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Alpha Floor

My Analysis of the Boeing 777 FBW System

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DISCLAIMER: I don't claim to be an expert on the FBW, The Boeing 777 or any aspect of life for that matter. I really just "learn by doing" and like to share my findings with the community because sometimes they happen to be useful to other people as well. If you happen to find inaquracies, inconsistencies or outright false statements, I'd very much like you to let me know them so that I can correct them.

 

Hello all,

 

After a video I recorded on the Phugoid Mode of the 777 I started what became a really interesting discussion.

 

That discussion fueled my curiosity to further learn about the Boeing 777 Fly-By-Wire system. I've recorded a series of videos analyzing different scenarios and aircraft responses to control inputs, both in Normal Law and in Direct Law.

 

Since it turned out to be rather long, I included an index in the description of the video. If you watch it in YouTube, you'll be able to click on each index item directly.

 

And here's the video:

 

Index:

 

00:00 - Introduction

01:50 - FBW Philosophy on the 777

03:43 - System Description

10:36 - PMDG FBW bug on the PFD

 

12:50 - Trim Change in Normal Law (constant Thrust)

15:50 - Trim Change in Direct Law (constant Thrust)

17:54 - Thrust Change in Normal Law (constant Trim)

 

21:45 - Roll in Direct Law

23:25 - Roll in Normal Law

 

25:37 - Lowering the Landing Gear in Normal Law

27:17 - Lowering the Landing Gear in Direct Law

 

31:11 - Approach to Stall in Normal Law

37:58 - Stall in Direct Law

 

41:17 - Failures (Secondary Law, Non-Normals...)

 

References:

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Things I donn't yet understand which are open for discussion:

 

  • Why does this 777 Full Flight Simulator not behave as I expected in a turn? I expected the nose to drop during the turn.
  • Why didn't the aircraft really "yaw" when I tried assymetric thrust with the TAC selected off. Now, you notice the rudder is being deflected on the synoptic so I assume the PFCs are in fact correcting the yaw, the question now is, why is the TAC switch for if the PFCs are doing the same function?
  • Is the external model accurately representing the position of the flight controls when the ACEs or hydraulics are failed? See the fully down deflected left elevator here.
  • Why wasn't the RUDDER TRIM indication crossed out in amber after the failure of its 3 ACEs? See video here. The trim wasn't working as you'd expect, but I understand it should be crossed out on the synoptic. See for instance the below figure showing the Stab Trim crossed out indicating that it's inoperative.

UOEbCoT.png

Note to moderator: Image is freely available from content downloaded from SmartCockpit.

 

I also attach this useful image that shows the system, ACEs, Hydraulics and backup systems:

Interestingly enough, there's cable backup to the no.4 and 11 spoilers, but it won't work if the ACEs are inoperative. The stabilizer trim will still work using the alternate trim controls on the pedestal as you can see here.

XPcTvsD.png

 


Fascinating.  Thanks Jamie!

 

Thanks a lot for watching!

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Superb video!

It shows how great the PMDG T7 is modeled and how deep the system programming is!

In the normal every day operation, one does only scratch the surface of things implemented in this fantastic simulation!

Thanks for your effort!

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Why does this 777 Full Flight Simulator not behave as I expected in a turn? I expected the nose to drop during the turn.

The 777 FBW is designed so that for normal bank angles it isn't necessary to compensate for nose drop. If you go beyond 30 deg bank you will need to pull the nose up to maintain height. The full flight sim video you linked to shows correct behaviour.

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The 777 FBW is designed so that for normal bank angles it isn't necessary to compensate for nose drop. If you go beyond 30 deg bank you will need to pull the nose up to maintain height. The full flight sim video you linked to shows correct behaviour.

 

So does this mean that the PMDG 777 is not turning correctly? Because without manual correction the aircraft will descend when rolled. See here.

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So does this mean that the PMDG 777 is not turning correctly? Because without manual correction the aircraft will descend when rolled. See here.

I'm sure this pitch compensation is supposed be modelled but I haven't tried it much myself. A lot depends on how stabilised you are before starting the turn and whether you make any inadvertant pitch inputs while making the roll input. Increasing the FBW pitch deadband in the CDU might help.

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I'm sure this pitch compensation is supposed be modelled but I haven't tried it much myself. A lot depends on how stabilised you are before starting the turn and whether you make any inadvertant pitch inputs while making the roll input. Increasing the FBW pitch deadband in the CDU might help.

