ThirtyEcho
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FBW? WHY?

Wed Jun 03, 2009 8:37 am

We HAD to replace hydraulically boosted cables and pushrods with FBW computer controlled systems? Please tell me why.

What was so damned wrong with something that had worked for 80 years, from Bleriot to the 747, that we had to take the aviator out of the loop and impose LAW on his/her every move?

Yes, there were hydraulic failures but, barring serious flak damage, any two 15 year old girls out of the mall have the strength to wrestle that pig to the runway.
 
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Starlionblue
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RE: FBW? WHY?

Wed Jun 03, 2009 8:50 am

First of all, you are talking about two different things. Computer controlled flight and FBW are not the same thing. I will repeat what I said in another thread today:

- Fly by wire. A means by which control surfaces are signaled with electrical impulses as opposed to wires and pulleys.
- Computer controlled flight. A means by which a computer controls the flight path. This is completely independent from fly by wire. Nothing stops a computer from controlling an aircraft with cables and pulley. In fact it happens every day in aircraft like the 747-400, where the autopilot controls the surfaces.
- "Computer interpreted flight" . A means by which computers not only control the surfaces during automated flight, but also interpret pilot commands. In this case, for example, a roll command is not sent directly to the surfaces, but stick side deflection is interpreted as a "desire" by the pilot to roll, and the surfaces are deflected in order to roll the plane in compliance with pilot desire. Surface deflection is not necessarily in proportion to stick deflection.
- Envelope protection. A further development on "computer interpreted flight" by which the computers not only interpret commands but protect the aircraft from commands that may damage it or create an unsafe condition like a stall.


As for hydraulic failures, you're talking yet another different thing. Most surfaces are still hydraulically actuated, even if they are electronically signaled.



As for the reasons for FBW:
- Cost effectiveness. Ask a maintenance guy how much work it is to align all those cables.
- Robustness. All those cable (metal) runs are much more prone to damage than electronic signaling wires.
- Flexibility. If the design changes, even slightly, you may have to redesign the entire system from yoke to ailerons. But with FBW, this does not entail redesigning the cable runs.


As for the reasons for computer interpreted flight:
- Allows more efficient flight, for example by flying closer to the "edge of the envelope".
- Allows envelope protection.

[Edited 2009-06-03 01:59:14]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
MD11Engineer
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RE: FBW? WHY?

Wed Jun 03, 2009 9:01 am

Don't forget the weight issue: All the bellcranks, pulleys, rods and cables can add up to a ton on a big aircraft.

Jan
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Starlionblue
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RE: FBW? WHY?

Wed Jun 03, 2009 9:46 am



Quoting MD11Engineer (Reply 2):
Don't forget the weight issue: All the bellcranks, pulleys, rods and cables can add up to a ton on a big aircraft.

Totally forgot about that!


I think in 50 years the question will be formulated "FBW, why would you want anything else?"  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
PhilSquares
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RE: FBW? WHY?

Wed Jun 03, 2009 10:08 am



Quoting ThirtyEcho (Thread starter):
We HAD to replace hydraulically boosted cables and pushrods with FBW computer controlled systems? Please tell me why.

Well, let's take your logic(?) one step further. Why did we do away with just good old fashioned control surfaces? After all, who needs hydraulically powered flight controls, it' just another thing to go wrong! We could use the same logic to look at the evolution of glass cockpits..after all what was wrong with the old steam gauge presentation!

I would argue it's the evolution of systems that has driven FBW. But the current generation of FBW aircraft isn't really FBW. There are still hydraulic actuators that move the flight controls. Only the cables and the associated plumbing have been removed. But, the upside is the increased flight envelope protection that is obtained, the increased reliability, the weight savings, the list goes on and on.

Personally, I can't wait until there is a true FBW system that used electric actuators to move the flight controls. Now there will be some substantial weight savings and increased reliability.
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Faro
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RE: FBW? WHY?

Wed Jun 03, 2009 10:21 am



Quoting Starlionblue (Reply 3):
I think in 50 years the question will be formulated "FBW, why would you want anything else?"

To avoid electro-magnetic interference from the system; if you are a sub hunter with sensitive detection and tracking sensors, that's a critical consideration. For information the following link to data on the Kawasaki P-1 maritime patrol aircraft, the first production fly-by-light aircraft in the world. Interesting to see it has indigenous Japanese turbofan engines too.

http://en.wikipedia.org/wiki/Kawasaki_P-1

Faro
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Starlionblue
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RE: FBW? WHY?

Wed Jun 03, 2009 10:36 am



Quoting Faro (Reply 5):
To avoid electro-magnetic interference from the system; if you are a sub hunter with sensitive detection and tracking sensors, that's a critical consideration.

Well yes. In THAT case.  Wink
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rheinwaldner
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RE: FBW? WHY?

