Airworthiness Directives; Various Sikorsky-Manufactured Transport and Restricted Category Helicopters

We are superseding Airworthiness Directive (AD) 98-26-02 for certain Sikorsky Aircraft Corporation (Sikorsky) Model S-61A, D, E, L, N, NM, R, and V helicopters. AD 98-26-02 required determining whether the main rotor shaft (MRS) was used in repetitive external lift (REL) operations, performing a nondestructive inspection (NDI) for cracks, replacing any unairworthy MRS, and establishing retirement lives for each REL MRS. This new AD retains some of the requirements of AD 98-26- 02 but determines a new retirement life for each MRS, expands the applicability to include additional helicopters, and requires removing from service any MRS with oversized dowel pin bores. This AD was prompted by the manufacturer's reevaluation of the retirement life for the MRS based on torque, ground-air-ground (GAG) cycle, and fatigue testing. We are issuing this AD to prevent MRS structural failure, loss of power to the main rotor, and subsequent loss of control of the helicopter.