Summary
The FAA proposes to supersede two existing airworthiness directives (AD) that apply to certain Model 747-100, 747-200B, and 747- 200F series airplanes. The existing ADs currently require inspections to detect fatigue-related skin cracks and corrosion of the skin panel lap joints in the fuselage upper lobe, and repair if necessary. One of the existing ADs, AD 94-12-09, also requires modification of certain lap joints and inspection of modified lap joints. The other AD, AD 90- 15-06, requires repetitive detailed external visual inspections of the fuselage skin at the upper lobe skin lap joints for cracks and evidence of corrosion, and related investigative and corrective actions. This proposed AD would reduce the maximum interval of the post-modification inspections, and adds post-repair inspection requirements for certain airplanes. This proposed AD results from reports of cracking on modified airplanes. We are proposing this AD to detect and correct fatigue cracking and corrosion in the fuselage upper lobe skin lap joints, which could lead to rapid decompression of the airplane and inability of the structure to carry fail-safe loads.