Wed Jun 03, 2009 12:14 pm



Quoting PhilSquares (Reply 4):
Personally, I can't wait until there is a true FBW system that used electric actuators to move the flight controls. Now there will be some substantial weight savings and increased reliability.

Why is it not done until now? Electric motors e.g. control elevation and direction of 35mm Oerlikon cannons. This task is very demanding in terms of forces, speed and accuracy. I can not imagine that control surfaces require more of these. Maybe the weight of motors is an issue. Also redundancy could be tricky.

Regarding the thread-question: Though FBW can mean a lot and in the pure sence only means one aspect I assume the OP means the Airbus flight control. Key feature is IMO the safety aspect. You can easily list a number of recent accidents that would not have happened if a proper FBW flight envelope protection would have been in place (TK AMS, Colgan Air, Spanair ...). Maybe even the Airbus flight envelope protection could be enhanced in the light of these accidents (e.g. against erroneous high lift device configurations = flaps retraction).
 
PhilSquares
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RE: FBW? WHY?

Wed Jun 03, 2009 12:29 pm



Quoting Rheinwaldner (Reply 7):
Also redundancy could be tricky.

You just answered your own question.

Quoting Rheinwaldner (Reply 7):
assume the OP means the Airbus flight control. Key feature is IMO the safety aspect.

Care to elaborate? All the accidents you cite are not FBW aircraft. Are you saying there is a safety implication to the Airbus vs. Boeing FBW?
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rheinwaldner
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RE: FBW? WHY?

Wed Jun 03, 2009 1:47 pm



Quoting PhilSquares (Reply 8):
Quoting Rheinwaldner (Reply 7):
assume the OP means the Airbus flight control. Key feature is IMO the safety aspect.

Care to elaborate? All the accidents you cite are not FBW aircraft. Are you saying there is a safety implication to the Airbus vs. Boeing FBW?

No, I don't even know the Boeing FBW so well. I explicitely said a "proper FBW" because I am not even sure whether the current Airbus implementation would save all three accidents.
- The Airbus throttle probably had applied full power in case of the AMS 738 after stick shaker.
- I don't know whether Airbus prevents taking off with wrong configuration (Spanair).
- I don't know whether Airbus prevents setting wrong configuration (Colgan). However the optimum recovery from stall is something which the Airbus system delivers. The captains wrong input probably would have been corrected by the flight envelope protection. On the Airbus the pilot just pulls the stick and the "computers" do the best possible recovery from the low energy situation.

Still I would never claim that Boeing's approach is not save, because it is save. What I meant is that a FBW system could be designed (probably more advanced than today systems) that would have prevented all those three accidents.
 
PhilSquares
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RE: FBW? WHY?

Wed Jun 03, 2009 2:08 pm



Quoting Rheinwaldner (Reply 9):
No, I don't even know the Boeing FBW so well. I explicitely said a "proper FBW" because I am not even sure whether the current Airbus implementation would save all three accidents.

First of all, both are "proper FBW" systems. Secondly, you are comparing accidents that were in conventional flight control aircraft with FBW systems. The FBW system is an integrated system that also includes the throttles. So, in the case of the TK accident it's pretty difficult to answer.

Also, I think your description of the Colgan accident is incorrect. First of all, the accident report hasn't been released, so I don't know where you are coming up with the "wrong configuration". I can assure you if you are in an Airbus, and select Flaps 0 while at Vref, you the system can't prevent that and it can't get you out of the stall that you would be in very quickly.

Finally, no FBW system prevents departing with the improper configuration. I think short of having the thrust levers locked at the idle position, you are going to have a tough time with that one. That's why there is a takeoff configuration warning system.
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EMBQA
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RE: FBW? WHY?

Wed Jun 03, 2009 2:21 pm



Quoting PhilSquares (Reply 4):
Well, let's take your logic(?) one step further. Why did we do away with just good old fashioned control surfaces?

Dope and fabric...!!??

Nothing sucks worse then changing, then rigging flight control cables.
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474218
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RE: FBW? WHY?

Wed Jun 03, 2009 3:26 pm



Quoting PhilSquares (Reply 10):
The FBW system is an integrated system that also includes the throttles. So, in the case of the TK accident it's pretty difficult to answer

FBW does not have to be an integrated system. It is entirely possible to have FBW engine controls and cable/pushrod flight controls or cable/pushrod engine controls and FBW flight controls.

Fly by wire simply means that the input to the servo is accomplished by means of an electrical input in lieu of mechanical input. Once the signal gets to the servo everything is the same.
 
PhilSquares
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RE: FBW? WHY?

Wed Jun 03, 2009 3:42 pm



Quoting 474218 (Reply 12):
FBW does not have to be an integrated system. It is entirely possible to have FBW engine controls and cable/pushrod flight controls or cable/pushrod engine controls and FBW flight controls.

