Flight Director Systems — Page 340, Lesson 393

I want to walk you through the Flight Director System — the FDS. This is a system that was originally developed as an aid for the pilot during landing. Think about that: the whole reason it exists is to help you put the aircraft on the runway. What it does is give you steering and attitude signals on a single instrument, and that single instrument reduces your workload. Instead of scanning several gauges and mentally computing what to do, the FDS presents the answer directly. Now, here's an important piece of history that explains why the FDS matters so much today. As autopilots became more advanced, the signals produced by the FDS could be coupled to the autopilot. That coupling allowed the autopilot to perform more complex tasks. So the FDS isn't just a pilot aid — it's the brain that can drive the autopilot through demanding manoeuvres. Let me give you the big picture of what the FDS integrates. With an FDS, information about the attitude, heading, and flight path of the aircraft can be integrated with navigation information. The output is either easy-to-interpret visual instructions for the pilot, or input to the autopilot, or both. So you have one system taking aircraft state data, mixing in navigation data, and producing either a picture for you or a command for the autopilot. To bring the terminology of the FDS and the autopilot together, we describe the FDS as having two "channels." The first channel is the roll channel. The second is the pitch channel. You'll learn much more about channels in the autopilot section, but for now, just hold onto that: roll and pitch are the two axes the FDS commands. Now, where does the FDS get its information? There are several possible sources. First, the Pitot-Static system or the Air Data Computer — the ADC. That gives you air data like speed and altitude. Second, the VHF Nav receiver, which allows input from VOR beacons or the Instrument Landing System — the ILS. Third, the Flight Management System and the Inertial Navigation or Reference System. The FDS also requires attitude and directional information. On older, electromechanical systems, that came from the gyro-magnetic compass and a vertical gyro system. More modern aircraft use the Inertial Navigation/Reference System — the INS or IRS — in place of a vertical gyro. And those modern systems can feed navigation data directly into the FDS and autopilot combination. That figure shows a typical electromechanical flight director system — the older generation, where you can see the gyro-driven instruments physically doing the work. Now let's move to the components of a typical modern FDS. The first component is the Electronic Attitude Director Indicator — the EADI. This is a fairly standard artificial horizon providing pitch and roll information. The "Attitude" part of the name comes from that — it's an attitude display. The "Director" part comes from the instrument's ability to display demand information from the flight director system using Flight Director Command Bars. Those command bars come in two main forms. Both indications are intuitive and essentially the same. In one form, you have "wires" that cross, and you fly to the point where the wires cross. In the other form, you have wedges, and you fly to the point between the wedges. Either way, you're satisfying the demand from the FDS by flying to that specific point on the display. Those two figures show the two forms of the command bars — the crossing wires and the wedges. The second component is the Primary Flight Display — the PFD. The PFD is part of an Electronic Flight Instrument System, the EFIS display. It brings all of the information required to fly the aircraft onto one display. It has an EADI normally surrounded by speed, altitude, and vertical speed tapes, and often a compass display incorporating some minimal navigation information. It also has an area used for annunciating flight director, autopilot, and autothrottle modes and status. That area is known as the Flight Mode Annunciator — the FMA. So let me tie this together. The FDS takes attitude, heading, and flight path data, integrates it with navigation information, and produces either visual commands for you or inputs for the autopilot. On a modern aircraft, you see the result on the PFD — the EADI in the centre with the command bars telling you where to fly, surrounded by your speed, altitude, and vertical speed tapes, and the FMA up top telling you what modes are active. That's the whole system in one display.