Electronic Instrumentation — Page 546, Lesson 654

Let’s pick up right where we left off — we’ve been looking at how the EICAS organizes its messages, and now I want to walk you through the finer points of how those messages are managed, and what happens when a display unit itself fails. First, the distinction between a caution and an advisory. In the EICAS message list, an advisory is always indented one space to the right of a caution. That’s the visual cue — the indentation tells you at a glance which category a message belongs to, even before you read the text. Now, the master warning and caution lights. These are located adjacent to the display units, and right there with them you have two switches: a ‘CANCEL’ switch and a ‘RECALL’ switch. Here’s the key operating rule: pushing the ‘CANCEL’ switch removes only the caution and advisory messages from the display. The warning messages cannot be cancelled — that’s a hard limitation. Warnings stay until the condition clears. The ‘RECALL’ switch does the opposite job: it brings the caution and advisory messages back into the display. And when you do that, the word ‘RECALL’ appears at the bottom of the display, so you know you’re looking at recalled messages rather than live ones. Now, how do messages leave the display on their own? A message is automatically removed when the associated condition no longer exists. And here’s the neat part about the list mechanics: when a message is deleted, all the messages below it move up one line to fill the gap. So the list always stays tight — no blank lines. When a new fault occurs, its message is inserted on the appropriate line of the display. That insertion can push older messages down one line. For example, if a new caution message comes in, all existing caution and advisory messages move down one line to make room. So the list is dynamic — it shifts up when things clear, and shifts down when new faults arrive. Now, what if there are more messages than can be displayed at one time? The whole list forms what’s termed a ‘page’. The lowest message is removed from view, and a page number appears in white on the lower right side of the list. That white page number tells you there’s more than one page of messages. If there’s an additional page, you can display it by pushing the ‘CANCEL’ switch — and here’s the important detail: warning messages are carried over from the previous page. So warnings are never lost when you page through; they persist across pages. Now let’s move to a different failure scenario — display unit failure. Suppose the lower display unit fails while secondary information is being displayed on it. What happens? An amber alert message appears at the top left of the upper display unit, and the information that was on the failed lower unit is transferred to the upper unit. The format of this transferred display is referred to as ‘compact’ — that’s the technical name for this reorganized layout. You can remove the compact format by pressing the ‘ENGINE’ switch on the display select panel. But there’s another consequence of a display unit failure: it causes the function of the panel ‘STATUS’ switch to be inhibited. That means the status page format cannot be displayed. So when you lose a display unit, you lose the ability to call up the status page — that function is locked out until the failure is resolved. Let me tie that together for you. The message management system has three layers: warnings that can’t be cancelled, cautions and advisories that can be cancelled and recalled, and automatic removal when conditions clear. The list shifts up and down as messages are deleted and inserted. When the list overflows, it pages, with warnings carried over. And when a display unit fails, you get the amber alert at the top left, the compact format on the surviving unit, and the status page function inhibited. Take a look at Figure 39.6 — that shows you the ‘compact format’ display, so you can see exactly how the information is reorganized when the lower unit fails.