Summary
We propose to adopt 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 proposed AD is prompted by reports of corrosion on the main gearbox (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 proposed actions 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.