Airworthiness Directives; Sikorsky Aircraft Corporation (Sikorsky) Helicopters

We are adopting a new airworthiness directive (AD) for certain Sikorsky Model S-76A, B, C, and D helicopters. This AD requires inspecting the tail rotor drive shaft (TDS) flange-to-shaft attachment hardware for correct assembly and correct torque of the fasteners. If there is a discrepancy, this AD requires, before further flight, applying an index mark to the flange and TDS, inspecting the flange and shaft for a crack, fracture, wear, and certain measurements, and replacing any part that does not meet the approved criteria before further flight. This AD is prompted by a partial loss of tail rotor drive resulting in a forced landing. The actions specified by this AD are intended to prevent failure at the flange-to-shaft attachment, loss of a tail rotor drive, and subsequent loss of control of the helicopter.