PMDG 777 FBW and weird Flight Controls behaviour?

[kevinh 409]

I'm not sure if I would like it or not. I'd have to see the implementation of this bug in the real aircraft. It sounds to me like a different application of the trim system, which might or not might work.

The FBW bug doesn't exist in any real aircraft. My suggestion was to set a speed bug as a reference to show the trimmed speed for demo purposes. It wouldn't do anything else.

[ahsmatt7 1,199]

The FBW bug doesn't exist in any real aircraft. My suggestion was to set a speed bug as a reference to show the trimmed speed for demo purposes. It wouldn't do anything else.

what you just mentioned is exactly what i was talking about. You just explained it differently and better. I guess I was explaining a moving bug, which is still different than your idea but whatever. I'm on a 2 day trip right now and the latest thing I want to do is talk about airplanes.

[777simmer 117]

To understand trim you have to understand what makes an aircraft fly.

What happens to the control surfaces when you move the yoke left/right/up/down and why it it turns when you roll/etc.

You have to understand what happens to control surfaces when you trim.

 

You have to be thought this by a flight instructor or you can teach yourself by reading PPL books, and then you have to go out and try how things FEEL and react on a real airpane when you perform different maneuvers.

You will then begin to develop a FEEL for what aerodynamics do and how they work.

 

Although FSX can help in many ways...you can not develop a FEEL for aerodynamics with a computer based system that has no feedback.

You can be taught to change the position of a trim reference bug ofcourse.....but that is not the same as developing a FEEL for the aircraft.

The outcome is the same though.

 

The idea is that you look outside and learn to develop a FEEL for the aircraft as a PPL student.

Look outside and see where the horizon is inreference to your cockpit....what level looks like and what a turn looks like.....without needing an attitude indicator!

Or at least, that was the general believe back when I learned flying 20 years ago.

A trim ref bug would only be distracting to this way of teaching.

Things are changing though, and now many pilots are being taught how to become an IFR trained airline pilot with an Airbus/Boeing  simulator from day one...apperantly without having ever touched a Cessna.

No Cessna....no real pilot....haha :-)

 

The display of TRS will show a digital confirmation of being out of trim....the pillot can then hit the trim switch untill TRS = actual speed.

But without study of basic aerodynamics you will have no idea what the flight control surface just did and thus you have no real added knowledge of how things work.

All you have, is a cheat mode that helps you achieve what should come natural to any pilot (in my book).....trim by FEEL!

(I am not saying you have to know all the time what all your flight control surfaces are doing or otherwise you are not a good pilot.....the 777 is constantly being augmented by moving flapperons and slats,  spoilers and auto stab trim.....and most of the time I could not care less what surface is doing what......but I can not support the claim that a TRS bug will help you understand aerodynamics.)

 

Is it necessary that you can trim by feel?

No...airbus decided it is not, they build in an autotrim system, and it works fine too (I never flew an airbus though so I cant say if it is nice or not, but most pilots I know who change from Airbus to Boeing are pretty annoyed by the fact they have to trim the whole time).

But Boeing decided that they wanted to make a Fligh By Wire (FBW) aircraft that FEELs like what an old world war 2 bomber  pilot was used to (conventional aircraft).

Personally I like to have to trim and feel....in my opinion it is one more thing that makes you aware of what is happening to your aircraft (hey I need to trim.....I must be getting faster/slower).

Just the same I like the visual feedback a Boeing pilot gets throught the movement of the AT and the movement of the control column/wheel when the AP is connected.

I like that....but it is not required (Airbus).

 

As someone mentioned, the pushing and pulling that is required by FSX to keep the airplane in a certain attitude....and the trimming by feel that we do to trim this constant manual input away, is similar to the real thing.

 

The problem with the PMDG777 is that when PMDG decided that they wanted to simulate the FBW system as close as possible to the real thing, they tried to sort of simulate a simulation in a simulation.

They have tried to simulate the artificial feel unit that is responsible for the simulated(artificial) forces that a real 777 pilot FEELs.

