PMDG 777 FBW and weird Flight Controls behaviour?

[777simmer 117]

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

 

 

Pretty good, but also pretty long explanations and videos......I already took more time for this allways interesting subject that will never go to bed I am sure, haha

I just cant spend too much more time on it right now.

Sorry.

 

The one thing I would like to pass along to you is that in all your explanations and videos you are comparing/prooving your findings from your reading material and knowledge to the PMDG777.

And although it is a good simulation of the real thing.....it is not the real thing.

And you have to be carefull with that when you delve this deep into its systems!

Because when you get a little thing wrong, this deep into the matter, that is when people like me, lurking in the dark go....gotcha ;-)

 

For example, to claim there is not much difference between secondary and direct mode on the 777 is maybe a step too far to conclude from books and PMDG777 sim alone.

I find direct mode quite a bit more difficult than secondary mode....at least on the level D simulator that was my conclusion (never had that problem in real life).

 

Keep up the good work though.

[aceridgey 330]

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.

 

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 ;-)

 

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!

I would love that option!

 

Hi there,

 

I certainly keep the indicated trim position on, i've never had it offf. Still find the NGX more comofrtable to hand fly,, by a lot

[Alpha Floor 237]

The one thing I would like to pass along to you is that in all your explanations and videos you are comparing/prooving your findings from your reading material and knowledge to the PMDG777.

And although it is a good simulation of the real thing.....it is not the real thing.

 

Yes, you're exactly right. I wan't to believe that it's at least close enough to the real thing that I can establish conclusions based on the PMDG aircraft. Sometimes I just "jump into the pool without checking if there's water first"

 

 

For example, to claim there is not much difference between secondary and direct mode on the 777 is maybe a step too far to conclude from books and PMDG777 sim alone.

 

Yeah. I haven't flown the real aircraft in its 3 modes so I can't really tell. But from my experience trying the PMDG 777 and from what I can read in the FCOM V2, which has a chapter for "Normal Law" and then a chapter for both "Secondary and Direct Law", without really making a distinction among those two, leads me to believe that indeed there's little difference in handling qualities among the degraded modes. 

 

Also, I quote from FCOM 9.20.9:

 

Airplane handling qualities are approximately the same [in the direct mode] as in the

secondary mode.

 

 

So the only thing I have left to really fool-proof my statements is to go on to a Level-D FFS and try it in a scientific method kind of way. Unfortunately I don't have that chance.

 

And thanks for your input Rob, I find your ideas very interesting!

[Alpha Floor 237]

I found an article by a Boeing engineer, Bob Yeh, which shows the following table:

 

 

The only difference between Secondary and Direct seems to be the Yaw Damper and that comes with an "asterisc" as well.

[777simmer 117]

You mean that this supports your conlcusion that there should not be much difference between secondary and direct mode?

 

Look, its all relative isnt it...maybe one feels it is harder to fly in direct mode than the other.

 

My opinion is that direct mode in the level D sim needs your attention.

 

Although it is a few years ago I remember my approach in that mode even today!

Canarsy VOR Rwy 13R (in the level D sim)

I had not anticipated how crappy and slow the airplane reacts in direct mode so I blew right through final afer passing the VOR and is was quite a struggle to get back in a safe position to land on that curved final approach.

 

Sure, if you have a 10nm final to line up, then I guess direct mode is not that hard.

But on that curved final approach.....it sucked....and I sucked.

 

Also dont forget that we are still talking about a digital aircraft here.

How the flight controls move around all depends on how the internal calculation are done.

Calculate differently (update the software for example) and the outcome is that you are flying an aircraft that feels totally different than before!

 

In secondary mode you still have the PFCs calculating the final output to the flight controls.

Neither you nor I know what exactly goes on in those PFCs.....but it seems logic to conclude that if they are still connected and doing something in secondary mode....then there must be a reason for this. They must be having an effect in the output signal to the flight controls other than just the jaw damper.

Three PFCs still connected, providing feedback to the ACEs.....only for jaw dampening......I dont believe that.

 

The table you found does not show the whole picture, I promise you.

Just like my technical manual does not explain all.

The ammount of info for each and every system available to technicians is so much more than what we have available to us.

And then there is even so so so much more available to the actual engineers who build it!

You cant conclude from our books and and a few other documents that the PFCs do nothing other than calculating yaw dampning corrections in secondary mode.

 

In direct mode you loose all PFCs and ALL the calculations they do (so not just the yaw dampner but whatever else they calculate as well)!

In direct mode it is like sending raw data to the flight controls....and I found that quite noticable.

 

But it could have been my imagination....I could have just had a bad day.

[Alpha Floor 237]

You mean that this supports your conlcusion that there should not be much difference between secondary and direct mode?

 

No, not at all Rob. I'm not trying to prove you wrong or anything, even if I sound like that.

 

I'm just showing the information I could gather for us to comment on it

 

One thing is the theory, then there's practice. It's not the first time that a technical manual says "in theory A and B are very similar" but then in practice they are not. You have had experience with the 777 FFS, I haven't, so I believe what you say!

