Airworthiness Directives; Boeing Model 737-100, -200, -200C, -300, -400, and -500 Series Airplanes

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The FAA is adopting a new airworthiness directive (AD) for all Boeing Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. This AD requires repetitive inspections of the stiffeners at left buttock line (LBL) and right buttock line (RBL) 6.15 for cracks; and replacement of both stiffeners with new, improved stiffeners if any stiffener is found cracked. This AD also allows replacement of both stiffeners at LBL and RBL 6.15 with new, improved stiffeners, which terminates the repetitive inspections. This AD is prompted by reports of cracks in the stiffeners at LBL and RBL 6.15 on the rear spar of the wing center section. We are issuing this AD to detect and correct cracks in the stiffeners at LBL and RBL 6.15, which could result in damage to the keel beam structure and consequently reduce the capability of the airplane to sustain flight loads.