Summary
The FAA is revising an earlier NPRM for an airworthiness directive (AD) that applies to certain Boeing Model 747 series airplanes. The original NPRM would have superseded an existing AD that currently requires inspecting to detect cracking in certain lower lobe fuselage skin lap joints, doing repetitive inspections for cracking at certain fastener locations having countersunk fasteners, and replacing countersunk fasteners with protruding head fasteners at certain fastener locations. The original NPRM proposed to replace a previous high-frequency eddy current (HFEC) inspection method with a new HFEC inspection method, add a one-time inspection for cracking of certain airplanes, and terminate the adjustment factor for the inspection compliance times based on cabin differential pressure. The original NPRM also included an inspection at an additional lap joint. The original NPRM resulted from reports of fuselage skin cracks found at certain countersunk fastener locations in the upper row of lap joints near the wing-to-body fairings, and from a report that the presence of Alodine-coated rivets could cause faulty results during the required inspections using the optional sliding probe HFEC inspection method specified in the existing AD. This new action revises the original NPRM by including inspections at additional lap joint locations and by removing inspections at certain other lap joint locations. We are proposing this supplemental NPRM to prevent reduced structural integrity of the fuselage.