Electronic Instrumentation — Page 546, Lesson 651

I want to walk you through the EICAS control panel and its alert message system. We're looking at the heart of how the crew interacts with engine and airframe monitoring. Let's start with the controls on the display select panel. 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. The other positions are for the manual selection of left or right computers. So you have three positions: AUTO, LEFT, and RIGHT. In AUTO, the system runs on the left computer as primary, and if that fails, it swaps to the right one without you touching anything. The manual positions let you force a specific computer if you need to. Next, the Displays Brightness Control. This is a dual-knob arrangement. The inner knob controls the intensity of the displays, and the outer knob controls brightness balance between displays. So the inner knob sets the overall brightness level, and the outer knob adjusts the balance so one display isn't brighter than the other. Then we have the Thrust Reference Set Switch. 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. EPR is Engine Pressure Ratio, and N1 is the low-pressure compressor speed, typically shown as a percentage. So you pull the inner knob, rotate it to move the reference cursor to the desired thrust setting, and the outer knob selects which engine or engines that cursor applies to. Finally, the Maximum Indicator Reset Switch. 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 the system flags an exceedance automatically, and once the parameter is back within limits, you use this switch to clear the alert from the display. Now let's move to the alert message system. The system continuously monitors a large number of inputs — typically over 400 — from engine and airframe systems' sensors, and it will detect any malfunctioning of systems. If this should occur, 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. 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 Level A is the most urgent: red text, master warning lights, and an audible alarm. 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 Level B is amber, has a caution light, and gives you a double tone — not a continuous alarm like Level A, but still demanding your attention and a planned action. Level C is an Advisory requiring crew awareness. Also displayed in amber, but no caution lights or aural tones are associated with this level. So Level C is the lowest urgency: amber text, no lights, no sound — just information you should be aware of. The messages appear on the top line at the left of the display screen, as shown in Figure 39.5. That's where you'll see them stacked, up to 11 at a time, ordered by urgency. So to tie it together: the control panel gives you computer selection, display brightness, thrust reference setting, and alert reset. The alert system itself monitors over 400 inputs and presents up to 11 messages in three urgency levels — red warnings with lights and sound, amber cautions with lights and a double tone, and amber advisories with no lights or sound. That's the full picture of how EICAS keeps you informed.