So here’s the key idea I want you to hold onto — Page 348, Lesson 399

Let’s pick this up right where the flight director system starts doing its real work — computing commands for turns, climbs, and descents. So here’s the key idea I want you to hold onto. The FDS — that’s the Flight Director System — can work in two very different ways. It can use the raw data, or it can use what we call computed information. Let me make that distinction crystal clear, because it’s the backbone of everything that follows. Raw data is the direct, unprocessed signal coming from the navigation or attitude sources — the basic information about where you are and what your aircraft is doing. Computed information, on the other hand, is what the FDS derives from that raw data. It takes the rate of change of deviation of the incoming signal. That phrase is important — the rate of change of deviation. It’s not just how far off track you are; it’s how fast that off-track error is changing. Here’s the logic the FDS uses. 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 track. So think about it: if you’re drifting off the desired track and the deviation is growing quickly, the system knows that a simple correction won’t cut it. It has to command a turn that will bring you back onto the track and intercept it smoothly, not overshoot it. That’s the whole point of computed information — it anticipates, rather than just reacts. Now, where does that computed command go? The FDS can signal it to the pilot via the flight director bars — those are the command bars on the attitude indicator that you fly to — or it can signal directly to the autopilot. Either way, the same computed information drives the command. And here’s a critical safety point. This becomes important if the FDS fails. Because if the FDS fails, the computed information will no longer be reliable or available. But — and this is the key — the raw information may still be available and used. So even if the flight director’s computed commands disappear, you can still fall back on the raw data to fly the aircraft. That’s a fundamental redundancy concept in instrument flying. Now, before we go mode by mode, I want to flag something about the structure of what we’re about to cover. Many of the FD modes — the flight director modes — are common with the autopilot modes. So where they’re similar, the description here is fairly basic, and the full description lives in the autopilot chapter. Where the FD mode differs from the autopilot mode, the description here will detail those differences. So don’t expect a full treatment of every mode in this section — just the differences and the essentials. Now let’s talk about how the FDS tells you whether the information it’s giving you is reliable. This is the FD Fail Indications section, and it’s all about flags. On electromechanical displays — the older-style instruments with moving needles and mechanical flags — warning flags are used. These are physical flags that pop into view to tell you something has failed. Here’s the first one. If the vertical gyro — that’s the gyroscope that provides the attitude reference — or if any other vertical referencing system fails, or if its power supply fails, then a “Gyro” warning flag will pop into view. And it will normally appear on the ADI — that’s the Attitude Director Indicator, the primary attitude instrument. Now, there’s a second set of failures. If the FDC fails — that’s the Flight Director Computer — or if the instrument amplifier fails, or if the ADI itself fails, then a different flag appears. This one is labelled either “ATT” for attitude, or “FD” for flight director. And again, this will normally be on the ADI display. So you have two distinct flag families. “Gyro” for the vertical reference system or its power. “ATT” or “FD” for the flight director computer, the amplifier, or the ADI itself. Each tells you a different part of the system has failed. Now, one more flag, and this one’s about glide slope. If the glide slope information is unreliable — either due to poor signal strength or failure of the system — a “GS” flag will appear. And here’s the placement detail: it appears in front of the raw glide slope scale, wherever that scale appears. That could be either to the side of the ADI or on the HSI — the Horizontal Situation Indicator. So let me tie this together. The flags are your failure annunciators. “Gyro” for the vertical reference. “ATT” or “FD” for the flight director computer, amplifier, or ADI. And “GS” for unreliable glide slope. Each one tells you exactly which part of the system you can no longer trust, and each appears in a specific location so you know where to look. Before we move on to the individual modes, let me just make sure the raw-versus-computed distinction is solid, because it’s the foundation. Raw data is what the sensors give you directly. Computed information is what the FDS derives from that raw data — specifically from the rate of change of deviation — to command smooth intercepts. And if the FDS fails, you lose the computed commands but you keep the raw data. That’s your fallback. Now, I want to show you what these mode annunciators actually look like on the PFD — the Primary Flight Display. Let me bring up the flight mode annunciator figure for you.