Originally Posted by
RVDT
Even more interesting if they had a HYD FAIL - probably uncontrollable unless you get HYD OFF fairly quickly. Pre-TO checks? HYD TEST?
IMHO, this would not necessarily result in an immediate loss of control, but would be dependent upon the situation when loss of hydraulics occurred. One thing is for certain, there would have been an imbalance in the control loads with cyclic displacement under these levels of accumulator charge if the cutoff switch wasn't activated immediately.
I am not surprised that the accumulators had low nitrogen pressure. It’s not that unusual, especially in aircraft that lack adequate maintenance and attention. However, at this level of charge/imbalance, a pilot should have detected a noticeable jolt in the controls during the performance of a proper hydraulic check.
However, the yaw load compensator charge is especially alarming at 1/3 of the prescribed level, it would have significantly increased the pedal loads if hydraulics were lost.
Per my (above) referenced accident report, pedal loads change significantly between (1) normal operation, (2) hydraulics off with, and (3) without yaw load compensator (YLC). However, the pedal load is also dependent upon the amount of pedal displacement.
The normal AStar hydraulic system operates at 40 bar. Note in the attached diagram, the pressure relief valve for the YLC actuates at 55 bar. To fully understand the YLC operation you should understand why this is set at a much higher pressure:
