Electronic Flight Information Systems — Page 297, Lesson 358

Right, let's get into the heart of the Electronic Flight Information System — specifically, the symbology you'll see on the EHSI, the Electronic Horizontal Situation Indicator. This is the display that replaces the old mechanical compass rose and gives you your navigation picture. I want to walk you through the symbols you'll see, what they're called, and crucially, in which modes they appear. The display isn't a single fixed picture; it changes depending on what you're doing — whether you're in PLAN, MAP, or on a VOR or ILS approach. Let's start with the Distance Display. You'll see this as either "200 NM / 4.4 NM" or "DME 124 / DME 24.6". This shows distance in nautical miles. In PLAN and MAP modes, it's the distance to the next FMC waypoint. In VOR or ILS modes, it's the distance to the tuned navaid, as read from the DME. Next is the Heading Indicator, shown as "263 HDG". This is a reference for orientation. In all modes — PLAN, MAP, VOR, ILS — it tells you that the number under the pointer is a heading. The box around it indicates your actual heading. Now, here's a critical detail: this heading is referenced to magnetic north between 60° South and 73° North latitude. But if you're flying above those latitudes — in the polar regions — it automatically switches to true north. That's a key operational point. Then we have the ETA Display, shown as "0835.4z". This appears in PLAN and MAP modes. It gives you the Estimated Time of Arrival at the active waypoint, calculated from your current ground speed. Now, the Selected Heading Marker. This is the heading you've dialled into the MCP — the Mode Control Panel. It appears in PLAN, MAP, VOR, and ILS modes. A dotted line extends from the marker to the aeroplane symbol, so you can track where that marker is even when it's out of view. Note that this dotted line doesn't appear in plan mode. Let's talk about the compass roses. There are two types. The Expanded Compass Rose shows about 70° of the full 360° available. It's used in PLAN, MAP, VOR, and ILS modes. The compass data comes from the selected IRS — the Inertial Reference System. The Full Compass Rose shows all 360°, and it's used in Full VOR and Full ILS modes. Again, data comes from the selected IRS. Now, the Present Track Line and Range Scale. This appears in MAP, VOR, and ILS modes. It predicts the ground track that will result from your present heading and the winds. The displayed range mark is one-half of the actual selected range — so if you've selected 40 NM, the mark shows 20 NM. The Aeroplane Symbol appears in two forms. In MAP, VOR, and ILS modes, it's a triangle, and your position is at the apex — the point of the triangle. In Full VOR and Full ILS modes, it's a different symbol, and your position is at the centre. Let's look at waypoints. The Active Waypoint — shown as "AMBOY" — appears in MAP and PLAN modes. It represents the waypoint you're currently navigating to. The Inactive Waypoint — also shown as "AMBOY" — represents a navigation point that makes up your selected active route, but isn't the one you're heading to right now. The Altitude Range Arc appears in MAP mode. When it intersects with the track line, it predicts the point where your reference altitude will be reached. This is a vertical navigation aid. The Trend Vector is a prediction of your directional trend at the end of 30, 60, and 90 second intervals. It's based on bank angle and ground speed. Here's the detail: when your selected range is greater than 30 NM, you see three segments. On the 20 NM scale, you see two. And on the 10 NM scale, you see just one segment. Finally, the Active Route and its modifications. The active route is shown with continuous lines between waypoints. Active route modifications — changes you've made but not yet activated — are shown with short dashes. When you activate the change in the FMC, those short dashes become a continuous line. Inactive routes are shown with long dashes between waypoints. Let me show you what this looks like in practice. — here's the expanded compass rose with a VOR frequency selected. — and here's the full compass rose with an ILS frequency. — and the expanded version with ILS. — and finally, the MAP display, which is the mode you'll use normally, in conjunction with your flight plan data. That's the core symbology. Each symbol has a specific meaning, a specific mode it appears in, and a specific operational purpose.