Investigation Summary
The accident was traced to the failure of main rotor blade serial number 5434. The blade fractured 102 inches outboard from the center of rotation. Investigators found that the fatigue crack initiated at the lower external back-wall radius of the spar, specifically at a deep scratch nearly perpendicular to the longitudinal axis. This scratch was a remnant of the original manufacturing process where 60-80 grit abrasive paper was used to finish the spar surface. Although the blade had been refinished with finer 240-grit paper during a 1958 repair following a 'quickstart' incident, the deeper original scratches beneath the bonded pockets remained. The Civil Aeronautics Board (CAB) found that Sikorsky's quality control relied on visual inspections rather than comparison blocks, leading to surface finishes that were significantly rougher (50-60 RMS) than the 40 RMS specified in engineering drawings. At the time of failure, the fatigue zone covered approximately 57 percent of the spar's cross-sectional area.
Final Conclusions
The Civil Aeronautics Board determined the probable cause was the structural disintegration of the helicopter in flight, caused by a fatigue fracture of a main rotor blade spar. The fatigue crack originated from a surface stress riser created by improper finishing during manufacture. The investigation also noted that a 1958 'quickstart' incident, which caused severe torque and mechanical lockage, may have contributed to the stress on the rotor system, although the blade had been inspected and repaired afterward. The blade failed at 1,786 service hours, well below its FAA-approved life of 2,450 hours.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Main Rotor Blade Spar
The primary structural 'D' shaped member of the rotor blade. A fatigue crack in this component led to the separation of the blade tip.
Tail Cone
The rear section of the fuselage supporting the tail rotor; it separated due to extreme vibrations and loads following the main rotor failure.
Interesting Facts
- 01The fatigue crack originated from 60-80 grit sanding scratches that were deeper than allowed by design specifications.
- 02The failed blade had previously survived a 'quickstart' incident in 1958 that caused a mechanical lockage and broke the rotor scissors.
- 03The outer 19 feet of the failed rotor blade was found 1,900 feet behind the tail cone's location.
- 04The CAB found the spar surface finish was 50-60 RMS, exceeding the 40 RMS limit required by engineering drawings.
- 05The helicopter was flying at an assigned altitude of 1,500 feet above the ground in clear weather.
Safety Actions & Advisories
Manufacturing Process Refinement
Sikorsky transitioned from 60-80 grit paper to 240-grit paper for finishing spar surfaces to ensure a smoother finish and reduce stress risers.