Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to certain Boeing Model 747-100 and -200 series airplanes. The existing AD currently requires repetitive inspections for cracking of the station 800 frame assembly, and repair if necessary. This proposed AD would retain the repetitive inspection requirements of the existing AD, but would expand the area to be inspected. This proposed AD also would reduce the initial inspection threshold, remove the adjustment of the compliance threshold and repetitive interval based on cabin differential pressure, and add airplanes to the applicability. This proposed AD results from several reports of cracks of the station 800 frame assembly on airplanes that had accumulated fewer total flight cycles than the initial inspection threshold in the existing AD. We are proposing this AD to detect and correct fatigue cracks that could extend and fully sever the frame, which could result in development of skin cracksthat could lead to rapid depressurization of the airplane.