Airworthiness Directives; Eurocopter France Model AS-365N2, AS 365 N3, EC 155B, EC155B1, SA-365N, N1, and SA-366G1 Helicopters

This amendment revises an existing airworthiness directive (AD) for Eurocopter France (Eurocopter) Model AS-365N2, AS 365 N3, EC 155B, EC155B1, SA-365N, N1, and SA-366G1 helicopters that currently requires inspecting the main gearbox (MGB) base plate for a crack and replacing the MGB if a crack is found. This amendment increases the time intervals for inspecting the MGB base plate and includes minor editorial changes throughout the AD. This amendment is prompted by crack growth tests that indicate that the inspection intervals can be increased without affecting safety. The actions specified by this AD are intended to detect a crack in an MGB base plate and prevent failure of one of the MGB attachment points to the frame, which could result in severe vibration and subsequent loss of control of the helicopter.