Summary
This document proposes the adoption of a new airworthiness directive (AD) for Enstrom Helicopter Corporation (EHC) Model TH-28 and 480 helicopters. The AD would require establishing a life limit for certain upper and lower main rotor hub plates of 5000 hours time-in-service (TIS), creating a component history card or equivalent record, and replacing each main rotor hub plate (hub plate) having 5000 or more hours TIS with an airworthy hub plate. This proposal is prompted by a recent reliability-based stress analysis that indicates a 5000-hour TIS life limit should be imposed on certain hub plates. The actions specified by the proposed AD are intended to prevent failure of a hub plate, loss of control of the main rotor, and subsequent loss of control of the helicopter.