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United Airline
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### How Does Fly-by-wire Work?

How does fly-by-wire work? So there isn't any hydralics at all?

PEK777
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### RE: How Does Fly-by-wire Work?

It flies via a wire.

BMI727
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### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Thread starter):So there isn't any hydralics at all?

There is, it's just all done via computers rather than cables or electromechanical means.

 Quoting United Airline (Thread starter):How does fly-by-wire work?

Basically it changes the way the pilot interacts with the aircraft.

In a traditional control scheme, pulling back on the stick commands the elevators to deflect up, moving the stick left causes the left aileron to deflect upwards and the right to deflect downwards, and so forth.

In a fly by wire aircraft pulling back on the stick doesn't actually do anything with the elevators directly. Rather than commanding the elevators to deflect, it commands a pitch rate. Moving the stick to the left will command a roll rate.

Think of it like a large lecture hall. You might want all the lights on for people to hear a lecture, have almost all the lights off if you are showing a video, and something in between if people are going to be taking notes during a presentation. Controlled like a non-fly by wire aircraft, you'd have to stand at the panel and switch individual lights on and off until the room is as bright as you want it. If it was like a fly by wire plane, you'd just control how bright you want the room and the computer system would work out which lights should be on and which should be off to achieve the commanded result. One aspect of this is that the system could theoretically be able to compensate for burned out bulbs. I can't find the article at the moment, but I think NASA or someone has flown a testbed to try and compensate for the loss of a control surface.
Why do Aerospace Engineering students have to turn things in on time?

David L
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### RE: How Does Fly-by-wire Work?

Strictly speaking, what you're describing is Fly-by-wire "Plus". As long as the control inputs are transmitted to the actuators electrically then you have Fly-by-wire. However, as you say, modern practice can and does place varying degrees of processing and computation between the controls and the actuators.

 Quoting United Airline (Thread starter):So there isn't any hydralics at all?

In its most basic form, FBW replaces the mechanical cables and rods that link the controls on the flight deck with the control surface actuators. The actuators can still move the surfaces hydraulically on a FBW aircraft.

 Quoting United Airline (Thread starter):How does fly-by-wire work?

That depends on the aircraft and manufacturer... and on how much detail you want to know.

United Airline
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### RE: How Does Fly-by-wire Work?

So which one is better? The hydralic system or FBW?

The A380 is FBW for sure. What about the B 777/787? B 737NG? B 747-8?

David L
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Joined: Tue May 18, 1999 2:26 am

### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 7):So which one is better? The hydralic system or FBW?

FBW does not (necessarily) replace hydraulics...

 Quoting BMI727 (Reply 5):Quoting United Airline (Thread starter):So there isn't any hydralics at all? There is, it's just all done via computers rather than cables or electromechanical means.
 Quoting David L (Reply 6):In its most basic form, FBW replaces the mechanical cables and rods that link the controls on the flight deck with the control surface actuators. The actuators can still move the surfaces hydraulically on a FBW aircraft.

Here's another Wikipedia "starter"...

http://en.wikipedia.org/wiki/Fly-by-wire

 Quoting United Airline (Reply 7):The A380 is FBW for sure. What about the B 777/787? B 737NG? B 747-8?

The Airbus 320, 330, 340, 350 and 380 and the Boeing 777 and 787 are FBW, the 748 is partially fly-by-wire and the A300, A310 and 737NG are not. If you consider when each was introduced, you'll see a pattern.

BMI727
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### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 7):So which one is better? The hydralic system or FBW?

That's like asking whether an engine or a transmission is better.

In most cases, you need both. The fly by wire system transmits, and usually determines via software based on control inputs, how the control surfaces move. The hydraulics take those signals and actually move the control surfaces. Fly by wire airliners have hydraulic systems basically the same as any other. You can also use a direct electrical drive on the control surfaces (I don't think any airliners or business jets have that yet) or use a hydrostatic actuator which is basically a miniature electrically powered hydraulic system which I think the G650 uses as a backup system.
Why do Aerospace Engineering students have to turn things in on time?

bueb0g
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### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 7):So which one is better? The hydralic system or FBW?

Did you read the posts before? Fly-by-wire does not replace hydraulics, only the control cables that go from the cockpit to the hydraulic (or electric as the case may be) actuators.

 Quoting United Airline (Reply 7):The A380 is FBW for sure. What about the B 777/787? B 737NG? B 747-8?

