Eurocopter AS 350 B3 N808LF incident near Temple, Texas

Probable cause

The pilot’s failure to reposition the yaw servo hydraulic switch to the “on” position during the pretakeoff hydraulic system check, which resulted in a complete lack of hydraulic boost to the tail rotor system and increased the load required to move the control pedals and led to the pilot’s subsequent inability to manipulate the control pedals and his loss of yaw control.

NTSB analysis

After an uneventful emergency medical services flight, the commercial pilot landed the helicopter at an airport to refuel for the return flight to the helicopter's base. The pilot reported that his preflight checks were normal and that, immediately after takeoff, the helicopter started a counter-clockwise yaw. He added that the antitorque pedals were locked in the neutral position and felt jammed. He attempted to correct the rotation but was unable to do so. After the helicopter rotated several times, he descended it from about 15 to 4 to 5 ft and then executed a hovering autorotation. The helicopter hit the ground on its right front skid and then slid upright to a stop.

A postincident examination of the helicopter revealed no anomalies with its tail rotor and hydraulic systems. However, the yaw servo hydraulic switch on the collective was found in the "off" position even though the pretakeoff dual hydraulic system check requires that the yaw servo hydraulic switch be moved to the "off" position and then moved back to the "on" position before takeoff. After this incident, the helicopter manufacturer issued a safety information notice, which stated that, if the pilot fails to move the yaw servo hydraulic switch back to the "on" position before takeoff, there will be a complete lack of hydraulic boost to the tail rotor system because, before this part of the check, the yaw load compensator would have been discharged to verify proper operation of the hydraulic accumulator test switch and valve. This situation could be perceived by the pilot as a tail rotor control failure due to the increased load required to move the control pedals. If not quickly identified and corrected, this situation could lead to a loss of helicopter control, as occurred during the accident flight.

Text from the NTSB. Open the NTSB docket for CEN14IA329 for the full record.