Summary
We propose to adopt a new airworthiness directive (AD) for MDHI Model 500N, 600N, and MD900 helicopters to require determining the cure date for each NOTAR fan blade tension-torsion strap (T-T strap), establishing a calendar-time retirement life for certain T-T straps, reducing the retirement life of certain T-T straps, marking each T-T strap with the expiration date, creating a component record card for each T-T strap, and revising the airworthiness limitations section of the maintenance manual to reflect the changes to the retirement life. This proposal is prompted by a report from the T-T strap manufacturer that, over a period of time, moisture may reduce the strength of a T-T strap. The proposed actions are intended to prevent failure of a T-T strap, loss of directional control and subsequent loss of control of the helicopter.