Electrically controlled, and hydraulically operated? (A320 question)
 

Firstly, when saying this, just to make sure I understand it fully.
It basically says that it is electrically controlled in that the signals are sent through computers, which then give feedback, but the operation and movement is actuated hydraulically. Correct?

I believe most things work this way on the 320, however I remember coming across one or two things that were electrically controlled and pneamatically operated.
I don't remember what these things were (if someone can?), nor how exactly it works when its pneumatically operated.

Anybody please explain?

Bleed valves

Pneumatically operated - It's simplistic but I just think of a spring loaded mechanism that won't function until a certain pressure threshold is reached.

electro-hydraulic v electro-pneumatic
 

When it is said that a component is electrically controlled but hydraulically actuated it simply means, as you said, an electrical signal is sent to the controlling element of the component and this action allows hydraulic pressure to actuate the component. The same principle applies to those components which use pneumatic supply to actuate the component, i.e, electrically controlled, pneumatically actuated. As nncO says, bleed air valves are a classic example.

Packs are a good one, electrically controlled pneumatically operated, if the electrical operation fails they run automatically therefore fail-safe.

Ie packs selected off (electrical signal) engines running bleed pressurized and turn all the generators off packs will run.

Electrically controlled or signalled but the muscle is hydraulic or pneumatic.

So without a electrical signal the muscle is designed to usually fail open or fail closed or fail on fail off.

And with electrics but no muscle (hyd/pneumatic failure) nothing moves.

I wonder if you loose all hydraulic systems, will the surfaces be locked-in place or they will be freefloating moving?

I mean either you don't have any pump working or if you have hyd leak and loose all pressure. I think these 2 situations are different.

Thank you so much!

The 747 classics and probably modern, had electro/pneumatic driven L/E devices, pneumatic hydraulic pumps and reversers on the JT-9's

)
 

David36

With any control surface that is hydraulically actuated (it does not matter what the signalling system is) then with the aircraft servicable but shut down one would not expect to be able to manually adjust the control surface position (try it for yourself!) as the fluid provides a 'hydaulic lock'. With no fluid this lock would not be present.

So the surfaces will be freefloating only if there's a total hyd loss due to fuel leak, right?

Sorry if that was not clear enough.

Freefloating - mmmm. There will be some friction in the mechanism which will vary with the particular actuator. Whether your hand could move it easily or a 2kt breeze bang it about will depend on said friction.

David36: Look at any A320 on the ramp after a few minutes of engine shutdown and you will usually see both ailerons drooping down under gravity and, if it's windy, then the rudder will be blown downwind. No loss of fluid involved but no system pressure.

In that case, I figure out that a total hyd loss also brings a flutter risk situation assuming freefloating surfaces.

Gents,
Extract from A320 AMM for your perusal.


Ailerons
Each aileron can be powered by two servocontrols signalled by two Elevator and Aileron Computers (ELAC) and supplied from different hydraulic systems
In the event of a failure, the ailerons become automatically controlled by the ELAC 2 (roll) and the associated servocontrols switched to the active mode, the others being now damped. If a multiple failure condition causes the loss of the control of the two servocontrols of an aileron, the servocontrols automatically switch to the damping mode. This operating mode is also automatically engaged in the event of loss of pressure.

Rudder
One servoactuator is normally operating, the other being by-passed. A spring rod is provided to center the actuators when both of them are depressurized.


Elevators
In the event of the loss of control of the two servocontrols of the elevator, the servocontrols are automatically switched to a centering mode and hold the surface in the neutral position (Electrical control loss). In the event of the loss of the two hydraulic systems supplying the servocontrols of one elevator, the damping mode becomes automatically engaged.

Spoilers
In the event of an electrical failure, the associated surface is hydraulically held down. In the event of a hydraulic failure, the servocontrol is hydraulically locked in one direction to prevent the surface from raising. In both cases the control of the symmetrical surface is automatically inhibited.