question about 777 flight controls

[AnQrKa]

I was wondering if the 777 fbw controls allow for manual reversion. Are the surfaces connected by cables like the 767 or are they like the 320/330/340 family. Is/will the 787 the same?

regards

[showtime777]

Reversion takes places in stages. The normal mode is of course "normal mode"

The reversion process is as follows:

1. Secondary mode
2. Direct mode

The above modes still require electric power. When it is not available, the mechanical backup you refer to is used to control the plane.

It consists of cables from the flight deck to selected spoilers, and the stabilizer. As far as I know, there are no cables to the rudders.

Pardon me if only the last bit answers your question. Kinda felt I needed to paint the full picture.

Perhaps an airbus driver could fill in the details on the bus though i don't think there are cables linking the sidestick to the surfaces. Still required electricity of some sort to drive the flight control system (batteries etc in the event of total generator failure).

[Cough]

Airbus -

Rudder is conventional so you use that and the trim wheel for pitch.

[ALK A343]

If my memory serves me right mechanical back up control for the rudder and pitch trim on the A320 was a requirement to get it certified by the FAA back then. The early A330/A340s have the same arrangement, although there is no more direct mechanical link from the A340-500 and -600 onwards. I am however not familiar with the new arrangement, maybe somebody else can explain that.

[EMIT]

Manual reversion means that muscle power only moves controls through mechanical connection between the steering tools and the control surfaces.

This sort of steering does not exist anymore in any big modern aircraft!

Always there is need for some sort of power system to drive the control surfaces - usually hydraulic power.

Moving the steering tools delivers a signal to the powered actuators to set the control surfaces.

Now you get the distinction between conventional control systems and fly by wire:
Conventionally, the position of the steering tools is linked mechanically to the actuators.
With fly by wire, the steering tool position is one of the parameters that is used to compute the required control surface position and electrical wires are used to transmit the command to the actuators.

Mechanical backup is not the same as manual reversion! Mechanical backup still needs (hydraulic) power to actually move the controls.

[kijangnim]

Greetings

Cables have gone, and there are NO cables on the B767/757 upwards
they have installed a RAT to ensure mini Hydraulic for flight controls :ok:

[Denti]

Dammit, now i have to live with the fact that the 737 is not a big modern aircraft. Thats hard to take :}

[CV880]

The only control surfaces mechanically connected to the flight deck on a 777 are a pair of flight spoilers and the horizotal stab trim system.
Airbus FBW originally had mechanically connected stab trim and rudder but AI eliminated the mechanically signalled rudder and replaced it with an independant electrically signalled backup system.
The control surfaces still need hydraulic power to operate ie. there is no reversion to manual (tab) operation on loss of hydraulics as on a DC8 ailerons or rudder for example.

[stilton]

Thank you for pointing that out EMIT.

I am constantly surprised at the number of people who misunderstand what manual reversion is.

And even more so that some think a wide body aircraft's flight controls could be moved without hydraulic assistance !


My guess is that the 737 is the last remaining production transport with true manual reversion.


I will never forget doing manual reversion practice in the 727 sim, that was work !

[ALK A343]

Fully Agree with EMIT,

My previous post may have been misleading. As mentioned earlier on the initial Airbus FBW design the mechanical back up was only used to transmit a mechanical signal to the hydraulic actuators for the rudder and pitch trim thus bypassing all flight control computers. It was intended as a temporary configuration in case you loose all flight control computers and before resetting some of them.
No FBW Airbus can be flown without hydraulics.

[leewan]

No FBW Airbus can be flown without hydraulics.Not true. The A380 has back-up electrical actuators for the flight controls. And most, if not all, FBW Boeings also can't be flown without hydraulics.
I started a thread regarding this a few months ago.
http://www.pprune.org/engineers-technicians/366186-777-back-up-actuator.html

[Joetom]

The 380 back ups are local elec/hyd units, they need elec supply and need to maintain their own hyd fluid, if too much fluid is lost, it will not work.

So even the 380 needs so called hyds/hyd pressure.

Of interest, Airbus are doing research to control aircraft using the fuel trim system, pls see link below.
.
Airbus researches weight-shift control system (http://www.flightglobal.com/articles/2009/04/08/324993/airbus-researches-weight-shift-control-system.html)
.

[ALK A343]

leewan,

You are right, I rephrase my statement. No FBW Airbus except the A380 can be flown without hydraulics.
Although, as Joetom states in his post that actually the electric actuators also need their own hydraulic fluid.
But as far as the main hydraulic systems are concerned your statement is correct.