Fly by wire simply means that the input to the servo is accomplished by means of an electrical input in lieu of mechanical input. Once the signal gets to the servo everything is the same.

You really should go back and re-read my post since you are completely off base with respect to what I wrote.

FBW as it has evolved today, in not just the flight controls. It's also the autothrottles. You can't have AOA Crit protection without control of the thrust.

What I wrote had nothing to do with what you referenced!
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474218
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RE: FBW? WHY?

Wed Jun 03, 2009 11:15 pm



Quoting PhilSquares (Reply 13):
You really should go back and re-read my post since you are completely off base with respect to what I wrote.

FBW as it has evolved today, in not just the flight controls. It's also the autothrottles. You can't have AOA Crit protection without control of the thrust.

What I wrote had nothing to do with what you referenced!

I am sorry I was responding to what you wrote below.

Quoting PhilSquares (Reply 10):
The FBW system is an integrated system that also includes the throttles.

FBW is not a integrated system but a method of operating the flight controls it covered in Chapter 27 of the MM. Autothrottles are found in Chapter 22 of the MM, part of autoflight. While they work together they are separate systems.
 
lowrider
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RE: FBW? WHY?

Wed Jun 03, 2009 11:18 pm



Quoting EMBQA (Reply 11):
Nothing sucks worse then changing, then rigging flight control cables.

No? What about working inside fuel tanks?
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roseflyer
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RE: FBW? WHY?

Thu Jun 04, 2009 4:03 am

It is a big argument, especially since the two most popular commercial aircraft are completely opposite. The A320 is FBW, while the 737 is cable driven.

The 737 is thoroughly evolved and perfected design that is very reliable with experts in place refining systems so that they have one of the best cable driven control systems. The A320 was a pioneer and has proven a reliable and safe aircraft.

FBW offers things that cable driven systems can't. It is more reliable since backup systems can be created with adequate separation and redundancy over cables. A steel cable is only reliable to a degree and electrical control when done properly can be more reliable. But you get to the point of what is safe enough, and as of now, both can be. FBW is lighter, can be more reliable, and is a lot less expensive to install. It's where we are going and in my mind, it is a good thing. If I had my way, we'd have more of the 737 go to FBW.
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rheinwaldner
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RE: FBW? WHY?

Thu Jun 04, 2009 8:32 am



Quoting PhilSquares (Reply 10):
First of all, both are "proper FBW" systems.

Ok I didn't want to put down the existing systems. I wanted to express a "FBW that optimizes safety possibly even beyond what we have today".

Quoting PhilSquares (Reply 10):
Secondly, you are comparing accidents that were in conventional flight control aircraft with FBW systems.

Yes, only such incidents are candidates to study potential safety improvements by FBW.

Quoting PhilSquares (Reply 10):
The FBW system is an integrated system that also includes the throttles. So, in the case of the TK accident it's pretty difficult to answer.

That is my understanding too. Some confusion in this thread was about the definition of a FBW system.

Quoting PhilSquares (Reply 10):
So, in the case of the TK accident it's pretty difficult to answer.

After stick shaker the Airbus throttle would never reclaim idle thrust I assume. Setting and maintaining full power is an automatic feature of the Airbus implementation in this case (I know that you know that even much better than I, please correct me if I am wrong).

Quoting PhilSquares (Reply 10):
Also, I think your description of the Colgan accident is incorrect.

That is possible. My analysis based on the understanding that the FO in the middle of a stall retracted the flaps and the captain pulled the yoke instead of pushing it first.

Quoting PhilSquares (Reply 10):
That's why there is a takeoff configuration warning system.

That is indead a sufficent solution.
If the runway is long enough and the airplane is able to get airborne the FBW would help to climb out at the best possible climb rate. Dealing with a plane close or at stall should be easier. There is another recent incident (EK in Melbourne) where an aircraft with FBW was saved in similar conditions (here the flaps were ok but the weigth was entered false). In theory the pilot really only had to pull the stick fully all the time and the computer would have made sure that the best possible climb out was performed. I wonder whether it was really the case (full backward deflection of the stick) in MEL.

Quoting PhilSquares (Reply 10):
I think short of having the thrust levers locked at the idle position, you are going to have a tough time with that one.

IMO all parameters to do this completely automatic are known to the aircraft (selected runway -> length, conditions -> Vr, weight). A flight computer theoretically has all the information to determine whether the take off with the current configuration works. A complete automatic setting of high lift devices (or at least proposing with a simple acknowledge by the crew) would therefore be possible in theory.
I don't think however that current systems go that far.
 
PhilSquares
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RE: FBW? WHY?

Thu Jun 04, 2009 12:14 pm



Quoting Rheinwaldner (Reply 17):
After stick shaker the Airbus throttle would never reclaim idle thrust I assume. Setting and maintaining full power is an automatic feature of the Airbus implementation in this case (I know that you know that even much better than I, please correct me if I am wrong).

