It’s derived from the rate of change of deviation of the incoming signal — Page 348, Lesson 399

Let’s start with the core idea of how the Flight Director System, the FDS, actually works, because everything else hangs off this. The FDS takes raw data — that’s the basic, unprocessed signal from the navigation or attitude sensors — and it computes something called computed information. This computed information is the key. It’s derived from the rate of change of deviation of the incoming signal. Let me unpack that phrase, because it’s the heart of the system. Imagine you’re flying and you’ve drifted off your desired track. The raw data tells you how far off you are — that’s the deviation. But the FDS doesn’t just look at how far off you are; it looks at how fast that deviation is changing. That’s the rate of change. If the deviation is increasing quickly, you’re diverging fast. If it’s decreasing, you’re converging back. So the FDS takes that rate of change, computes it, and then signals a command. That command goes one of two ways: either to the pilot, through the flight director bars on the attitude indicator, or directly to the autopilot. Either way, the logic is the same. Here’s the clever part. As the rate of change of deviation from the desired track increases, the FDS computes that, in order to intercept the track correctly — rather than fly straight through it — it must indicate a turn onto the track. So instead of waiting until you’re back on track and then turning, it anticipates. It commands a turn before you reach the track, so you intercept it smoothly rather than overshooting. That’s what gives you those smooth turns, climbs, and descents. Now, why does this matter if the FDS fails? Because of the distinction between computed and raw information. If the FDS fails, the computed information is no longer reliable or available — but the raw information may still be available and usable. So you can still fly the approach or the navigation using the raw data, even though you’ve lost the computed commands. That’s a critical redundancy point for you as a pilot. One more thing before we go into the modes: many of the flight director modes are common with the autopilot modes. Where they’re similar, the description is fairly basic, with the full detail in the autopilot chapter. Where the FD mode differs from the autopilot mode, the description will detail those differences. So keep that in mind — we’re not going to re-derive everything for the FD; we’re focusing on where it’s the same and where it’s different. Now, let’s talk about FD fail indications — how the system tells you its information is unreliable. On electromechanical displays, warning flags are used. These are physical flags that pop into view. First, if the vertical gyro or another vertical referencing system fails, or its power supply fails, a “Gyro” warning flag pops into view, normally on the ADI — that’s the Attitude Director Indicator, the primary attitude instrument. Second, failures of the FDC — that’s the Flight Director Computer — or the instrument amplifier, or the ADI itself, are indicated by a flag labelled either “ATT” for attitude or “FD”. Again, this normally appears on the ADI display. Third, if glide slope information is unreliable — due to poor signal strength or failure of the system — a “GS” flag appears in front of the raw glide slope scale, wherever that scale appears, either to the side of the ADI or on the HSI, the Horizontal Situation Indicator. So to summarise the flags: “Gyro” for vertical reference failure, “ATT” or “FD” for flight director computer or amplifier or ADI failure, and “GS” for glide slope unreliability. Each one tells you exactly which part of the system has lost integrity, so you know what you can trust and what you can’t. That’s the foundation. Now we can move into the individual flight director modes themselves.