Airworthiness Directives; Eurocopter France Helicopters

We are adopting a new airworthiness directive (AD) for all Eurocopter France (Eurocopter) Model SA-365N, SA-365N1, AS-365N2, AS 365 N3, EC 155B, EC155B1, SA-365C, SA-365C1, SA-365C2, and SA-366G1 helicopters. This AD requires inspecting portions of the main gearbox (MGB) for the presence of sealing compound and corrosion. This AD was prompted by reports of corrosion on the main MGB casing lower area between the two servo-control anchoring fitting attachment ribs. An investigation determined that the corrosion was associated with sealing compound on the lower part of the fitting/casing attachment. The actions in this AD are intended to detect corrosion on the MGB casing, which could lead to a crack, failure of the MGB, and subsequent loss of control of the helicopter.