A new airworthiness directive for the MD Helicopters MD500N, MD600N and MD900 requires determining the cure date for each Notar fan blade tension-torsion strap (T-T strap). The purpose is to establish a calendar-time retirement life, mark each T-T strap with the expiration date, create a component record card and revise the airworthiness limitations section of the maintenance manual. It was prompted by a report that over time moisture may reduce the strength of a T-T strap, potentially causing loss of directional control and subsequent loss of control of the helicopter. The AD is effective March 15.