Summary
We propose to adopt a new airworthiness directive (AD) for certain Model 747-100, 747-100B, 747-100B SUD, 747-200B, 747-200C, 747- 200F, 747-300, 747-400, 747SR, and 747SP series airplanes. This proposed AD would require repetitive detailed inspections of certain overwing intercostal webs, and related investigative and corrective actions if necessary. This proposed AD results from reports of cracks in overwing intercostal webs. We are proposing this AD to detect and correct such cracking, which could grow and result in a severed intercostal. If an intercostal is severed, cracks could develop in the adjacent frame structure and skin, resulting in a rapid loss of cabin pressure.