Incident Description
During the fourth pass of an aerial application flight, the pilot reported sensing a high-frequency, medium-strength vibration in the tail rotor pedals. Shortly thereafter, the helicopter lost directional and yaw control. While executing an emergency descent, the helicopter collided with oak trees estimated to be between 30 and 80 feet tall.
Postaccident Examination
A postaccident examination revealed that the end caps on both main rotor blades had separated from the blades. Additionally, the outboard portions of both tail rotor blades had separated. Metallurgical examination was conducted on the main rotor blade end caps and the tail rotor blades.
Main Rotor Blade End Caps
Both main rotor blade end caps exhibited damage at the leading edge. One end cap showed a visible impact mark on the leading edge and was compressed chordwise toward the trailing edge. The attachment screw holes on this end cap were pulled and torn, and wooden chips were found entrapped in the folds of the deformed metal. In contrast, all attachment screw holes on the other main rotor blade end cap were undamaged.
Tail Rotor Blades
Examination of the tail rotor blades using a low-power binocular microscope did not reveal any indication of paint transfer in the damaged areas. Similarly, no paint transfer was observed on the main rotor blade end caps.
Probable Cause
No probable cause was determined from the available information.