Airworthiness Directives; Bell Helicopter Textron, Inc. Model 204B Helicopters

This amendment adopts a new airworthiness directive (AD) for Bell Helicopter Textron, Inc. (BHTI) Model 204B helicopters that requires replacing any main rotor mast assembly (mast), part number (P/ N) 204-011-450-001, within 25 hours time-in-service (TIS). This amendment is prompted by the crash of a restricted category Model UH-1B helicopter due to failure of a mast, P/N 204-011-450-001. The same mast P/N is used on the Model 204B helicopters. The actions specified by this AD are intended to prevent failure of the mast and subsequent loss of control of the helicopter.