 

Also I really enjoy reading your experience on the Canarsie app

 

 

 

Neither you nor I know what exactly goes on in those PFCs.....but it seems logic to conclude that if they are still connected and doing something in secondary mode....then there must be a reason for this. They must be having an effect in the output signal to the flight controls other than just the jaw damper.

 

I agree fully.

 

It's just difficult to specifically point out what the difference is. But there must be one.

[777simmer 117]

Also I really enjoy reading your experience on the Canarsie app

 

 

 Glad someone enjoys my misery, lol

[Alpha Floor 237]

 Glad someone enjoys my misery, lol

 

Yes, haha, I meant it in a good way.

 

That's why I replied to someone back in this thread that the 777 is meant to be flown in Normal Law for a reason. Direct Law is really an emergency, even though the aircraft is flyable, it doesn't handle nearly as well as a 737 which is designed to fly like it is. The 737 is non-FBW, but it still has protections and flight control computers that help the pilots. The 777 in Direct Law has no aids at all (or barely)...

[FlyChuck 3]

I'm sorry to bring this old thread back to the surface. I find that the pmdg777 is not nice to hand-fly because of its strange (and in my opinion unrealistic) flying characteristics. I found this from an online FCOM;

 

 

Normal Mode Pitch Control

Overview

In the normal mode, airplane pitch control characteristics are like

conventional airplanes. Unlike conventional airplanes, the control column

does not directly position the elevator in flight. The control column

commands the PFCs to generate a pitch maneuver. The PFCs automatically

position the elevator and the stabilizer to generate the commanded maneuver.

The PFCs constantly monitor airplane response to pilot commands and

reposition the elevator and stabilizer to carry out these commands. Airplane

pitch responses to thrust changes, gear configuration changes, and turbulence

are automatically minimized by PFC control surface commands.

The PFCs also provide compensation for flap and speedbrake configuration

changes, and turns up to 30° of bank. The PFCs automatically control pitch to

maintain a relatively constant flight path. This eliminates the need for the

pilot to make control column inputs to compensate for these factors. For turns

up to 30° of bank, the pilot does not need to add additional column back

pressure to maintain altitude. For turns of more than 30° of bank, the pilot

does need to add column back pressure.

When the autopilot is not engaged, as airspeed changes, the pitch control

system provides conventional pitch characteristics by requiring the pilot to

make control column inputs or trim changes to maintain a constant flight path.

Manual trim is necessary only when changing airspeed. Manual trim is not

necessary when changing configuration.

Pitch Trim

Primary Pitch Trim

Primary pitch trim is controlled by the dual pitch trim switches on each

control wheel. Both switches must be moved to command trim changes. The

primary pitch trim switches are inhibited when the autopilot is engaged. Pitch

trim does not move the control column.

FLT CONTROLS

Sec. 6.9 Page

8

Rev. 11/01/00 #5

Continental

777

Flight Manual

In the normal mode, primary pitch trim operates differently on the ground

than it does in flight. On the ground, the stabilizer is directly positioned when

the pilot uses the pitch trim switches. In flight, the pitch trim switches do not

position the stabilizer directly; they provide inputs to change the trim

reference speed. The trim reference speed is the speed at which the airplane

would eventually stabilize if there were no control column inputs. Once the

control column forces are trimmed to zero, the airplane maintains a constant

speed with no column inputs. Thrust changes result in a relatively constant

indicated airspeed climb or descent, with no trim inputs needed unless

airspeed changes.

When pilot trim inputs are made, the PFCs analyze the command and generate

signals to move the elevators to achieve the trim change, then moves the

stabilizer to streamline the elevator. Stabilizer motion may also automatically

occur to streamline the stabilizer and elevator for thrust and configuration

changes.

 

So the FBW compensates for:

-any configuration change

-thrust changes

-turbulence

-back pressure requirements in turns of up to 30° bank. (so in a 777 no back pressure is needed for a normal turn)

 

It doesn't compensate for:

-any speed change

(-back pressure requirements in turns of over 30° bank)

 

When I fly the pmdg 777 (with a MS ffb2 joystick, in normal flight, so no aerobatics or steep turns or anything):

-It's sometimes not clear whether or not you are in trim. I can fly significantly out of trim (as seen by the trim speed bug), but it feels in trim (stick neutral, no force), apart from erratic pitch movements and flaring problems.

-It won't put in back pressure in turns

 

I hope this makes sense

[skelsey 765]

Hi Nicolas,

 

I don't know about the B777, but what I can say is that you should be very wary when comparing the flying characteristics to the FCOM. The A320 FCOM makes the same claim about automatic pitch compensation in turns and for configuration changes etc, but in reality that is very far from being the case -- small pitch corrections are still necessary in turns, it still balloons when flaps are extended etc etc. So the FCOM is not necessarily a true representation of how the system performs in real life, and I imagine that PMDG will have modelled the system based on actual pilot feedback rather than purely on the FCOM.

