FBW explained

[multicom123]

Hi,

I have done a search, but cant seem to find a simple explanation of what Fly-by-wire is, and how it works.... please help

Thanks!

[The African Dude]

Hiya Multicom,

When aircraft were first being built, when you moved the control column it pulled a cable attached to the control surface (eg. aileron, elevator), which made it move and allowed you, as the pilot, to control the aeroplane.

Later, as aeroplanes flew faster, physically moving these control surfaces became harder because of the increased forces on them from the faster moving air around them. So, we engineer types designed them with pneumatic/hydraulic controls to do all the work, to move the surfaces for you. These were still actuated by the pilot's control column.

But now, modern technology has advanced to the point where computers can monitor almost every aspect of the aircraft for you.... and these electronics were encorporated into the systems that moved the control surfaces. So, when the pilot makes a control input in the cockpit, eg. moves the control column forwards, this is identified by the associated cockpit technology as an electrical signal, and will be sent as electrical inputs to the hydraulics and/or motors. They then move the elevator down the amount that the electrical inputs tell them to.

Also, the cockpit technology recieves feedback from the hydraulics and/or motors as to how much force is needed to move the control surface. As an example of why that's useful, the electrical information is translated through motors attached to the control column enabling the pilot to 'feel' what the aeroplane is doing. It all just allows for much easier, safer (and yet more detailed) control and management of the aeroplane you are flying.

Replacing cables with wires has coined the phrase 'Fly-by-Wire'.

It's late so that might have come out not only garbled but inconcise.. but I hope it helps you :-)

Andy.

[Lu Zuckerman]

Another point of view.

To: multicom123

On a true FBW system the cockpit controls are connected to signal generators commonly called LVDTs (Linear Variable Differential Transformers) or RVDTs (Rotary Variable Differential transformers). The LVDTs and the RVDTs put out a signal that is proportional to the movement of the controls. The electrical output can be DC positive or, DC negative. The polarity is tied to which way the control is moved and which way the LVDT/RVDT is displaced. This signal is in most cases usually +/- 0 to 5 Volts DC. The signal is processed by the Flight Control Computer and is sent to the correct servo or servos. In the case of the ailerons the processed signal will tell the appropriate servo to either extend or retract moving the left and right ailerons either up or down. In some cases the aileron will have a LVDT or RVDT attached to it or in other cases the LVDT is on the servo. In either case the movement of the servo or the aileron will send a signal that is equal in proportion to the control movement but opposite in polarity. This feed back signal is digested by the computer and measured against the original input. When the two signals are equal in voltage but opposite in polarity the movement stops.

In some installations the servo will have both mechanical input capabilities and electrical input capabilities but this is not true fly by wire. When the servo has electrical input capabilities any deflection of the controls will pass through the computer causing the servo valve to displace. In this case the servo valve or the power piston will have a feedback LVDT. In this case when the feed back signal is equal in voltage but opposite in polarity the computer senses this and the servo valve is returned to the null position. The controls will maintain their position until the control input is either reversed or increased and the whole thing starts all over again.

Some FBW systems will use the inputs from the Air Data Computer as well as the monitoring capabilities of the FCC. Between the two the pilot will be restrained from making rapid movements of the controls in order to minimize the stresses on the airframe. This feedback can be in the form of force feel mechanisms, which restrict the pilot from making rapid inputs by increasing the force necessary to deflect the controls.

In the case of how much force is required to deflect the control servos the hydraulic system which in most cases has a variable displacement constant pressure pump the system will generate only that pressure that is required to move the control surface. Even if the pump and system are rated at 3000 PSI if it takes only 1200 PSI to move the surface that is what the pump will put into the system.

[multicom123]

cheers

Thanks alot for the replies... much appreciated.

Can anyone sum it up nicely in just a few sentences? Thats really what Im looking for.

Thanks again

[Specnut727]

There's no direct mechanical (or hydraulic) connection from the joystick to the control surfaces. Only electrical/electronic black magic, wires and mirrors, etc, etc, but hopefully not too much smoke !!!

