Bolkow: BO-105-LSA3; Sheared Tail-Rotor Coupling Bolts; ATA 6520 “During the course of an accident investigation,” writes an FAA inspector, “that resulted in one fatality, it was noted the bolts connecting the tail rotor drive shaft to the input ‘Thomas Coupling’ (on the intermediate gear box) were sheared. The shearing took place just below the grip length area of the bolt. The sheared bolts were still in the flange of the gearbox Thomas coupling. The threaded portions of the bolts were recovered (with the nuts installed) from the wreckage, along with other hardware. It was noted the bolt holes in the flange on the tail rotor drive shaft were elongated. This would seem to indicate an under-torque condition which—if gone unnoticed—could likely lead to shearing of (these) bolts. This would result in loss of tail rotor control and an accident. I suggest a one time inspection of these bolts for proper torque. If any bolts are found to be under-torqued, an inspection of the bolts for signs of metal fatigue would be in order.” (Bolt P/N: LN9082-8X18. Intermediate gearbox P/N: 4639002001).
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