Summary
We propose to adopt a new airworthiness directive (AD) for all Eurocopter France (EC) Model SA-365N, SA-365N1, SA-366G1, AS-365N2, AS 365 N3, EC 155B, and EC155B1 helicopters. This proposed AD was prompted by the discovery of a cracked main rotor mast nut. This condition, if not corrected, could lead to complete failure of the mast nut, resulting in failure of the rotor mast and loss of control of the helicopter. This proposed AD would require replacing the main rotor mast nut with an airworthy main rotor mast nut. We are proposing this AD to prevent failure of the main rotor mast and subsequent loss of control of the helicopter.