Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to Sikorsky Aircraft- manufactured Model S-64E and S-64F helicopters, that currently requires initial and repetitive inspections of the main gearbox assembly second stage lower planetary plate (plate) for cracks, and removal and replacement of the plate if cracks are found; and daily inspections of certain main transmission oil filter packs for magnesium chips, and removal and replacement of the main transmission if chips are found. The AD also provides for reworking and re-identifying the plate, as well as establishes a retirement life for the plate, including those that have been reworked and re-identified. This action would require, for Model S-64E helicopters, inspections and rework of the plate and establishes a new retirement life for the plate. This proposal is prompted by the type certificate holder's reports that four plates were discovered to have cracks, three of which had been reworked in accordance with the existing AD. The actions specified by the proposed AD are intended to add another plate to the applicability of the AD, remove the requirements of AD 77-20-01 for the Model S-64F and prevent failure of the plate on the Model S-64E due to fatigue cracking, which could lead to failure of the main gearbox and subsequent loss of control of the helicopter.