Airworthiness Directives; MD Helicopters, Inc. Model 600N Helicopters

This amendment supersedes an existing airworthiness directive (AD) for the MD Helicopters, Inc. (MDHI) Model 600N helicopters, that currently requires inspecting both upper tailboom attachment fittings, nut plates and both angles for a crack or thread damage, and repairing or replacing any cracked or damaged part. That AD also requires replacing the upper right tailboom attachment bolt with a new attachment bolt, and if the upper right attachment bolt is broken, replacing the three remaining attachment bolts with airworthy bolts. Adding a washer to each bolt and modifying both access covers is also required. Thereafter, inspecting the upper tailboom attachments and repairing or replacing any cracked part is required by that AD. That AD was prompted by the discovery of a cracked attachment bolt on a helicopter. This AD requires those same actions, plus installing additional inspection holes in the aft fuselage skin panels and inspecting the upper and lower tailboom attachment fittings, the upper longerons, and the angles and nut plates for cracks. It also requires, within a specified time, replacing the upper right tailboom attachment fitting, painting the inspection area, and replacing existing nut plates. Additionally, it requires inspecting the attachment bolts for any damage or wear. This amendment is prompted by an accident involving a Model 600N helicopter. The actions specified by this AD are intended to prevent failure of the tailboom attachment fittings, separation of the tailboom from the helicopter, and subsequent loss of control of the helicopter.