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

This amendment supersedes an existing airworthiness directive (AD) for MD Helicopters, Inc. (MDHI) Model 600N helicopters. The AD currently requires modifying the fuselage aft section within the next 24 months to strengthen the tailboom attachment fittings and upper longerons. This amendment retains the current required modification and adds the following requirements: Repetitively inspecting the tailboom attachment fittings; installing inspection holes and attachment bolt washers; modifying both access covers; and replacing broken attachment bolts. These additional requirements were inadvertently omitted from the current AD. This amendment is prompted by the need to correct our mistake and continue the repetitive inspections of the tailboom attachment fittings until the fuselage aft section is modified. This series of ADs is prompted by an accident due to failure of the tailboom attach points resulting in loss of control of the 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.