Electronic Flight Information Systems — Page 297, Lesson 356

Right, let's pick this up with the PLAN mode on the EHSI. This is a distinct display mode you'll select when you want to review your route before you commit to flying it. In PLAN mode, the EHSI shows a static map background. The key word there is static — the map itself doesn't move or rotate with your aircraft's heading. Instead, the active route data is orientated to true north. So the map is always drawn with north at the top, regardless of which way the aircraft is pointing. Now, here's the workflow. Any changes to the route — say you want to add, delete, or modify a waypoint — you select those changes at the keyboard of the flight management computer, the FMC. The display of those proposed changes is shown on the EHSI, so you can check them before they are entered into the FMC. It's a preview function. You verify the change looks correct on the display, and only then do you commit it. The top portion of the EHSI — that's the compass rose area — remains the same as in the map mode. So you still get your heading reference up top. This mode allows the pilot to review the planned route by using the FMC/CDU LEGS page. CDU is the Control Display Unit — that's the keyboard and screen unit you use to talk to the FMC. The LEGS page is the page that lists the legs of your flight plan, the sequence of waypoints and the tracks between them. There are two important inhibitions in PLAN mode. First, weather radar display data is inhibited — you won't see any weather returns on this display. Second, no wind speed or direction information is shown. So this is purely a route-planning picture, not a tactical navigation or weather picture. Now, there's a subtle trap here that I want you to be very aware of, because it's a classic confusion point. The arc of compass shown at the top of the display is "real time" — that means it reflects your aircraft's current actual heading, live. But the planning section below that is north orientated. So the top compass arc and the map below are not necessarily aligned with each other. Here's the example from the text: the heading from TOBIX to LOGAN is approximately 020°, NOT 155°. So if you're looking at the map and you think the route looks like it's about 155°, you'd be wrong — the map is north-up, so the true track from TOBIX to LOGAN is about 020°. The 155° figure is a distractor, a false reading you might get if you mistakenly interpret the map using the real-time compass arc. Always remember: in PLAN mode, the map is north-up, and the compass arc is real-time heading. They're two separate references. Let me show you what this looks like. Now, let's move on to Data Source Selection. In the type of system we've been describing, there are means provided whereby the pilots can, independently of each other, connect their respective display units to alternate sources of input data. This is done through a data source switch panel. So the captain and the first officer each have their own display units, and each can independently select which source feeds their displays. The alternate sources you can select from include: the left or right Air Data Computer — that's the ADC, which computes airspeed, altitude, and other air data; the Flight Management Computers, the FMCs; the Flight Control Computers, the FCCs; and the Standby Inertial Reference Systems, the IRSs. The point of this is redundancy and flexibility. If one source fails, or if you want to cross-check one source against another, you can switch your display to a different source without affecting the other pilot's displays. Next, let's talk about Failure Annunciation. When data signals fail — for example, from systems such as the ILS, the Instrument Landing System, or the radio altimeter — that failure is displayed on each EADI and each EHSI. The EADI is the Electronic Attitude Director Indicator, the primary flight display. The failure is shown in the form of yellow flags 'painted' at specific matrix locations on their CRT screens. CRT is the cathode ray tube — the display screen itself. So you get a yellow flag appearing at a specific position on the screen to tell you that particular data source has failed. In addition to those flags, fault messages may also be displayed. For example, if the associated flight management computer and weather radar range disagree with the control panel range data, the discrepancy message 'WXR/MAP RANGE DISAGREE' appears on the EHSI. So WXR is weather radar. If the range selected on the control panel doesn't match what the FMC and the weather radar are actually using, you get that explicit text message on the display. Finally, let's look at the colour conventions used on the navigation displays. These are the standard colours you'll see on each HSI, the Horizontal Situation Indicator, depending on the EFI Control Panel switch selection. EFI is Electronic Flight Instrumentation. Symbols can be displayed with different colours, but the general colour presentation is as follows. GREEN, abbreviated G, is used for engaged flight mode displays and dynamic conditions. So if a mode is actively engaged, it's green. WHITE, abbreviated W, is used for present status situation, scales, and armed flight mode displays. So things that are set up and ready, but not yet active, are white. MAGENTA, abbreviated M — that's the pink colour — is used for command information, pointers, symbols, and the fly-to condition. So when you're being commanded to fly somewhere, that command is magenta. CYAN, abbreviated C — that's blue — is used for non-active and background information. So the stuff that's there for context but isn't the active focus is cyan. RED, abbreviated R, is used for warning. Red is always the highest level of alert. AMBER, abbreviated A, is used for cautionary information, faults, and flags. So the yellow flags we just talked about — those are amber in this colour scheme. And BLACK, abbreviated B, is used for blank areas and display "off" conditions. So the colour code is your quick visual shorthand: green for what's active, white for what's armed, magenta for what you're commanded to do, cyan for background, red for warning, amber for caution, and black for blank or off. That's PLAN mode, data source selection, failure annunciation, and the colour conventions. Let's keep going.