Summary
We are revising an earlier proposed airworthiness directive (AD) for certain Boeing Model 737-600, -700, and -800 series airplanes. The original NPRM would have required an inspection of the free flange of the lower stringers of the wing center section for drill starts, and applicable related investigative and corrective actions. The original NPRM resulted from drill starts being found on the free flange of the lower stringers of the wing center section during a quality assurance inspection at the final assembly plant. This action revises the original NPRM by expanding the inspection area to include the free flange, the vertical web, and the fillet radius between the vertical web and the free flange. We are proposing this supplemental NPRM to prevent cracks from propagating from drill starts in the free flange, vertical web, and radius between the free flange and vertical web of the lower stringers of the wing center section lower stringers, which could cause a loss of structural integrity of the wing center section and may result in a fuel leak.