 

I'm pretty sure the aircraft will descend if it's rolled, but I will try it out again as you say incresing that deadband to be certain. I also encourage you to try and share your findings!

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Why didn't the aircraft really "yaw" when I tried assymetric thrust with the TAC selected off. Now, you notice the rudder is being deflected on the synoptic so I assume the PFCs are in fact correcting the yaw, the question now is, why is the TAC switch for if the PFCs are doing the same function?

 

Could be to do with airspeed. Try the same experiment at V1 and see what happens... To my knowledge TAC is mostly to help with takeoff and landing on one engine. At higher speeds, the rudder and PFC may well have enough authority to deal with some of it.

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Why didn't the aircraft really "yaw" when I tried assymetric thrust with the TAC selected off. Now, you notice the rudder is being deflected on the synoptic so I assume the PFCs are in fact correcting the yaw, the question now is, why is the TAC switch for if the PFCs are doing the same function?

 

I'm answering myself here:

 

The TAC responds to "thrust asymmetries" applying rudder trim.

The PFCs respond to "slip or skid" applying rudder deflection. So with TACs off and one engine inop, the PFCs will apply rudder as if the aircraft was turning, compensating for the yaw.

 

I guess the TAC is inherently faster because it doesn't care about the ball, it just compensates for thrust asymmetry instantaneously, there should be a better response in case of engine failure at V1 for instance. The TAC prevents the yaw from happening in the first place while the PFCs just correct an existing yaw, so to speak. Now if you're flying one engine, the TAC will apply the rudder trim and then on top of that the PFCs will apply aditional rudder for coordinating a turn.

 

If you want to see the aircraft yawing to thrust asymmetries, you'll have to disconnect the PFCs (which will also disconnect the TAC)

 

 

Could be to do with airspeed. Try the same experiment at V1 and see what happens... To my knowledge TAC is mostly to help with takeoff and landing on one engine. At higher speeds, the rudder and PFC may well have enough authority to deal with some of it.

 

 

See above :)

 

If I could only edit the previous post and add something like "answered in post #11"...

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Hi Jaime,

 

do you have these ?

 

http://www.citemaster.net/get/db3a81c6-548e-11e5-9d2e-00163e009cc7/R8.pdf  ( on the 777 FBW )

 

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf ( on the 777 FBW )

 

http://www.dcl.hpi.uni-potsdam.de/teaching/depend10/18_examples.pdf  (PFC's code is written in ADA !!!! Whow !!! )

 

http://www.ieee-cqr.org/2014/ETR-RT/Yeh_IEEE-ETR-RT-2014_Ultra Reliable_12May2014.pdf  ( on the FBW )

 

http://www.flight.org/the-boeing-777-thrust-asymmetry-compensation-tac  ( on TAC )

 

and... BTW - Excelllent Thread!

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http://www.dcl.hpi.uni-potsdam.de/teaching/depend10/18_examples.pdf (PFC's code is written in ADA !!!! Whow !!!)!

Yep. ADA was pretty much the sole language used for any mission and/or safety-critical avionics software in military and high-end civilian applications - especially in the 1990s. ADA is still widely used, but there are other options today.

 

Did you notice the microprocessors used in the block diagram of the PFC? Intel 80486! (Takes me back to Windows 3.1 days, when my top-of-the line desktop PC had a 486 CPU, and a whopping 16 MEGAbytes of RAM.

 

I have seen the circuit cards contained in the Honeywell flight guidance computer that was (and still is) part of the SPZ8400 avionics system on a Gulfstream IV. It uses 8086 CPUs - the same microprocessor that powered the original 1982-vintage IBM PC-XT!

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Yep. ADA was pretty much the sole language used for any mission and/or safety-critical avionics software in military and high-end civilian applications - especially in the 1990s. ADA is still widely used, but there are other options today.

 

Did you notice the microprocessors used in the block diagram of the PFC? Intel 80486! (Takes me back to Windows 3.1 days, when my top-of-the line desktop PC had a 486 CPU, and a whopping 16 MEGAbytes of RAM.

 

I have seen the circuit cards contained in the Honeywell flight guidance computer that was (and still is) part of the SPZ8400 avionics system on a Gulfstream IV. It uses 8086 CPUs - the same microprocessor that powered the original 1982-vintage IBM PC-XT!

