Airworthiness Directives; MD Helicopters Inc.

We are adopting a new airworthiness directive (AD) for MD Helicopters Inc. (MDHI) Model 500N and 600N helicopters with certain rotating cone assemblies installed. This AD requires establishing a life limit of 10,000 hours time-in-service (TIS) on these rotating cone assemblies. This AD was prompted by the determination that MDHI created rotating cone assemblies with new dash numbers but incorrectly failed to identify them as life-limited parts. The actions are intended to prevent operation of rotating cone assemblies past their life limits, failure of the rotating cone assemblies, loss of directional control, and subsequent loss of control of the helicopter.