So we’re in the middle of the EFIS chapter — Electronic Flight Information… — Page 283, Lesson 340

ays powered from an aircraft’s DC power supply, via pilot-controlled switches. So we’re in the middle of the EFIS chapter — Electronic Flight Information Systems. Let’s pick up with the power and the signal path, because that’s what ties the whole system together. First, the power source. The EFIS units are powered from the aircraft’s DC power supply — that’s the direct current bus, the same 28-volt DC you’ll find across the aircraft. And the key point here is that this power is routed through pilot-controlled switches. So the crew has direct control over whether the EFIS is energised or not. That’s a simple but important fact: the pilot can isolate the system from the DC bus. Now, the interface. The connection between the EFIS units, the data busses, and the other aircraft systems is shown in Figure 22.1 — that’s the multi-crew EFIS units and signal interfacing diagram. That brings us to the heart of the system: the Symbol Generators, or SGs. Let me define this properly, because it’s the central brain of the whole EFIS. The Symbol Generator provides the analogue, discrete, and digital signal interfaces between the aircraft’s systems, the display units, and the control panel. So it’s the translator — it takes signals from the aircraft systems, converts them into the right format, and drives the displays. But it does more than that. It also performs symbol generation monitoring, power control, and the main control functions of the EFIS overall. So the SG isn’t just a signal converter — it’s the supervisor. It monitors the symbols it generates, it controls the power to the displays, and it runs the overall control logic of the EFIS. The interfacing between the card modules of an SG is also shown in Figure 22.1. So the SG itself is built from card modules — individual circuit boards — and the diagram shows how those cards talk to each other internally. Now let’s talk about the Display Units themselves. There are two types you’ll see. The first is the Cathode Ray Tube — the CRT — that’s the older, bulky glass tube technology. The second is the Liquid Crystal Display — the LCD. And here’s the key contrast: LCDs have the advantage of being smaller, and they generate less heat, so they need less cooling. That’s a real operational benefit — less heat means less demand on the aircraft’s cooling system, and smaller means more flexibility in panel layout. Now, a very practical point about the PFD and the ND — the Primary Flight Display and the Navigation Display. These two are usually identical units. Why? To facilitate spares commonality. If the PFD and ND are the same physical unit, you only need to stock one spare part. And they’re often interchangeable with the systems display units — that’s the EICAS or the ECAM. EICAS is the Engine Indication and Crew Alerting System, and ECAM is the Electronic Centralised Aircraft Monitor. So the same display unit can be swapped between the PFD, the ND, and the systems display — that’s a huge maintenance advantage. Now, the Colour Display System. Here’s something you need to know right away: there is no set colour standard. Colour displays may vary slightly between aircraft types and manufacturers. So don’t memorise a universal rule — memorise the typical system. In a typical display system, five colours are usually assigned for the display of the many symbols, failure annunciators, messages, and other alphanumeric information. And there’s a sixth colour — RED — which is reserved for weather, the WXR, the weather radar returns. Let me walk you through the five colours and what they mean. WHITE is used for the display of present situation information. That’s the current state of the aircraft — the basic situational data. GREEN is used for display of present situation information where contrast with white symbols is required, or for data having lower priority than white symbols. And critically, green is also used for engaged autoflight modes. So when the autopilot or autothrottle is actually engaged, that mode annunciation comes up in green. MAGENTA is used for all ‘fly to’ information — that’s the command guidance. Flight director commands, deviation pointers, active flight path lines. Magenta tells you where to go, what to fly to. So the logic is: white and green show you where you are, magenta shows you where you need to go. And red is reserved for weather. That’s the colour logic of the EFIS display system. Now, the excerpt cuts off there, but that’s the core of what you need: the power path through pilot-controlled switches, the Symbol Generator as the central interface and supervisor, the CRT versus LCD trade-off, the spares commonality between PFD, ND, and systems displays, and the colour coding — white for present situation, green for contrast or lower priority or engaged autoflight modes, magenta for fly-to information, and red for weather.