Correct, but it works the same way in a Boeing FBW.

Quoting Rheinwaldner (Reply 17):
That is possible. My analysis based on the understanding that the FO in the middle of a stall retracted the flaps and the captain pulled the yoke instead of pushing it first.

Yes, but we don't know if that caused a secondary stall or just what effect that had on the aerodynamics. That is more of a CRM issue than anything else.

Quoting Rheinwaldner (Reply 17):
That is indead a sufficent solution.
If the runway is long enough and the airplane is able to get airborne the FBW would help to climb out at the best possible climb rate. Dealing with a plane close or at stall should be easier. There is another recent incident (EK in Melbourne) where an aircraft with FBW was saved in similar conditions (here the flaps were ok but the weigth was entered false). In theory the pilot really only had to pull the stick fully all the time and the computer would have made sure that the best possible climb out was performed. I wonder whether it was really the case (full backward deflection of the stick) in MEL

My guess would be it was full deflection to unstick at such a low speed. Boeing would work the same way. In fact, even on the 400 you have the same ability, just you have to do it manually. You just pull to the max energy line (moustache)

Quoting Rheinwaldner (Reply 17):
IMO all parameters to do this completely automatic are known to the aircraft (selected runway -> length, conditions -> Vr, weight). A flight computer theoretically has all the information to determine whether the take off with the current configuration works. A complete automatic setting of high lift devices (or at least proposing with a simple acknowledge by the crew) would therefore be possible in theory.
I don't think however that current systems go that far.

No they aren't. The error in MEL was caused by a 100 ton gross weight error. The FMC only knows what is input, so if there is an error in the gross weight the resultant FMC calculations will be in error.
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rheinwaldner
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RE: FBW? WHY?

Thu Jun 04, 2009 2:01 pm



Quoting PhilSquares (Reply 18):
FMC calculations will be in error.

Possibly even the envelope protection would not work properly. Would be even more amazing if the "pull-the-stick-fully"-trick still worked. Calculate the envelope protection based on real four-dimensional movement should be a lot more tricky than to rely on assumptions about "known" figures like weight, lift, drag, thrust. It would mean that the control system is able to derive the actual flight characteristic from the flown flight profile. This would be the perfect full control loop. It would even cope with such weird things (that have happened in reality) as impaired structure, separated engines, actual thrust vs. commanded thrust mismatches and other seldom issues.
 
prebennorholm
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RE: FBW? WHY?

Fri Jun 05, 2009 1:16 am



Quoting Starlionblue (Reply 3):
I think in 50 years the question will be formulated "FBW, why would you want anything else?"

50 years? Ask the designers today, they have said that already for a long time.

The last large non-FBW plane: Boeing 767, designed late 70'es, first metal cut 1979, rolled out and first flight 1981, certificated and service entry 1982. For more than 30 years no all new, large airliner has been designed without FBW.

Large western non-FBW passenger transport planes still in production: Boeing 737, and very little more.

Plus Boeing 767 being produced at a trickle, mostly freighters, and the rest mostly due to 787 delays. And 20 Boeing 748i on order for LH. Period.

FBW is not some new and fancy alternative way to control large airliners. For decades it has been THE way - the only way - to design controls of new large airliners. It is almost as old as the transistor replacing the radio tube. The cancelled Canadian CF-105 Arrow fighter was designed with FBW, while Britain also in the 50'es tested it on an Avro Vulcan. The first really oparational man carrying crafts using FBW were the Apollo Moon Lander and the Concorde.
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Starlionblue
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RE: FBW? WHY?

Fri Jun 05, 2009 1:58 am



Quoting Prebennorholm (Reply 20):

Quoting Starlionblue (Reply 3):
I think in 50 years the question will be formulated "FBW, why would you want anything else?"

50 years? Ask the designers today, they have said that already for a long time.

Well yes. But to be clear, I meant ANY aircraft, not just airliners.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
happylandings
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RE: FBW? WHY?

Fri Jun 05, 2009 6:45 am



Quoting PhilSquares (Reply 10):
I can assure you if you are in an Airbus, and select Flaps 0 while at Vref, you the system can't prevent that and it can't get you out of the stall that you would be in very quickly.

The A330/340 inhibit slats retraction when under a certain speed and/or angle of attack. Therefore you would at least not enter a full stall, because you most probably are still below max. AOA of CONF1, I think. I'd have to check. If you happen to be close to or above max. AOA anyway, if you're in normal law, that smart airplane reduces pitch to maintain an AOA close to max. and also adds full power.

So, at least in theory, setting the flaps lever to Zero at Vref in normal law would in my opinion render you climbing with slats out, TO/GA thrust and at high AOA. If you kept the stick pulled to max. deflection, you would be climbing not only with high AOA but with max. AOA, but you WOULDN'T stall.
 
