Airworthiness Directives; Boeing Model 747-400 and 747-400D Series Airplanes

We are adopting a new airworthiness directive (AD) for certain Boeing Model 747-400 and 747-400D series airplanes. This AD requires repetitive inspections to detect cracks in the floor panel attachment fastener holes of the Section 41 upper deck floor beam upper chords, and related investigative and corrective actions if necessary. This AD results from reports of cracks found in the Section 41 upper deck floor beam upper chords. We are issuing this AD to detect and correct cracks in these chords, which could become large and cause the floor beams to become severed and result in rapid decompression or reduced controllability of the airplane.