Airworthiness Directives; MD Helicopters, Inc. Model 369D, 369E, 369F, and 369FF Helicopters

This amendment adopts a new airworthiness directive (AD) for specified MD Helicopters, Inc. (MDHI) helicopters with a tailboom modified according to Aerometals Supplemental Type Certificate (STC) SH5055NM or SH4801NM. This AD requires an inspection to identify the part number (P/N) of the bolts that attach the tail rotor gearbox to the tailboom and replacing any bolt of inadequate grip length with an airworthy bolt. This AD also requires determining the number of bolt threads protruding from each nutplate and adding an additional washer if more than four threads protrude. This amendment is prompted by the discovery that bolts of inadequate grip length were specified to attach the tail rotor gearbox to the tailboom. The actions specified by this AD are intended to prevent loss of a tail rotor gearbox due to attaching bolts of inadequate grip length and subsequent loss of control of the helicopter.