Airworthiness Directives; The Boeing Company Airplanes

We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 747-100, 747-100B, 747-100B SUD, 747-200B, 747-300, 747SR, and 747SP series airplanes. This AD was prompted by an evaluation by the design approval holder (DAH) indicating that the upper chords of the upper deck floor beams are subject to widespread fatigue damage (WFD). This AD requires repetitive inspections for cracks at the floor panel attachment fastener holes; repetitive inspections for cracks in the upper and lower chords of the upper deck floor beams at permanent fastener locations; repetitive inspections for cracks in certain repaired and modified areas; and related investigative and corrective actions if necessary. This AD also requires repetitive replacement of the upper chords of the upper deck floor beams, including pre-replacement inspections and corrective action if necessary; and post-replacement repetitive inspections and repair if necessary. We are issuing this AD to detect and correct fatigue cracking of the upper chords of the upper deck floor beams. Undetected cracking could result in large deflection or deformation of the upper deck floor beams, resulting in damage to wire bundles and control cables for the flight control system, and reduced controllability of the airplane. Multiple adjacent severed floor beams could result in rapid decompression of the airplane.