AD FAA-2013-0636

We are adopting a new airworthiness directive (AD) for Sikorsky Model S-70, S-70A, and S-70C helicopters. This AD establishes a new life limit based on a prorated formula for certain identified components (parts) installed on Model S-70, S-70A, and S-70C helicopters after being previously installed on certain military model helicopters. This AD was prompted by the discovery that certain parts have been interchanged between military helicopter models with different life limits and the possibility that these same parts can be interchanged with civilian models with different life limits. The actions are intended to establish a pro-rated in service life limit for each identified part to prevent fatigue failure of a part and subsequent loss of control of the helicopter.