Electronic Instrumentation — Page 546, Lesson 654

Let’s pick this up right where the EICAS message logic leaves off, because this is where the system’s behaviour really starts to matter in the cockpit. First, the distinction between a caution and an advisory. On the EICAS display, a caution message and an advisory message are not just different in severity — they are physically formatted differently. The advisory is always indented one space to the right. That indentation is your visual cue: if it’s pushed in one space, it’s an advisory, not a caution. So the format itself tells you the priority. Now, the master warning and caution lights. These are located adjacent to the display units, and right next to them you have two switches: a ‘CANCEL’ switch and a ‘RECALL’ switch. Here’s the critical logic. Pushing the ‘CANCEL’ switch removes only the caution and advisory messages from the display. Warning messages cannot be cancelled — that’s a hard limit. You can never clear a warning by pressing CANCEL. The ‘RECALL’ switch does the opposite: it brings the caution and advisory messages back into the display. And at the same time, the word ‘RECALL’ appears at the bottom of the display, so you know the messages you’re seeing have been recalled, not freshly generated. Now, how messages move. A message is automatically removed from the display when the associated condition no longer exists. So when a fault clears, its message disappears on its own. And here’s the neat part: when a message is deleted, all the messages that were below it each move up one line. The list closes up. When a new fault occurs, its associated message is inserted on the appropriate line of the display. That insertion can push older messages down one line. For example, a new caution message would cause all existing caution and advisory messages to move down one line. So the list is dynamic — new faults push down, cleared faults pull up. Now, what happens if there are more messages than can be displayed at one time? The whole list forms what is 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. So you know you’re looking at page one of a multi-page list. If there is an additional page of messages, it can be displayed by pushing the ‘CANCEL’ switch. And note this: warning messages are carried over from the previous page. Warnings are never lost when you page through — they persist across pages. Now let’s move to display unit failure. This is a specific failure case. If the lower display unit should fail when secondary information is being displayed on it, an amber alert message appears at the top left of the upper display unit, and the information is transferred to it. So the upper unit takes over the lower unit’s job. The format of this transferred display is referred to as ‘compact’. And it may be removed by pressing the ‘ENGINE’ switch on the display select panel. So you can clear that compact format by pressing ENGINE. But there’s a consequence. Failure of a display unit 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 — the STATUS switch is inhibited. Let me tie this together. The EICAS message system has a strict hierarchy: warnings can’t be cancelled, cautions and advisories can be cancelled and recalled. Messages move up when faults clear, down when new faults appear. When the list overflows, it pages, with the page number in white at the lower right, and warnings carry over between pages. And when a display unit fails, the system transfers the information to the remaining unit in a compact format, which you can remove with the ENGINE switch, but you lose the STATUS page function. That’s the complete logic of the EICAS message display and its failure behaviour.