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gnomegemini

Fly-By-Wire of the 777 and it's simulation

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Hi there,

 

I read some time ago the 777 has also fly by wire. As this term only say's the yoke has no physical connection to the surfaces I wanted to know whether Boeing has also a system like airbus where the plane stays in the last commanded attitude (autotrim called?). Or does central yoke mean zero deflection on surfaces unlike on airbus?

 

Also is this feature simulated?

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I wanted to know whether Boeing has also a system like airbus where the plane stays in the last commanded attitude (autotrim called?)

 

I think the 777 FBW system is more of a hybrid version of the A3XX system. Meaning that the flight controls are electically controlled but she doesn't have the roll/pitch restrictions that the A3XX birds have.

 

 

 


Also is this feature simulated?

 

I'm sure it would be.

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A very good explanation of the 777 FBW is here:

 

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

 

Concerning Trim, see " Pitch Control" (Chapter 11.8.1):

"The unique 777 implementation of maneuver demand and speed stability in the pitch control laws
means that:
• An established flight path remains unchanged unless the pilot changes it through a control column
input, or if the airspeed changes and the speed stability function takes effect.
• Trimming is required only for airspeed changes and not for airplane configuration changes."

 

Guy

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Trimming the 777 feels just like on a conventional airplane.

When for example you are constantly pushing to keep the planes pich where you want it, you trim nose down untill it feels good.

 

It is true that flap config changes/adding or reducing thrust/banking, etc do not require retrimming or manuel control wheel input or only very little input. Resulting forces due to those changes are automatically trimmed away.

 

However normally when you are extending flaps during approach, you are also slowing down. And speed changes do require you to retrim.

Same during climb while retracting flaps you are also accelerating.

So in practice on approach and during climb acceleration you will find yourself retrimming and correcting pitch pretty much as for any non fly by wire aircraft.

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Or does central yoke mean zero deflection on surfaces unlike on airbus?

 

Are you refering to the control surfaces of the airbus remaining at a fixed deflection or rapid movements to maintain attitude? as if you were to keep a constant deflection, the aircraft would continue to pitch/roll/yaw.

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As for comparing the 777 to an airbus....

 

Many pilots that transition from airbus to Boeing complain that they dont understand why they have to retrim the 777 after a speed change.

Some find it annoying even.

 

I understand the Airbus automatically trims away all forces you excercise on the stick.

Boeing does not. I think it is just a matter of preference on the manufacturers side.

 

The goal of the 777 fly by wire system is to keep the flight path as constant as possible.

Not necesarily pitch but flight path!

So extending flaps does change pitch but it tries to maintain your 3 degrees glide path.

It tries, but you still have to help along though.

You extend from flaps 5 to 20 and you WILL feel and see a ballooning effect that will make you deviate from the Glide Slope if you dont correct for it.

Just less than conventional airplanes do I guess.

 

When you are in 20 degrees bank descending turn, the aircraft will try to maintain that flight path. So I guess saying central yoke = zero deflection is not really correct.

It is correct as long as no outside forces are working on the aircraft.

But when the FBW system is correcting you will have flight control deflections even though the yoke is centered.

The Airbus I understand is way more stabilised (correct me if I am wrong cause I never flew an Airbus). You set 15 pitch down and 20 degrees bank, then let go of the stick and the airbus will maintain those values exactly. The 777 will try to stablise its path but not like an Airbus. You still have to work for your money so to say.

 

Basically you dont have to think much about the 777 FBW system, and is the nice thing about it (exspecially compared to Airbus that is quite complicated and goes through different modes during approach that you have to have a thorrow understanding of. At least that is what I have been told)

 

Just fly as always, trim as always, get established in that descending base turn and enjoy that somehow, miraclously it all seems to be a bit easier/more stable than on the B737 for example :-)

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In simplistic terms the bus is designed around a 1g load factor, the 777 and I assume also 787 basically trim the machine for you based on speed, therefore a more conventional approach.

