Summary
This document proposes superseding an existing airworthiness directive (AD) for MD Helicopters Inc. Model MD-900 helicopters. That AD currently requires inspecting the main rotor upper hub assembly drive plate attachment flange (flange), determining the torque of each flange nut (nut), and if a crack is found, before further flight, replacing the hub assembly. In addition to the current requirements, this action would require visually inspecting the outer surface of the flange at specified intervals and removing the drive plate and visually inspecting the flange for a crack at specified intervals and replacing any unairworthy hub assembly. This proposal is prompted by reports that cracks starting at the drive plate attachment holes were found in the main rotor hub. The actions specified by the proposed AD are intended to detect a crack in the flange and to prevent failure of the hub assembly, loss of drive to the main rotor, and subsequent loss of control of the helicopter.