Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to certain Boeing Model 747-100, -200, -300, and 747SP series airplanes. The existing AD currently requires certain inspections to find missing or alloy-steel taperlock fasteners (bolts) in the diagonal brace underwing fittings, and corrective actions if necessary. For airplanes with missing or alloy-steel fasteners, the existing AD also mandates replacement of certain fasteners with new fasteners, which constitutes terminating action for certain inspections. This proposed AD would expand the applicability to include additional airplane models and would require a new inspection to determine fastener material and to find missing or broken fasteners, and related investigative/corrective actions if necessary. This proposed AD is prompted by reports indicating that cracked fasteners made of A286 material were found on airplanes that had only fasteners made of A286 material installed in the area common to the diagonal brace underwing fittings. We are proposing this AD to prevent loss of the underwing fitting load path due to missing or damaged alloy-steel or A286 taperlock fasteners, which could result in separation of the engine and strut from the airplane.