 

As far as hard limitations go it will be debated for many years as to which is best, in my opinion both have their merits depending on the situation.

 

Airbus philophosy is to limit the machine based on its max performance in a given flight regime, eg full back stick and TOGA in windshear you get max safe performance.

 

Boeing says the pilot has the last say, full back Yoke & TOGA in windshear on the 777 and the houses get bigger, however the two guys up front know their machine well enough to apply the correct escape manoeuvre and apply the correct amount of elevator.

 

As far as "Is it simulated" This has been simulated in FS by many developers, and PMDG have been developing what they call 'Fly by software' perfectly for many years.

 

@Rob - Nothing complicated about Airbus philosophy if you understand it ;) In fact in normal law at 50ft RA the only change in feel is flare law, which blends the aircraft to a more direct (737 feel) to keep things more conventional.

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No the 777 does NOT trim based on speed.

It trims to maintain a flight path.

 

When you add thrust, the normal tendency of any aircraft with Engines mounted under the wings is that it will pitch up.

This momentum is counteracted by the 777 FBW system with elevator and stabiliser.

Ofcourse, if you keep that increased thrust setting long enough then the airplane will accelerate and as it is trimmed for a certain speed (like a conventional aircraft) at some point the aircraft will start a climb.

 

If you added thrust with the goal to accelerate while maintain your current flight path (be it straight and level or a 3degrees Glide slope,etc) then just as on a conventional aircraft you will need to trim nose down.

 

(or am I misunderstanding what you are saying)

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The FBW trims for speed, not sure I can put it any other way..

 

I think we may well have a misunderstanding here :)

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I think you're both right, but about different aspects of the FBW system. Here's my understanding of it:

 

In normal mode, use of the pitch trim switches on the yoke commands a speed, and the FBW will adjust the stabiliser as required to maintain that. On the other hand when the airplane is in a stable state (speed unchanged, and no pilot input) it's the flightpath that is being maintained when config changes are made.

 

Also, search for 'C*U' in the earlier-referenced document to learn a bit more about how pitch inputs are interpreted by the FBW.

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The 777 trim system works by the pilot setting the trim speed with the trim switches.

Maybe thats what you mean.

 

On a conventional aircraft, with the trim switch you change the elevator trim directly.

With the 777, each one second of pressing the trim switches down/up will reduce/increase the trim reference speed by about 10kt.

 

So If you are doing 210kt and you press the trim switches one second up, the trim reference speed in the primary flight computers have now been set to about 220kt.

The FBW system will set the elevator and stabiliser accordingly.

 

Now if you were to push on the yoke and dive and accelerate, what the FBW system does not do, is trim this push force away automatically (nor does a conventional aircraft). You have to tell the FBW system (with the trim switches) that you want to increase the trim reference speed.

 

So yes it trims for a given speed, but that is nothing special.

A Cessna's trim is set for a speed as well.

What is special, is that config changes/thrust changes/banking does not require you (or very little) to compensate with the yoke. The FBW system will change elevator and stabiliser for you.

 

EDIT: Hi Mark, I was typing this up as you replied.

I will have a look at that doc. sounds interesting!

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@Rob - Nothing complicated about Airbus philosophy if you understand it ;) In fact in normal law at 50ft RA the only change in feel is flare law, which blends the aircraft to a more direct (737 feel) to keep things more conventional.

"Flare Law"?!,no,direct pitch control

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In an Airbus flare the law blends from normal into direct. It's a process, not a 'hard' switchover.

 

The B777 is called 'speed-stable', that is, autotrim trims away all other forces to maintain the speed before the configuration was changed. If the T7 pilot wants to change speed, he/she has to re-trim the aircraft manually. (This, of, course, relates to A/P off!)

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In an Airbus flare the law blends from normal into direct. It's a process, not a 'hard' switchover.

