Accident Description
A helicopter conducting a visual flight rules (VFR) approach to a fueling site experienced a catastrophic failure when components of the main rotor system separated. Witnesses observed the separation, after which the helicopter entered an uncontrollable descent and impacted the ground.
Examination Findings
Post-accident examination revealed that one of the four main rotor blades had separated because the tension-torsion (T-T) strap securing the blade to the main rotor head failed. The failed T-T strap showed that the polyurethane protective coating had debonded from a portion of the strap, allowing corrosive agents—including carbon, sulfur, and chlorine—to infiltrate the strap structure. This corrosion led to fatigue cracking of the wires making up the strap. The fatigue cracks originated at corrosion pits and deposits introduced after the protective coating debonded.
Service History
The T-T strap had accumulated 7,815.3 flight hours and 35,971 flights at the time of failure. It had been in service since the helicopter was manufactured in 1987. Following the accident, a life limit of 120 months or 25,000 flights (whichever comes first) was established for T-T straps.