Remeber that without the artificial feel unit a real 777 pilot wheel never feel anything....not attitude changes, not out of trim conditions, nothing would provide a force on the yoke (it is all electrical signals/no cables) if it wasnt for the artificial feel unit.

(I am going out on a limb here a bit, who am I to say what PMDGs intentions are/where, but this is my conclusion and my understanding of how things work after many past discussions).

So this unit provides feedback forces to the pilot when when changing the airplanes attitude and also when out of trim....and all of a sudden you are sitting there fighthing an artificially introduced force and the airplane FEELs like a world war 2 bomber again:-)

(ok a way more sensitive and way less sluggish bomber.....but still conventional aircraft again).

 

The forces that a 777 pilot FEELs, when out of trim....are not realy there, aerodynamically speaking :-)

The airplane would not need trimming at all (just like an Airbus does not need it) if Boeing had not build in an artificial feel unit and an artifical out of trim feel system (dont remember what the real name of this system is).

Because as I understand it, the 777 does infact auto trim.....it will constantly move the stabilizer so that it becomes flush with the elevator.....this is autotrimming.

 

At that moment, when auto trimmed, you would not feel any forces any more if it wasnt for the artifical feel unit and the artificial out of trim feel system.

These two systems require you to exercise a contant force to the yoke to keep it CENTERED.

If you let go of the yoke with such an out of trim force...then the yoke would move out of center and thus the aircraft would pitch up or down.

It is this force that you have to trim away in the real aircraft....:::.so nothing changes aerodynamically (again, as far as I have come to understand the system).

 

But we dont have atifical motor driven yokes at home....or at least I dont.

So when we let go of the yoke, it goes back to center regardless....there is no out of trim force....there is only us trying to keep the airplane from pitching up/down.

 

So, to simulate an out of trim force (of an in-trim PMDG777) PMDG has.....and here comes the reason for why we have a PMDG speed ref bug......created a system that influences the effectiveness of your yoke vertical axis. (you could call it changed elevator bias)

(You can do the same with FSUIPC if you change the elevator bias insettings....or maybe it is not the same, I dont know, but it is similar to that I find).

 

I never understood how an out of trim force can be simulated by reducing my yokes elevator effectiveness.....just thinking about it now gives me a headacke again....so have given up on understanding how that should work....lets just accept it and work with it

 

So now, when out of trim, the effectiveness of your vertical yoke axis becomes less and thus you should be holding more and more yoke deflection as you get more and more out of trim.

The thing is that this changing of the elevator bias seems to work well for some and not so good for others.

Which is why PMDG has given us the TRS bug.....so that those who have difficulty to trim by FEEL....can revert to simply trimming the TRS bug to actual speed.

 

In my case, I cant trim the PMDG777 by FEEL at all times.

When way out of trim I can FEEL this....the bias has become so low and yoke effectiveness so small that I am holding sufficient elevator deflection that I can feel it (even on my fairly light Saitek Cessna Yoke).

But as soon as I get closer to my in trim speed the reduced elevator effectiveness bias becomes less and less and I feel nothing.

It is simply not possible to feel or notice a difference on my yoke between centered and 1cm deflection....the thing is too light.

Even with added springs in my modded yoke this effect is still pressent although less pronounced.

So basically I can not trim small out of trim situation away by feel alone.

 

Without the TRS speed bug I could not fly my PMDG777 version as the airplane keeps pitching up/down to find its in-trim speed (even if I am only 3kt away from being in trim)

Maybe it is something on my end like FSUIPC settings or the type of hardware I am using or who knows what.

I only described how my PMDG777 needs the TRS bug to explain why this bug is there in the first place......I do not want to hack this thread on how to improve my trim by FEEL experience.....we have been there and tried enough in the past.....I have come to accept how my PMDG777 works and with the TRS  bug I am quite happy with how it behaves.

 

But I hope my post helps explain why the PMDG777 comes with a TRS bug and why this bug does not help you understand aerodynamics better.

[Alpha Floor 237]

Great post Rob, very well put! Please allow me to comment on some points:

 

 

Because as I understand it, the 777 does infact auto trim.....it will constantly move the stabilizer so that it becomes flush with the elevator.....this is autotrimming.

