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

I want to walk you through the Electronic Flight Information System — EFIS — and specifically what I'm showing you here is the symbology and the logic behind the EHSI, the Electronic Horizontal Situation Indicator. This is the map display that sits in front of you on the instrument panel, and every symbol you see on it has a specific meaning and a specific condition under which it appears. Let me start with the frequency display, labelled G. This shows the frequency of the manually tuned navaid — the VOR or ILS you've dialled in. 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 not looking at a number, you're looking at the word AUTO, telling you the system has selected the frequency for you. Now, the drift angle pointer, labelled W. This indicates the aeroplane's present track. And it replaces the track line when a Full Rose mode is selected. So in the Full Rose presentation, this pointer takes over the job of showing you where you're actually tracking. Next, 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. MCP is the Mode Control Panel — that's where you set your target altitude. And the prediction is based on two things: your present ground speed and your aeroplane's vertical speed. So the system is looking at how fast you're climbing or descending and how fast you're moving over the ground, and it draws an arc showing you exactly where you'll level off. Now let's look at the position difference display, labelled W. This is a fascinating one because it's comparing three sources of position. The FMC — the Flight Management Computer — has its own computed position. And you have two Inertial Reference Systems, the L IRS and the R IRS, each with its own position. The NUMBERS on this display 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 seeing two numbers — one for the left IRS, one for the right. The ARROWS rotate through 360 degrees to indicate the relative bearing to the associated IRS present position. So the arrow points toward where that IRS thinks you are. And the L or R label indicates which IRS present position the displayed Position Difference corresponds to. So you can tell at a glance which inertial system is being compared. Now, this display is only 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 it's a warning display — it appears when one of the IRS positions has drifted too far from what the FMC believes, and you need to know about that disagreement. Let me move to the present track line and range scale, labelled W. This displays your present ground track based on aeroplane heading and wind. So it's not just your heading — it's where you're actually going over the ground, accounting for the wind pushing you. Here's a critical detail: the range numeric values are one-half the actual selected range. So if the display says 20, the actual range is 40. And with heading-up orientation, the track line will be rotated left or right at an angle equal to the drift angle. So if you're being blown right, the track line tilts right by exactly the drift angle. Now, the lateral deviation indicator bar and deviation scale, labelled M and W. This displays ILS or VOR course deviation. And here are the exact sensitivity values you need to know. For ILS, one dot equals 1 degree on the normal scale, and one dot equals half a degree on the expanded scale. For VOR, one dot equals 5 degrees. So the expanded ILS scale is twice as sensitive as the normal — half a degree per dot instead of a full degree. The selected course pointer and line, labelled W and M. This points to the selected course as set by the respective MCP course selector — the VOR/ILS course knob. So whatever course you've dialled in, this pointer shows you where that course lies on the display. Now, the To/From pointer, labelled W. This is 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 it's telling you the sense of the course — are you heading toward the VOR or away from it. And the TO/FROM symbol is displayed when VOR navigation is being used. Let me now cover the off-route waypoint, labelled C. This is shown in MAP and PLAN modes. When the WPT switch is ON, FMC database waypoints not used in the selected flight plan route are displayed. So these are waypoints that exist in the database but aren't part of your active route. And here's the range restriction: they're displayed only for HSI ranges of 10, 20, or 40 NM. So on the short-range settings, you see these extra waypoints. The airport symbol, labelled C, also in MAP and PLAN modes. This depends on the ARPT switch. When the ARPT switch is OFF, only the origin and destination airports are displayed. When the ARPT switch is ON, all FMC database airports within the MAP area are displayed. So you have a choice — just your two airports, or every airport in the database that falls within your map area. Finally, the airport identifier and runway, labelled W. This is available when the EHSI display range is 80, 160, or 320 NM — the long-range settings. And it's displayed if the airport has been selected as the origin or destination airport with a specific runway selected. So you see the airport's identifier and the specific runway you're using, but only on those long-range displays and only for your origin or destination. Let me pull this together. What you're looking at is a system that gives you three layers of information: navigation data like frequency and course, position awareness comparing your FMC against your inertial systems, and flight plan data like waypoints and airports — each with its own switch logic and range restrictions. The key discipline is knowing which symbol appears under which condition, because that tells you what the system is actually doing at any moment.