Summary
We propose to supersede an existing airworthiness directive (AD) that applies to certain Model 737-300, -400, and -500 series airplanes. The existing 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. Since we issued that AD, we have received reports of additional crack findings of the fuselage crown skin at the chem-milled steps. This proposed AD would add inspections for cracking in additional fuselage crown skin locations, and repair if necessary. This proposed AD would also reduce the inspection thresholds for certain airplanes, extend certain repetitive inspection intervals, and add airplanes to the applicability of the existing AD. We are proposing 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.