AD FAA-2011-0645

We are superseding an existing airworthiness directive (AD) for certain The Boeing Company Model 747 series airplanes. That 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 new AD shortens the interval for the repetitive inspections, requires modification for certain airplanes, and requires certain post-modification inspections for other airplanes. This AD was prompted by 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 issuing 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, which could result in sudden loss of cabin pressurization and the inability of the fuselage to withstand fail-safe loads.