Causes
Investigation attributed the cause of the fracture of the fan disk to a failure of United Airlines maintenance processes to detect an existing fatigue crack.[3] Post-crash analysis of the crack surfaces showed the presence of the penetrating fluorescent dye used to detect cracks during maintenance, indicating that the crack was present and should have been detected at a prior inspection. The detection failure arose from poor attention to human factors in United Airlines' specification of maintenance processes.
The crack in the fan disk was traced back to the Alcoa foundry from which the engine part was sourced. It turned out that there was a defect in elimination of gaseous anomalies during the purifying of the titanium disk ingot. An excess amount of nitrogen was in the material, causing a 'hard alpha inclusion' which cracked during forging and then fell out during final machining, forming a cavity with microscopic cracks at the edges. During the engine's normal running cycle, one of these cracks grew slowly each time the engine was powered up and brought to operating temperature, until it grew large enough for the disk to fail structurally. [3] Newer batches used much higher melting temperatures and a 'triple vacuum' process to eliminate these impurities.
The subsequent investigation and Airworthiness Directive also revealed several other fan disks already in service from the same batch of ingots which had started to exhibit the initial cracking symptoms of part failure.
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