History: United Flight 232
“United Airlines Flight 232, was a flight scheduled to fly from Stapleton International Airport in Denver to O’Hare International Airport in Chicago on July 19, 1989. It crash-landed at Sioux Gateway Airport in Sioux City, Iowa, after the failure of its tail engine caused the loss of all hydraulic control of the plane; more than half of those aboard survived.” - Britannica
Following is a really interesting article from 1989 about the technical roots of this accident.
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UPI ARCHIVES NOV. 2, 1989
Report: Cracked engine part made from flawed titanium
By JOHN PETERSON
SIOUX CITY, Iowa -- A crack in an engine part that may have caused the July crash of a United Airlines jet, killing 112 passengers, was made from a batch of titanium that had 'minor cracking flaws,' documents showed.
The National Transportation Safety Board Wednesday released documents that showed a manufacturer, TIMET Co. of Henderson, Nev., had problems with a log of titanium from which the cracked fan blade disk on Flight 232 was made.
Eight fan blade disks, including the suspected United part, were made from the same 6,000-pound log of titanium. The titanium was delivered to ALCOA, which forged the disk for General Electric.
ALCOA tested the titanium log and concluded the metal had 'minor cracking tendencies,' government records showed.
GE ultrasound testing found abnormalities in one of the eight disks before it left the GE engine plant in Evendale, Ohio, in 1971. An analysis of the disk, however, could not pinpoint the flaw.
Another disk, recalled after the Iowa accident, was found to contain a small flaw similar to the crash disk. That disk had been in service for nearly 10,000 engine on-off cycles.
United spokesman Robert Doughty said the flaws on the crash disk and the sister disk apparently formed when too much nitrogen settled in one spot during the manufacturing process of either the titanium or the disk.
The cracked disk, meanwhile, could have been detected a year before the tragedy if an August 1988 inspection of the DC-10 engine been more thorough, experts told the NTSB during a hearing Wednesday.
James Wildey II, a transportation safety board metallurgist, said the crack originatedfrom a component flaw in the disk made from the titanium almost 20 years ago.
The disk split while Flight 232 was en route from Denver to Chicago on July 19, severing hydraulic lines and rendering flight controls useless. The disk fell out of the engine, and the plane crashed on a runway while trying to land at Sioux Gateway Airport.
Chris Glynn, a GE engineer, testified the crack must have been present when United tested the rim of the disk using ultrasound in August 1988. That technology requires the emersion of the disk in water for 40 days.
'We feel very comfortable that the crack could have been found with ultrasound,' Glynn said.
Doughty said the airline tested the outside rim section of the disk, but not the area where the crack and the titanium imperfection were found. He said the crack found by transportation safety board inspectors was hidden on the surface of the disk and could not be seen by a visual inspection.
The last regular inspection of the disk by United mechanics was in February 1988 after it had gone through 14,743 on-off cycles, Doughty said. The normal life of a disk is 18,000 cycles.
'We were doing all we were required to do,' Doughty said about the last inspection. 'Ultrasound was supposed to be done at the original manufacturer and we were working under that assumption.'
Barry Montgomery, a lawyer for TIMET, said the metal company was not required by ALCOA to run ultrasound tests on its logs of titanium. Don Cooper, a TIMET vice president, testified the titanium log sent to ALCOA was a 'non-premium' quality, meaning the metals were melted twice, not three times.
Doughty admitted the phosphorescent method is much cheaper and more practical than ultrasound. The last regular inspection used the phosphorescent process, which could not detect cracks below the surface of the disk, Doughty said.
For further reading see: Britannica