CEN13FA331 · Eurocopter AS 350 B3

The instructor pilot reported that he and the pilot receiving instruction departed on a local night instructional flight to perform a simulated hydraulics failure maneuver in the helicopter. On shallow approach to the runway with hydraulic power off, the instructor told the pilot to fly the helicopter to a designated point on the airfield, at which point, the pilot was to perform a go-around. As the helicopter was "getting a little slow," the instructor again mentioned to the pilot that he needed to go around. As the pilot increased power, the helicopter's nose started rising, and the instructor told the pilot to get the nose down to increase the airspeed. The pilot remarked that it was hard to apply right pedal, so the instructor also manipulated the controls to assist him. At this point, the helicopter had started to turn left, and the instructor applied forward cyclic. The helicopter pitched nose down, and the instructor thought he might have overcompensated. The instructor felt the pilot applying aft cyclic, and the helicopter again pitched nose down, so he made an additional control input to level the helicopter. However, the helicopter started spinning left, developed extreme pitch attitude changes, and was not climbing. The instructor again attempted to level the helicopter, reduced the power to slow the rotation, and reapplied power to cushion the landing. The helicopter subsequently impacted terrain and rolled onto its right side.

Flight control continuity was established, and all observed separations were consistent with overload. No evidence of any preexisting failures of the helicopter's airframe or engine were found that would have precluded operational control of the helicopter. The recorded vehicle monitor data were consistent with impact and rollover occurrences, and no preimpact failures or overlimit events were recorded.

The flight manual notes that the hydraulic failure safety speed is from 40 to 60 knots and contains a caution to not attempt hover flight or any low-speed maneuver without hydraulic pressure assistance. The caution noted that the intensity and direction of the control feedback forces will change rapidly, which will result in excessive pilot workload, poor aircraft control, and possible loss of control. It is likely that the pilot receiving instruction failed to maintain adequate airspeed during the simulated hydraulics failure maneuver, which resulted in his loss of control of the helicopter and its subsequent impact with terrain.

Verbatim NTSB analysis and probable cause from the NTSB dataset