So you, as the pilot, have a direct hand in energising these displays — Page 283, Lesson 340

Let’s start with the power side, because that’s where this passage opens. The display units and the rest of the EFIS—the Electronic Flight Information System—are powered from the aircraft’s DC power supply, and the pilot controls that power through switches. So you, as the pilot, have a direct hand in energising these displays. Now, the heart of the system is the Symbol Generator, often abbreviated SG. I want you to think of the SG as the translator and manager of the whole EFIS. It provides three kinds of signal interfaces between the aircraft’s systems, the display units, and the control panel: analogue, discrete, and digital. Analogue signals are continuous electrical values, like a voltage that varies with a sensor reading. Discrete signals are simple on/off or open/closed states, like a switch position. Digital signals are the binary data that modern systems exchange. So the SG takes all these different signal types, converts them as needed, and routes them between the aircraft systems, the displays, and the control panel. But the SG does more than just pass signals. It also performs symbol generation monitoring—that is, it checks that the symbols it’s producing are correct. It handles power control, managing the electrical supply to the displays. And it carries out the main control functions of the entire EFIS. So the SG is not just a passive interface; it’s the active brain that generates the symbols you see, monitors its own output, and controls the power. The passage also mentions that the interfacing between the card modules of an SG is shown in Figure 22.1. That figure shows the multi-crew EFIS units and signal interfacing. Now let’s move to the Display Units themselves. These can be either Cathode Ray Tubes, CRTs, or Liquid Crystal Displays, LCDs. The LCDs have a clear advantage: they are smaller, and they generate less heat, which means they need less cooling. That’s a practical benefit in the cockpit, where space and cooling are at a premium. Here’s an important design point: the Primary Flight Display, the PFD, and the Navigation Display, the ND, 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 type of spare. And they are often interchangeable with the systems display units—that is, the EICAS, which stands for Engine Indication and Crew Alerting System, or the ECAM, the Electronic Centralised Aircraft Monitor. So one common display unit can serve multiple roles across the cockpit. Finally, let’s talk about the Colour Display System. I want to be clear: there is no set colour standard across the industry, so colour displays may vary slightly between aircraft. But in a typical display system, five colours are usually assigned for the many symbols, failure annunciators, messages, and other alphanumeric information. And there’s a sixth colour, red, reserved specifically for weather, abbreviated WXR. Let me walk you through the colours as they’re typically used. White is for the display of present situation information—that’s your current state, like your attitude or heading. Green is also for present situation information, but it’s used where contrast with white symbols is required, or for data that has lower priority than white symbols. Green is also used for engaged autoflight modes—so when the autopilot or flight director is actively controlling, that’s shown in green. Magenta is for all ‘fly to’ information: flight director commands, deviation pointers, and active flight path lines. So magenta tells you where to go, white and green tell you where you are, and red is strictly for weather. That’s the core of the EFIS architecture: DC power through pilot switches, the Symbol Generator as the interface and manager, display units that are common across roles, and a colour scheme that assigns meaning to what you see.