Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to Bell Helicopter Textron, Inc. (BHTI) Model 214ST helicopters, that currently establishes a mandatory retirement life of 50,000 high-power events for the main rotor mast (mast). This action would require changing the retirement life for the mast from high-power events to a maximum accumulated Retirement Index Number (RIN) of 140,000 and apply this RIN to an additional part numbered mast. This proposal is prompted by fatigue analyses and tests that show certain masts fail sooner than originally anticipated because of an unanticipated high number of takeoffs and external load lifts in addition to the deterioration in strength that occurs under other operating conditions. The actions specified by the proposed AD are intended to prevent fatigue failure of the mast, which could result in failure of the main rotor system and subsequent loss of control of the helicopter.