Incident Overview
A Hiller UH-12E helicopter was engaged in aerial application operations when it experienced an in-flight anomaly and subsequently collided with the ground during a forced landing. According to the pilot, while flying at an altitude of 15 to 20 feet AGL, a loud bang was heard, followed by helicopter shaking and yawing. The pilot recognized a loss of altitude and attempted to reduce the descent rate by raising the collective. The helicopter impacted the ground and caught fire. The pilot escaped the aircraft through a hole in the bubble.
Examination Findings
Post-accident examination concentrated on the main rotor hub assembly. Fracture of the hub was determined to have propagated through the inboard pin hole designated for the torsion/tension plate set. Six distinct fracture faces were identified in the examination, all associated with the pin hole. Two of these fracture faces exhibited discoloration with a distinctive banding pattern and smoothly curved crack arrest marks, indicative of fatigue crack propagation. The remaining two fracture faces showed no evidence of fatigue cracking; however, some corrosion was observed on the exposed portion of the pin.
Conclusion
The investigation documented the presence of fatigue cracking in the main rotor hub at the torsion/tension plate pin hole, along with corrosion on the pin. No causal determination was included in the report.