All of Lu's LVDT's, RVDT's, +/- signals, FCC, etc, just convert the mechanical signals from the joystick, to electrical signals, then back to mechanical signals to drive and control the position of the control surfaces. Then as Lu described, more similar equipment is needed to provide mechanical feedback to the pilot via the joystick. All the while, the FCC is in the middle of the system, having ultimate control.

I hope that helps.

Spec.

[Lemurian]

To :multicom 123
I beg to disagree with Lu and Specnut727.I do not know which airplane they are talking about but I suspect it is the Airbus as there is a mention of a "joystick".
Below is a simple schemat' of the Airbus FBW philosophy :

Pilot input->transmitter->Computer->Servo->Airplane (->return loop to Computer)
In this ,There is NO FEEDBACK to the pilot,whether mechanical or electrical.
For the system to work,one needs control laws :Basically,in normal law,in flight,the pilot flies a trajectory (a flight path) instead of just moving the control surfaces (ailerons/spoilers,elevators).This means that regardless of the speed,a pilot input will always cause the same airplane reaction,in other words,piloting means changing the load factor to suit your flight path.
I presume the FCC they are talking about refers to the flight computers.They do not have "ultimate control",in the contrary,
1)they provide the pilot the best possible performances of the aircraft,
2)keeping it inside its flight envelope,
3)they reduce the risks of over-control AND airframe overloads.
Sorry,that is the quickest explanation I could manage.
I hope it helped.
Regards.

[G-ALAN]

multicom123
Try this Explains very briefly in a few sentences

[Tinstaafl]

Another variant viewpoint:

NON-FBW: When the pilot moves the controls there's a direct link that transmits his/her force to the surface concerned. The link might be augmented by hydraulics/pneumatics/electrics/aerodymics however it's still the same principle. Powered controls will have some form of feedback to try to replicate the feedback present in non-powered controls

Note that moving the control is NOT the pilot's aim. Having the aircraft pitch/roll/yaw is what is intended. Those movements of the aircraft are what we understand our control inputs to 'mean'.

FBW: The pilot's movements of the controls are interpreted by a computer as a command to make the aircraft pitch/roll/yaw in accordance with some predetermined relationship to the control device (sidestick or whatever). The computer determines what control surfaces must be moved in order to make the aircraft pitch/roll/yaw as commanded by the sidestick.

Various protection levels can be built in to the programming whereby the computer will not respond to a control input that would involve exceeding an aircraft limit. It's also possible to progressively removes these safety barriers, leading to different levels of protections being active in different modes. Airbus uses terms like 'normal law', 'alternate law', 'direct law' etc. to distinguish between what modes of operation the computer is in.

It's also possible at the most basic level for the computer's calculations & protections to be bypassed, allowing the equivalent of an electrically direct connection between the sidestick & the control surface. Equivalent to a non-FBW that uses electrically powered controls. This would equate to 'direct law'.

So, in non-FBW the pilot decides what & how much control surface(s) movement must be, whilst in FBW a computer decides what & how much control surface(s) movemant must be made to effect the pilot's desired reaction of the aircraft.

[fruitloop]

Works well...as long as all the electrons keep running the right way !!

[John Farley]

Multicom 123

You have asked a good question. Some of the info you have been given here is not entirely accurate.

With fly-by-wire (FBW) the pilot is no longer connected to what we are used to thinking of as the aircraft ‘controls’ (elevators, rudders, ailerons, throttle, flaps, airbrakes, spoilers – or anything that can make the aircraft change the way it is flying.

Instead the pilot is connected to a computer and so instructs the computer what MANOEUVRE he wants to happen. (go up, go down, go left, go right, go slower, go faster)

The computer then moves whatever ‘control’ it thinks is appropriate to make that manoeuvre happen.

