Summary
The FAA proposes to supersede Airworthiness Directive (AD) 2021-20-10, which applies to certain Leonardo S.p.a. Model AB139 and AW139 helicopters. AD 2021-20-10 requires removing from service a certain part-numbered main gearbox (MGB) spherical bearing lock nut (lock nut) that is installed on certain part-numbered MGBs and replacing it with a newly designed MGB lock nut. AD 2021-20-10 also prohibits installing any MGB with the affected MGB lock nut and prohibits installing any affected MGB lock nut on any helicopter. Since the FAA issued AD 2021-20-10, it was discovered that a part number (P/ N) was incorrectly listed and that the applicability needed to be clarified. This proposed AD would retain the requirements of AD 2021- 20-10 and would clarify the applicability. The FAA is proposing this AD to address the unsafe condition on these products.