Electronic Instrumentation — Page 546, Lesson 651

I want to walk you through the EICAS display select panel and the alert message system. This is the heart of how the crew interacts with the engine and airframe monitoring on a modern transport aircraft. Let's start with the display select panel. This is the physical panel the crew uses to control what the EICAS displays show. It has four key controls. First, the Computer Select Switch. In the 'AUTO' position, it selects the left, or primary, computer, and automatically switches to the other computer in the event of failure. So think of it as a redundancy system — if the primary computer fails, the system automatically hands over to the backup without the crew doing anything. The other positions on the switch are for manual selection of left or right computers, giving the crew direct control if they need it. Next, the Displays Brightness Control. This has two knobs. The inner knob controls the intensity of the displays — that's the overall brightness. The outer knob controls brightness balance between displays — that's how you equalize the brightness across the different display units so they match. Then we have the Thrust Reference Set Switch. This one is a bit more involved. Pulling and rotating the inner knob positions the reference cursor on the thrust indicator display — either EPR or N1 — for the engine or engines selected by the outer knob. So the outer knob selects which engine you're setting, and the inner knob moves the reference cursor to the desired thrust setting on that engine's indicator. Finally, the Maximum Indicator Reset Switch. Here's the key concept: if any one of the measured parameters — for example oil pressure or EGT, which is exhaust gas temperature — should exceed normal operating limits, this will be automatically alerted on the display units. The purpose of the reset switch is to clear the alerts from the display when the excess limits no longer exist. So it's not a test switch or a mute — it clears the alert indication once the parameter has returned to normal. Now let's move to the alert message system. This is the brain of the EICAS. The system continuously monitors a large number of inputs — typically over 400 — from engine and airframe systems' sensors, and will detect any malfunctioning of systems. So it's constantly watching hundreds of parameters across the whole aircraft. If a malfunction occurs, appropriate messages are generated and displayed on the upper display unit in a sequence corresponding to the level of urgency of action to be taken. So the messages aren't just listed randomly — they're prioritized by how urgently the crew needs to act. Up to 11 messages can be displayed, and they come in three levels. Level A is a Warning, requiring immediate corrective action. These are displayed in red. Master warning lights are also illuminated, and aural warnings — for example a fire bell — from a central warning system are given. So this is the highest urgency: act now, and you get both visual and audible alerts. Level B is a Caution, requiring immediate crew awareness and future crew action. These are displayed in amber, and also by message caution lights. An aural tone is also repeated twice. So it's less urgent than a warning, but the crew must be aware and plan action — and you get a distinctive double tone. Level C is an Advisory, requiring crew awareness. Also displayed in amber. No caution lights or aural tones are associated with this level. So it's purely informational — the crew should be aware, but there's no light or sound to draw attention. The messages appear on the top line at the left of the display screen, as shown in Figure 39.5. Let me show you the functional diagram of the whole EICAS system so you can see how the sensors, computers, and displays all connect. And here's what the actual engine data displays look like on the screen. And this is the display select panel we just walked through. So to tie it together: the system watches over 400 inputs, prioritizes any malfunction into one of three urgency levels, and presents them on the upper display — red for warnings you must act on now, amber for cautions you must acknowledge and act on later, and amber advisories you just need to be aware of. The crew controls the computers, brightness, thrust reference, and alert clearing from the select panel.