 

The B777 is called 'speed-stable', that is, autotrim trims away all other forces to maintain the speed before the configuration was changed. If the T7 pilot wants to change speed, he/she has to re-trim the aircraft manually. (This, of, course, relates to A/P off!)

Ollie, Correct it is a blend into direct law, the bus stops trimming at 50ft RA the aircraft memorises pitch and slowly lowers the nose forcing you to flare the aircraft. On take off you are in direct law (ground mode) the aircraft then blends into normal law (flight mode)

 

Not quite sure what Capt is asking/stating? Am pretty sure he also keeps claiming to be an A320 captain :/

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His point might be that Boeing doesn't refer to FBW 'laws' but rather 'modes', but as you were talking about Airbusses (Airbii?) at the time I think the point was moot!

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(...)

Not quite sure what Capt is asking/stating? Am pretty sure he also keeps claiming to be an A320 captain :/

 

 

He claims to have been an A320 Captain for about 10 months - plus some time as an A320 F/O before that ...

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Many thanks for your answers. So the 777 has no autotrim system which makes it feels more like a real aircraft. :)

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It has an autotrim system that keeps the airspeed konstant, instead of the vertical acceleration like Airbus.

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Guys it really is a bit more complicated than a single sentence, and you want to be careful not to make a misleading statement by virtue of ommission of information.

The 777 FBW does have an auto trim element involved (although I don't think it's explicitly referred to as such), and C*U does take into account load factor at higher speeds.

My advice is to do some reading as per the links in this thread. You'll find it's a fascinating and worthwhile subject to look into, albeit a rather technical one. As recent events have demonstrated, this is a more complicated airplane than perhaps we give it credit for!

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No the 777 does NOT trim based on speed.

It trims to maintain a flight path.

 

The 777 trim system works by the pilot setting the trim speed with the trim switches.

Maybe thats what you mean.

 

So does it trim for speed or not? You want to make up your mind here?

 

So yes it trims for a given speed, but that is nothing special.

A Cessna's trim is set for a speed as well.

What is special, is that config changes/thrust changes/banking does not require you (or very little) to compensate with the yoke. The FBW system will change elevator and stabiliser for you.

 

Again, is it special or not? You correctly stated that trim on many aircraft won't account for config/thrust/attitude changes, so how can trim set speed in a Cessna without the FBW in the 777 that you say is necessary to achieve this? (It doesn't/can't)

 

I've never read a series of such confused statements before. Please, PLEASE if you don't know, or just as bad can't explain, then don't post.

 

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I've never read a series of such confused statements before. Please, PLEASE if you don't know, or just as bad can't explain, then don't post.

 

 

As already said, BEST is to READ carefully  the doc's given above:  :smile:

 

about the 777 FBW:

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf  (chap 11.8.1)

 

or

about the 777 Flight Controls:

http://www.smartcockpit.com/aircraft-ressources/B777-Flight_Controls.html  (chap 9.20.10)

 

Guy

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So does it trim for speed or not? You want to make up your mind here?

 

 

 

 

Again, is it special or not? You correctly stated that trim on many aircraft won't account for config/thrust/attitude changes, so how can trim set speed in a Cessna without the FBW in the 777 that you say is necessary to achieve this? (It doesn't/can't)

 

 

 

 

 

 

I've never read a series of such confused statements before. Please, PLEASE if you don't know, or just as bad can't explain, then don't post.

 

Has it ever occured to you that maybe it is explained quite well but you yourself are the problem?!

 

First:

I did make up my mind!

The pilot needs to trim for speed.

The 777 auto corrects for config changes/thrust changes/during banks/etc

 

Second:

You seem to have no understanding of what trim is if you believe a FBW system is required to trim out a Cessna!

 

Last:

I will be glad to try to explain the system to you 10 more times in all kinds of flavours.

Realy, I dont mind.

But you are going to have to come at me a little different please!

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Many thanks for your answers. So the 777 has no autotrim system which makes it feels more like a real aircraft. :)

Correct.

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