PhilSquares
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RE: FBW? WHY?

Fri Jun 05, 2009 7:23 am



Quoting Happylandings (Reply 22):
The A330/340 inhibit slats retraction when under a certain speed and/or angle of attack. Therefore you would at least not enter a full stall, because you most probably are still below max. AOA of CONF1, I think. I'd have to check. If you happen to be close to or above max. AOA anyway, if you're in normal law, that smart airplane reduces pitch to maintain an AOA close to max. and also adds full power

The 320 is the same, however there are situations where it won't work. In addition, if you are in an engine out situation, I can assure you there is not enough thrust available.
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rheinwaldner
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RE: FBW? WHY?

Fri Jun 05, 2009 10:53 am

I wrote this yesterday or so:

Quoting Rheinwaldner (Reply 17):
IMO all parameters to do this completely automatic are known to the aircraft (selected runway -> length, conditions -> Vr, weight). A flight computer theoretically has all the information to determine whether the take off with the current configuration works. A complete automatic setting of high lift devices (or at least proposing with a simple acknowledge by the crew) would therefore be possible in theory.
I don't think however that current systems go that far.

And within a day this is in the media:

Although Airbus will not say whether it is planning to develop a take-off performance monitoring system based on the ROW/ROP's capabilities, Jacob says that the system could be adapted for such a purpose. Take-off performance monitoring remains a holy grail that many have attempted to develop commercially without success, so if Airbus can provide a working system, it is likely to be universally welcomed.

from here: http://www.flightglobal.com/articles...-linked-pilot-systems-secrets.html
 
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zeke
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RE: FBW? WHY?

Fri Jun 05, 2009 10:56 pm



Quoting ThirtyEcho (Thread starter):
we had to take the aviator out of the loop and impose LAW on his/her every move?

When control engineers talk about "laws" they are referring to mathematical way of representing the control process. You can do this for any control process, i.e. riding a bike, driving a car, changing channels on a TV etc.

With the bike example, the rider applies differential force to the handle bars and that results in an acceleration of the handle bar in a particular direction, which in turn results in a change in direction of the movement of the whole bike. This is a control law, and can be drawn as a block diagram in a control system.

Control laws exist on EVERY aircraft, and even the 747 has different "laws", just the laws are not implemented using different hardware, e.g. the in flight change from outboard to inboard ailerons is an example of flight control law change.

What FBW enabled is more control laws to exist on the one aircraft without a lot of mechanical interlocks and additional hydraulics that would have been necessary without FBW.

The reason why we use FBW, it saves weight.
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PGNCS
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RE: FBW? WHY?

Sat Jun 06, 2009 12:16 am



Quoting ThirtyEcho (Thread starter):
Yes, there were hydraulic failures but, barring serious flak damage, any two 15 year old girls out of the mall have the strength to wrestle that pig to the runway.

When was the last time you flew a 737 in manual reversion?
 
prebennorholm
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RE: FBW? WHY?

Thu Jun 11, 2009 1:31 am



Quoting Zeke (Reply 25):
The reason why we use FBW, it saves weight.

That's one important reason. Envelope protection is another (whether it is hard limits as Airbus, or soft limits at Boeing).

But a third reason, which hasn't been covered very much on this thread, is performance gains.

Example: Drooped ailerons. I don't think they have been implimented on non-FBW planes. Mechanical drooped ailerons alone would be a formidable challenge.

Drooped ailerons has one nasty effect, they create a substantial "adverse yaw" which has to be countered by the rudder. This adverse yaw is highly dependent on the angle of attack, but not in a liniar way. On Airbus the aileron droop and the angle of attack are input to the control software which automatically calculates the correct rudder compensation to cancel out the adverse yaw. And any pedal input is mixed into that rudder compensation, especially relevant for crosswind landing.

Drooped ailerons has a significant positive effect on takeoff and landing performance. But taking the nasty adverse yaw effect out of it can hardly be imagined in a mechanical control system, while in software it is still no no-brainer, but doable.
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zeke
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RE: FBW? WHY?

Thu Jun 11, 2009 2:32 am



Quoting Prebennorholm (Reply 27):
That's one important reason. Envelope protection is another (whether it is hard limits as Airbus, or soft limits at Boeing).

Can be incorporated mechanically, which is basically what the old fashioned stick shaker was.

Quoting Prebennorholm (Reply 27):
Example: Drooped ailerons. I don't think they have been implimented on non-FBW planes. Mechanical drooped ailerons alone would be a formidable challenge.

Flaperons have been around for some time, mainly on light aircraft.
We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
 
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Jetlagged
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RE: FBW? WHY?

Thu Jun 11, 2009 3:11 am



Quoting Prebennorholm (Reply 27):
Example: Drooped ailerons. I don't think they have been implimented on non-FBW planes. Mechanical drooped ailerons alone would be a formidable challenge.

