Incident Overview
Shortly after takeoff, the #3 engine and its supporting pylon detached from the aircraft. The separation occurred as a result of the failure of the pylon inboard midspar support fitting. Investigation revealed that the fitting had been cracked for some time due to fatigue, and the crack had propagated to the point of structural failure.
Inspection Background
The failed fitting was subject to Federal Aviation Administration Airworthiness Directive 88-24-10 and Boeing Service Bulletin 3183. These directives required a visual inspection of the midspar fitting every 1,500 flight hours or 600 flight cycles, whichever occurred first. The inspection was designed to detect cracks in the fitting.
Inspection History
Records show that the inspection had been performed on the failed fitting on January 2, 1992. At the time of the failure, 821 flight hours and 328 flight cycles had elapsed since that inspection. Despite being completed on schedule, the crack was not detected, likely because the point of cracking was not visible for inspection from the wing.
Conclusion
The separation was caused by the failure of the pylon inboard midspar support fitting, which had developed a fatigue crack that was not detectable through the required visual inspections.