Summary
We propose to supersede Airworthiness Directive (AD) 2008-12- 04, which applies to certain Boeing Model 737-600, -700, -700C, -800, and -900 series airplanes. AD 2008-12-04 currently requires various repetitive inspections to detect cracks along the chem-milled steps of the fuselage skin, and to detect missing or loose fasteners in the area of the preventive modification or repairs, replacement of the time- limited repair with the permanent repair if applicable, and applicable corrective actions, if necessary, which would end certain repetitive inspections. Since we issued AD 2008-12-04, an evaluation by the design approval holder (DAH) has indicated that the upper skin panel at the chem-milled step above the lap joint is subject to widespread fatigue damage (WFD) if the modification was installed after 30,000 total flight cycles. This proposed AD would reduce the post-modification inspection compliance times, limit installation of the preventive modification to airplanes withfewer than 30,000 total flight cycles, and add repetitive inspections for modified airplanes. We are proposing this AD to detect and correct cracking of the upper skin panel at the chem-milled step above the lap joint, which could result in reduced structural integrity of the airplane.