Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to certain Model 747 series airplanes. The existing AD currently requires repetitive inspections for cracks of the fuselage skin lap splice between body station (BS) 400 and BS 520 at stringers S-6L and S-6R, and repair if necessary. This proposed AD would shorten the interval for the repetitive inspections, require modification for certain airplanes, and require certain post- modification inspections for other airplanes. This proposed AD results from reports of multiple adjacent cracks on an airplane, and a recent fleet-wide evaluation of widespread fatigue damage of skin lap joints, which indicated the need for revised procedures and reduced compliance times. We are proposing this AD to detect and correct cracking of the fuselage skin lap splice between BS 400 and BS 520 at stringers S-6L and S-6R. Such cracking could result in sudden loss of cabin pressurization and the inability of the fuselage to withstand fail-safe loads.