Helicopter crash; Servo Control Transparency

AS350 BA heli-skiing at Bear Creek Lodge gets into servo control transparency when pilot buzzes the lodge and then eases back on the cyclic, which loads up forces on the rotor disk. The aerodynamic forces on the blades now exceed the ability of the hydraulic servos to control them and servo control transparency occurs. We hear the intermittent beeping of the high rotor horn as the aircraft pitches right and into the mountain. No serious injuries. I’ll explain further. SCT happens at high disk loading and high speed. VNE in an ASatr is 155 kts minus 3kt/1000’ so if they are at 6,000’, it’s now at about 135 kts. In addition, adding a ski basket on the side causes more drag, so more pitch is require to reach that same 135 kts and that means you’ll hit the SCT limit even sooner. The hydraulic system has a pressure relief valve that opens and sends fluid back to the reservoir at 40 bars or about 600 PSI. The low pressure light and horn comes on at 30 bars or 450 PSI so that the working range. We found pilots complaining about one AStar where they were getting SCT in cruise flight, so the training department had the engineers test the pressure relief valve. They come from the factory with no life limit and no test interval. We had valves that were opening at 32-35 bars instead of 40, so those systems were running at 3/4 to 7/8 of what the pressure was supposed to be. As a result our company added something to the maintenance manual where these valves were checked on a regular basis to make sure they were running at 40 bars.