View Full Version : Boeing Vs Airbus Fly By Wire

22nd May 2002, 13:16
Greetings ladies and Gents,

I'm was just curious whether anyone out there knew the differences between the Boeing 777 Fly By Wire, compared to the Airbus's!!!

Is it only the fact that, should all hell break lose on the 777, it still has the ability to fly manually (Direct Mode)?? Whereas the Airbus doesn't??

I've looked at both of the Boeing and Airbus website for info, but was unable to find any real explanations!!

Would appreciate any info at all.


22nd May 2002, 14:41
When you say manually, do you mean without autopilot or without hydraulics? The Bus is always 'connected' to the autopilot to some extent in each law. If you mean hydraulically, nope, the Bus will fall out of the sky... unless you're good at using asymmetric thrust... but it's not all that difficult.

22nd May 2002, 16:36
Seem to remember reading somewhere that the 777 only has conventional control wires on the elevators and rudders, and then only for back-up. Roll control is totally electronic / hydraulic.

As for the Airbus, I think the Air Transat A330 that recently landed without engine power, after loosing all fuel, landed using 'Direct Law'. As far as I can gather, the auto-pilot went into some sort of mode where it did only as the pilot asked without any other intervention.

22nd May 2002, 16:41
Both the 777 and Airbus are useless without Hydraulics, You can move the yoke or side stick and nothing will happen as there is "NO" force to move the flight control surfaces.
Now a Electrical Power failure which completly disables the FBW computers will still leave you with Hydraulics so then all you have is mechanical back-up i.e. Cables, on the Airbus you have the Stab trim wheel and rudder (for roll control) and on the 777 you have the Elevator and some spoilers for roll control.
The major difference is the 777 is still flown with the yoke while the Airbus the sidestick cannot be used as it's not mechanically connected to anything.
But both A/C state the A/C is only flown this way until power is restored, even if by Batteries, as the redundency is built into the system a lot has to go wrong. Plus the RAT would have to fail ultimately to leave you on batteries alone.
The Airbus is flown in "NORMAL LAW" then ALTERNATE LAW 1, ALTERNATE LAW 2, DIRECT LAW (no protection ie stall.) and finally MECHANICAL BACK UP. and I'd imagine the 777 has a similar amount of redundency which ends up in MECHANICAL BACK UP.

A correction thanks to max motor, is the 777 uses a manual pitch trim for the stab (so no elevator) the same as the classic, I'd forgotten it was there as it was removed from the 747-400, this then leads to the second correction which is the 777 would be flow with the yoke for roll and not for pitch.

23rd May 2002, 01:37
From the Airbus manual : "Mechanical control of the THS (trimable horizontal stab) is available from the pitch trim wheel at any time provided green or yellow hydraulic is available. Mechanical control from the pitch trim wheel has priority over electrical control".

"Mechanical Back Up. Pitch: To control the aircraft during a temporary complete loss of electrical power".

Note: The rudder also has mechanical control.

23rd May 2002, 03:11
The boeing control's are linked and backdriven, so you see what is being commanded of the aircraft as well. (Something that Airbus was sorely lacking in, and yes I flew A320's for 3 years as a captain)

That gives you the ability to see just what your mate is doing, what the autopilot is attempting to do, and the throttles can be goosed to help them out. A much better system from a pilots point of view but it adds expense and weight. (worth it IMO)


23rd May 2002, 04:35
I admit I don't fly the 320 but the 330 and both the 330/340 have a Mechanically controlled rudder, by cables, now when I say this it means cables run from the pedals to the Hydraulic actuators and the same for the handwheels to the THS (and same for the 777 elevator and spoilers). I find it strange that the 320 has none of this ? surely it must have some sort of mechanical back up as I believe Airbus is just now considering removing all the mechanical back up for the 380 and possibly 345/6 due to reliability of current Airbus FBW systems.

See max motors correction regarding the 777 elevator (should be stab) as he is correct.

23rd May 2002, 12:11
Thanks heaps guys for the replies so far!!

So after reading all of the above posts. I would be right by saying that there is absolutely no difference between Boeing and Airbus, except for the level of redundancies??

Because by the sounds of it..... You are a goner with both types, without HYD Pressure!



23rd May 2002, 12:16

Just another question then about the HYD' on both aircraft!

What level of redundancies are on the HYD then? It's no good having all the backups for the electrics, if the hydraulics can fail!

I think I might change careers.... This is too confusing! Damnation

23rd May 2002, 16:33

Same as the A300-600, 747 DC-10, L1011 etc.

