Electronic Instrumentation — Page 546, Lesson 647

I want to walk you through the EICAS — the Engine Indication and Crew Alerting System — starting with its display units. This is the electronic successor to the old electromechanical engine gauges, and it's a core part of modern electronic instrumentation. Let's start with the display units themselves. These are the screens that present engine information to the flight crew. They provide a wide variety of information relevant to engine operation, and also to the operation of other automated systems on the aircraft. Now, here's a key technical point: these units use colour shadow mask CRTs — cathode ray tubes — along with associated card modules. And the functions of those CRTs and card modules are identical to the ones used in the EFIS units, the Electronic Flight Instrument System. So the same display hardware technology drives both your flight instruments and your engine instruments. Physically, the units are mounted one above the other, as you can see in Figure 39.2. Now, what does each unit show? The upper unit displays the primary engine parameters. Those are N1 speed — that's the low-pressure compressor or fan speed, expressed as a percentage — EGT, which is exhaust gas temperature, and the warning and caution messages. So the most critical, immediately-actionable engine data lives on top. In some cases, the upper unit can also display EPR, engine pressure ratio. Whether it does depends on the type of engines installed and on the methods used to process data by the thrust management control system. So EPR is not universal — it's a function of the engine type and how the thrust management computer handles the data. The lower unit displays the secondary engine parameters. Those are N2 speed — the high-pressure compressor speed — fuel flow, oil quantity, oil pressure and oil temperature, and engine vibration. Beyond the engine itself, the lower unit can also display the status of non-engine systems. For example, flight control surface positions, the hydraulic system, the APU — auxiliary power unit — and so on. It can also show aircraft configuration and maintenance data. So the lower screen is your secondary engine data plus a general systems status page. Here's a nice operational detail: the rows of 'V's shown on the upper display unit only appear when secondary information is being displayed on the lower unit. So those little V-shaped markers are a cue that tells you the lower screen is active and showing secondary data. Now let's talk about colour. The CRTs produce seven colours, and each colour has a strict, standardised meaning. This is a professional convention you must know cold. White is used for all scales, the normal operating range of pointers, and digital read-outs. So the basic, neutral data. Red is for warning messages, maximum operating limit marks on scales, and digital read-outs. Red always means danger — exceed a limit or a warning condition. Green is the thrust mode read-out and the selected EPR or N1 speed marks, or target cursors. So green shows you what thrust target you've selected. Blue is used for testing of the system only. You'll see blue only during self-test, never in normal operation. Yellow is for caution and advisory messages, caution limit marks on scales, and digital read-outs. So yellow is the step below red — caution, not warning. Magenta is used during in-flight engine starting, and for cross-bleed messages. Cross-bleed refers to transferring bleed air from one engine to start another. And cyan — that's a blue-green colour — is used for the names of all parameters being measured, for example N1, oil pressure, TAT — total air temperature — and so on, and for status marks or cues. So cyan labels what each read-out actually is. Finally, the displays are selected according to an appropriate display selection mode. That's the logic that decides which page or format appears on each unit. Let me show you the functional diagram so you can see how all these pieces connect. And here's the display select panel that lets the crew choose those display modes. So to tie it together: the upper unit is your primary engine parameters and alerting, the lower unit is secondary engine data plus systems status, and the colour coding gives you an instant, standardised read on the severity and meaning of everything on screen. That's the EICAS display system.