AFDS Modes Using Flight Director Only

[Guest frankathl]

I am learning a lot from these forums lately, particularly in relation to the use of the autopilot. Unfortunately, with greater knowledge comes the potential for greater confusion(more sophisticated confusion, I like to think :-), or maybe I was too ignorant to be very confused before :-(). I think I had(maybe still have) a flawed understanding of automatic flight, which is perhaps a legacy of the autopilot in the default planes(nothing much like the real thing, I know). As a result, I am somewhat confused about how the various modes of the AFDS function with the Flight Director ONLY, and then with BOTH the Flight Director AND the Autopilot engaged.Here are a few (sample)questions to indicate my confusion:1. Assume F/D ON, A/T ON, A/P OFF. What should happen when I set speed in the SPEED Selector window and push the SPEED switch to ON? IF the A/P were engaged, I would expect an increase in thrust and a pitch down to facilitate an increase in speed, but what happens with only the F/D engaged?2. Assume F/D ON, A/T ON, A/P OFF. When in the climb, at about 1000 AFE, I push the N1 Switch to ON. Since this is an A/T mode, it should engage, ie, actually reduce thrust to climb thrust? If so, is there any difference in functionality from when the A/P is switched ON?3. What about the other modes - LVL CHG, V/S, LNAV, VNAV, etc? What are the functional differences with F/D only, then with F/D and A/P?Now, I know there is a general answer which says that with the F/D only, you get pitch and roll guidance on the EADI, and, with A/P also engaged, the AFDS will automatically follow the F/D movements. However, this does not address the whole issue, as far as I can see. For example, with F/D only, if in VNAV how will I meet speed/altitude constraints(pitch guidance on the F/D?)?I realise I am struggling to express myself here, but I hope this post is not TOO confused? If it is, I will try to explain further when(if :-)) I get any replies.Thanks for your patience in reading all of this.BR,Frank

[michal 44]

Frank,Since you may be confused what FD is and what is not I suggest you read this excellent post by some retired 747 pilot here on Avsim. It gives you some interesting background in FD operation. I hope you find this reading rewarding. Then maybe someone else will address your specific questions.IS YOUR FLIGHT DIRECTOR A DUD? by Peter Edwards This is a rehash of a post I made here at Avsim some 18 months ago. Forgive me for revisiting this subject, but unfortunately nothing much has changed since then. My credentials are - retired airline pilot, having spent the last five years of my career as a captain on the B747-200 with a leading international airline, and having been a user of MS Flight Simulator in its various incarnations for some twelve years. First let's look at what a flight director's (FD) function is - it's to provide steering commands to the pilot to fly the aircraft manually just as the autopilot (AP) would in the same situation, and to this end it uses the same switching inputs as the AP. Its secondary role is to provide a monitoring function over the AP, in that if FD and AP are well integrated, the command bars stay centred throughout except in response to a new input until the aircraft catches up. An FD display is usually either a pair of "command bars" or "V-bars", the principle and display being the same whatever the aircraft type. The term "command bars" refers to the FD pitch and roll bars, not to ILS needles and such, which are situation indicators. If you use the AP all the time, you may not notice anything wrong with a bad FD as the indications can be subtle. This piece is mainly directed at people who enjoy handflying in IFR, in which case a good FD is a sine qua non. Many, if not most, of our simulated FDs are nothing of the sort. Gauge developers often seem to have little or no idea how they should function or what to aim for. Most seem to end up as situation indicators, which is pointless as we already have the raw data displays. If you treat this type as a steering aid you'll end up in real trouble. It's better to ignore them or, best of all, switchthem off. To put it another way, a bad FD tells you where an altitude/localiser/glideslope is, but a good one guides you there. How can you tell if you have a properly functioning FD? Fortunately MS, bless their hearts, got things right years ago with the FD on the default B737. It works correctly and well, so you can use it as a yardstick as well as see how a good FD should operate and what to look for in your add-ons. You need to set up a couple of situations, and you mustn't use the AP. First, fly along in FD altitude hold, manually, using your add-on panel/aircraft. Then deliberately offset yourself by, say, 1000ft from the FD held altitude. The pitch bar indicates a command to return to that altitude. But the dud bar stays deviated until you reach it regardless of the pitch you feed in. If you follow the bar, given time and space you'd probably reach the vertical. Now repeat the exercise on the default B737. The pitch bar deviates, just as before. But this time when you apply what the FD regards as an adequate control input the pitch bar will return to centre although you're still way off the preset altitude. Now all you do is keep the bar centred and it will direct you to that altitude at a reasonable rate of climb or descent and then smoothly command opposite control input as you approach it, so you can level off. The other exercise is on a manually flown ILS with the FD in approach mode. Start a fair way out to give yourself plenty of time and space - you needn't be on the glideslope. From a position of holding the centreline, offset yourself from the localiser (not the roll bar) further than full deflection of the ILS pointer or HSI display. Stay in approach mode. Your dud FD will command a turn to recover the localiser, but it won't control your roll angle which once again could exceed the vertical with the roll bar showing complete indifference until you actually reach the localiser (and zoom through it). Try the same thing in the default B737. This time, when you roll into the correcting turn, when you reach a normal bank angle (about 25 deg) the FD roll bar will come back to centre (or past, if you've over-rolled). If you keep it centred, your bank angle will be held at 25 deg until you reach a reasonable intercept angle, when the roll bar will move the opposite way, commanding you to roll out of the turn and continue straight on until approaching the localiser centreline. At this point it will direct you to turn onto the localiser and smoothly put you on the centreline. You can do the same exercise in relation to the glideslope or, if you're feeling clever, both at once. Once set up, this way you can check out your FD in five minutes or less. In my experience, though, if the altitude capture mode doesn't work properly, the rest won't either. Bear in mind that with an FD, the fact that the needles are centred or V-bars aligned doesn't necessarily mean that you're on any sort of navaid centreline (though you can be) - it just indicates that you're doing the right thing to get there. When an FD is functioning correctly, all you have to do is keep the bars centred (or the V-bars aligned with the aircraft symbol) with occasional glances at the ILS pointers to make sure things are OK. The FD makes IFR handflying far easier, but it must work properly and many panel developers aren't producing the goods. To return to what I said at the start - the FD should provide steering commands only. It is not a situation display - for that you must look at the little pointers at the edge of the ADI. At the moment many developers seem to think an FD only needs to be a repeat of the ILS pointers in a different format - there's far more to it than that. MS got it right, some five years ago, and it's high time everyone else did. For those of you who've stuck with me so far, remember - these days you're paying good money for many of these add-ons, but you can't take it for granted they'll work properly because of that - some freeware examples are fine, some payware ones amateurish, to put it mildly. Things won't change until you start complaining. So - save or copy this page, tell your friends, spread the word, and check out the FD that comes with your shiny new Cheyenne or whatever. If you think it doesn't function correctly, ask for it to be put right. A final thought - despite its importance, I've never seen this failing picked up in a website review, so perhaps reviewers should do some homework and a more searching job in this regard. Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Jet Tech 31]

