page_title: United Airlines Flight 232
section_title: Failed component
hierarchical_section_title: United Airlines Flight 232 / Investigation / Failed component
caption_attribution_description: English: The recovered fan disk from UAL 232, showing the in-flight fracture which led to the engine failure and eventual crash landing
context_page_description: United Airlines Flight 232 was a regularly scheduled United Airlines flight from Denver to Chicago, continuing to Philadelphia. On July 19, 1989, the DC-10 serving the flight crash-landed at Sioux City, Iowa, after suffering a catastrophic failure of its tail-mounted engine, which led to the loss of many flight controls. At the time, the aircraft was en route from Stapleton International Airport to O'Hare International Airport. Of the 296 passengers and crew on board, 112 died during the accident, while 184 people survived. The crash was the fifth-deadliest one involving the DC-10, behind Turkish Airlines Flight 981, American Airlines Flight 191, Air New Zealand Flight 901, and UTA Flight 772. Despite the deaths, the accident is considered a prime example of successful crew resource management because of the large number of survivors and the manner in which the flight crew handled the emergency and landed the airplane without conventional control.
context_section_description: The investigation, while praising the actions of the flight crew for saving lives,  later identified the cause of the accident as a failure by United Airlines maintenance processes and personnel to detect an existing fatigue crack. The Probable Cause in the report by the NTSB read as follows: 
  The National Transportation Safety Board determines that the probable cause of this accident was the inadequate consideration given to human factors limitations in the inspection and quality control procedures used by United Airlines' engine overhaul facility which resulted in the failure to detect a fatigue crack originating from a previously undetected metallurgical defect located in a critical area of the stage 1 fan disk that was manufactured by General Electric Aircraft Engines. The subsequent catastrophic disintegration of the disk resulted in the liberation of debris in a pattern of distribution and with energy levels that exceeded the level of protection provided by design features of the hydraulic systems that operate the DC-10's flight controls.
  Post-crash analysis of the crack surfaces showed the presence of a penetrating fluorescent dye used to detect cracks during maintenance. The presence of the dye indicated 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.
  Investigators discovered an impurity and fatigue crack in the disk. Titanium reacts with air when melted, which creates impurities that can initiate fatigue cracks like that found in the crash disk. To prevent this, the ingot that would become the fan disk was formed using a "double vacuum" process: the raw materials were melted together in a vacuum, allowed to cool and solidify, then melted in a vacuum once more. After the double vacuum process, the ingot was shaped into a billet, a sausage-like form about 16 inches in diameter, and tested using ultrasound to look for defects. Defects were located and the ingot was further processed to remove them, but some contamination remained. GE later added a third vacuum-forming stage because of their investigation into failing rotating titanium engine parts.
  The contamination caused what is known as a hard alpha inclusion, where a contaminant particle in a metal alloy causes the metal around it to become brittle. The brittle titanium around the impurity then cracked during forging and fell out during final machining, leaving a cavity with microscopic cracks at the edges. For the next 18 years, the crack grew slightly each time the engine was powered up and brought to operating temperature. Eventually, the crack broke open, causing the disk to fail.
  The origins of the crash disk are uncertain because of significant irregularities and gaps, noted in the NTSB report, in the manufacturing records of GE Aircraft Engines (GEAE) and its suppliers.  Records found after the accident indicated that two rough-machined forgings having the serial number of the crash disk had been routed through GEAE manufacturing. Records indicated that Alcoa supplied GE with TIMET titanium forgings for one disk with the serial number of the crash disk. Some records show that this disk “was rejected for an unsatisfactory ultrasonic indication”, that an outside lab performed an ultrasound inspection of this disk, that this disk was subsequently returned to GE, and that this disk should have been scrapped.  The FAA report stated, “There is no record of warranty claim by GEAE for defective material and no record of any credit for GEAE processed by Alcoa or TIMET”.   
  GE records of the second disk having the serial number of the crash disk indicate that it was made with an RMI titanium billet supplied by Alcoa.  Research of GE records showed no other titanium parts were manufactured at GE from this RMI titanium billet during the period of 1969 to 1990.  GE records indicate that final finishing and inspection of the crash disk were completed on December 11, 1971.  Alcoa records indicate that this RMI tita
caption: The fracture is clearly visible in the recovered fan disk from the center engine of UAL 232.
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