What happened
Shortly after takeoff, the number three engine and its associated pylon separated from the aircraft.
The investigation
Investigators determined that the separation was caused by the failure of the pylon inboard midspar support fitting. This component had developed a crack due to fatigue, and the crack had been present for some time prior to the incident. The specific location where the cracking occurred was not visible for inspection from the wing.
The fitting in question is subject to FAA Airworthiness Directive 88-24-10 and Boeing Service Bulletin 3183, which mandate visual inspections of the midspar fitting every 1500 flight hours or 600 cycles. Records show that the most recent inspection of this failed fitting was completed on January 2, 1992, which was 821 flight hours and 328 cycles before the failure occurred.