Primary finding
Probable cause
Foreign object damage sustained to the main rotor blades due to aerial tree trimming.
Investigator assessment
Analysis narrative
While conducting an aerial tree-trimming flight with an external load aerial saw, the pilot heard a loud “thump” and felt a collision. A heavy vibration was felt, and the pilot landed in a nearby cornfield without further incident. Upon landing substantial damage was found to the main rotor blades with embedded pieces of tree bark and vegetation found in the blades. The pilot reported that there were no mechanical malfunctions that contributed to the accident. The aerial saw was suspended about 120 ft below the helicopter. The operator noted that this distance is normally adequate to prevent foreign object damage (FOD) to the main rotor blades.