Summary
We are revising an earlier proposed airworthiness directive (AD) for all Boeing Model 747 airplanes. The original NPRM would have required inspecting for cracks in the left- and right-side Stringer 11 longeron adjacent to the horizontal stabilizer pivot bulkhead, and related investigative and corrective actions if necessary. The original NPRM resulted from a report of a crack found in the right-side Stringer 11 longeron horizontal flange, adjacent to the horizontal stabilizer pivot bulkhead, during a routine maintenance inspection. This action revises the original NPRM by reducing the compliance times for certain airplanes. We are proposing this supplemental NPRM to detect and correct fatigue cracking of the longeron, which can propagate and cause damage to the adjacent horizontal stabilizer pivot bulkhead. This damage could result in loss of structural integrity and consequent inability of the bulkhead to carry flight loads, which could adversely affect controllability of theairplane.