Summary
We propose to supersede an existing airworthiness directive (AD) for Sikorsky Model S-76A, B, and C helicopters. The existing AD currently requires inspecting the main rotor lower bifilar arm assembly (bifilar arm assembly) for a crack, and if there is a crack, replacing the bifilar arm assembly. The AD also requires a one-time test for the correct torque on the lug nuts, and if necessary, conducting torque stabilization tests. Since we issued that AD, Sikorsky has developed a terminating procedure for the inspections required by the existing AD. This proposed AD would retain the requirements of that AD, and would require replacing the main rotor hub (MRH) pilot with a different part- numbered MRH pilot, which would be terminating action for the requirements of the AD. The proposed actions are intended to prevent failure of a bifilar lug, damage to the main rotor control system, and subsequent loss of control of the helicopter.