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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.
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.
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.
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.
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.
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.
