Summary
The FAA is revising an earlier NPRM for an airworthiness directive (AD) that applies to certain Model 747 airplanes. The original NPRM would have superseded an existing AD that currently requires repetitive inspections of the body station (BS) 2598 bulkhead, and corrective actions if necessary. The existing AD also currently requires a terminating modification for the repetitive inspections and a post-modification inspection of the modified area. The original NPRM proposed to continue to require those actions using revised service information. For certain airplanes, the original NPRM proposed to require new repetitive inspections, an interim modification, and post- interim modification inspections. For certain airplanes, the original NPRM also proposed to require replacing any previously repaired aft inner chord and reinstalling the terminating modification. The original NPRM resulted from reports of cracked aft inner chords on airplanes after certain requirements of the existing AD were done. This new action revises the original NPRM for airplanes that are converted to the Model 747-400 large cargo freighter (LCF) configuration by reducing the threshold and repeat intervals of certain post-modification inspections. This new action also revises the original NPRM for all airplanes by proposing that certain inspections of the upper aft outer chords and diagonal brace attachment fittings, flanges, and rods continue after the terminating modification. We are proposing this supplemental NPRM to prevent fatigue cracking of the BS 2598 bulkhead structure, which could result in inability of the structure to carry horizontal stabilizer flight loads, and loss of controllability of the airplane.