 

It does auto-trim for everything other than speed-changes, yes. First it moves the elevators, then it moves the stabilizer to streamline (cancel) the elevators.

 

 

so nothing changes aerodynamically

 

Well, something changes indeed. If you're pushing the control in an out-of-trim condition and then release it, the column will go to neutral and the aircraft will pitch up. In the back the elevators and stabilizer do indeed move. There are aerodynamic effects. Did I not understand what you meant here properly? Sorry if that's the case.

 

 

 

In my case, I cant trim the PMDG777 by FEEL at all times.

 

Exactly! It just doesn't work like in the real aircraft with our "cheap" "low quality" "jerky" controllers.

 

 

 

Without the TRS speed bug I could not fly my PMDG777 version as the airplane keeps pitching up/down to find its in-trim speed (even if I am only 3kt away from being in trim)

 

The real aircraft has a feature pilots call "BLIP TRIM". If you're within 5 kt of your actual TRS and push the trim switches in any direction, the TRS will jump to the actual speed. I guess this makes trimming easier.

 

 

if Boeing had not build in an artificial feel unit and an artifical out of trim feel system (dont remember what the real name of this system is).

 

Those are just called "Feel Units", like you said

 

The system diagram is as follows:

 

 

Source here.

[aceridgey 330]

I just flew a circuit with the flight computers disconnected and I have never ever had the pmdg 777 fly so nicely...

 

I feel like I have much more control of the aircraft without it trying to bubble up and down all the time ....

 

 

Is there any way to disable the PMDG FBW system without disconnecting the overhead switches? I would much rather fly like this.,

[Alpha Floor 237]

Is there any way to disable the PMDG FBW system without disconnecting the overhead switches? I would much rather fly like this.,

 

Yes, you can go to the Failures page of the FMC and select either: "PFCs to Secondary" or "PFCs to Direct"

 

In Secondary and Direct the aircraft flies pretty much the same way. If you choose direct, it's really the same effect as disconnecting the PFCs in the overhead.

 

Might I add that the Direct Law of the 777 is even more "direct" than that of a 737 for instance, because some compensations lke the Yaw Damper are built-in into the PFCs.

 

Also I don't agree that the plane flies so nicely in Direct. Flying in Normal Law is a pleasure for me, requires really minimal input, handles like an S-Class. Maybe you're over-controlling, used to conventional aircraft, and that's why the aircraft is bubbling up and down all the time (i.e., pilot induced oscillations)?

[aceridgey 330]

Yes, you can go to the Failures page of the FMC and select either: "PFCs to Secondary" or "PFCs to Direct"

 

In Secondary and Direct the aircraft flies pretty much the same way. If you choose direct, it's really the same effect as disconnecting the PFCs in the overhead.

 

Might I add that the Direct Law of the 777 is even more "direct" than that of a 737 for instance, because some compensations lke the Yaw Damper are built-in into the PFCs.

 

Also I don't agree that the plane flies so nicely in Direct. Flying in Normal Law is a pleasure for me, requires really minimal input, handles like an S-Class. Maybe you're over-controlling, used to conventional aircraft, and that's why the aircraft is bubbling up and down all the time (i.e., pilot induced oscillations)?

 

So is secondary similar to say the NGX? Rather than direct?

[Alpha Floor 237]

So is secondary similar to say the NGX? Rather than direct?

 

In both Secondary and Direct the aircraft will fly in the same way (maybe not exactly the same, but pretty much so). Also you won't have any protections and won't be able to engage the AP (just in case you want to fly a flight flying by hand all the time). The difference is that in Secondary the PFCs are online and generating the Control Laws, while in Direct the PFCs are offline altogether.

 

And no, it's not exactly the same as the 737. The 737 has a Yaw Dampening System that dampens Dutch Roll. If you fly the 777 in Direct or Secondary, there's no Dampening and no Stability Augmentation. As I said, it's even more "direct" than flying a conventional aircraft.

