AD FAA-2011-0914

We are superseding an existing airworthiness directive (AD) for certain Model 737-300, -400, and -500 series airplanes. That AD currently requires repetitive external non-destructive inspections to detect cracks in the fuselage skin along the chem-mill step at stringers S-1 and S-2 right, between station (STA) 827 and STA 847, and repair if necessary. This new AD adds inspections for cracking in additional fuselage crown skin locations, and repair if necessary. This new AD also reduces the inspection thresholds for certain airplanes, extends certain repetitive inspection intervals, and adds airplanes to the applicability of the existing AD. This AD was prompted by reports of additional crack findings of the fuselage crown skin at the chem- milled steps. We are issuing this AD to detect and correct fatigue cracking of the fuselage skin panels at the chem-milled steps, which could result in sudden fracture and failure of the fuselage skin panels, and consequent rapid decompression of the airplane.