Summary
We propose to supersede an existing airworthiness directive (AD) that applies to certain The Boeing Company Model 747 series airplanes. The existing AD currently requires inspection of the fuselage skin lap splice between body station (BS) 340 and BS 400 at stringers (S)-6L and S-6R, and repair, if necessary. Since we issued that AD, analysis results indicated that the protruding head fastener modification and related post-modification inspections currently required are not adequate to prevent cracking at the upper row of fasteners in the stringer 6 lap joint before the cracks reach a critical length. This proposed AD would add new repetitive inspections for cracking in the stringer 6 skin lap splice, which would terminate the existing inspections; and eventual modification of the lap splice, which would terminate the repetitive inspections; post-modification inspections; and corrective actions if necessary. This proposed AD would also add airplanes to the applicability. We are proposing this AD to detect and correct cracking at the upper row of fasteners in the stringer(s) 6 lap joint, which could result in a sudden loss of cabin pressurization and the inability of the fuselage to withstand failsafe loads.