 

My advice: Fly the aircraft the way it's supposed to, believe me, it's really smooth and easy to control. By all means experiment and see how it flies in direct, but for the sake of experimentation and system awareness. When flying "on the line", use the Normal Law. I recommend you have a look at this thread and watch my video analysis on the FBW with Normal-Direct comparisons)

[aceridgey 330]

In both Secondary and Direct the aircraft will fly in the same way (maybe not exactly the same, but pretty much so). Also you won't have any protections and won't be able to engage the AP (just in case you want to fly a flight flying by hand all the time). The difference is that in Secondary the PFCs are online and generating the Control Laws, while in Direct the PFCs are offline altogether.

 

And no, it's not exactly the same as the 737. The 737 has a Yaw Dampening System that dampens Dutch Roll. If you fly the 777 in Direct or Secondary, there's no Dampening and no Stability Augmentation. As I said, it's even more "direct" than flying a conventional aircraft.

 

My advice: Fly the aircraft the way it's supposed to, believe me, it's really smooth and easy to control. By all means experiment and see how it flies in direct, but for the sake of experimentation and system awareness. When flying "on the line", use the Normal Law. I recommend you have a look at this thread and watch my video analysis on the FBW with Normal-Direct comparisons)

I see what you mean thats ffor the explination

 

However,

 

I do get frustrated when the aircraft bobbles up and down and it's very difficult to fly without frustration

[Alpha Floor 237]

 

 

I do get frustrated when the aircraft bobbles up and down and it's very difficult to fly without frustration

 

If you feel like the aircraft is bobbling, just let go of the controls and see what happens.I think this might be an over-controlling issue. On the 777 the principle of "minimal input" applies, just as in an Airbus (which doesn't mean they fly the same way)

 

Also have a look at the videos that the OP posted in the first message, the ones of the EVA Pilot in the Full Flight Simulator.

[kevinh 409]

The problem with the PMDG777 is that when PMDG decided that they wanted to simulate the FBW system as close as possible to the real thing, they tried to sort of simulate a simulation in a simulation.

 

They have tried to simulate the artificial feel unit that is responsible for the simulated(artificial) forces that a real 777 pilot FEELs.

 

Remeber that without the artificial feel unit a real 777 pilot wheel never feel anything....not attitude changes, not out of trim conditions, nothing would provide a force on the yoke (it is all electrical signals/no cables) if it wasnt for the artificial feel unit.

 

(I am going out on a limb here a bit, who am I to say what PMDGs intentions are/where, but this is my conclusion and my understanding of how things work after many past discussions).

 

So this unit provides feedback forces to the pilot when when changing the airplanes attitude and also when out of trim....and all of a sudden you are sitting there fighthing an artificially introduced force and the airplane FEELs like a world war 2 bomber again:-)

 

(ok a way more sensitive and way less sluggish bomber.....but still conventional aircraft again).

 

 

 

The forces that a 777 pilot FEELs, when out of trim....are not realy there, aerodynamically speaking :-)

 

The airplane would not need trimming at all (just like an Airbus does not need it) if Boeing had not build in an artificial feel unit and an artifical out of trim feel system (dont remember what the real name of this system is).

 

Because as I understand it, the 777 does infact auto trim.....it will constantly move the stabilizer so that it becomes flush with the elevator.....this is autotrimming.

 

 

 

At that moment, when auto trimmed, you would not feel any forces any more if it wasnt for the artifical feel unit and the artificial out of trim feel system.

 

These two systems require you to exercise a contant force to the yoke to keep it CENTERED.

 

If you let go of the yoke with such an out of trim force...then the yoke would move out of center and thus the aircraft would pitch up or down.

 

It is this force that you have to trim away in the real aircraft....:::.so nothing changes aerodynamically (again, as far as I have come to understand the system).

 

 

 

But we dont have atifical motor driven yokes at home....or at least I dont.

 

So when we let go of the yoke, it goes back to center regardless....there is no out of trim force....there is only us trying to keep the airplane from pitching up/down.

