Summary
This document revises an earlier proposed airworthiness directive (AD) for Sikorsky Aircraft Corporation (Sikorsky) Model S- 76A, B, and C helicopters. That AD proposed to require inspecting each installed HR Textron main rotor servo actuator (servo actuator) for a high rate of leakage and for contaminated hydraulic fluid and reducing the time-in-service (TIS) interval for overhauling each servo actuator. That proposal was prompted by a National Transportation Safety Board (NTSB) Safety Recommendation issued in response to an accident involving a Model S-76 helicopter. In the NTSB Recommendation, the performance of an HR Textron servo actuator was questioned as a result of piston head seal leakage and piston head plasma spray flaking. Since the issuance of the initial proposal, and based on further information obtained from the accident investigation, the comments to the proposal, and other test and service history data since we issued the initial proposal, we continue to believe that servo actuator pistons may experience piston head seal leakage and plasma spray flaking, but have determined that the full scope of the initial proposal is unnecessary. We believe that the piston head seal leakage and plasma spray flaking can be addressed by leakage rate inspections and replacement of the current servo actuator pistons with an improved design not as susceptible to plasma spray flaking. Therefore, we are revising the proposed rule by removing the requirement to inspect the hydraulic fluid for contamination; removing the requirement to reduce the interval for overhauling an affected servo actuator from 3,000 to 2,000 hours TIS; revising the initial inspection time; and removing the 600 hours TIS repetitive hydraulic fluid leak inspection. We are proposing to add a 2,250 hours TIS hydraulic fluid leakage inspection and to add a requirement to either install a new design servo actuator or replace the servo actuator pistons when there is excessive leakage or upon reaching a certain time interval. These actions 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.