Bell 206B N6181A accident near Rockport, Washington

Probable cause

The pilot's failure to maintain helicopter control while on the ground, which resulted in mast bumping.

NTSB analysis

The commercial pilot reported that, after landing the helicopter on a flat area in remote, mountainous terrain, she reduced the throttle to the idle position and the left front seat passenger exited the helicopter. The pilot reported that, while the passenger was securing the door, the helicopter started to oscillate laterally. The pilot adjusted the flight controls and increased the throttle, and the lateral oscillations stopped. The passenger stated that the helicopter "bounced and lurched" while he was securing the door but that the tail rotor did not come in contact with any objects or terrain. The pilot subsequently shut down the engine and conducted a postflight inspection, which revealed substantial damage to the transmission system. The pilot stated there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

Damage observed on the pylon mount assembly and main rotor mast was consistent with a mast bumping event, during which the static stops contact the mast due to excessive rotor blade flapping. During ground operations with the rotor turning, the main rotor may be affected by wind gusts and flap to its limits, resulting in a light static stop-to-mast contact. In such an event, mast bumping may manifest itself as a light shudder felt throughout the helicopter. The more extreme the flapping, the more severe the shudder. Mast bumping can also occur during ground operation if the cyclic is incorrectly positioned or is moved sufficiently to tilt the rotor disc to an extreme position.

Text from the NTSB. Open the NTSB docket for WPR16LA188 for the full record.