In the nineties, at BHL Alister Gordon, introduced an amendment to the 212 flight test schedule to include a double hydraulic failure. His reasoning was that if an aircraft had a single failure on a rig, it could still be flown back, on the assumption that a second failure would be controllable. It wasn't, the aircraft each time went into a right hand dive and turn, and the pilot urgently said turn it back on, or words to that effect.
The amendment was withdrawn after Bell said ,"stop being so stupid, your going to kill yourselves".
With regard to the S76 tail control aft quadrant spring centring system, this was fitted to put the system to neutral in the event of a cable brake. It would have no effect on a servo runaway.