Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to Schweizer Aircraft Corporation Model 269A, A-1, B, and C, and TH-55A helicopters, with a certain main rotor transmission ring gear (ring gear) installed. This proposal would require inspections of the ring gear teeth for pitting, wearing, cracking or corrosion, and replacement of the ring gear if such ring gear teeth surface deterioration is found; and would also require creating a main rotor transmission component log card, if none is available, and making a notation on the main rotor transmission component log card if a ring gear is changed. This proposal is prompted by reports of failures of the ring gear due to single tooth distress as a result of improper gear tooth spacing during the manufacturing of the ring gear. The actions specified by the proposed AD are intended to prevent failure of the ring gear, loss of drive to the main rotor gearbox, and a subsequent forced landing.