Summary
We propose to supersede Airworthiness Directive (AD) 2012-22- 02, which applies to certain The Boeing Company Model 747-400, -400D, and -400F series airplanes. AD 2012-22-02 currently requires measuring the web at station (STA) 320 and, depending on findings, various inspections for cracks and missing fasteners, web and fastener replacement, and related investigative and corrective actions if necessary. Since we issued AD 2012-22-02, it was determined that there were no inspection or repair procedures included for airplanes with a STA 320 crown frame web thickness less than 0.078 inch, or greater than or equal to 0.084 inch and less than or equal to 0.135 inch. This proposed AD would require, for certain airplanes, replacement of the web, including related investigative and corrective actions if necessary. We are issuing this AD to prevent complete fracture of the crown frame assembly, and consequent damage to the skin. Such damage could result in in-flight decompression of the airplane.