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Flight Director

(Text posted by someone else – posts got mixed up)

Pardon my ignorance, but what is an FD? I understand from these lines that it cannot be used when the autopilot is active, so it must be some kind of “partial autopilot”? What does the acronym actually stand for? What are the pros and cons of it? When is it typically or best applied, or not?

Administrator
Shoreham EGKA, United Kingdom

I’ll volunteer an explanation of my own. Unlike an autopilot, FD retains the pilot in the control loop. Essentially, it’s a feedback device with an indicator usually overlaid on the AI. It behaves somewhat similarly to an ILS indicator in that you need to keep the moving pointer in the centre; however, unlike an ILS, which shows an instantaneous position of the aircraft, it computes the flight dynamics ahead of the time and shows you how to set the flight controls so that the aircraft would converge to the correct trajectory. This way, it eliminates the effect of time lag in aircraft response to control inputs, providing for a smoother, more stabilised trajectory, reducing the likelihood of pilot-induced oscillations. Obviously, its effect is greater for heavy aircraft.

LKBU (near Prague), Czech Republic

Or:
Imagine the Autopilot is ON but the servos disconnected and you are the one who has to set the controls in a way that you keep the FD symbol s at the right place to follow the computed trajectory. This A/P mode is called Flight Director. You use the A/P computer but your MUSCLES replace the servos to follow the symbol on the screen, or the mechanical Flight Director

Last Edited by Flyer59 at 24 Jan 12:43

The Flight Director is the TWO MAGENTA colored wedges in the attitude indicator. In this case the AP is ON. If you HANDFLY the Flight Director the wedges turn GREEN and the “AP” on top of the PFD is replaced with “FD”. When the FD is ON all you have to do is keep the two (green) wedges aligned with the yellow triangle and you will follow the 3D (!) trajectory.

If you switched to FD now the wedges would become green and the FD would show you how to follow the active GPSS route from GPS1. YOu can also use the FD in other modes like Approach, Nav.

Last Edited by Flyer59 at 24 Jan 12:54

It has probably already been said before: A flight director is an autopilot, that replaces mechanical servos with the muscles of the pilot. The same commands are given to the pilot either via standard crosspointers or the V-bar shown above by Alexis that would be given to the servos in case of autopilot engagement.

In an installation with FD and AP, the FD is always on when the autopilot is on – it is nothing but a graphical output of the autopilot computer. In some installations you can remove the FD indicators from your instruments/screens, but the FD remains on in the background.

Flying by FD is very similar to playing a video game: All one has to do is to keep (in the illustration above – the colours vary) the yellow trinangle inside the magenta triangle and the aircraft will do exactly what has been set on the mode selector (e.g. altitude hold, heading hold, ILS). Flying on flight director is actually so easy that we don’t teach it to students. It would be an expensive waste of precious training time.

EDDS - Stuttgart

Yes, once you KNOW THAT IT EXISTS it’s pretty easy. But can you remember the times when all this was new to you? I had no idea what the FD actually IS for some time after I had my IR … My opinion: You spend 30.000 Euros for the IR, this information should be included ;-))

My opinion: You spend 30.000 Euros for the IR, this information should be included ;-))

I briefly demonstrate it in the prodedure trainer (ours is configured as a Piper Seminole with King autopilot and (electromechanical!) flight director bars) and let each student fly an ILS or two. But the real training is flown on raw data. And there is no FD in any of our training aircraft – and even if, it would be the first thing that most examiners take away on the checkride.

But can you remember the times when all this was new to you?

Yes, that was when I first flew a Cessna 421 in 1992. But back then, a type rating was required for each series of piston twins and our type rating instructor showed it to us.

Last Edited by what_next at 24 Jan 13:17
EDDS - Stuttgart

Yes, that IS the right way to handle it. I was not talking about the checkride. Although MY examiner even let me use the GPS for track on the NDB approach (“you have to use what you have”) … which really made it pretty simple ;-)

Last Edited by Flyer59 at 24 Jan 13:16

Although MY examiner even let me use the GPS for track on the NDB approach (“you have to use what you have”)

They do that here. But try the same thing on a checkride in the UK (I go there once per year for my Citation recurrent training): "Does this chart say “RNAV overlay” anywere? No? So why do you still have that FMS track on your screen? "

Last Edited by what_next at 24 Jan 13:22
EDDS - Stuttgart

What a wealth of response, and so quick, and curiously and unusually unisono!
Thanks to all!

EBZH Kiewit, Belgium
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