Flight Director Systems — Page 340, Lesson 395

I want to walk you through the Flight Director System chapter, and we're starting with the Electronic Horizontal Situation Indicator, or EHSI. Now, you may already be familiar with the gyro-magnetic compass. Think of the HSI as a gyro-magnetic compass display, but with a lot more information packed onto it. Let me break down what you'll see on it. First, there's the Course Deviation Indicator, or CDI bar. That's the bar that shows you whether you're left or right of your selected course. Alongside it, you'll see a series of dots representing deviation in degrees. The scale of those dots varies with the type of display, so don't memorize a specific number of degrees per dot—it depends on the system. There's also a from/to pointer, which tells you whether you're flying toward or away from the station or waypoint. You'll have a selected course window, where you read the course you've dialed in, and a DME display of range, which gives you your distance in nautical miles. Finally, there's a heading bug, which is a marker you set to your desired heading. Now, here's an important distinction. In older systems, the course selection is done directly on the HSI using an attached knob. But the system we're referring to in 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 itself, you set your course and modes from this central panel. The system we're referring to also uses a Navigational Display, or ND. Now, the ND, like the Primary Flight Display, or PFD, is a more flexible display. But it's able to show "classical" representations of an HSI. So even on a modern glass cockpit, the ND can present that traditional HSI picture you'd have seen on an electromechanical instrument. Take a look at Figure 25.4 for the HSI display and Figure 25.5 for the EHSI to see the difference. 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. On older aircraft, this information could come in the form of analogue outputs from the Air Data Computer, the ADC, and the Vertical Gyro, the VG. Current systems will be purely digital. But here's a key 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, and/or to the autopilot, as required. Now, let's talk about the optional components. Flight Director Systems use other components depending on what generation they belong to. First, there's the Instrument Amplifier or Symbol Generator. Where information needs to be displayed on electromechanical instruments, the signals require amplification to drive the associated motors. That's what the instrument amplifier does. But on EFIS-fitted aircraft—that's Electronic Flight Instrument System—the FDC output can be fed directly to the symbol generators for the Primary Flight Display and Navigational Display units. So no separate amplifier needed there. Next, we have the Vertical Gyro, or the INS/IRS. In older or smaller aircraft types, normally without any INS or IRS, 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 this? 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, being in electrical form, can be fed to any other items requiring attitude information—for example, the autopilot. So it's not just for your display; it's a shared source of attitude data. Now, more modern and larger aircraft may use data from their INS or IRS—that's Inertial Navigation System or Inertial Reference System—to replace that vertical gyro reference. So instead of a dedicated remote gyro, they tap into the inertial data they already have. Let me pause there. We've covered the EHSI display, the Flight Director Computer, and the optional components—the amplifier or symbol generator, and the vertical gyro versus INS/IRS. Take a look at Figure 25.1, which shows a typical electromechanical flight director system, to see how all these pieces connect together.