Every Airbus aircraft A320 and onwards is FBW, 777 and 787 are, 747-8 is partially.

Have to agree with the others here... This is something that's so simple to search it's pointless posting it here. This forum is for questions that can't be answered by a wiki search.
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Starlionblue
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### RE: How Does Fly-by-wire Work?

I came up with this classification a few years ago. It is hardly "official" but it helps the discussion.

-Fly by wire. A means by which control surfaces are signaled with electrical impulses as opposed to wires and pulleys. Examples: F-16, 777, 330/340.

- 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. Example: Any aircraft with an autopilot.

- "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. Example: F-16, 330/340.

- 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. Example: 330/340.

Putting it in Airbus terms, these are more or less equivalent to:
Direct law
Autopilot
Alternate law
Normal law

 Quoting BMI727 (Reply 5):In a traditional control scheme, pulling back on the stick commands the elevators to deflect up, moving the stick left causes the left aileron to deflect upwards and the right to deflect downwards, and so forth. In a fly by wire aircraft pulling back on the stick doesn't actually do anything with the elevators directly. Rather than commanding the elevators to deflect, it commands a pitch rate. Moving the stick to the left will command a roll rate.

As David L points out, this is not really fly by wire, but an extension of it. In the strictest sense, FBW just means electronic signaling.

 Quoting United Airline (Reply 7):So which one is better? The hydralic system or FBW?

As mentioned, FBW and hydraulics are neither opposites nor mutually exclusive. Surfaces are still typically hydraulically actuated. FBW replaces the control rods and cables with electronic signals.

Changing the question to FBW or mechanical signaling, FBW saves on weight, construction and maintenance. It also easily allows simple implementation of computer aided flight, stability and envelope protection. Compare an old school inline and separate mach trimmer with a modern system that has the functionality built in.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

United Airline
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### RE: How Does Fly-by-wire Work?

Sorry for being misleading. I mean which one is better. FBW or non-FBW......

thegeek
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### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 12):Sorry for being misleading. I mean which one is better. FBW or non-FBW......

FBW is better in most applications. FBW is lighter and would have other advantages too. Downside is the "cool" failure modes which don't really apply in a 737. Bigger planes have no real advantage for non-FBW, which is why the 767 will be the last wide body without it.

David L
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Joined: Tue May 18, 1999 2:26 am

### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 12):I mean which one is better. FBW or non-FBW......

I thought I'd given you a clue here:

 Quoting David L (Reply 8):The Airbus 320, 330, 340, 350 and 380 and the Boeing 777 and 787 are FBW, the 748 is partially fly-by-wire and the A300, A310 and 737NG are not. If you consider when each was introduced, you'll see a pattern.

If non-FBW was better than FBW then the B787, A350 and A380 would probably not be FBW.

Starlionblue
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### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 12): Sorry for being misleading. I mean which one is better. FBW or non-FBW......

As I mentioned above...

 Quoting Starlionblue (Reply 11):FBW or mechanical signaling, FBW saves on weight, construction and maintenance. It also easily allows simple implementation of computer aided flight, stability and envelope protection. Compare an old school inline and separate mach trimmer with a modern system that has the functionality built in.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

flyingturtle
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### RE: How Does Fly-by-wire Work?

 Quoting BMI727 (Reply 9):You can also use a direct electrical drive on the control surfaces (I don't think any airliners or business jets have that yet)

At least there are alternate flap actuators that purely work electrical. The trimmable horizontal stabilizer is also electrical - but this jackscrew-based stuff is sloooow. But they hardly count as control surfaces.

David
Reading accident reports is what calms me down

ferpe
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### RE: How Does Fly-by-wire Work?

 Quoting United Airline (Reply 7): The hydralic system or FBW?

The hydraulic system has nothing to do with controlling the aircrafts flying path, it is a convenient form of power amplifier. For a small and slow aircraft (read Cessna 172 or other general aviation planes) the human muscle force is adequate to move the surfaces needed to control the airplane. Fly faster (ie fighters) or make the plane bigger and faster (ie jet airliner) and the muscle forces are no longer adequate to deflect the rudder surfaces. You then need to amplify your force, the most convenient amplifier to date (which is strong and fast enough in the cramped space available in e.g. a wing for moving the aileron) is the hydraulic booster circuit. It works the same way as your cars servo amplified steering, it makes it easier to move the surfaces. In the original implementations the aircraft hydraulic servo had the same principle as the car servo, it amplified the force from the yoke in the cockpit, if the hydraulic pressure went you could still move the surface but it required a lot of force. To make analogy with the car steering. the yoke and the steering wheel is your input device, the cables or rods and your steering axle transmits your force and the servo amplfies that force so you can move the rudders or the wheels with ease.

