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

This amendment adopts a new airworthiness directive (AD) for Sikorsky Aircraft Corporation (Sikorsky) Model S-76A, B, and C helicopters that requires inspecting each installed Woodward HRT (formerly HR Textron) main rotor servo actuator (servo actuator) for a high rate of leakage and replacing each affected servo actuator with a servo actuator containing a newly re-designed servo actuator piston. This amendment is prompted by a National Transportation Safety Board (NTSB) Safety Recommendation issued in response to an accident involving a Model S-76C helicopter. In the NTSB Safety Recommendation, the performance of a servo actuator piston upon reaching 3,000 hours time-in-service (TIS) was questioned as a result of piston head seal leakage and piston head plasma spray flaking. The actions specified by this AD are intended to prevent degraded servo actuator performance as a result of piston head seal leaking and plasma spray flaking, which could result in subsequent loss of control of the helicopter.