The FAA is proposing to adopt a new airworthiness directive for Eurocopter France SA365N, SA365N1, SA366G1, AS365N2, AS365N3, EC155B and EC155B1 helicopters. It was prompted by the discovery of a cracked main-rotor mast nut that, if not replaced, could lead to complete failure of the mast nut, failure of the rotor mast and loss of control of the helicopter. The proposed AD would require replacing the main-rotor mast nut with an airworthy main-rotor mast nut.