Boeing 767, Airbus A300, Airbus A310. All have drooped ailerons. None are FBW.

Quoting Prebennorholm (Reply 27):
Drooped ailerons has one nasty effect, they create a substantial "adverse yaw" which has to be countered by the rudder.

I've not noticed this flying A300 simulators and I rarely bother to touch the rudder pedals!
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
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Jetlagged
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RE: FBW? WHY?

Thu Jun 11, 2009 3:23 am



Quoting ThirtyEcho (Thread starter):
We HAD to replace hydraulically boosted cables and pushrods with FBW computer controlled systems? Please tell me why.

What was so damned wrong with something that had worked for 80 years, from Bleriot to the 747, that we had to take the aviator out of the loop and impose LAW on his/her every move?

Yes, there were hydraulic failures but, barring serious flak damage, any two 15 year old girls out of the mall have the strength to wrestle that pig to the runway.

Your perception of what FBW is a bit skewed. FBW does not work in the way you appear to think. The pilot is still firmly in the loop in a FBW aircraft. The envelope protections only come in at the extremes.

Even Bleriot would have trouble landing a 747 without hydraulics (no manual reversion). Those controls aren't merely boosted, they are fully powered.

FBW represents progress. It's an improvement and offers significant advantages.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
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Starlionblue
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RE: FBW? WHY?

Thu Jun 11, 2009 6:03 am



Quoting Jetlagged (Reply 29):
I've not noticed this flying A300 simulators and I rarely bother to touch the rudder pedals!

It might have one of 'em newfangled yee-haw dampers fitted. And you gots 'em on plenny o' planes aside from 'em suspeecious Frenchy types.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mandala499
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RE: FBW? WHY?

Thu Jun 11, 2009 1:28 pm



Quoting PGNCS (Reply 26):
When was the last time you flew a 737 in manual reversion?

Slap on single engine and a jammed rudder trim on top... my leg was numb after that ride... lucky it was a sim.  Smile
When losing situational awareness, pray Cumulus Granitus isn't nearby !
 
474218
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RE: FBW? WHY?

Thu Jun 11, 2009 1:38 pm



Quoting Jetlagged (Reply 29):
Boeing 767, Airbus A300, Airbus A310. All have drooped ailerons. None are FBW.

The L-1011-500 has active outboard ailerons that react to wing loading and the L-1011-500 uses cables and push rods.
 
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SEPilot
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RE: FBW? WHY?

Thu Jun 11, 2009 2:03 pm



Quoting PhilSquares (Reply 4):
Personally, I can't wait until there is a true FBW system that used electric actuators to move the flight controls. Now there will be some substantial weight savings and increased reliability.

Unfortunately, the lightest and most compact linear actuators available are still hydraulic. Electric linear actuators are not only heavier and bulkier for the same output as hydraulic, but they are also much more expensive. And it has been a moving target; as electric actuators have been improved, so have hydraulic ones. I have first hand experience with this; I was involved in the design of the world's first totally computer controlled grinding machine and one of the design goals was to eliminate hydraulics. The first machines we built did in fact have no hydraulic system, but it wasn't long before we found that there were many tasks for which the best, cheapest, and most reliable actuation means was hydraulic. Then we decided to go from ball slide ways to hydrostatic ways, and all excuse for avoiding hydraulics disappeared. I suspect with aircraft the weight and power issues are predominant. Hydraulics also have an overwhelming advantage when dealing with redundancy, and that is when a hydraulic system malfunctions it does not lock up, whereas electric ones almost always do. A system for moving flight controls with two or three independent electric actuators would have to be designed so that the failure of any one of them would not lock the surface, and also that no combined failures would leave the control surface totally free. This is very, very hard to do. So that is why hydraulic actuators are likely to remain for some time to come.

Quoting Zeke (Reply 25):
The reason why we use FBW, it saves weight.

Doesn't it also reduce maintenance? Although I will concede that the weight issue is probably the most important.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
474218
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RE: FBW? WHY?

Thu Jun 11, 2009 4:02 pm



Quoting ThirtyEcho (Thread starter):
We HAD to replace hydraulically boosted cables and pushrods with FBW computer controlled systems? Please tell me why.

Hydraulically boosted control systems like those found in the 737 differs from the fully powered controls systems like those found in the 767 or the 777's FBW system.

A very basic discription follows:

The 737 system has cables and pushrods that attach directly to the control surface. The cable/pushrod system is hydraulically boosted (like power steering) to make the crews job easier. However, if the hydraulic boost system lost, the aircraft can still be controlled. But this will take a much higher inputs from the flight crew.

The 767 uses cables and pushrods to make inputs to a control servo. The servo then supplies hydraulic power to actuators that move the control surface. If all hydraulic power is lost, all control is also lost.

