Summary
We propose to supersede an existing airworthiness directive (AD) that applies to certain The Boeing Company Model 747-400 and -400D series airplanes. The existing AD currently requires repetitive inspections to detect cracks in the floor panel attachment fastener holes of the Section 41 upper deck floor beam upper chords, and corrective actions if necessary; and repetitive post-repair and post- modification inspections, and corrective actions if necessary. Since we issued that AD, an evaluation by the design approval holder (DAH) indicated that certain upper chords of the upper deck floor beam are subject to widespread fatigue damage (WFD). A replacement was developed to support the airplane's limit of validity (LOV) of the engineering data that support the established structural maintenance program. This proposed AD would add repetitive inspections of Section 44 upper deck floor beam upper chords, and corrective actions if necessary; repetitive post-repair and post-modification inspections, and corrective actions if necessary; and replacing the upper deck floor beam upper chords. We are proposing this AD to detect and correct fatigue cracking in certain upper chords of the upper deck floor beam, which could become large and cause the floor beams to become severed and result in rapid decompression or reduced controllability of the airplane.