Since decades one has skipped the amplifier principle on large aircraft and one now does a complete transfer from pilot yoke displacement to hydraulic movement of the rudder surface and if you have no hydraulics you can't move the surface, therefore your have redundant hydraulics circuits on such planes. The displacement of the yoke can be transmitted via mechanical means (cables, rods, pulleys...) or electrical means (see above), the force then either gets amplified by the servo or totally replaced as described. Thus your signaling of the rudder movements and how you get the force to move them is 2 separate things. On can also envision electrical servo amplifiers, they take more space therefore they have typically been used on small to mid size planes for less demanding amplifications of force.

[Edited 2013-04-28 11:05:38]
Non French in France

SKC
Posts: 166
Joined: Sat Oct 06, 2012 12:48 pm

### RE: How Does Fly-by-wire Work?

 Quoting Starlionblue (Reply 8):I came up with this classification a few years ago. It is hardly "official" but it helps the discussion. -Fly by wire. A means by which control surfaces are signaled with electrical impulses as opposed to wires and pulleys. Examples: F-16, 777, 330/340. - 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. Example: Any aircraft with an autopilot. - "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. Example: F-16, 330/340. - 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. Example: 330/340. Putting it in Airbus terms, these are more or less equivalent to: Direct law Autopilot Alternate law Normal law

Except that's not correct, unless I'm just not reading it correctly.

Normal law is the state in which all computerized flight envelopes and protections are functioning properly.

Alternate law is operations in a degraded computerized state. Some computers are inop or not working as they should, so some protections are not available.

Direct law is the lowest/minimum level of computerized flight. There's barely any inputs from computers and you have very little to no flight envelope protections.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.

Starlionblue
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### RE: How Does Fly-by-wire Work?

 Quoting SKC (Reply 15):Except that's not correct, unless I'm just not reading it correctly. Normal law is the state in which all computerized flight envelopes and protections are functioning properly. Alternate law is operations in a degraded computerized state. Some computers are inop or not working as they should, so some protections are not available. Direct law is the lowest/minimum level of computerized flight. There's barely any inputs from computers and you have very little to no flight envelope protections.

I think that's what I said. However I did note that this is "more or less" equivalent to the statements I made.

The problem is that someone says FBW and people immediately jump to "Airbus 320". FBW can be much more basic. For that matter, nothing stops you from incorporating envelope protection on a mechanically signaled airplane with cables and pushrods. Mach trimmers and stick pushers are forms of envelope protection and they were around long before FBW.

My definitions above are, in my opinion, a decent way to classify FBW so that when we talk about FBW and its components we are on the same page.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

wingscrubber
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### RE: How Does Fly-by-wire Work?

Fly-by-wire is meant to be taken to mean 'control-by-wire', whereby hydraulic servo-actuators are commanded electronically rather than mechanically with pushrod or cable, but the misconception is that 'fly-by-wire' aircraft are also 'power-by-wire', which is not the case. (Fly-by-light is another variation I'd come across, which is fly-by-wire with fibre optic cables)

POWER-by-wire is the next step, where you do away with the distributed hydraulic systems and replace the hydraulic actuators with servo-electric or electro-hydraulic actuators, but even modern airliners like 787 and A380 don't have this implemented across the board yet.

787 does have electric brakes though, for instance, and A380 has some backup EHAs (electro-hydraulic actuators), but they both still have extensive hydraulic plumbing, even if the technology is started to creep in incrementally.

OEMs do want to get rid of hydraulics completely though and replace everything with electronics, but the trade studies show time and again that it's not weight competitive or as power-efficient, but that said expect to see more EHAs and EMAs in the pipeline just because that's the trend flight controls are heading.

The perception in doing this is that you do away with the maintenance headaches and failure modes associated with hydraulics, but the reality is that you just shift the criticality to the electrical system/flight control laws instead.
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SKC
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### RE: How Does Fly-by-wire Work?

For anyone who wants all the info on Airbus' flight control laws, see this link:

http://www.airbusdriver.net/airbus_fltlaws.htm
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.

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