Frank,Well, Micheal has got the FD portion covered pretty well so let me confuse you a bit more :-)The Mode control panel controls the AFDS system which includes the Flight Director, Auto Pilot and Auto Throttle systems. Selecting the FD switch on and selecting a Pitch or Roll mode on the Mode Control Panel (MCP) will cause the flight director guidance bars to give you steering commands. The left side of the MCP contains most of the Autothrottle modes which are SPD, N1 and FL CH (and technically Vnav). With the Auto Throttle switch armed, Selecting the N1 mode will make the throttles move to the selected N1 target (T/O, Climb, cruise, GA) while selecting SPD will attempt to maintain the selected speed in the MCP Speed window by changing the thrust settings. FL CH mode is a bit tricky since it is trying to maintain the Dialed in speed on the MCP while climbing or decending to the selected Altitude on the MCP. If your flying by hand in this mode you can get a lot of thrust changes to try to keep up with your changes in pitch. Same goes for Vnav as it will try to maintain the selected Speed in the FMC for the particular phase of flight.Well that should make things clear as mud. Welcome to the world of Heavy Metal. RegardsPaul Gollnick :-cool Technical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.comhttp://www.precisionmanuals.com/images/forum/devteam.jpg

[Guest frankathl]

That's a very interesting piece, Michael, and very good in explaining the basic function of the Flight Director. I confess I hadn't thought very much before about exactly how the F/D works, and the captain makes this very clear. Thanks,Frank

[Guest frankathl]

Actually, what you say, Paul, is clear enough to me. Thanks!Further questions, though:1."selecting SPD will attempt to maintain the selected speed in the MCP Speed window by changing the thrust settings."This makes sense as A/T is engaged. At the same time, the F/D is giving pitch guidance, is it not? What happens now if the A/P is engaged? I presume it will follow the pitch guidance on the F/D as a means of changing speed? So, to summaraize:SPEED selected ON with F/D only egaged, manages thrust to alter speed, and gives pitch guidance via the Flight DirectorSPEED selected ON with F/D & A/P engaged, manages thrust and changes aircraft pitch to alter speed.Is this correct?2. I gather from what you've said that all A/T modes are independent of BOTH the F/D and the A/P? That is to say, you can, for example, push N1 to ON to transition to, say, climb thrust, even without the F/D connected?BR,Frank

