Just a few thoughts...
Think of a "reversible flight control system" as one in which, if you were to move the control surface by hand, you'd see the corresponding cockpit control move in unison (i.e. a fly-by-wire control system would NOT be a reversible-type).
With most hydraulically-assisted flight controls, remember that the pilot is not making a direct input to the flying surface; he/she is only moving a lever on the servo, which in turn operates a valve and allows the hydraulic force multiplication to move the control surfaces.
In the event of a hydraulic system failure (i.e. loss of hydraulic pressure) in the AS350B2 and similarly-equipped models, a locking pin in the servo drops into place, making the servo become a solid/direct link between the flight controls and aerodynamic surfaces, i.e., a reversible control system. This allows continued manipulation of the controls during a hydraulic failure, albeit with increased control loads and aerodynamic feedback.
Now, if the conditions exist, such that the aerodynamic / gyroscopic / accelerative forces are greater than the forces that can be generated by the normal hydraulic mechanical advantage, then you have overpowered (not broken) the hydraulic system, and have reached the "servo control reversibility limit." Since the flight conditions required to generate enough force to overcome the hydraulic system are generally extreme and acute, then its a good bet that your attempts to control the aircraft at that point will seem futile as well (since hydraulic pressure has not been lost, just met with equal resistance, the locking pins have not dropped). Remove the aggravating condition causing increased loads (relax backpressure, etc.), and the hydraulic system happily goes back to doing its job.
Is this a design flaw? I'll ask another question... is the maneuvering speed in an airplane a cheap attempt at covering up a design flaw? Certainly not. It's an "aerodynamic safety valve" that exists so you don't compromise the structural integrity of your aircraft by making large control inputs that can generate forces and moments capable of breaking things. As I understand it, the same idea exists behind the "servo control reversibility limit."
If a pilot told me the design maneuvering speed was dumb because someday they may need to be able to pull more G to save their own ass, and how dare the manufacturer allow the airplane to stall instead of giving them what performance they asked for (you know, the kind of performance capable of folding the wings in half), the blank stare on my face would be priceless.
Just like an airplane wing stalls when the maximum angle of attack is exceeded, no matter what its airspeed, attitude, etc., "jack stall / servo transparency / control reversibility limit" occurs whenever the hydraulic system's maximum force is exceeded by the loads imposed on the rotor system no matter what the altitude, attitude, airspeed, etc.
I hope my long-winded post gave some insight to an interesting topic.
Regards,
Frank Lombardi
Last edited by rotorwash4944; 2nd Feb 2011 at 03:47.