LAX07GA217 · Eurocopter France AS350 B3

The certificated flight instructor (CFI), who was seated in the left seat, reported that he was demonstrating practice hydraulic-off emergency procedures, as the second pilot, who was seated in the right seat, had an upcoming check ride where he would be required to conduct such maneuvers. While on the downwind leg of the traffic pattern, the CFI configured the helicopter to an airspeed of 60 knots (kts) with the hydraulics turned off. As he maneuvered the helicopter onto final approach, the cyclic became stiff, with increasing force required to manipulate it. The control forces on the cyclic were becoming increasing harder to overcome and difficult to move, while the collective remained in the neutral position. The CFI was trying to hold the cyclic in the forward right position and was using a large amount of physical force to do so. As the helicopter slowed it began to drift to the left of the intended approach site and the CFI attempted to move the cyclic to correct the drift, but could move the control due to the stiffness. He attempted to accelerate and instructed the second pilot to restore the hydraulics, knowing that only the collective for the right-seated pilot had the hydraulics switch. The second pilot never restored the hydraulics. Seconds later the helicopter climbed to about 20 feet above ground level (agl) and the CFI had no control. The helicopter rolled to the left and impacted terrain. A review of the flight manuals revealed that the pilots followed the proper procedures to perform a practice hydraulic-off emergency procedure and maintained the correct airspeeds. The second pilot stated that he never turned the hydraulics back on because he recalled a warning in training that doing so at such a low altitude could result in the pilot unintentionally over-controlling the cyclic and the helicopter crashing. Anecdotally the Safety Board is aware of other pilots that share the same belief; however, the Rotorcraft Flight Manual does not specifically address the issue. An examination of the wreckage disclosed that the left lateral hydraulic servo was rigged out of limits (0.106 inches), though the helicopter's manufacturer stated that the anomaly would not have a noticeable affect in the capabilities or handling of the helicopter. The servo accumulators were examined and tested. Upon disassembly of the longitudinal servo, investigators observed that the liner exhibited a bulging area about 1 inch from the end, similar to mushroomed deformation. The piston was removed from the liner revealing that about 1/2 of the Teflon white piston pad was displaced from the piston seal groove. The anomaly could not be definitively attributed to a preimpact condition. Additionally, in spite of extensive analysis by the manufacturer, the significance of the anomaly is not yet fully understood at this time.

Verbatim NTSB analysis and probable cause from the NTSB dataset