Flight Director Systems — Page 340, Lesson 393

I want to walk you through the Flight Director System — this is the start of a new section, so we're building from the ground up. Let's begin with what the FDS actually is. The Flight Director System was originally developed as an aid used by the pilot during landing. That's its historical root. The core idea is that the FDS provides the pilot with steering and attitude signals on one single instrument, and that consolidation is what helps reduce his workload. Instead of scanning multiple instruments and mentally computing what to do, the FDS puts the answer on one display. Now here's an important evolution. 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 also a source of command signals for the autopilot. Let me give you the integrated picture. With an FDS, information about the attitude, heading, and flight path of the aircraft can be integrated with navigation information. The output of that integration is either easy-to-interpret visual instructions for the pilot, or input to the autopilot, or both. So the FDS is essentially a computer that takes aircraft state and navigation data, and produces commands. Now, terminology. To bring FDS and autopilot terminology together, it's usual to describe the FDS as having two "channels." The first channel is the roll channel, and the second is the pitch channel. You'll learn more about channels in the autopilot section — for now, just hold onto that two-channel concept. Where does the FDS get its information? There are several possible sources. First, the Pitot-Static system or the Air Data Computer — that's the ADC. Second, the VHF Nav receiver, which allows input from VOR beacons or the Instrument Landing System. Third, the Flight Management System and the Inertial Navigation/Reference System. The FDS also requires attitude and directional information. On older, electromechanical systems, that attitude and directional data came from the gyro-magnetic compass and a vertical gyro system. More modern aircraft use Inertial Navigation/Reference System information — that's INS/IRS — in place of a vertical gyro. And those modern systems are able to feed the navigation data from the INS/IRS into the FDS/Autopilot combination. That figure shows a typical electromechanical flight director system — you can see how the components interconnect. Now let's move to the components of a typical modern FDS. The first component is the Electronic Attitude Director Indicator, or EADI. This is a fairly standard artificial horizon providing pitch and roll information. The "Attitude" part of the name comes from that — it gives you attitude. 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. One form is a set of "wires" — two bars that cross. The other form is a set of wedges. Both indications are intuitive and essentially the same. The pilot is required to "fly to" either the point where the wires cross, or the point between the wedges, in order to satisfy the demand from the FDS. So if you fly the aircraft so the command bars are centered, you're doing what the flight director wants. That figure shows the two forms of command bar display. The second component is the Primary Flight Display, or PFD. The PFD is part of an Electronic Flight Instrument System — that's EFIS — and 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, or FMA. So to summarize the key terms you need to hold onto: FDS, the two channels — roll and pitch — the data sources including Pitot-Static/ADC, VHF Nav, FMS, INS/IRS, the EADI with its flight director command bars in wire or wedge form, and the PFD with its tapes and the FMA. That's the foundation of the flight director system.