Airworthiness Directives; Bell Helicopter Textron, Inc. Model 412, 412EP and 412CF Helicopters

This amendment adopts a new airworthiness directive (AD) applicable to Bell Helicopter Textron, Inc. (BHTI) Model 412, 412EP, and 412CF helicopters. This action requires inspecting and measuring the thickness of certain main rotor yoke assemblies. This AD also requires adding 500 hours time-in-service (TIS) to the total time for main rotor yoke assemblies that measure below 0.478-inch thickness and noting the measurement and added TIS on the component history card or equivalent record. This amendment is prompted by a report of an emergency landing due to severe main rotor vibration on a BHTI Model 412 helicopter. Subsequent fatigue analysis indicates that the main rotor yoke assembly (yoke) does not have the anticipated service life when manufactured below 0.478-inch thickness. The actions specified in this AD are intended to prevent a fatigue failure of the yoke, loss of a main rotor blade, and subsequent loss of control of the helicopter.