Summary
We propose to adopt a new airworthiness directive (AD) for Sikorsky Model S-70, S-70A, and S-70C helicopters. This proposed AD would establish 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 proposed AD is 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 proposed 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.