Summary
The FAA proposes to adopt a new airworthiness directive (AD) for certain Leonardo S.p.a. (Leonardo) Model AB139 and AW139 helicopters. This proposed AD was prompted by a report of several occurrences of a cracked main gearbox (MGB) spherical bearing lock nut (lock nut). This proposed AD would require removing from service a certain part-numbered MGB lock nut that is installed on certain part- numbered MGBs and replacing it with newly designed MGB lock nut. This proposed AD would also would prohibit installing any MGB with the affected MGB lock nut and would prohibit installing any affected MGB lock nut on any helicopter. The FAA is proposing this AD to address the unsafe condition on these products.