Electronic Instrumentation — Page 546, Lesson 647

I want to walk you through the EICAS system — the Engine Indication and Crew Alerting System — starting with the display units themselves. This is the heart of what you'll be scanning in the cockpit, so let's build it up properly. First, the display units. These are the screens that give you a wide variety of information relevant to engine operation, and also the operation of other automated systems. They use colour shadow mask CRTs — that's cathode ray tube technology — and associated card modules whose functions are identical to those of the EFIS units, the Electronic Flight Instrument System units you've already met. So the hardware is the same family; the EICAS displays are just dedicated to engine and systems data. The units are mounted one above the other, as you can see in Figure 39.2. That physical stacking matters because it defines which screen shows what. The upper unit displays the primary engine parameters. Those are N1 speed — that's the fan speed, the low-pressure compressor speed — EGT, which is exhaust gas temperature, and warning and caution messages. So the top screen is your go-to for the critical engine health numbers and any alerts. Now, in some cases this upper unit can also display EPR — engine pressure ratio — depending on the type of engines installed and on the methods used to process data by the thrust management control system. So EPR isn't always there; it depends on the engine fit and how the thrust management system handles the data. The lower unit displays the secondary engine parameters. That's N2 speed — the high-pressure compressor speed — fuel flow, oil quantity, oil pressure and oil temperature, and engine vibration. So the lower screen carries the supporting engine data. But it doesn't stop there. The lower unit can also display the status of non-engine systems — for example, flight control surface positions, the hydraulic system, the APU, the auxiliary power unit — together with aircraft configuration and maintenance data. So the lower screen is your systems and configuration page as well. Here's a neat detail: the rows of 'V's shown on the upper display unit only appear when secondary information is being displayed on the lower unit. Those 'V's are a cue that tells you the lower screen is showing secondary data — so you know the lower unit isn't blank or on a different page. Now let's talk colour. The CRTs produce seven colours, and each colour has a strict meaning. This is a professional standard you must know cold. White is used for all scales, the normal operating range of pointers, and digital read-outs. So the basic instrument markings are white. Red is for warning messages, maximum operating limit marks on scales, and digital read-outs. Red is the highest priority — it's the danger colour. Green is for the thrust mode read-out and selected EPR or N1 speed marks, or target cursors. So green shows you what thrust mode you're in and where your target is set. Blue is for testing of the system only. You'll only see blue during a self-test. Yellow is for caution and advisory messages, caution limit marks on scales, and digital read-outs. So yellow is a step down from red — it's caution, not warning. Magenta is used during in-flight engine starting, and for cross-bleed messages. So when you're starting an engine in the air, or dealing with cross-bleed operations, magenta appears. And cyan — that's the light blue — is used for the names of all parameters being measured, for example N1, oil pressure, TAT — that's total air temperature — and status marks or cues. So cyan labels what you're looking at. Finally, the displays are selected according to an appropriate display selection mode. That's the key point — you don't just see everything at once; you select what's shown based on the mode you're in. Let me show you the functional diagram so you can see how these units connect into the system. And here's the engine data displays themselves. So to tie it together: upper screen for primary engine parameters — N1, EGT, warnings and cautions, possibly EPR. Lower screen for secondary engine parameters — N2, fuel flow, oil quantity, pressure, temperature, vibration — plus non-engine systems status, configuration, and maintenance data. Seven colours, each with a fixed meaning. And the 'V's on the upper display tell you the lower unit is showing secondary information. That's your EICAS display foundation.