What happened
On July 7, 2021, a Bell 212HP helicopter, registration N911KW, was performing an external load flight near Weed, California, as part of a US Forest Service water drop contract. While approaching a lake to retrieve water using a 100-foot long line and bucket, the pilot experienced a sudden, significant vibration described as a once-per-minute rotation.
At the time of the vibration, the aircraft was approximately 400 feet above ground level. The pilot noted that the radar altimeter was set to 200 feet. Although the pilot observed the helicopter descending and attempted to abort the retrieval by applying forward cyclic and raising the collective, the descent continued.
Unable to stop the descent and wishing to avoid overspeeding the engines or transmission, the pilot performed a forced landing on the water. The pilot did not jettison the long line and bucket, intending to fly away from the site. After the skids touched the water, a final attempt to take off failed. As the pilot reduced the throttles to idle to ditch the aircraft, the helicopter rolled to the right, causing the main rotor blades to strike the water. The aircraft subsequently sank. The pilot was not injured.
The investigation
Post-accident examinations of the engine powerpack and the 240-gallon water bucket revealed no mechanical malfunctions that would have prevented normal operations. However, investigators found that the rod end bearings for the pylon dampers were outside of the allowable tolerance. Specifically, the right aft damper bearing play was 0.006 inch and the left aft damper bearing play was 0.016 inch, exceeding the maximum allowable radial play of 0.004 inch. Excessive radial clearance in these bearings can reduce the effectiveness of the dampers in counteracting pylon vibration.
ADS-B data indicated that at the final recorded data point, the bucket was approximately 35 feet above the water and the helicopter was descending at approximately 768 feet per minute. Calculations showed that the bucket was configured to hold approximately 1,966 pounds of water, which would have increased the helicopter's gross weight to approximately 10,116 pounds.