Airworthiness Directives; Turbomeca S.A. Turboshaft Engines

We propose to adopt a new airworthiness directive (AD) for all Turbomeca S.A. Arriel 1D and 1D1 turboshaft engines with a pre- modification (mod) TU357 gas generator module (M03), installed. This proposed AD was prompted by reports of divergent rubbing between the piston shaft small diameter labyrinth and the rear bearing support. This proposed AD would require removing the pre-modification (mod) TU357 gas generator module (M03) and replacing with a part eligible for installation. We are proposing this AD to prevent failure of the labyrinth seal and engine, in-flight shutdown, and loss of control of the helicopter.