Fly-by-wire question

[MadsDad]

I know that on a fly-by-wire aircraft the flight controls are all controlled by a computer (e.g. you push a rudder pedal and a computer decides how far the rudder itself should be moved and then operates the valves to the hydraulics to do so) but are all the controls activated by computers.

I was thinking particularly of the brakes and if there is some sort of back-up in case the computer failed when the aircraft was landing, but then I thought about steering etc.

(The question actually crossed my mind half-way through a particularly bad landing at Luqa last week).

[speedbird_heavy]

Skyservice A320 C-FTDF nearly went off the end of the runway at Cardiff after a computer error. On the first approach, the computers failed and had to be re-set so the captain performed a go-around. On the second approach, the computers failed just before touchdown and becasue of this the crew couldnt enggage any braking systems (spoilers, reverse thrust or auto brake) and the only way the aircraft was brought to a halt was by the captain engaging the parking brake. Blew 3 main gear tyres, took out several runway lights and stopped less than 5m from the end of the runway.

[oompa loompa]

I'm not sure the computers failing on an A320 would cause a loss of braking; the normal system is usually brake by wire, but the alternate is often (depending on type, includes A320 anyway) a hydromechanical system.

Therefore, if the pilot had to select park brake to stop, then more than the computers failed. Possibly a pedal jam? Would still need to jam both sets of pedals though...

Sounds like they were extremely unlucky to have got into this sort of situation, as there would have had to be multiple failures all at the same time.

I'm not familiar with the boeing brake control systems, so can't comment on them, I expect they would be similar?

[Capt. Inop]

Famous last words on Airbus flightdeck:

Why di it do THAT?
and
It has never done THAT before.. :}

You can still fly a Boeing with all electrical power lost,
cant do that in a bus..

[Spearing Britney]

Uh-oh more ill informed rubbish, I fear this thread is going straight to hell (for hell read jetblast)...

[MadsDad]

Thank you gentlemen.

The answer seems to be 'brake by wire but mechanical backup, with the paking brake mechanical'. The bit about the parking brake being purely mechanical did sound a bit odd at first but thinking about it it does have to work when the aircraft is powered down so perfectly logical (also reminds me of the comment on the David Gunson tape of 'you, as co-pilot, should make sure the handbrake is off. Otherwise when landing we will get a smell of burning rubber followed immediately by 18 loud bangs').

As to the 'fly with total loss of electrical power' comment I would have thought that on a large aircraft (and I am speaking from a possition of total ignorance here) that no electrics = no pumps = no hydraulics = can't move controls because of the mechanical forces involved.

[Sky Wave]

Only a wannabie so I may be speaking out of turn. however my understanding is that in the event of a total electrical failure and an engine failure the RAT will provide a small amount of hydraulic and electrical power to run flight controls and instruments. Failing that on the Bus the stab trim and rudder have mechanical connections giving some sort of control in the event of no electrics and no hydraulics. Any bus pilots can feel free to correct me if I've got it wrong.

[Ka8 Flyer]

Hi guys,

well first of all, regarding the statement
"no elec power => no pumps => no hydraulics" is not quite correct, as you still have engine driven pumps (they don't require electrical power, and will be powered as long as the engine are running or windmilling)
So, yes, a B767 can be flown without electrics. Even further, a B737 can be flown without hydraulics, as the flight controls are directly connected to the control surfaces.

The functioning of the RAT depends on aircraft. While on the Bus it does power elec/hyd, it will only power one hydraulic system on the B767. On the MD-11 I believe you have the option of choosing hyd or elec from the RAT.
Some B767 fans may now say, hey, if the C Hyd Sys is powered by the RAT then the HMG (hydraulic motor driven gen) can power its busses. Unfortunately, the RAT only supplies the primary flight control surfaces thus no additional electrics from the RAT.

Regarding total loss on the glass-bus (elec & hyd), yes, rudder and manual pitch trim will still work. Its even taught in the sim (triple hyd failure is not taught on the 767).

Hope that helped,
Regards,

Mark

[Tree]

The parking brake control is just an electrical switch powered by the DC bus in normal configuration and the Hot 1 Batt bus in Emer Elec Configuration.

Cheers, Tree

[Wino]

In 1997 it was a problem for the Airbus.
There were a series of computer failures leading to over runs.
The Yellow sysyem (standby breaking system) had a seperate problem that was making itself useless after a failure of the green system

There was a temporay fix (blue page in the fcom) that required you to hold the autobrake switch for atleast 3 secs when selecting autobrakes for landing. The software fix came later and that restriction is no longer in effect though some operators may still do it out of habit and not bothering to purge the procedure. That problem was leading to a double LGCIU failure.

The problem with the yellow system brakes as I recalled had to do with moisture collecting somewhere on a cold skin or valve and freezing the valve. After the aircraft landed the valve would warm up and the problem would go away, so when ever anyone performed a brake check they weren't seeing the problem. The only time the problem was rearing its head was when the yellow system brakes were REALLY needed. I seam to recall the fix for the yellow system was simply drilling a hole in something on the yellow system so that water could drain and ICE wouldn't interfere with the valve and the system.

The Two unrelated problems (in seperate area's of the brake system) I believe lead to 3 or four hull losses but no fatalities. It took a while to find the Ice problem as it was one of those self curing problems, untill it happened at the wrong time. The one I remember was the Airtours or someone in Turkey. New airplane and engines all full of dirt when it went offf the end... Summer of 97 or 98 I think it was (It was an Airworld summer for me)

OOMPALOOMPA. Yellow system is not hydromechanincal. It is also brake by wire. Just a different wire...

Cheers
WIno

[Cuban_8]

Yet more mis-information about the Scarebus being promulgated by those who choose to comment without being in possession of the facts.

1. Airbus design philosophy allows the a/c to be flown without electrical power through the use of the rudder and THS (both mechanical in nature)

2. As Oompa Loompa correctly stated, only "Normal" braking is a fly-by-wire, computer controlled system. Alternate braking, in both modes, is hydro-mechanical - the only computer input being the anti-skid function, if operative. The "Cardiff" situation suggests that far more than a BSCU computer was at fault, that is, if the facts are correct.

3. Airtours have not experienced an over-run event with the A320.

Regards,

Cuban_8

[ant1]

Regarding fly by wire I would say that aircraft controls are like computer controls: they send electrical signals to a computer that does "a lot of thinking" before resending those inputs to the servo actuators.

I think in the 737 (300-400 series) normal operation, control column inputs are previously sent to some kind of computer that doesn't use such a complex logic as in FBW but "modulates" to some extent those inputs. Correct me if I'm wrong.

Can anybody elaborate a bit more on 737 manual reversion?

:)

[Ka8 Flyer]

"I thing in the 737 (300-400 series) normal operation, control column inputs are previously sent to some kind of computer that doesn't use such a complex logic as in FBW but "modulates" to some extent those inputs. Correct me if I'm wrong."

Okay then let me correct you ;-)

Seriously, the 737 flight control system doesn't modulate a thing. Deflection at a given control column position will be the same regardless of what system is powered.
Its basically the same as the power steering in your car (at least the older types).
The force required to move the column is reduced - that's it. Just as you can steer a car without power steering, you can fly the 737 without hydraulics. You'll just have bad muscle ache :-)
(Actually its not that difficult if you have a little experience with it - just don't forget to trim!

Regards,

Mark