ATPL · Navigation · Instrumentation Atpl Ground Training Series… lesson — Page 283, Lesson 337

Let’s start at the very top of this chapter, because the Electronic Flight Instrument System — EFIS — is the backbone of everything you’ll see on the modern flight deck. I want you to think of EFIS as the system that takes attitude and navigation information and presents it to you, the pilot, on two electronic display units. The whole point is that this format is easier to read and less likely to be misinterpreted than some of the older mechanical instruments. That’s the core promise of EFIS — clarity and reduced ambiguity. Now, a key point: in terms of its basic functions and the number of display units, EFIS is fully integrated with digital computer-based navigation systems. It uses colour Cathode Ray Tube — CRT — or Liquid Crystal Display — LCD — types of Attitude Director Indicator and Horizontal Situation Indicator. So the ADI and HSI, which you’ll know from conventional instruments, are here rendered as electronic displays. The system is extremely advanced, not just in physical construction, but in how much attitude and navigational data it can present to the flight crew. Let’s break down the units of a system. Just like a multi-crew conventional flight director system, a complete EFIS installation is made up of left — Captain — and right — First Officer — systems. Each system in turn is comprised of five components. First, the Electronic Attitude Director Indicator, or EADI, which is also called the Primary Flight Display, or PFD. That’s your attitude reference. Second, the Electronic Horizontal Situation Indicator, or EHSI, also called the Navigation Display, or ND. That’s your navigation picture. Third, the Control Panel — that’s how you select what’s displayed. Fourth, the Symbol Generator, abbreviated SG. This is the brain that generates the symbols you see on the displays. And fifth, the Remote Light Sensor Unit — that adjusts display brightness based on ambient light. Now, here’s an important redundancy feature. A third, centre symbol generator is incorporated. Its drive signals can be switched to either the left or right display units in the event of failure of their corresponding symbol generators. So if the left symbol generator fails, the centre one can take over for the left display. Same for the right. The signal switching is accomplished within the left and right symbol generators themselves, using electromechanical relays. These relays are powered from the aircraft’s DC power supply, via pilot-controlled switches. So you, the pilot, have control over which symbol generator drives which display. Let me show you the physical layout. That figure shows the multi-crew EFIS units and how they interface with each other and with the data busses. You can see the left and right systems, the centre symbol generator, and how the signal paths can be switched. And here’s the control panel. That’s where you make your selections for what appears on the displays. So, to summarise what we have: a complete EFIS is two independent systems — Captain and First Officer — each with an EADI/PFD, an EHSI/ND, a control panel, a symbol generator, and a remote light sensor. Plus a centre symbol generator for redundancy, with relay-based switching under your control. That’s the architecture. Next, we’ll look at how you actually use the displays — the modes like Expanded ILS, Map Mode, and Plan Mode — and how the system handles data source selection and failure annunciation.