Flight Director Systems — Page 340, Lesson 395

I want to walk you through the Flight Director System chapter now, starting with the display side of things. Let's begin with the Electronic Horizontal Situation Indicator, or EHSI. You're already familiar with the gyro-magnetic compass, I assume. An HSI is essentially a gyro-magnetic compass display, but it adds a lot of navigation information on top of it. Let me break down what you'll see on it. There's a Course Deviation Indicator bar, which we call the CDI bar — that's the needle that shows you how far left or right you are from your selected course. Alongside that, there's a series of dots representing deviation in degrees. Now, the scale of those dots varies with the type of display, so don't fixate on a specific number of degrees per dot — it changes. You also have a from/to pointer, which tells you whether you're flying toward or away from the station or waypoint. There's a selected course window, where you read the course you've dialed in, and a DME display of range — DME being Distance Measuring Equipment, so that's your distance to the station. And there's also a heading bug, which is a marker you set to remind yourself of a desired heading. Now, here's an important distinction. In older systems, you did the course selection directly on the HSI itself, using an attached knob. But the system we're going to refer to throughout this chapter uses a remote, centralized flight director mode control and autopilot panel. That panel is called the Autoflight Mode Control Panel — abbreviated AMCP, or simply MCP. So instead of reaching to the HSI, you manage course and mode selection from this central panel. The system we're referring to also uses a Navigational Display, or ND. Like the Primary Flight Display, or PFD, the ND is a more flexible display — it can show you many different formats — but importantly, it's able to show "classical" representations of an HSI. So even on a modern glass cockpit, you can still get that traditional HSI picture on the ND. Let me show you what these look like. That's the classical HSI display, and is the EHSI version. Now, let's move to the heart of the system — the Flight Director Computer, or FDC. This is where all the information is gathered and processed. Think of it as the brain. On older aircraft, this information came in the form of analogue outputs from the ADC and the VG. ADC is the Air Data Computer, and VG is the Vertical Gyro. Current systems, though, are purely digital. And here's a practical point: older aircraft that have been refitted may have analogue inputs going into a digital converter, so that modern displays and autopilot systems can be used. So the FDC takes all this raw data, processes it, and sends outputs to the symbol generators for the EADI and EHSI — that's the Electronic Attitude Director Indicator and the Electronic HSI — and/or to the autopilot, as required. Now, flight director systems use other components depending on what generation they belong to. Let me walk you through the optional components. First, the Instrument Amplifier or Symbol Generator. Where information needs to be displayed on electromechanical instruments — the older, mechanical gauges — the signals require amplification to drive the associated motors. That's what the instrument amplifier does. But on EFIS-fitted aircraft — EFIS being the Electronic Flight Instrument System — the FDC output can be fed directly to the symbol generators for the PFD and ND units. So no separate amplifier needed; the symbol generators handle it. Second, the Vertical Gyro or INS/IRS. In older or smaller aircraft types, normally without any INS or IRS — Inertial Navigation System or Inertial Reference System — the reference to the vertical for the Artificial Horizon part of the ADI is provided by a remote vertical gyro system. Let me explain what "remote" means here. It simply means that the gyro that acts as the artificial horizon is not contained in the instrument in the panel in front of the pilot. Instead, it can be anywhere on the aircraft, normally near the center of gravity, the C of G. The information derived from it is fed to motors that drive the display of the ADI — the Attitude Director Indicator. Now, why would you want that? There are two benefits. First, the gyro can be near the aircraft's center of gravity, and therefore provide a more accurate display of the aircraft's attitude. Second, this data is in electrical form, so it can be fed to any other items that require attitude information — for example, the autopilot. More modern and larger aircraft, however, may use data from their INS or IRS to replace that vertical gyro reference entirely. Let me show you a typical electromechanical flight director system so you can see how all these components connect. So to tie it together: the FDC is the processor, taking inputs from the air data computer, the vertical gyro or INS/IRS, and your selected course and heading from the MCP. It computes the commands, then sends them out — either to the symbol generators for your displays, or to the autopilot to actually fly the aircraft. That's the core architecture of a flight director system.