None of the wide bodies can be flown with a TOTAL loss of hydraulics.

What is different is that a TOTAL loss of electicity (Battery dead too, which might happen long enough over water) but an otherwise sound aircraft might leave the airplane unflyable. Right now in AIrbus and boeing there is mechanical backup of the valves to move atleast a few flight controls and the stab. What airbus is proposing is to remove that backup feature as well, so a serious electicial problem which takes the aircraft down to battery power more than 30 minutes from landing will result in the loss of the aircraft.

Electrical faults are often an aging aircraft issue, and airbus hasnt really be flying the FBW aircraft longenough to make this sort of judgement call that they are trying now. The problem will be with the aircraft start to get 20 years old and the insulation on various wires starts to crack.


23rd May 2002, 16:38

I'm not sure your actually reading my posts, i.e. the first line of my first post.

Plus I think your confused as to what Mechanical Back up means as you wrote:

" Therefore there is no MECHANICAL link between the rudders or the trim wheel, what in fact they have is a linkage to HYD shuttle cock valve which directs the pressure in the ordered direction".

There is a mechanical link between the rudder pedals and trim wheel, via cable runs to the Rudder Actuator and THS jackscrew which is the "linkage" (mechanical) you mention in the second sentence .:confused:

Which is what I said in my second post.

Note I never said any flight control was mechanically ACTUATED like say the 707 or 717 which use this system for some flight controls via servo tabs.

I've just spoken to a mate on the 320 who has flown the 330 as well and he said the systems are almost identical in regard to mechanical back up.

Sorry to ask this silky but do you fly the 320 or just have access to the FCOMs.

23rd May 2002, 16:56

Just as you said there is plenty of redundancy for control (electrical or mechanical) of the flight controls, there is just as much for actuation via hydraulics.
With 3 hydraulic systems and 7 pumps plus a RAT (A330) your fairly safe and a few other things help as well.

A good example for hydraulic redundancy is one of a 747 out of SFO which took out the lights at the end of the runway (due to incorrect V speeds) and the damage took out 3 of 4 hydraulic systems yet the A/C managed to dump fuel for an hour and then land.
Note the Classic and -400(except flap, manual stab trim and rudder trim which were changed to electric) flight control systems are an entire system of what the airbus uses as mechanical back up, but obviously a lot more complicated for redundancy.

An unlucky case is the JAL 747 which had an explosive decompression through the rear pressure bulkhead which effectively blew the fin apart and with it went all four systems which were feeding the rudders, they managed to fly for a while using differential thrust, but lost the battle. As a result I think, but can't remember resulted in the installation of hydraulic fuses prior to the RPB which prevent massive fluid flows, these are already installed in the wheel well area due to the proximity of spinning wheels and foreign objects.

The Sioux city DC-10 was somewhat of a success.

max motor
23rd May 2002, 20:48
Just a couple of points to add:

The 777 elevators are purely fly by wire. There are however manual pitch trim levers that allow mechanical trimming of the horizontal stabiliser. This is the only “mechanical” method of controlling the aircraft in pitch. (two spoilers are available in roll).

The 777 ‘Direct Mode’ is effectively an analogue fly by wire mode. In direct mode electrical signals from the yoke/pedal are fed directly to the control servos. They are not digitised and modified by control laws in the flight control computers as they are in Primary and Secondary modes.


crusin level
24th May 2002, 15:20
Surely the truth is in the flying.
Having flown both types,the difference to me seems that the 777 fbw is transparent whilst the airbus always gives you constant reminders its a computer game!

27th May 2002, 13:00
Greetings once more,

To all of you who have replied, Thankyou very much. It's help to get an appreciation of the way the two work.

Cheers! :)

29th May 2002, 22:12
The original question Boing 777 versus Airbus was misunderstood by all those who tried to reply.

That is my understanding anyway.

Let me try to rephrase the question:

Is the understanding correct that the Boeing 777 way of flying by wire also includes the ability to fly the airplane in direct mode wihtout the computer overruling the steering input of the pilot.???

Much of the reservations many people have about the airbus filosophy is due to the fact that the captain of the airplane does not have the last say.

The computer is the pilot in command

The captain can only use steering input approved be the computer programmer at Airbus Industries.

As far as I understand the captain is the pilot in command on a Boeing airplane. He can use any steering without being overruled by the computer if direct mode is selected.

Please I would like a Boeing 777 specialist to comment...