[michal 44]

>At the same time, the F/D>is giving pitch guidance, is it not? It is.>I presume it will follow the pitch guidance on>the F/D as a means of changing speed? Speed mode only works with A/T engaged. There is no chasing speed through pitch change.>SPEED selected ON with F/D only egaged, manages thrust to>alter speed, and gives pitch guidance via the Flight Directoryes.>SPEED selected ON with F/D & A/P engaged, manages thrust and>changes aircraft pitch to alter speed.again, speed mode is only active with A/T on.>can, for example, push N1 to ON to transition to, say, climb>thrust, even without the F/D connected?F/D is always "conncted" even if its commands are not displayed. A/P always follows F/D (think of F/D as an internal black box which is at the heart of A/P). Also apparently Boeing does not recommed flying with A/T engaged while A/P is disengaged. Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Guest frankathl]

Thanks, Michael, for the further clarifications. See further queries below.>>At the same time, the F/D>>is giving pitch guidance, is it not? >>It is.>>>I presume it will follow the pitch guidance on>>the F/D as a means of changing speed? >>Speed mode only works with A/T engaged. There is no chasing>speed through pitch change.What, then, is the purpose of the pitch guidance? From what you say above, SPEED is an A/T only mode, and does not care whether the A/P is engaged or not(nor the F/D, for that matter, although, as you say, the Flight Director is always on).>>>SPEED selected ON with F/D only egaged, manages thrust to>>alter speed, and gives pitch guidance via the Flight>Director>>yes.>Same point here - the pitch guidance seems unnecessary?>>SPEED selected ON with F/D & A/P engaged, manages thrust and>>changes aircraft pitch to alter speed.>>again, speed mode is only active with A/T on.>>>can, for example, push N1 to ON to transition to, say, climb>>thrust, even without the F/D connected?>>F/D is always "conncted" even if its commands are not>displayed. A/P always follows F/D (think of F/D as an internal>black box which is at the heart of A/P). >I see. So, turning the F/D on means just to display its steering commands on the EADI.>Also apparently Boeing does not recommed flying with A/T>engaged while A/P is disengaged.> >Thanks once again, Michael!>Michael J.>WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB>Raptor,1GB PC3200,Audigy 2

[victorlima01 2]

Just to make things a little harder :) I don't know if my real-world experiences (limited to a few rides in the jumpseat) are consistent with Michael's last post. I say this because I clearly remember the captain of a triple seven disconnecting the A/P right after glideslope capture and hand-flying all the way down while using the A/T to manage the approach speed. BTW I believe this is standard operating procedure for this company. And takeoffs are done almost 100% of the times with A/T on, F/D engaged and A/P off correct? Sorry if I misunderstood your post if by any chance you were referring to 737NGs only. And if so, than should I disengage the A/T along with the A/P when I handfly an approach? Cheers,Victorhttp://www.precisionmanuals.com/images/forum/800driver.jpg

[Guest boxjockey99]

FrankA lot of people get all up tight about the difference between A/P and F/D functions and understandably so since the lines between the two are becoming more and more blurred with integrated auto flight systems such as the system fitted to the NG. I was one of those people until I actually thought about it and watched what the thing actually did. Prior to flying the 737 (in real life) I flew the Fokker F27-500/600 3 of which had Sperry auto pilot/ flight directors fitted which were an ideal way to see what was going on in a basic A/P & F/D system. The way it works for all intents and purposes is that the F/D computer is the brains of the operation. The F/D tells you the pilot where to put the aircraft using command bars on the attitude indicator. So from this base we can then add the autopilot which blindly will follow what the F/D commands. SO by you setting the flight director you are indirectly telling the autopilot what to do. Now before I get flamed by various people for what I have just said, yes I do know that the 737 can be flown on the autopilot with the F/D turned off. I use the F27 Sperry system as an example to try and simplify the way it all works 99.9% of the time. On the F27 you didn't have the option, you had to have the F/D engaged to get the a/p to function. The 737 has a F/D switch on the MCP but consider this (and I'm sure Ian will correct me on this one) that switch simply turns the bars on and off on the EADI, the F/D computer is still working and doing its thing in the background. This is why you can fly the 737 on the A/P with the F/D switch in the OFF position.The AFDS on the 737 is a different beast in many ways but the functionality in the main is the same but we now have the added fun and games associated with A/THR and FMC controlled modes such as LNAV and VNAV. Again people get up tight about the modes etc and it is easy to go down that road but you need to look at what the AFDS is trying to acheive and how it is able to acheive it in order to see what is going on. I'll try and answer the queries you posted in an easy way so even we mere mortals can understand them. 1) When you hit speed with the F/D off the a/thr will try to get to the speed you have set by increasing thrust up to max N1 for that phase of flight (CLB or CRZ normally). If you don't pitch down as the flight director would be commanding if it were on then the A/THR simply assumes the responsibility and increases power accordingly. 2) no there is no difference in the functionality when the A/P is engaged, all the A/P does is follow the F/D bars and the A/THR will do the same as it would with you driving it!3) Same as with the A/P engaged I'm afraid, the a/THR and F/d behave completely the same whether the A/P is engaged or notyou ask whether you will still make alt constrains in VNAV with the A/P disengaged well guess what, yes the F/D is providing commands to follow the Path if in VNAV PTH and therefore assuming you follow the bars then you should be on the profile! this is why there are certain rules that A/THR logic follows in climb / descent modes. In all climb and descent modes (except V/S which is rarely used) the A/THR will drive to a datum point namely full climb power in the climb or idle in the descent. This means that the path is therefore always being descided by the pitch of the aircraft.In the climb LVL CHG and VNAV, A/THR will drive the power to full CLB (or R-CLB if you reduced thrust) power and the aircraft will pitch to maintain the selected speed. If it gets slow it will lower the nose and if it gets fast it will raise the nose. Thrust reamins constant and so pitch is the only thing determining the climb speed and path.In the descent A/THR will drive the thrust levers to idle thus you control either the speed (in LVL CHG or VNAV SPD) or the path (in VNAV PTH) by pitching and again the thrust remains constant at idle!I think that clarifies it and I apologise if I have repeated what other posters have stated alreadyAll the best and please feel free to ask me any more questionsKris