The 777 uses an electrical signal to make inputs to a control servo. The servo then supplies hydraulic power to the actuators that move the control surface. If all hydraulic and/or electrical power is lost all control is lost.

The reasons why FBW has become the standard have all ready been covered.
 
tdscanuck
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RE: FBW? WHY?

Thu Jun 11, 2009 4:16 pm



Quoting SEPilot (Reply 34):
Doesn't it also reduce maintenance?

Yes. Rigging flight controls is a long, tedious, and somewhat continuous process, since the rigging position sets the baseline for the closed-loop feedback on the position.

On a FBW system, you have a direct sensor on the flight control position and close the loop that way. Once the surface is properly mounted and balanced, it's basically self-rigging. You also get rid of all the cables, pulleys, cable guides, connectors, etc., etc.

Tom.
 
474218
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RE: FBW? WHY?

Thu Jun 11, 2009 4:24 pm



Quoting Tdscanuck (Reply 36):
Yes. Rigging flight controls is a long, tedious, and somewhat continuous process, since the rigging position sets the baseline for the closed-loop feedback on the position.

I have never worked on a FBW system, but they have to be a closed loop system too. So how is feed back from the surface to the servos accomplished, mechanically or electrically?
 
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Starlionblue
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RE: FBW? WHY?

Fri Jun 12, 2009 12:59 am



Quoting 474218 (Reply 37):
I have never worked on a FBW system, but they have to be a closed loop system too. So how is feed back from the surface to the servos accomplished, mechanically or electrically?

My guess is electrically. Also on Airbi there is no real feedback to the stick AFAIK. It just "hardens" but not in relation to surface deflection. I could be wrong.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
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RE: FBW? WHY?

Fri Jun 12, 2009 3:30 am



Quoting 474218 (Reply 37):
I have never worked on a FBW system, but they have to be a closed loop system too.

They are.

Quoting 474218 (Reply 37):
So how is feed back from the surface to the servos accomplished, mechanically or electrically?

Electrically. There's no way to mechanically close the loop because the input to the system is electrical.

Boosted or pure-hydraulic systems have mechanical feedback (the PCU will track the true position of the end of the cable), which is why rigging is such a big deal. If the end of the cable isn't in the right spot, the surface won't be where you think it is based on the control position.

Quoting Starlionblue (Reply 38):

My guess is electrically. Also on Airbi there is no real feedback to the stick AFAIK. It just "hardens" but not in relation to surface deflection. I could be wrong.

That's true for FBW Boeing's too.

Tom.
 
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Faro
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RE: FBW? WHY?

Fri Jun 12, 2009 9:34 am



Quoting 474218 (Reply 37):
I have never worked on a FBW system, but they have to be a closed loop system too. So how is feed back from the surface to the servos accomplished, mechanically or electrically?

No shielding is 100% effective: with a direct lightning strike, there must be a diminutive transient spike that wriggles itself into the control loop. I wonder is the feedback effect of this spike too small to be detected by the pilots in manual flight?

Faro
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474218
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RE: FBW? WHY?

Fri Jun 12, 2009 1:18 pm



Quoting Tdscanuck (Reply 39):
Electrically. There's no way to mechanically close the loop because the input to the system is electrical.

The connection between the servo and the actuator is hydraulic, not electrical.

When the servo tells the actuator to move the control surface a specific amount, there has to be feedback from the surface to the servo that says, it has moved where it was commanded to move, no more no less.

So what I am asking is the feedback from the control surface to the servo mechanical of electrical
 
tdscanuck
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RE: FBW? WHY?

Sat Jun 13, 2009 3:57 am



Quoting Faro (Reply 40):

No shielding is 100% effective: with a direct lightning strike, there must be a diminutive transient spike that wriggles itself into the control loop. I wonder is the feedback effect of this spike too small to be detected by the pilots in manual flight?

I suspect not...if the systems is designed well, the size of the transient spike that gets through the shielding should be a great deal smaller than the command signal.

Quoting 474218 (Reply 41):
Quoting Tdscanuck (Reply 39):
Electrically. There's no way to mechanically close the loop because the input to the system is electrical.

The connection between the servo and the actuator is hydraulic, not electrical.

True.

Quoting 474218 (Reply 41):
When the servo tells the actuator to move the control surface a specific amount, there has to be feedback from the surface to the servo that says, it has moved where it was commanded to move, no more no less.

So what I am asking is the feedback from the control surface to the servo mechanical of electrical

It could be either. Hydraulic and mechanical runs are relatively heavy, so I suspect you'd only use mechanical or hydraulic from the servo to the actuator if they were physically colocated.

It's also possible there's no feedback from the servo to the actuator...you could do it as feedback from the surface position to the servo command. That has the advantage of hiding any servo issues inside the control loop, but it probably makes the control law more tricky. You could probably get away with it if the servo response is significantly faster than the control surface though.

