Flight Director Systems — Page 348, Lesson 402

I want to walk you through the Flight Director System now, and we're starting with the failure indications and the command-bar concept, then we'll get into the take-off mode. First, let's talk about how the system tells you something is wrong. If you lose VOR, LNAV, or LOC information — whether it's poor reception, unreliable signal, or a complete loss — that's indicated by a "NAV" flag, and that flag is normally located on the HSI, the Horizontal Situation Indicator. So you're looking at your HSI and you see that NAV flag pop up, you know your navigation source is compromised. The HSI also has a power failure flag. That flag indicates loss of power to the instrument itself, or to the compass gyro. And there's a third condition it shows you: if the compass system is operating in Directional Gyro mode — DG mode — that means the magnetic reference has been lost, and the compass is now essentially acting as a Directional Indicator, a DI. So you're flying on gyro-stabilised heading only, no magnetic reference. Now let's move to the core concept of command attitude changes. When you're flying level, you select an attitude that, for the given airspeed, will achieve level flight. Here's the key relationship: at low airspeed, you need a pitch angle of several degrees nose-up to avoid descending. That's because you don't have enough speed to generate lift without raising the nose. To help you keep a constant pitch angle, the FD command bars can be offset vertically. That gives you an intuitive aiming point — you fly the aircraft to keep the bars in a certain position, and that maintains your pitch. This same technique works for keeping constant pitch angles during climbs and descents, not just level flight. How do you move the bars? You move the Pitch Trim knob or wheel — and which one you have depends on the age of the design. You adjust it until the command bars are in the desired position. But here's an important limitation: the pitch trim system is inhibited whenever any other pitch mode is active. So you can only use that trim to position the bars when no other pitch mode is commanding. Now, the Flight Director Take-off Mode. Initially, both flight director systems should be switched on prior to starting the take-off roll. The FD take-off mode is engaged by pressing either of the TO/GA switches on the throttles — TO/GA meaning Take-Off / Go-Around. The AFDS annunciation — that's the Auto Flight Director System annunciation — reads "TO/GA". Here's the sequence of commands. The initial FD commands are for 10 degrees nose-down pitch and wings level. Then, at 60 knots IAS — indicated airspeed — the FD command changes to 15 degrees nose-up and wings level. So you get that transition from nose-down to nose-up as you accelerate. There's also a way to engage the FD system during take-off even if the FD switches are off. You press the TO/GA button after 80 knots IAS but before 2000 feet or 150 seconds after lift-off, and the command bars will automatically appear. So that's your window: after 80 knots, before 2000 feet altitude, or within 150 seconds of lift-off. And after lift-off, the FD provides pitch commands. That's where the excerpt cuts off, but that's the foundation — the failure flags, the command-bar pitch concept, and the take-off mode sequence. Let me show you the mode annunciators so you can see how these commands are displayed. And here's the altitude acquire and altitude hold modes, which relate to how the FD manages pitch in different phases. So to tie it together: the NAV flag and power failure flag tell you when your navigation or instrument data is unreliable, the command bars give you a visual pitch target you can trim into position, and the take-off mode gives you a specific sequence — 10 degrees nose-down initially, then 15 degrees nose-up at 60 knots, with that 80-knot/2000-foot/150-second window to engage it even with the switches off.