 

That's very interesting, and yes, the computing power of a modern jetliner is lagging some 30 years behind the state of the art. Just imagine what an aircraft with the power of a modern gaming PC could do. It would throw one of the pilots out of the window and replace him by a dog. You know how it goes from here :P :smile:

Hi Jaime,

 

do you have these ?

 

http://www.citemaster.net/get/db3a81c6-548e-11e5-9d2e-00163e009cc7/R8.pdf  ( on the 777 FBW )

 

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf ( on the 777 FBW )

 

http://www.dcl.hpi.uni-potsdam.de/teaching/depend10/18_examples.pdf  (PFC's code is written in ADA !!!! Whow !!! )

 

http://www.ieee-cqr.org/2014/ETR-RT/Yeh_IEEE-ETR-RT-2014_Ultra Reliable_12May2014.pdf  ( on the FBW )

 

http://www.flight.org/the-boeing-777-thrust-asymmetry-compensation-tac  ( on TAC )

 

and... BTW - Excelllent Thread!

 

José hi! Many thanks for these references!

 

The 4th link is apparantly broken. The 2nd link I referenced it in my video and used it during my research indeed, that whole book "Avionics Handbook" is very good by the way.

 

On the TAC, I answered myself in post 11 and the 5th link confirmed what I said! Thanks for the link, I included it to the description of the video.

 

Links 1 and 3 seem very interesting, I will read them! Thanks agains for the links José!

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Awesome topic! Thanks for sharing the video

 

Danke sehr Klaus! Glad you liked it!

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I'm sure this pitch compensation is supposed be modelled but I haven't tried it much myself. A lot depends on how stabilised you are before starting the turn and whether you make any inadvertant pitch inputs while making the roll input. Increasing the FBW pitch deadband in the CDU might help.

 

 

I'm pretty sure the aircraft will descend if it's rolled, but I will try it out again as you say incresing that deadband to be certain. I also encourage you to try and share your findings!

 

So, I've been running further tests and must conclude that there IS turn compensation. The aircraft might not maintain the altitude EXACTLY and you might observe some vertical speed. HOWEVER; you can see the STAB TRIM adjusting to contain the altitude loss/gain.

 

Also, if you roll into the turn smoothly the system will have more time to adjust than if you roll the aircraft abruptly.

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Things that remain unanswered:

  • Why doesn't the RUDDER TRIM indication cross out in amber on the synoptic when all systems, and the rudder as well, are failed (see image below)?
  • Accuracy of the external model with failed hydraulics/ACEs. I'm inclined to believe this is an FSX limitation. In any case, as long as the aircraft behaves as it should I don't care too much about the exterior model. I fly the plane from the cockpit anyway.

 

Things that remain to be tested:

  • Spoilers 4 and 11 operation with all hydraulic systems failed. These spoilers are mechanically linked to the control wheel and, together with the STAT TRIM through the ALTN TRIM on the pedestal would render the plane somewhat flyable in a full hydraulic loss together with RAT failing to deploy. Now, I can't seem to test this for 2 reasons: 1) Failing all four ACEs won't do it because the linkage of spoilers 4 and 11 still requires ACE operation, and 2) Failing all hydraulics will deploy the RAT and I can't seem to prevent the RAT from unlocking.

 

Some odd behavior I just discovered. Playing with the failures, I got to this point. Wheel fully deflected to the left, no. 4 spoiler fully up both in the synoptic and on the external model, but the aircraft is NOT rolling at all:

 

yNEeEP5.jpg

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Great video! And thanks for all the explanations! This cleared a lot of confusions I had regarding the FBW system on the 777. 

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Great video! And thanks for all the explanations! This cleared a lot of confusions I had regarding the FBW system on the 777. 

 

Thank you for watching! Glad you liked it!

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For everyone interested, I wrote an article regarding the 777's FBW on my LinkedIn page here. I think people that followed this thread will enjoy my article.

 

It should be publicly accessible, but if not, I will upload it somewhere as .pdf and post a link.

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Will certainly give it a read!

 

Your contribution has been, just for itself and the enthusiasm and dedication you have put on it, the single reason for me to re install FSX, and the PMDG 777 :-)  - this, from someone that mainly uninstalls.... means a LOT :-)

 

Thank You Jaime! Keep the excellent work!

 

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Thank You Jaime! Keep the excellent work!

 

Thanks José! But I'm much afraid that from now on till November or so I won't, or at least shouldn't, devote so much time to "playing in the sim" because university (hahaha)

 

Thanks again!

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