Electronic Flight Information Systems — Page 297, Lesson 356

Let's pick up with the PLAN mode on the EHSI. This is the mode you'd use on the ground or during cruise to review your route before you commit to it. In PLAN mode, the EHSI shows a static map background, and the active route data is oriented to true north. That's a key point — the map is fixed, and the route is drawn relative to true north, not to your aircraft's heading. Any changes to the route can be selected at the keyboard of the flight management computer, and the display shows on the EHSI so you can check the changes before they're entered into the FMC. So it's a preview function — you see what you're about to load before you actually load it. The top portion of the EHSI remains the same as in map mode. So the upper part of the display, where you see your heading and other primary data, doesn't change. What changes is the lower planning section. This mode lets you review the planned route using the FMC/CDU LEGS page. LEGS is the page on the control display unit that shows the individual legs of your flight plan — each waypoint and the track between them. So you can cross-check the route on the EHSI against the LEGS page. Now, two important limitations in PLAN mode. First, weather radar display data is inhibited — you won't see weather returns. Second, no wind speed or direction information is shown. So this is purely a navigation planning display, not a tactical weather or wind display. Here's a critical point I want you to remember, because it's a classic trap. The arc of compass shown at the top is "real time" — it reflects your current heading. But the planning section below is north-oriented. So if you're looking at a route from TOBIX to LOGAN, the heading shown on that planning section is approximately 020°, NOT 155°. The 155° might be your current heading on the compass arc, but the route line is drawn relative to true north. Don't confuse the two. Now let's move to data source selection. In this type of system, means are provided so the pilots can, independently of each other, connect their respective display units to alternate sources of input data through a data source switch panel. So the captain and first officer can each select different sources for their own displays. The sources you can select include the left or right Air Data Computer (ADC), the Flight Management Computers (FMC), the Flight Control Computers (FCC), and the Standby Inertial Reference Systems (IRS). So if one source fails, you can switch your display to an alternate source without affecting the other pilot's display. Now, failure annunciation. When data signals fail from systems like the ILS or the radio altimeter, that failure is displayed on each EADI and EHSI in the form of yellow flags 'painted' at specific matrix locations on their CRT screens. So you get a yellow flag at a specific position on the screen indicating which signal has failed. In addition, 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 if the FMC and weather radar are set to different ranges than what's selected on the control panel, you get that specific message. Finally, let's cover the navigation display symbols and their colour coding. These symbols can be displayed on each HSI depending on the EFI Control Panel switch selection. The general colour presentation is as follows: GREEN (G) is for engaged flight mode displays and dynamic conditions. So things that are actively happening. WHITE (W) is for present status situation, scales, and armed flight mode displays. So what's currently set, and modes that are armed but not yet engaged. MAGENTA (M), which is pink, is for command information, pointers, symbols, and fly-to conditions. So the guidance you need to follow. CYAN (C), which is blue, is for non-active and background information. Things that are there but not currently relevant. RED (R) is for warning. The most critical. AMBER (A) is for cautionary information, faults, and flags. So the yellow flags we mentioned earlier would fall under this category. BLACK (B) is for blank areas and display "off". So areas of the screen that are intentionally blank, or when the display is switched off. So when you're looking at an EHSI, the colour tells you the priority and nature of the information. Green is active, white is set or armed, magenta is command, cyan is background, red is warning, amber is caution, and black is blank or off. That's the PLAN mode, data source selection, failure annunciation, and the colour coding for the navigation displays.