Fly By Wire uses hydraulics. Control surfaces are moved by hydraulics, but commands from the control-column are carried to the control-surface via electrical signal. The electrical sygnal commands the hydraulic system to move the control surface. The hydraulic system still moves the control-surface.
Fly-by-Wire does not require digital computer's. Analog one's work just fine as well. The CF-105 Arrow, which was built in 1958, had Fly By Wire. It was the most advanced aircraft of its time. The control colum input is analog, the computers convert it into a digital signal. That goes to the control surfaces, then translates back to analog before reaching the actuators (I think). They call this Direct-mode. The A-320 has direct mode only for the stab, and the rudder. Nothing else.
All fly-by-wire is not the same. Even digital fly-by-wire isn't. There's unaugmented fly-by-wire. That is the computer simply does whatever the hell the pilot tells it to do. Augmented fly-by-wire is usually always digital in nature, but it makes small adjustments to the control-surfaces to stabilize the aircraft. Many aircraft that used fly-by-wire such as the F-16 are longitudinally unstable (it would cartwheel backwards in mid-air if the computer didn't make constant adjustments, CL is in front of the CG, which makes for very interesting results), the computer stabilizes it. Longitudinally unstable aircraft tend to be very agile. Additionally, airliners can profit from this system in turbulent weather, the elevators are constantly being adjusted to avoid the plane from getting bopped up and down. The L-1011 (I'm not certain if this is FBW or not, I don't think it is)-500 has active control ailerons which tie into accelerometers, which measure vertical movements. They counter it by deflecting the ailerons up in updrafts to keep the wings from being excessively deflected to ease the loading on the wings. FBW planes tend to be quite stable even in turbulent weather. The 777 doesn't require back elevator to make turns; simply turn the yoke, and it automatically kicks in the elevator. I think this only works up to 25-35 degrees though.
The real thing about FBW that gets all these Boeing vs. Airbus debates is called Performance Envelope Protection. It's a safety system so to speak, which is designed to keep the aircraft from stalling, overspeeding, over-g-ing, or from potentially going upside down. It's also used to prevent the aircraft from taking on excessive descent rates. There are two ways to do this, and this makes all the difference between Boeing and Airbus.
Boeing uses soft-limits, and Airbus uses hard-limits.
Soft Limits are limits that are programmed in by the manufacturer in an arbitrary fasion. When the captain or pilot flying attempts to exceed these limitations, the computer will resist the pilots inputs, in addition to sounding many annoying alarms and warnings. However the pilot possesses the urge to override this. This could be done with a switch in the cockpit, or in the 777's case, by a good hard throw of the wheel!
The 777 has a bank-limitation of 35-degrees, which can be overridden to 66-degrees. Once you attempt to exceed this, the computer automatically starts attepting to roll the aircraft the opposite way, by a 30-degree counter-wheel movement. IF the pilot holds the stick where it is, it would be felt as quite a bit of resistance. However he or she is capable of overriding it by simply overpowering the system. Not really recommended, but there are circumstances where this feature can be quite useful.
Hard-Limits are used by Airbus. Requires a more-complicated code. The airbus system has pre-programmed limits in it's system as well. But unlike Boeings, there is NO override to them. The computer makes the final call. This is what upsets many Boeing fans
. Limits for the A-320 include a maximum pitch Angle of 30 degrees, maximum bank-angle of 67.5 degrees, maximum vertical-acceleration of 2.5 G's and -1 G's. It also will not allow the aircraft to descend past a certain altitude without the gear down, will not allow go-around mode to be engaged below 100 feet AGL, climb past a certain altitude if it senses problems in the pressurization system (including in-secure doors). The pilot could pull and pull on his big-ole stick all he wants, and the plane will not exceed these limits. No matter what.
Each one has their own advantages. The Boeing design relies on pilot competence, the fact that these guys are very well trained, have thousands of hours logged, and really don't want to end up dead! The Airbus design kind of relies on pilot incompetence and sort of acts as a computerized baby-sitter for them. It does have some safe-advantages in the fact that the aircraft simply can't be stalled, overspeed, overstressed, or rolled-inverted (unless they get a really nasty wake turbulence encounter
). Boeing's advantage is that the aircraft is protected, but in an emergency the captain or pilot can take the aircraft to it's absolute limits.
To be honest, I agree with the Boeing method. Having a PPL myself, I think that having spent quite a bit of money (particularly the fact that much of it was my family's money!) on a PPL, undergoing the training involved, I think that I'd actually like to fly the aircraft without being interrogated by a committee of computers. Why bother spending all that money learning how to fly if the computer's gonna do whatever it wants regardless what you do! I think the pilot should be in ultimate control. Simply put, computers are not yet advanced enough to be able to handle unique emergencies. Put a computer in control of Fl- 232 when it blew it's fan-disk and it would have kept rolling until it went onto it's back and dove straight into the ground probably going supersonic when it did it! Now put a human being behind it and it's still iffy, but Captain Haynes managed to do it. The fact that he had 33 years experience with United, and 4 years of Marine training (he was a Marine Corps flight instructor), nearly 30,000 hours of flight time logged, and 7,000 of that being in the DC-10, combined with an experienced crew, and a DC-10 captain named Dennis Fitch (check-captain), may have had just a *little* to do with it
. Even when computers do get that advanced, I'd still like to be the one who makes the final call anyway. The 777 is also more backed-up in case of an electrical failure, with 3 of the flight spoilers and the inboard aileron still backed up via mechanical means, with stab-trim remaining, and manual-rudder control. Hey, it's NEVER EVER supposed to fail, but just in case, I still like that extra back-up it offers. Additionally, when the throttles make a power-adjustment, the throttles move, which I personally prefer.
Fly By Power: I think you mean Power by Wire. That is different in the way that the actuators themselves are electrical in nature. Not hydraulic.