Airworthiness Directives; Sikorsky Aircraft Corporation Model S-76A, B, and C Helicopters

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This amendment adopts a new airworthiness directive (AD) for the Sikorsky Aircraft Corporation (Sikorsky) Model S-76A, B, and C helicopters. This action requires certain inspections of the main rotor lower bifilar arm assembly in the attachment area around the lower bifilar lugs for a crack. If a crack is found on any bifilar lug, this AD requires replacing the bifilar arm assembly with an airworthy bifilar arm assembly. If no crack is found, this AD requires a one-time test for the required torque on the lug nuts and, if necessary, applying the required torque and conducting the torque stabilization tests. This amendment is prompted by four reports of cracked bifilars. The actions specified in this AD are intended to prevent failure of a bifilar lug, damage to the main rotor control system, and subsequent loss of control of the helicopter.