[zubart 5]

Victor,I'll just add the the fire.>Just to make things a little harder :) I don't know if my>real-world experiences (limited to a few rides in the>jumpseat) are consistent with Michael's last post. I say this>because I clearly remember the captain of a triple seven>disconnecting the A/P right after glideslope capture and>hand-flying all the way down while using the A/T to manage>the approach speed. BTW I believe this is standard operating>procedure for this company. During the Boeing 737NG training they do teach if the A/P is off then the A/T should be off but like you witnessed in the jumpseat, most line pilots tend to do exactly what he did. It's easier. And takeoffs are done almost 100%>of the times with A/T on, F/D engaged and A/P off correct?Yes. The normal takeoff in a 737NG is both F/D's on and A/T engaged. Always. >Sorry if I misunderstood your post if by any chance you were>referring to 737NGs only. And if so, than should I disengage>the A/T along with the A/P when I handfly an approach? >>>Cheers,>Victor>>>http://www.precisionmanuals.com/images/forum/800driver.jpgViAloha,Floyd

[michal 44]

Great input. Thanks Kris.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Guest frankathl]

Apologies for my delay in returning here!Thank you, Kris, for a full explanation and very clear post. No need to apologise for repeating anything as it serves to confirm some things I

[Qavion 19]

>The way it works for all intents and purposes is that the F/D computer is the brains of the operation. The F/D tells you the pilot where to put the aircraft using command bars on the attitude indicator. It's probably much easier for the folks to understand if it is described in the way you say, Kris.Unfortunately, when you go a bit deeper into the subject, you may find a technical manual which shows you a block diagram of the internals of a Boeing aircraft autopilot computer which puts the "FD processing" slightly outside of the brain (see example below). However, flight control surface commands and FD commands both, basically, come from a common source.http://members.ozemail.com.au/~b744er/FCCProcessing.gifYou may also see statements in some Boeing aircraft manuals which say, during some ILS intercepts, the (autopilot controlled) aircraft may not exactly follow the FD bars (but this is no cause for concern).As you say, switching off the FD, for the most part, will not affect the autopilot. You're basically only switching off the displays.Anyway, thanks for the excellent summary :-) Cheers.Ian.

[Guest boxjockey99]

IanStop please stop I am getting flashback to the groundschool course and the annoying german accented english woman who does the voice over... it sends a shiver down my spine LOL!!Great diagram, I'll keep that somewhere safe, one day I might be good enough to actually understand half the names on it! I had forgotten that sometimes the A/P seems to ignore the F/D bars (a prime example being Malaga in spain on the ILS for 14 where the A/P seems to ignore the glide completely which is always fun! In some situations it is not uncommon to have the 2 flight directors saying two different things as well which is another entertaining situation. Thanks for the clarification though just goes to show how much better engineers know these aircraft. Had my first look in the E&E bay a copuple of days ago which was like stepping into a different world, too many looms of wire and black boxes and entirely too few buttons to play with! Happy trailsKris

[Guest frankathl]

Thanks, Ian! That's a very interesting diagram!BR,Frank