Summary
This document proposes to revise an existing airworthiness directive (AD) for the specified MD Helicopters, Inc. (MDHI) helicopters. That AD currently requires identifying 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. That AD also requires adding an additional washer if more than four threads protrude from the nutplate. This action would contain the same requirements as the existing AD but would reduce the applicability to only certain tailboom serial numbers and parts modified in accordance with either Supplemental Type Certificate (STC) SH5055NM or SH4801NM. The action would also correct a typographical error and clarify that a slippage mark needs to be reapplied to each bolt regardless of the outcome of the required torque test. This proposal is prompted by the need to reduce the applicability and to correct and clarify other portions of the existing AD. The actions specified by the proposed AD are intended to prevent loss of a tail rotor gearbox due to bolts of inadequate grip length and subsequent loss of control of the helicopter.