Summary
We propose to supersede an existing airworthiness directive (AD) for MD Helicopters, Inc. (MDHI) Model MD900 helicopters. The existing AD requires a visual inspection, and if necessary, an eddy current inspection of the main rotor lower hub assembly (lower hub) for a crack. If a crack exists, the AD requires replacing the lower hub with an airworthy lower hub before further flight. Because that AD was immediately effective, we did not include in its requirements certain long-term actions that did not necessitate adoption prior to public comment. This proposed AD would require those long-term actions. This proposed AD would require the same inspections as the existing AD but would also require recurring inspections and replacing the lower hub with an airworthy lower hub. We are proposing this AD to detect a crack in the lower hub and prevent failure of the lower hub and subsequent loss of control of the helicopter.