PHI Bell 407 accident

The commercial pilot was conducting a nonscheduled, cross-country passenger flight. He reported that, while en route, he detected an in-flight vibration and made a precautionary landing on an oil platform. As he was shutting down the engine, the vibration increased, and he initiated an emergency shutdown using the rotor brake.

Postaccident examination revealed that a tip block and weights had separated from one of the tail rotor blades. Cracks were noted on the tail rotor gear box, which was detached from the tailboom support casting. The casting was entirely fractured and exhibited cracking consistent with overstress separations. The left-side attachment studs were fractured and exhibited reversed bending fatigue fractures. The casting also had cracks at the two forward stud hole locations, and stud hole elongation was noted at the two aft stud hole locations.

The separation of the tip block from the tail rotor blade resulted from an incomplete bond area due to a waffle pattern in the adhesive and contamination of the bond line by repair debris. About 50% of the adhesive surface had smooth and glossy surfaces consistent with voids and lack of contact between the adhesive and the tip block, which was likely due to insufficient vacuum pressure being applied while curing the adhesive.

The blade tip exhibited worn separation surfaces and the presence of dark material, consistent with engine exhaust, indicating that a crack had formed along the flat side bond line and then progressed until the degraded bond strength was exceeded by operational loads, and the tip block separated. This resulted in a violent rotor imbalance and induced sufficient loads to crack the gearbox attachment studs and produce rapid fatigue cracking in the left-side attachment studs and to crack and nearly fracture the gearbox support assembly.

The tip block had been repaired about 65 hours before the flight. According to the helicopter manufacturer, after the repairs were made, the blade passed the postbond pull test of 1,320 lbs, which was equal to the load on the tip block at maximum tail rotor rpm, and it was returned to service. Given the postaccident condition of the adhesive, the postrepair test procedures were not adequate to detect the insufficient adhesive bonding, which resulted in the separation of the tip block from the tail rotor blade. After the accident, the helicopter manufacturer revised its approved repair and inspection procedures. The changes included, in part, a revision to the cure cycle process to use only positive pressure (not vacuum pressure) during the cure cycle that cures the adhesive that bonds the block on the blade tip and an expansion of its postrepair inspection procedures.

Verbatim NTSB analysis and probable cause from the NTSB dataset