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

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The FAA proposes to adopt a new airworthiness directive (AD) for certain Boeing Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. This proposed AD would require repetitive detailed and high frequency eddy current inspections to detect cracks of the backup intercostals and the upper sill of the forward airstair doorway, and applicable corrective actions. This proposed AD would also provide for an optional terminating action, which would eliminate the need for repetitive inspections. This proposed AD results from a report indicating that cracks were found in the backup intercostals and upper sill web of the forward airstair doorway. We are proposing this AD to detect and correct fatigue cracking of the backup intercostals and upper sill web of the forward airstair doorway, which could result in a rapid loss of cabin pressure.