Originally Posted by
Condor
However there must come a point where the aerodynamic forces at the elevator will be stronger than the hydraulic powered controls (as per the well known servo transparency / jack stall in the rotary world) - has anyone any idea what sort of forces we are talking about in this fixed wing scenario?
Sorry, didn't see this before. The answer is, absolutely massive forces required to backdrive the PCUs in any but severe system failure cases.
Consider that because the controls are powered, there's no real design need to aerodynamically balance the controls, so the PCUs can overcome the large hinge moments associated with large control deflections at high speeds, and also can maintain control positional accuracy against the risk of fluttter all the way to VD and beyond. Then there's the fact that to backdrive a hydraulic actuator you've usually got to overpressure the cylinders enough to blow off a protective valve or some such (or just blow the seals) and you're probably talking forces an order of magnitude greater than those which can snatch the controls from a human hand.
Perhaps in the event of mutliple hydraulics failures you might get the system power down enough. But that's one Hell of a bad day you're having ....