by National Transportation Safety Board

“ The first fatigue region was found in lower wing skin, on a flat, 90-degree plane relative to the skin surface, and extended from a rivet hole between stringer 14 and 15, aft for about 0.6 inch, before transitioning to overstress. The region had been covered by a 9.6-inch-wide doubler that extended from the forward edge of stringer 14 to the aft edge of stringer 15. ”

“ According to Lockheed Study LG81ER0148, the forward underside region of the center wing, at CWS 185.5, was known to be susceptible to fatigue. However, the location was slightly aft of the area examined. In addition, the study cited that the critical size for a crack growing in the wing skin alone was 1.72 inches, while the critical size for a crack growing in the wing skin and stringer together was 0.86 inch. ”

“ Although the supporting structure was fractured or consumed by fire, the engines retained their relative positions on the right wing. The persistent odor of ammonia was detected in and about the No. 3 engine. ”