Fly by wire is NOT the replacement of mechanical control runs with electric cables to pass the message. (That is mere electric signalling of commands and has been around for much longer than computer based FBW)


NB
In fact the terms used to describe FBW systems are different and very specific to avoid the sort of confusion that arises over loose use of words like ‘control’ or ‘controls’ There are three main elements:

Inceptors – the things the pilots move with their hands and feet

Motivators – the things that make the aeroplane do something (elevators, rudders, ailerons, throttle, flaps, airbrakes, spoilers and so on are examples of motivators)

Control laws – the software package that sits in the flight control computers and turns the pilots inceptor commands into suitable signals to the motivators and the feedback systems that make the whole ‘closed loop’ system work

Do not worry about whether the motivators are powered by hydraulics or electrics as that does not affect the price of FBW fish

[Dan Winterland]

To reduce the risk of failures, there is usually more than one system running in parralel. The airbus (I gather) has one system based on Intel technology, another on Motorola technology and another which was designed by Airbus itself. They all talk to each other and come up with a joint solution and two good systems will disregard an errant one. If this lot fails, it then reverts to direct law which is as described, moves the controls proportional to the displacement of the control colum. A bit like a conventional control system.

One advantage of FBW is that you can build artificial stability into an aircraft. The Airbus is controlled by selecting an attitude with the sidestick, the kit will hold it in that attitude until you make another input. This is different to a conventional control system which will rely soley on aerodynamic stability to maintain an attitude. You can also build in certain safety features such as the Airbus' 'Alpha Floor Protection' which basicly prevents the aircraft stalling.

Fighters frequently make use of FBW. A fighter is more manouevreable if it is unstable. The F16 is highly manouevreable but is so aerodynamicly unstable, it is impossible to fly manually.

[swh]

Airbus has some good reading in their Flight, Airworthiness, Support and Technology (F.A.S.T.) archives on the topic

[Blacksheep]

Nothing new - Just another step along the way

'Fly by Wire' is very much a matter of degree. Even a B757 or B767 can be considered as partially fly-by-wire when under automatic control - the aircraft flys a trajectory computed by the Flight Management System using electronic computers to maniplulate the aircraft control surfaces according to a set of rules embedded in the Flight Control Computers' software. The combination of the 'Yaw Dampers' 'Automatic Pilot' 'Automatic Throttle' and "Flight Management Sytem', known as the Automatic Flight Control System or AFCS flys and navigates the aircraft under human management oversight. Movements of the pilots controls result from AFCS back driving them but the pilot can take over direct control as and when he sees fit.

Originally all aircraft control inputs were from the pilot. As aircraft developed some complicated functions were designed to be accomplished automatically - Yaw Damping, Rudder Ratio Changing and Auto-Mach Trim, for example - so as not to distract the human pilot. As automatic pilots evolved into automatic flight control systems or AFCS the capabilities of the systems increased until the AFCS could undertake almost all of the control functions needed to move an aircraft from A to B, using navigational data from a Flight Management System. The AFCS takes electronic inputs from external sensors, compares these to the required flight path fed from the FMS and resolves these into vertical and horizontal manouvering commands, within manouvering limits embedded in the AFCS software.

In the developments described above the pilot can still over-ride the system manually and the pilots controls remain within the loop, being back-driven by control surface movements. In true Fly-by-Wire (FBW) the pilots controls become just one more electronic input into the flight control system. The FBW computers add the pilots control input into the equations, process them through the control law filters and produce an aircraft manouver that is as close as possible to what the pilot intended, without exceeding the control laws and either departing from controlled flight or overloading the aircraft structure. The pilot cannot override the control law part of the system and the pilot's controls are not moved or back-driven by control surface movements.

The next development is to remove the pilot from the system altogether. For public transport operations this will involve more psychological effort than technical input. For military applications unmanned air combat vehicles are already in use and will develop considerably further during the coming century in ways that are just becoming imaginable today. I expect that there will always remain a need for some human pilots though - in the military for battle zone decision making and in civil aviation for psychological reasons.

[fruitloop]

"In the developments described above the pilot can still over-ride the system manually and the pilots controls remain within the loop, being back-driven by control surface movements."

Sorry to burst ya bubble but this statement is not 100% correct.
After seeing what a high temp bleed air leak can do to a harness of what was originally 26 wires (redundancy)and ended up as "1" my views of FBW went up in smoke !!

[Blacksheep]

You should see what happens when a floor collapses onto the control cables underneath too! Mechanical controls are just as susceptible to external causes as FBW command lanes and they don't need a majority vote from three seperate lanes either - in a typical dual mechanical control installation, pull either wire and down she goes...