Flight Director Systems — Page 352, Lesson 412

Right, let's pick this up with the Flight Director System. We've already covered the basic command bars and how the flight director generates pitch and roll commands. Now I want to walk you through the more advanced scheduling and the dual-system architecture you'll find on large transport aircraft. First, let's talk about the radio altimeter's role in gain scheduling. The radio altimeter is a system that relies on no ground signals — it measures actual height above the terrain directly. Because it gives you accurate height information, the flight director can schedule its gain — that is, adjust its sensitivity — in relation to the actual aircraft height. This is a key advantage: the actual scheduling can be phased in gradually, as opposed to the stepped method you'd get from a timing or marker system. So instead of the command sensitivity jumping in discrete steps as you pass certain altitudes, the radio altimeter lets the system blend the change in smoothly and continuously. Now, gain scheduling or adaptation can be made to occur for any change in the flight regime. The purpose here is to reduce the demands from the flight director to ensure adequate safety. In other words, as the flight conditions change — say, during approach or flare — the system adjusts its own responsiveness so it doesn't command aggressive manoeuvres that could compromise safety. Now let's move to the dual Flight Director System, or dual FDS. On large aircraft, you typically have two FDS fitted — one for each pilot. Each system can be used to monitor the indications of the other. This monitoring arrangement is called the FD Comparator, or FD Monitor. Here's how the comparator works. It monitors the command bar positions on both systems. If it senses a difference between the two FDS of approximately 1 to 4 degrees of pitch, and/or 3 to 9 degrees of roll, the command bars are removed — they disappear from the displays. When the difference returns to within those limits, the FD command bars reappear. So the comparator is essentially a disagreement monitor that hides the commands when the two systems don't agree. Now, FD comparison is not always active. It's only active during certain modes of FD operation. There are two conditions. First, both FD switches must be on, and neither autopilot may be engaged. Second, it only operates in either the TOGA mode — that's Take-Off/Go-Around — or the APP mode, which is Approach, and only below 800 feet radio altitude. FD comparison is also inhibited — that is, deliberately switched off — for several reasons. It's inhibited on the ground, or when either FD is affected by an electrical bus transfer. It's also inhibited by failure of either an FD sensor or an FD itself. Now, having two FDS also gives you redundancy. If one system should fail, as long as the associated display equipment is serviceable, both sets of displays can, if necessary, be run from one FDC — that's the Flight Director Computer. On electromechanical instruments, this is often simply a switch. When operated, that switch splits the FDC output from the serviceable FDS instrument amplifier and feeds it into both sets of ADIs and HSIs — the Attitude Director Indicators and Horizontal Situation Indicators. So one healthy computer drives both pilots' displays. To further aid redundancy, the flight instrument and FD power — and that of navigational information sources — may also be separate. And on EFIS-equipped aircraft — that's Electronic Flight Instrument System — there's a spare symbol generator as well, to retain extra redundancy. So you have multiple layers of backup: the comparator for monitoring, the switch for reconfiguring displays, separate power sources, and a spare symbol generator on EFIS. Let me show you the overall architecture of this dual system. That figure illustrates how the two FDS, the comparator, and the display switching all fit together. So to summarise: the radio altimeter gives smooth, height-based gain scheduling; the FD comparator monitors the two systems and hides command bars on disagreement; and the dual architecture provides redundancy through display switching, separate power, and a spare symbol generator on EFIS.