View Full Version : Flight Director - Basics

16th Nov 2003, 10:35
Very general question here -

What is the basic function of the FD? How does the Flight Director differ from the AP?

16th Nov 2003, 11:15
In very simple terms, the AP flies the plane the way you direct.

The FD only shows you, on your artificial horizon, what the autopilot would do if it were flying. You therefore direct it thesame way as the autopilot, but you have to do the flying yourself, using the FD as a guide.

16th Nov 2003, 15:29
With many older designs, the flight director was certainly not used for takeoff, due in most part, to false indications.
Having said this, with more modern designs, the flight director can be very useful for detailed standard instrument departures.
Flight director use is mainly driven by the companies policy...and pilots dis-trust of the indications, usually directed to older types.

Flap operator
16th Nov 2003, 18:35
Which is why, on a multi crew aircraft, many SOP's have the flight director selected on the non-handling pilot's display, which is selected by the handling pilot after takeoff having assertained that the data being displayed is valid.

16th Nov 2003, 18:40
I like the gist of Phoenix' reply. But I'll add my 2c worth (as usual! :p )

An autopilot pitches, rolls and trims the aircraft to maintain the desired flight path.

A flight director commands YOU to pitch or roll the aircraft to maintain the desired flight path.

Both the AP and the FD get their 'instructions' from the Mode Control Panel. The MCP selections might be IAS in a climb or a descent, levelling off at a preset ALT, or VNAV input from the FMS (FD pitch bar) as well as holding a HDG, a VOR radial or flying an LNAV path (FD roll bar).

On receipt of their instructions, and AP will try to do what you have asked for on the MCP all by itself, and a FD will instruct you how to push/pull/turn to acheive the desired flightpath.

The FD will do this within limits (BAe146 example) being: FD roll bar will command up to 25deg AoB normally, reducing to 10deg AoB when established on a LLZ beam. FD pitch will command up to 20 NU. And I think a +1.5g/-0.5g limit too (back to the books, ITCZ!)

So when a FD bar moves, it is saying "pitch up to maintain that speed/altitude, thats it, ok pitch it down, now start rolling to the right to intercept the localiser, more, more, ok roll off the bank..... etc!

Raw data is FD off.

Another example.

Learning to fly IF, you probably heard your instructor breaking the task down into.. where am I, where do I want to be, what do I need to do to get there..

Raw data, your instrument panel is simply telling you how things are and where you are right at this very moment (or, really, a fraction of a second ago, once you account for lag in some instruments). = Where am I?

Adding a MCP then 'tells' the panel where you want to go. = Where do I want to be?

The FD then tells you to pitch and roll the aircraft to match your MCP selections. = What do I need to do to get there?

Not inviting argument or finer points on FD vs attitude and performance flying --- save those for later!

17th Nov 2003, 04:30

Just a minor correction... In a modern FMS-driven airplane, the FD also tells the AP what to do to make the airplane go where/how the MCP and FMS want the airplane to go. While you may follow the FD bars on the ND when hand-flying, the AP also follows the signals that move those bars when it is connected.

When "FD" shows on the ND, the FD expects you to do the work. When "COMMAND" shows, the AP is taking the commands from the FD.

G-SPOTs Lost
18th Nov 2003, 06:27
My Training Captain told me to imagine myself as a carbon based autopilot actuator when flying the FD....... :p

20th Nov 2003, 04:10
When manually flying would you always use the flight director (providing it was available) or would you turn it off? It's fun to fly it yourself and good to practice but although the FD helps you why use it? If you feel you need it, or are struggling under a high work load/fatigue,etc why not use the autopilot? If you want to practice hand flying, or are just doing it for fun why use the flight director?

Perhaps I'm missing the point. Is there a safety reason in that it acts as a back up to your personal judgement, an operational reason as it commands a more efficent way to fly than leaving it to pilot judgement, or just good practice to keep the autopilot commands in sync with your intentions even though it is not being used?

I'm a curious chap and these questions probably arise out of a lack of knowledge of companies' opinons on when and how much manual flying is permitted or encouraged.

Many thanks, Skut.

20th Nov 2003, 06:49
If you hand fly the climb, for example, and you want F/D assistance, you could select "Vertical Speed" mode and enter whatever rate you wish to climb at, say 1500 feet per minute, and the F/D command bars would show you the pitch angle on the Attitude Indicator in order to maintain that. Or if you want to climb at a specific airspeed, you could select "Indicated Airspeed" mode and the command bars would give you the pitch attitude to maintain that. But the F/D will not protect you from a stall, if the airplane is not able to maintain proper airspeed at the selected 1500 feet per minute climb rate. Likewise, the F/D commands will not protect you from a stall during the climb if you follow a specific vertical speed command that is not commensurate with the airplane's weight at a given altitude.

21st Nov 2003, 12:58
Your post may be correct for older FDs (& APs) but the majority of newer designs will gradually decrease the command as performance drops to eventually limit you to (say) a minimum of 50 feet per minute in CLIMB mode. Your speed should not decay towards stall unless you're really at the performance limit.

In most modern systems, the FD and AP are all in the same card (or more correctly, pair of cards). The crew set up the Horizontal & Vertical modes on the Guidance Panel and select the NAV source (which could be the FMS) if not in HDG. This is used to produce the FD data. The AP, if engaged, then takes the FD output which is also sent to the displays so that the crew can monitor it (or,of course, use its guidance to fly manually).