Let me start with what EFIS actually is — Page 283, Lesson 337

I want to walk you through the Electronic Flight Instrument System — EFIS — because this is the heart of how modern flight decks present attitude and navigation information to you. Let me start with what EFIS actually is. The Electronic Flight Instrument System presents attitude and navigation information to the pilot on two electronic display units, in a format that is easier to read and less likely to be misinterpreted than some older mechanical instruments. That's the whole point of it — clarity and reduced misinterpretation. Now, the system is generally referred to as 'EFIS'. It is fully integrated with digital computer-based navigation systems, and it utilizes colour Cathode Ray Tube — CRT — or Liquid Crystal Display — LCD — types of Attitude Director Indicator and Horizontal Situation Indicator. Let me unpack those two acronyms, because they matter. The Attitude Director Indicator, or ADI, is the display that shows you your aircraft's attitude — pitch and bank. The Horizontal Situation Indicator, or HSI, is the display that shows you your position relative to navigation aids and your heading. In EFIS, these are not separate mechanical instruments; they are rendered on electronic screens. The CRT is the older colour tube technology, and LCD is the modern flat-panel liquid crystal display. Both are used in EFIS. So the system is extremely advanced, not only in terms of physical construction, but also in the extent to which it can present attitude and navigational data to the flight crew. Now let's look at the units that make up a complete EFIS installation. Just like a multi-crew conventional flight director system, a complete EFIS installation is made up of left — Captain — and right — First Officer — systems. So you have two independent systems, one for each pilot. Each system in turn is comprised of five components. Let me list them carefully: First, the Electronic Attitude Director Indicator, or EADI — also called the Primary Flight Display, or PFD. This is the attitude display. Second, the Electronic Horizontal Situation Indicator, or EHSI — also called the Navigation Display, or ND. This is the navigation display. Third, the Control Panel. This is where you select what you want to see and how. Fourth, the Symbol Generator, abbreviated SG. This is the computer that generates the symbols and images you see on the displays. Fifth, the Remote Light Sensor Unit. This senses the ambient light in the cockpit and adjusts the display brightness accordingly. Now, here's a critical redundancy point. A third — centre — symbol generator is also incorporated. Why? So that its drive signals may be switched to either the left or right display units in the event of failure of their corresponding symbol generators. In other words, if the left symbol generator fails, the centre one can take over and drive the left displays. Same for the right. Now, how is that switching accomplished? The signal switching is accomplished within the left and right symbol generators, using electromechanical relays. Those relays are powered from the aircraft's DC power supply, via pilot-controlled switches. So the pilot has switches to select which symbol generator drives which display, and the actual switching happens through relays inside the left and right symbol generators. Let me show you the physical layout of this multi-crew EFIS system and how the signal interfacing works. And here's the control panel you'd use to manage these displays. So to summarize what we have: EFIS is a fully integrated, digital, computer-based display system. Each pilot has an EADI or PFD for attitude, an EHSI or ND for navigation, a control panel, a symbol generator, and a remote light sensor unit. And there's a third centre symbol generator for redundancy, with switching done by electromechanical relays powered from the DC bus through pilot-controlled switches. That's the foundation. From here, we'll move into the specific display modes — things like Expanded ILS, Map Mode, and Plan Mode — and how you select data sources and handle failures. But first, make sure you're comfortable with these five units and the redundancy scheme, because everything else builds on it.