 

This is all true, but it's also true of any irreversible powered flight control system in aircraft such as the 747. It's not just a feature of FBW aircraft. Without artificial feel units there would be no aerodynamic feedback to the pilot in the 747 either. So I don't see why it should have created a problem for simulating the 777 flight controls and trimming in particular.

 

So, to simulate an out of trim force (of an in-trim PMDG777) PMDG has.....and here comes the reason for why we have a PMDG speed ref bug......created a system that influences the effectiveness of your yoke vertical axis. (you could call it changed elevator bias)

 

(You can do the same with FSUIPC if you change the elevator bias insettings....or maybe it is not the same, I dont know, but it is similar to that I find).

 

 

 

I never understood how an out of trim force can be simulated by reducing my yokes elevator effectiveness.....just thinking about it now gives me a headacke again....so have given up on understanding how that should work....lets just accept it and work with it

 

This is the main problem with trimming in the PMDG 777. I think it makes manual flying and trimming harder, not easier. I wish the reduced elevator effectiveness could be made a user option so I could switch it off. Reducing elevator effectiveness has a negative effect on handling quality. Not a problem if you are near the trim reference speed, but if you've changed airspeed without trimming you can get to the point where the elevator will only respond in one direction. I'm convinced this is why some people have problems hand flying this otherwise fantastic sim.

 

Spring forces in hobby joysticks and control yokes at full deflection are so light that attempting to increase the force by reducing elevator effectiveness is never going to do much, and any increased force is offset by reduced control effectiveness.

[JRBarrett 3,070]

Spring forces in hobby joysticks and control yokes at full deflection are so light that attempting to increase the force by reducing elevator effectiveness is never going to do much, and any increased force is offset by reduced control effectiveness.

Interesting to note that the force feedback provided by the artificial feel units in most aircraft with hydraulic-powered flight controls, is also accomplished using springs. The tension on the springs, (and thus the amount of resistance the pilot feels in the yoke) is controlled by hydraulically actuated lever mechanisms whose position is directly proportional to the deflection of the control surfaces.

 

That sort of spring-driven artificial feel system is not applicable to Airbus models of course...

[777simmer 117]

Great post Rob, very well put! Please allow me to comment on some points:

 

 

 

It does auto-trim for everything other than speed-changes, yes. First it moves the elevators, then it moves the stabilizer to streamline (cancel) the elevators.

 

 

 

Well, something changes indeed. If you're pushing the control in an out-of-trim condition and then release it, the column will go to neutral and the aircraft will pitch up. In the back the elevators and stabilizer do indeed move. There are aerodynamic effects. Did I not understand what you meant here properly? Sorry if that's the case.

 

The whole thing is quite complicated.

And if its not then we (me) usually will make it more complicated than it is ;-)

The lack of proper guidance material does not help either.

 

But I meant what I said

 

I have come accept  that the 777 autotrims the stabilizer even for speed.

(And even though I flew the 777 for more than 13 years I have no proof of this)

 

When the PMDG777 first came out it did just that....it auto trimmed.

Things were rectified and we got what we have now (which is better)

How could have PMDG gotten it wrong?.....I clearly had to trim for speed in the real thing.

Well because the more you read into all sorts of manuals....the less clear it becomes unless you are the architect of the system I guess.

 

I am browsing the Technical Manual of the 777 right now (its not the complete thing but has more info than any other manual we have) and when I read that I again conclude that the 777 does NOT autotrim for speed....that it needs manual trim to correct an out of trim situation.

Just like you say.

 

And yet my feeling tells me I am just interpreting the manual incorrectly or I just dont have all the info to back up my thesis.

Why?

Because it STILL seems to me the 777 autotrims even for speed and all we get in the cockpit is a simulated out of trim feel.

If you ask me it is allways trimmed....it just FEELs out of trim throught the artificial feel unit.

What has brought me to this conclusion?

Well....I have to take a D-tour.....

The Left Elevator is powered by the Left and Center hydraulical system.

The Right Elevator is powered by the Right and Left hydraulical system.

So, if both Left and Right hydraulical system would fail....you would end up with only the Left Elevator being powered through the Center hydraulical system.

