Summary
This document proposes adopting a new airworthiness directive (AD) for the Sikorsky Model S-70A and S-70C helicopters. This proposal would require an ultrasonic test (UT) inspection of the tail gearbox output bevel gear (gear) for a crack. If you find a crack, replacing the gear with an airworthy gear before further flight would be required. This proposal is prompted by three gear cracking incidents, one of which resulted in the tail rotor separating from the helicopter. The actions specified by this proposed AD are intended to detect a crack in the gear to prevent a tail rotor separating, loss of tail rotor control, and subsequent loss of control of the helicopter.