Bell 212 accident near Dardanelle, California, August 25, 2018

The NTSB's narrative final · quoted from the NTSB record

The pilot was conducting water bucket operations when he heard a one-per-revolution whispering sound emanating from the helicopter. He elected to return to the helibase and accepted the Air Attack controller’s offer to escort him back. While en route to the helibase, at an airspeed about 60 knots, the whispering sound evolved into a vibration that progressively worsened. The pilot then elected to land as soon as possible at a nearby location. Upon arrival to the precautionary landing site, the vibration momentarily stopped and was followed by a loud bang, an aggressive vertical vibration, and a loss of control. The helicopter impacted steep mountainous terrain and rolled once onto its left side. The occupants of the escort helicopter observed the accident helicopter nose over about 20° to 30°, followed by a 360° flat turn before it impacted the terrain. Examination of the wreckage revealed that multiple components of the main rotor control system had failed along with a span-wise failure of one of the main rotor blades. The drive link associated with the failed rotor blade had failed in overload at its lower lug and swashplate trunnion bearing. The outer race of the trunnion bearing exhibited brinelling consistent with axial movement of the inner section of the bearing. The inner section of the trunnion bearing separated from the rotating swashplate along with the lower lug of the drive link. Gouges and impact marks found on the arms of the non-rotating swashplate were likely made from the lower lug of the drive link as the inner section of the trunnion bearing began to pull out of the outer race. Contact of the lower lug with one of the arms of the non-rotating swashplate likely resulted in the overload failure of the drive link. These liberated components were not recovered. The in-flight failure of the swashplate drive link resulted in the loss of control of the associated main rotor blade. The uncontrolled blade, free to move out of track with the other controlled blade, resulted in at least a partial loss of control of the main rotor system. The associated vibrations from the out-of-track rotors likely resulted in the failure of the swashplate support and the total loss of control of the helicopter.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.