"The instructor simulated a hydraulic system failure by pressing the HYD TEST (hydraulic test) button. When the student pilot began to experience control difficulties, the instructor took over and asked the student to turn the hydraulics back on. The student mistakenly selected the HYD OFF button, by which time the helicopter was uncontrollable in yaw. The instructor managed to reset the HYD TEST button, but hydraulic power was not restored. He was unable to prevent the rotation continuing so elected to place the machine on the ground before the situation worsened. The helicopter rolled onto its side upon touching down. Hydraulic failure is normally simulated in the AS350 series by depressing the HYD TEST button. This depressurises the system, and the three accumulators in the roll and pitch circuits will continue to provide hydraulic assistance long enough for the pilot to reduce speed to the safe operating range of between 40 and 60 knots. Once the accumulator pressure has been exhausted, the aircraft is effectively under manual control, and the pilot will feel significant loads on the controls. The flight manual actions for hydraulic failure are firstly to reduce collective pitch and adjust the airspeed to between 40 and 60 knots, and secondly to operate the HYD OFF switch on the collective lever. This switch serves to eliminate any residual pressure on the servo pistons, minimising the mechanical loads required to move the control linkages. It also cancels the warning horn, which sounds when hydraulic pressure is lost. The B2 model has an additional accumulator in the yaw control circuit. Operating the HYD TEST button depressurises this accumulator immediately, but in the case of an actual hydraulic failure, this accumulator remains pressurised indefinitely (according to the flight manual), when the collective switch is operated and the HYD TEST Button is NOT pushed. Simulating hydraulic failure in the B2 model by using the HYD TEST button will result in control loads being felt immediately in the yaw circuit, with the normal delay in the roll and pitch circuits. There is a note in the emergency section of the flight manual that states: "Do not press the HYD TEST button; this will cause immediate depressurisation of the accumulator and the resulting control loads may be heavy." In this occurrence, when the instructor simulated hydraulic failure by means of the HYD TEST button, the student would have immediately felt control loads though the yaw pedals but normal forces on the cyclic and collective until the accumulator pressures discharged. By the time the latter occurred, he should have had the speed within the recommended range and operated the Hydraulic Cut-Off switch on the collective lever. However, the collective switch was not reset to ON in this case, and the instructor did not reset the HYD TEST button until after control difficulty was experienced. This action did not immediately restore the hydraulic system to its normal operation as the HYD OFF switch was still in the OFF position and the T/R accumulator remained discharged."
This is one area where it can come unglued. Could be entirely unrelated as to what happened in this incident but it is worth putting forward to address the conjecture here!
southcoast sez "there are parts of the envelope where the forces are extremely high and unpredictable (ie beyond the rated strength of control runs in the yaw axis". The contol runs in a 350 don't look much different downstream of the servo, in fact have a look at the size of the bolt at the end of the bellcrank attached to the pitch change mechanism??
Go try an A109E it will lift you out of the seat nearly.
"Uncontrollable yaw and pitch oscillations ending up on its side after attempting to come to a hover in a degraded hydraulic mode" which would be outside the procedures in the RFM no?? Caveat Emptor.
Another area for potential problems is under OR overpressure in the accumulators which has been discussed here before. Underpressure is obvious and overpressure leaves the accumulator with less oil in it.