Airworthiness Directives; Enstrom Helicopter Corporation Model F-28, F-28A, F-28C, F-28F, 280, 280C, 280F and 280FX Helicopters

This amendment supersedes an existing airworthiness directive (AD) for Enstrom Helicopter Corporation (EHC) Model F-28, F-28A, and 280 helicopters. That AD currently requires inspecting the main rotor shaft (shaft) for a crack or other evidence of damage until appropriately modifying or replacing the shaft with an airworthy shaft at specified time intervals. This amendment adds EHC Model F-28C, F- 28F, 280C, 280F, and 280FX helicopters and establishes life limits after which all unmodified shafts must be retired. This amendment requires determining the radius of the shaft fillet, certain visual and dye-penetrant inspections before further flight, and replacing certain main rotor transmissions. This amendment is prompted by the failure of a shaft on an EHC Model F-28A helicopter due to a fatigue crack. The actions specified by this AD are intended to prevent shaft failure and subsequent loss of control of the helicopter.