Summary
This document revises an earlier proposed airworthiness directive (AD) for the specified Eurocopter France (Eurocopter) model helicopters that would have required, for bevel gears with more than 6,600 hours time-in-service (TIS), inspecting the bevel gear for a crack using a borescope within 50 hours TIS, and thereafter at intervals not to exceed 150 hours TIS. That proposal was prompted by a crack that was detected on a bevel gear during a main gearbox teardown inspection. This action revises the proposed rule by requiring the borescope inspection at intervals not to exceed 150 hours TIS or 1,000 torque variation cycles (cycles) for helicopter operations involving frequent torque variations, whichever occurs first. This action is prompted by an analysis of the crack growth rate, which indicates that the growth rate is higher in helicopters with operations involving a torque variation frequency of 4 or more cycles per hour. The actions specified by this proposed AD are intended to detect a bevel gear crack and prevent failure of the bevel gear, loss of torque to the main rotor system, and subsequent loss of control of the helicopter.