Flight Director Systems — Page 352, Lesson 412

Let’s pick this up right where the flight director’s scheduling logic leaves off. We’ve been talking about how the flight director decides how aggressively to command the aircraft — that’s the gain scheduling. Now I want to show you the third way it can be scheduled, and then we’ll move into what happens when you have two flight director systems on board. The third scheduling method is the radio altimeter. Unlike the timing system or the marker beacon system, the radio altimeter relies on no ground signals at all — it measures actual height above the terrain directly. Because it knows the true height, it can give accurate scheduling in relation to actual aircraft height. And here’s the key advantage: because it’s continuous, the actual scheduling can be phased in gradually, as opposed to the stepped method of the timing or marker systems. So instead of the flight director’s demands jumping in discrete steps as you pass a marker or a time point, the radio altimeter lets the demands blend in smoothly as you descend. Now, one more general point about gain scheduling before we leave it: gain scheduling or adaption can be made to occur for any change in the flight regime. The purpose is to reduce the demands from the flight director to ensure adequate safety. So whenever the flight regime changes — speed, configuration, altitude — the system can adapt its gain to keep the commands safe and flyable. Now let’s move to the dual FDS — the dual flight director system. On large aircraft, you typically have 2 FDS fitted, one for each pilot. Each system can be used to monitor the indications of the other. That monitoring function is called the FD Comparator, or the FD Monitor. Here’s how the comparator works. It monitors command bar positions. The command bars are the moving bars on the ADI that tell the pilot where to pitch and roll. If the comparator 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. The FD command bars reappear when the difference returns to within limits. So the comparator is essentially a disagreement monitor: if the two systems disagree beyond a threshold, it takes the commands away rather than let the pilot chase conflicting guidance. But FD comparison is only active during certain modes of FD operation. There are two conditions. First, both FD switches must be on, and neither autopilot engaged. Second, it only operates in either the TOGA mode — that’s Take-Off/Go-Around — or the APP mode — that’s Approach — and only below 800 ft RA, that’s radio altitude. So below 800 feet on the radio altimeter, in TOGA or APP, with both FDs on and no autopilot — that’s when the comparator is live. Now, FD comparison is inhibited — switched off — for several reasons. It’s inhibited on the ground. It’s inhibited when either FD is affected by electrical bus transfer — that’s when power is being switched between electrical buses. And it’s inhibited by failure of either a FD sensor or an FD itself. So if one side’s sensor or computer fails, the comparison is disabled rather than giving you a false disagreement. Now, having two FDS also gives you redundancy — a backup. If one system fails, 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 the Horizontal Situation Indicators. So one good computer drives both pilots’ displays. To further aid redundancy, the flight instrument and FD power — and that of the navigational information sources — may also be separate, so a single power failure doesn’t take everything down. And finally, on EFIS-equipped aircraft — that’s Electronic Flight Instrument Systems — there is a spare symbol generator to retain extra redundancy. The symbol generator is what actually draws the symbols on the electronic displays, so having a spare one means you can keep the displays alive even if the primary generator fails. So to tie it together: the radio altimeter gives you smooth, height-based gain scheduling; the dual system gives you monitoring through the FD Comparator, with specific activation conditions and inhibition cases; and the redundancy — through the FDC switch, separate power, and the spare symbol generator on EFIS — keeps you flying even when something fails.