Now imagine only the Left Elevator moving about as you maneuver around.

That elevator is pretty big....surely that would cause some role when only one side moves up and down no?

And yet, I have never noticed any roll movement during any such sim sessions.

So my conclusion was that the horizontal stabilizer probably moves automatically (autotrim for speed) and the elevator would be flush again.

Thus no roll.

But maybe I am wrong....maybe I am just adapting the system to my needs because I cant make any sense of it all anymore.

Or because I am fed up with it....and just want to fly.

It could be that I am soooo good at trimming that I am never out of trim (much) and that is the reason for no roll in the hydraulic fail scenario.

 

 

Anyway, the only thing in my technical manual that supports my conclusion is the mention of "elevator off load function"

This function offloads the elevator by moving the stabilizer as soon as the elevator is extended for more than two seconds, so that the elevator is flush again.

Other phrases in my manual, such as " the pimlot moves the horizontal stabilizer with the trim switches" contradict my conclusion.

[Alpha Floor 237]

Rob, maybe you find my analysis on the FBW of the 777 useful, I think it might shed some light. Also, have a look at the references, particularly the first two ones.

 

An Analysis of the Boeing 777 FBW System

 

There have also been odd conditions reported on the PMDG external model in this very thread. I also reported an odd NO ROLL yet aileron/spoiler deflected condition when playing with the failures, here it is:

 

[777simmer 117]

 

The real aircraft has a feature pilots call "BLIP TRIM". If you're within 5 kt of your actual TRS and push the trim switches in any direction, the TRS will jump to the actual speed. I guess this makes trimming easier.

 

 

 

Those are just called "Feel Units", like you said

 

The system diagram is as follows:

 

 

The PMDG777 has BLIP TRIM as well.

If you display the TRS bug you can see it at work.

Here on my end it does not allways work...it works like 70% of the time.....further increasing the difficulty to trim by feel.

 

 

I did not mean the feel units themselves....I know they are called artificial feel units, haha.

I meant the artificial out of trim feel system....or software I guess......not sure how that system is called.

I vaguely remember PMDG commenting on that a long long time ago as well.....that there is a system or software responsible for simulating out of trim forces that would otherwise no be there.

Could have mixed things up thought...it has been a while this whole thing.

So is secondary similar to say the NGX? Rather than direct?

 

The real 777 is way easier to fly than the 737....way more stabile due to all the automatic flight path corrections taking place.

 

In secondary mode the 777 becomes more like a 737.

 

In direct mode the 777 is quite a bit harder to fly (very sluggish) than in direct mode and thus also quite a bit harder to fly than a B737.

 

By far the hardes of all is the B737 manual reversion though......cables only, no hydraulical systems available......this is incredibly heavy and I have often wondered how female pilots fly that.

Maybe I am just not as strong as I think ;-)

I see what you mean thats ffor the explination

 

However,

 

I do get frustrated when the aircraft bobbles up and down and it's very difficult to fly without frustration

 

That bobbeling up and down is what I get if I fly without the TRS bug.

 

The airplane keeps going up and down even if you are only 1kt out of trim.....I am not kidding.

 

Try it with the cheat mode for a while and see if the bobbing/pitching up and down stops if you keep hitting the trim switch so that that TRS bug is allwas and allways and allways exactly precisely equal to you ACTUAL speed.

Note: ACTUAL speed is not the speed where your target speed bug is at.....ACTUAL speed is what your speedtape reads!

Note: even with the AT engaged......ACTUAL speed is not where your target speed bug is at.......ACTUAL speed is still what your speedtape reads! (the AT is often off by a few knots and thus retrimming is required!

 

 

This is the main problem with trimming in the PMDG 777. I think it makes manual flying and trimming harder, not easier. I wish the reduced elevator effectiveness could be made a user option so I could switch it off. Reducing elevator effectiveness has a negative effect on handling quality. Not a problem if you are near the trim reference speed, but if you've changed airspeed without trimming you can get to the point where the elevator will only respond in one direction. I'm convinced this is why some people have problems hand flying this otherwise fantastic sim.

 

 

I would love that option!