Summary
This document proposes adopting a new airworthiness directive (AD) for Sikorsky Aircraft Corporation (Sikorsky) Model S-76A, S-76B, and S-76C helicopters. The AD would require 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 would also be required. The action would also require a recurring track-and-balance of the tail rotor. Finally, the proposed AD would require installing a vertical stabilizer modification kit, which would be terminating action for the requirements of the AD. This proposal is prompted by 26 reports of fatigue cracks in the aft spar assembly web and outer caps. The actions specified by the proposed 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.