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

We are adopting a new airworthiness directive (AD) for Sikorsky Aircraft Corporation (Sikorsky) Model S-76A, S-76B, and S-76C helicopters. This AD requires an initial and recurring inspections of the tail rotor vertical stabilizer aft spar assembly (aft spar assembly) for a crack, loose or working fasteners, and corrosion, and, if any are found, further inspections of the vertical stabilizer forward spar assembly (forward spar assembly). Repairing or replacing any unairworthy part before further flight is also required. This AD also requires a recurring track-and-balance of the tail rotor. Finally, this AD requires installing a vertical stabilizer modification kit, which is terminating action for the requirements of the AD. This AD is prompted by 26 reports of fatigue cracks in the aft spar assembly web and outer caps. The actions specified by this AD are intended to detect and correct an unbalanced or out-of-track tail rotor, which could lead to increased vibrations, a fatigue crack, loss of a portion of the vertical stabilizer and subsequent loss of control of the helicopter.