What happened
During a sling operation in a grassy pasture area, the pilot of a Hughes 3CO9D helicopter was hovering approximately 100 feet above the ground to release seismic equipment. While positioning a bag on a 50-foot lanyard, the aircraft suddenly experienced violent cyclic vibrations, causing the nose to pitch downward. Despite the pilot's attempt to level the aircraft with full aft cyclic, the helicopter descended heavily, striking the ground in a nose-down, left-side-low attitude. The impact caused the tail boom to separate in flight and resulted in the main rotor blades curling upon contact with the earth. The fuel tank ruptured, leaking fuel into the cockpit. The pilot sustained one injury involving a collapsed lung, chemical burns, and various abrasions, and required medical transport to a local hospital.
The investigation
Investigators examined the wreckage and the recovered main rotor blade, which had separated from the aircraft and was found 1,200 feet from the crash site. The examination of the green blade revealed chord-wise cracking just outboard of the lower attachment fitting, with the spar and skin showing signs of fatigue-related failure. Further inspection of the remaining four blades in the assembly revealed that all blades exhibited similar cracking or micro-cracking in the same location.
Technical analysis identified that a batch of non-conforming doublers used during production had been used despite an initial rejection. These components featured a crease in their curvature, which created inconsistent adhesive thickness and potential disbonding during assembly. The investigation also confirmed that the aircraft's maintenance, weight and balance, and recent inspections were all within regulatory requirements, and the weather conditions were clear.
Findings
- The primary cause of the loss of control was the fatigue failure and separation of a main rotor blade.
- The failure was driven by a combination of reduced load transfer due to adhesive disbonding and significant residual stresses introduced by non-conforming doublers during the assembly process.
- Severe vibrations resulting from the blade separation caused the tail boom to detach from the fuselage.
- The pilot's injuries were a result of the heavy lateral and vertical forces experienced during the ground impact.