[FlyChuck 3]

Hi Simon,

 

Thank you for your input. I'm sure that what you say is the case. Would you agree however that the a320 at least tries to trim out any forces? For example, if you turn left, the nose drops a bit for which you correct (and go back stick neutral?). The aircraft is by then in trim while turning. 

 

I've been doing an experiment and it seems pitch correction is simulated, as long as you don't try to be smart and add back pressure yourself.

 

 

Ignore the first 10 seconds of this one :rolleyes: I'm not adding backpressure.

https://youtu.be/GlT8Hzh8bKI

 

And in this one I am adding backpressure when the nose starts dropping slightly.

https://youtu.be/nLOV7zEN7iI

[kevinh 409]

 

I'm sorry to bring this old thread back to the surface. I find that the pmdg777 is not nice to hand-fly because of its strange (and in my opinion unrealistic) flying characteristics. I found this from an online FCOM;

So the FBW compensates for:

-any configuration change

-thrust changes

-turbulence

-back pressure requirements in turns of up to 30° bank. (so in a 777 no back pressure is needed for a normal turn)

 

It doesn't compensate for:

-any speed change

(-back pressure requirements in turns of over 30° bank)

 

When I fly the pmdg 777 (with a MS ffb2 joystick, in normal flight, so no aerobatics or steep turns or anything):

-It's sometimes not clear whether or not you are in trim. I can fly significantly out of trim (as seen by the trim speed bug), but it feels in trim (stick neutral, no force), apart from erratic pitch movements and flaring problems.

-It won't put in back pressure in turns

 

I hope this makes sense

 

Firstly, PMDG didn't simulate the extra load applied by the FBW as you exceed limits. It simply isn't possible with hobby equipment. You need a professional standard control loading system to be able to include that, Even with a force feedback stick you won't feel any change. As I understand it PMDG FBW reduces control authority at the extremes, so the sim becomes less responsive if you try and exceed limits.

 

If you are perfectly in trim with zero wind and turbulence and apply a pure roll input the FBW simulation will do a good job of maintaining pitch attitude in the turn. No need for any back pressure to maintain pitch attitude. If you are slightly out of trim or you apply some pitch input with the roll (easily done) then it will drift off horizontal or follow the small pitch input you made.

 

Following up on what Simon said, in ideal conditions the aircraft will maintain pitch attitude while banking up to 30 deg. Any atmospheric disturbance will through it off, and because it is maintaining 1g flight not pitch angle, if the aircraft pitches up slightly in turbulence it will maintain that new pitch attitude, it won't return to level.

 

Some people think FBW is like flying on rails when in fact it is very dynamic. An A320 will overspeed by quite a margin before speed protection brings it back. It doesn't just hit a wall at the barber pole. People talk about "hard" protections in the 'bus, when they are in fact quite soft and can be exceeded dynamically for a short period.

[FlyChuck 3]

Firstly, PMDG didn't simulate the extra load applied by the FBW as you exceed limits. It simply isn't possible with hobby equipment. You need a professional standard control loading system to be able to include that, Even with a force feedback stick you won't feel any change. As I understand it PMDG FBW reduces control authority at the extremes, so the sim becomes less responsive if you try and exceed limits.

 

If you are perfectly in trim with zero wind and turbulence and apply a pure roll input the FBW simulation will do a good job of maintaining pitch attitude in the turn. No need for any back pressure to maintain pitch attitude. If you are slightly out of trim or you apply some pitch input with the roll (easily done) then it will drift off horizontal or follow the small pitch input you made.

 

Following up on what Simon said, in ideal conditions the aircraft will maintain pitch attitude while banking up to 30 deg. Any atmospheric disturbance will through it off, and because it is maintaining 1g flight not pitch angle, if the aircraft pitches up slightly in turbulence it will maintain that new pitch attitude, it won't return to level.

 

Some people think FBW is like flying on rails when in fact it is very dynamic. An A320 will overspeed by quite a margin before speed protection brings it back. It doesn't just hit a wall at the barber pole. People talk about "hard" protections in the 'bus, when they are in fact quite soft and can be exceeded dynamically for a short period.

 

Hi Kevin,

 

Could you please check the two videos of my previous post and see what you make of it?

[kevinh 409]

Hi Kevin,

 

Could you please check the two videos of my previous post and see what you make of it?

I suppose the lesson from the second video is don't fight it. I don't think the real thing would do that. It looks to me that the pitch change is coming from thrust changes. As you pull back thrust starts to reduce, which will tend to make the nose drop. The FBW should always compensate for that but the PMDG version has two modes of operation depending on the threshold value you set in the CDU. It basically stops autotrimming if you make a large pitch input. It could be made to work properly, but I doubt PMDG will revisit FBW control laws again. It behaves well most of the time. Best thing is to forget the FBW exists and fly it like a 737.

[FlyChuck 3]

I wasn't aware that the pmdg version only trims when the column is almost neutral. But that seems to be the case indeed.Thanks