Hydraulic Flight Controls - Basic Question

[Dunhovrin]

OK - I'm displaying my ignorance here. None of my notes or books have an answer to this simple question regarding the positioning of flight controls that have infinite positions (ie ailerons, elevs, rudder, spoilers). How do the actuators or whatever move the control surface to where the flying control demands it to be? I can understand things like flaps or gear which have set positions and can be moved with hydraulic switches controlling double jacks. But when I wiggle the controls slightly what happens to make the control surfaces move just that amount and not more?

Am I missing something really simple?

[Dan Winterland]

Yep, there is a follow up mechanism. You move the control column, the valve opens, the jack moves the surface, the surface moves the follow up mechanism until it's position matches the input, the valve closes.

Although I'm not sure why you've been pondering this. Has life been so dull recently with all that time off?

But remember, on your new jet, if there's no yellow writing on the screen - there's nothing to worry about. There's no prizes for getting creative with the checks!

[basil fawlty]

You need to know how the servo valve on the PFCU works. This is very difficult to explain without a diagram of the PFCU. Hopefully your textbooks have one. Very basically, the pilot moves the control column, on "conventional" controlled aircraft this moves the appropriate flight control cable which in turn moves the piston in the servo valve on the PFCU. Hydraulic pressure is then ported such that the main ram moves in the appropriate direction relative to the casing. The casing itself is the output lever to which the flight control surface is connected, and therefore the control surface will deflect in the required direction. Fluid on the other side of the balanced ram is free to return, via ports in the servo, to the reservoir return line. Due to the fact that the main ram is attached at one end to the airframe, the hydraulic pressure has to move the entire PFCU body, and if there is no change in position of the servo valve piston (i.e. no fresh input from the pilot) the movement of the PFCU body will itself close off the outlet ports of the servo valve, and hydraulic fluid will cease to flow at the correct control surface deflection. In this situation the main ram is hydraulically locked, with fluid of equal pressure on both sides, and neither the control surface or PFCU body will move, until of course the equilibrium is altered by a new control input moving the servo piston again.
Hope that helps!!!

[Dunhovrin]

Thanks chaps. I think I was over-complicating the bit between the control wheel and the jacks. I guess a lump of metal string provides suffucient flexability. Of course WIWOS you could pull the rudder cables while sat on the karzii.

Thanks Dan for the jibe... I have lost interest in this topic now I have become a child of the magenta line. How do you spell VNAV?