Electronic Flight Information System (EFIS) — Page 298, Lesson 293

I want to walk you through the Electronic Flight Information System — EFIS — and specifically what you're seeing on that EHSI, the Electronic Horizontal Situation Indicator, in its various display modes. This is the page that breaks down every symbol, pointer, and annunciation you'll see, and I want you to know exactly what each one is telling you. Let's start with the VOR/ILS frequency display, labelled (G). This shows the frequency of the manually tuned navaid. But here's the key detail: if the VHF Nav radio is in the auto-tune mode, the word 'AUTO' is displayed in place of the frequency. So you're never guessing whether you're on a manually selected frequency or an automatically tuned one — the display tells you. Next, the Drift Angle Pointer, labelled (W). This indicates the aeroplane's present track. Now, it replaces the track line when a Full Rose mode is selected. So in the full compass rose presentation, this pointer takes over that job. Now, the Altitude Range Arc, labelled (G). This is a prediction tool. The intersection of the arc with the track line is the predicted point where the MCP altitude will be reached. And that prediction is based on two things: present ground speed and aeroplane vertical speed. So the arc is showing you where, along your current path, you'll capture that altitude you've set on the Mode Control Panel. Let's move to the MAP Position Difference Display, labelled (W). This one has three parts. The NUMBERS indicate the Position Difference in nautical miles between the FMC's present position and the L IRS and R IRS present positions, respectively. So you're comparing the Flight Management Computer's position against each Inertial Reference System. The ARROWS rotate through 360° to indicate the relative bearing to the associated IRS present position. And the L or R tells you which IRS present position the displayed Position Difference corresponds to. So you always know which system you're looking at. Now, there's a failure condition here. This display is shown when the Position Difference of the L IRS and/or R IRS exceeds the Position Difference limits detected by the FMC or the EFIS Symbol Generator. So if one of those inertial systems drifts too far from the FMC's position, you get this warning. Next, the MAP Present Track Line and Range Scale, labelled (W). This displays the present ground track based on aeroplane heading and wind. Here's a critical detail: the range numeric values are one-half the actual selected range. So if you've selected a 20 NM range, the numbers on the scale read 10. And with heading-up orientation, the track line will be rotated left or right at an angle equal to the drift angle. So the line isn't just straight up the screen — it's angled by the drift. Now let's look at the Lateral Deviation Indicator Bar, labelled (M), with its Deviation Scale, labelled (W). This displays ILS or VOR course deviation. And the scale calibration matters: for ILS, 1 dot equals 1° on the Normal Scale, and 1 dot equals 1/2° on the Expanded Scale. For VOR, 1 dot equals 5°. So you're reading how far off the course centreline you are, in degrees per dot. The Selected Course Pointer and Line, labelled (W) and (M), points to the selected course as set by the respective MCP course selector — that's the VOR/ILS selector on the Mode Control Panel. This appears in the EXP ILS and EXP VOR modes. Then there's the Selected Course Pointer, labelled (W), and the To/From Pointer, labelled (W), for the FULL VOR and FULL ILS modes. The pointer again points to the selected course from the MCP. And the TO/FROM symbol is displayed when VOR navigation is being used. That's the To/From Annunciation, labelled (W), and it's operative in VOR Mode only. It indicates whether or not the selected course, if intercepted directly and tracked, would take the aircraft TO or FROM the station. So you know your direction of travel relative to the VOR. Now, the Off-route Waypoint, labelled (C), in MAP and PLAN modes. When the WPT switch is ON, FMC database waypoints not used in the selected flight plan route are displayed. But here's the limitation: displayed only for HSI ranges of 10, 20, or 40 NM. So outside those ranges, you won't see them. Finally, the Airport Identifier and Runway, labelled (W), in MAP and PLAN modes. This is available when the EHSI display range is 80, 160, or 320 NM. It's displayed if the airport has been selected as the origin or destination airport with a specific runway selected. And there's the ARPT switch: when it's OFF, only origin and destination are displayed. When it's ON, all FMC database airports within the MAP area are displayed. So you've got a complete picture of what each symbol means, which mode it appears in, and what the limits are. That's your EFIS display decoded.