Basically, it's a question of whether they closed the entire control loop at once or used successive loop closure. The latter is more common because it's a lot easier to do, but with modern control design tools the former is also possible.

Tom.
 
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77west
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RE: FBW? WHY?

Sat Jun 13, 2009 1:05 pm

Kind of like saying why use 2 engines instead of 4.

Money.

Consumers want cheaper flights, fuel costs a lot, so we reduce empty weights and decrease fuel burn.

FBW systems are one way of reducing the weight of aircraft as well as simplifying and removing many repair and maintenance jobs.

A 777 that rolled out today does it's assigned job just as well as the 707 did in it's day. Gets the pax from A to B. It is their operating environment that changed, resulting in the electric jets of today.
77West - AW109S - BE90 - JS31 - B1900 - Q300 - ATR72 - DC9-30 - MD80 - B733 - A320 - B738 - A300-B4 - B773 - B77W
 
mandala499
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RE: FBW? WHY?

Sat Jun 13, 2009 9:50 pm



Quoting 474218 (Reply 41):
When the servo tells the actuator to move the control surface a specific amount, there has to be feedback from the surface to the servo that says, it has moved where it was commanded to move, no more no less.

What the Airbus FBW does on feedback????

The flow...
Sidestick or Autopilot computer sends input to Flight Control Computers.

These FCCs intepret these inputs and sends a computer order to the actuator who will move the control surface.

The control surface then moves, and a sensor picks up the position and feeds it back to the Flight Control Computers.

The feedback here, is from control surface position sensor back to the flight control computer... not the sidestick.

Redo loop from FCC downstream until new input is given.
Stick Input is Load demand input (G load) for pitch and roll rate on roll.

The Flight Control Computers on 330/340 consists of:
3 PRIM Computers (Flight Control Primary Computers), and
2 SEC Computers (Flight Control Secondary Computers).

The system picks 1 of the PRIMs as the Master PRIM (P1). It processes the orders and sends them to the other computers (P1, P2, P3, S1, S2) which will then execute them on their related servo-control. (If one computer is unable to execute the orders sent by the master, another computer executes the task of the affected computer (except for spoiler control)).

The flight Control Computers A320 family consists of:
2 ELACs (Elevator Aileron Computer)
3 SECs (Spoilers Elevator Control)
2 FACs (Flight Augmentation Computer)

Mandala499
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zeke
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RE: FBW? WHY?

Sun Jun 14, 2009 5:07 am



Quoting Faro (Reply 40):
No shielding is 100% effective: with a direct lightning strike, there must be a diminutive transient spike that wriggles itself into the control loop.

I think you will find at some stage in the loop they will use optical isolation. And lightning strikes should normally stay on the outside of the aircraft.
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Faro
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RE: FBW? WHY?

Sun Jun 14, 2009 10:16 am



Quoting Zeke (Reply 45):
I think you will find at some stage in the loop they will use optical isolation.

Very interesting, I've never heard of this before. Like a back-to-back electrical to optical to electrical transducer? I don't quite see how this would isolate/dampen/eliminate a spike though. Also begs the question of whether pure fly-by-light would afford any significant gains in weight/reliability/performance as of the state of today's technology...

Faro
The chalice not my son
 
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zeke
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RE: FBW? WHY?

Sun Jun 14, 2009 10:34 am



Quoting Faro (Reply 46):
I don't quite see how this would isolate/dampen/eliminate a spike though.

With the use of an opto-isolator, the circuit signals would be electrically isolated from each other, which I would imagine would be very useful in high noise areas.

Quoting Faro (Reply 46):
Also begs the question of whether pure fly-by-light would afford any significant gains in weight/reliability/performance as of the state of today's technology...

I think the A380 already makes use of fiber-optics, as it also allows for higher bandwidth.
We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
 
NoWorries
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RE: FBW? WHY?

Sun Jun 14, 2009 2:13 pm



Quoting Faro (Reply 46):
Very interesting, I've never heard of this before. Like a back-to-back electrical to optical to electrical transducer? I don't quite see how this would isolate/dampen/eliminate a spike though. Also begs the question of whether pure fly-by-light would afford any significant gains in weight/reliability/performance as of the state of today's technology...



Quoting Zeke (Reply 47):
With the use of an opto-isolator, the circuit signals would be electrically isolated from each other, which I would imagine would be very useful in high noise areas.

Opto-isolators are quite common -- essentially they eliminate any signal bleed through across systems. Think of it as an LED on one side and a phto-diode or phot-transistor on the other side. One very common usage is low voltage control circuits that must control/detect high voltage loads.

I'm not sure, but in an airframe it might not be possible to achieve complete isolation since all systems ultimately interconnect at power distribution -- though it seems likely the effects would be minimal.

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