flight plan data to give you that complete situational picture. So let me tie this together. Every symbol on that EHSI is telling you something specific: where you are, where you're going, when you'll get there, what the wind is doing to you, and what your route status is. The letters — M for Marker, W for Window, G for Graphic, C for Colour — tell you how that symbol is rendered. The modes tell you when each symbol appears. And the numbers — like the 70° expanded rose, the one-half range mark, the 30/60/90 second trend vector — are the precise values you'll be tested on and, more importantly, that you'll use every day in the cockpit. Learn this legend cold, and the display becomes a second language.