Bell 206 N607RA accident near Basin City, Washington

Probable cause

The pilot's improper execution of a slowing and turning maneuver, which resulted in a loss of control at an altitude that was too low for recovery.

NTSB analysis

The pilot had completed agricultural spray applications in the helicopter and was flying a "rinse-load" to clean the product hopper tank. The wind was from the southeast about 3-5 knots. The pilot reported that he conducted a brief spray run about 50 mph and then applied left pedal input and slightly lowered the collective control to assist him in looking to his left to view the spray pattern; the helicopter immediately began "shaking" and turning rapidly to the left. The pilot pulled up on the collective control and applied right pedal to stop the left yaw. The helicopter continued to "shudder," and the pilot noticed that the helicopter was at a low airspeed and descending. He attempted to regain airspeed, and when the helicopter was about 5 to 10 ft above the ground, he attempted to level the helicopter before the skids contacted the ground. The helicopter impacted the ground and caught fire immediately; a significant portion of the helicopter was destroyed or damaged by the fire.

Postaccident examination of the remaining wreckage did not reveal any preimpact mechanical malfunctions or failures that would have precluded normal operation. The pilot's event description, combined with the benign weather and lack of any physical evidence of mechanical failure, suggest that the accident was pilot induced. When the pilot simultaneously climbed and slowed the helicopter, he also initiated a left yaw by applying left antitorque pedal. That left yaw was amplified when the pilot lowered the collective. At this point, the helicopter was still decelerating, which resulted in airflow changes across the rotor disc, a loss of translational lift, and the beginning stages of vortex ring state, which likely caused buffeting that the pilot reported as a "shudder." Increasing the collective to stop the left yaw also likely increased the stalled area of the rotor disc, strengthening the vortex ring, decreasing rotor efficiency, and increasing descent rate. Because of the helicopter's low altitude, there was insufficient ground clearance for recovery before ground contact.

Text from the NTSB. Open the NTSB docket for WPR18LA214 for the full record.