Crash Study Blames Inspection Failures
United Airlines failed to detect a crack in a key engine part that ultimately caused the deaths of 112 passengers in a crash near Sioux City, Iowa, last year, the National Transportation Safety Board concluded Thursday.
The crack, on a fan disk inside one of the DC-10’s three jet engines, went undetected by United even though it had been present since the engine’s manufacture in 1972, the board said, adding that United overestimated the reliability of its inspection process.
The NTSB suggested that General Electric’s aircraft engine subsidiary should share partial responsibility for the crash because the crack, which was a little more than an inch long and about half an inch deep, originated from a previously undetected defect in the metal used to make the disk.
In an effort to avoid future mishaps, the board issued recommendations calling for better record-keeping by manufacturers, stepped-up research into automated inspection processes and development of backup flight control systems for emergency use when conventional systems fail.
United Flight 232 crashed in July, 1989, after the cracked fan disk disintegrated, severing the DC-10’s hydraulic systems and causing the flight crew to lose control of the plane. The board investigated the incident for more than a year.
“This flaw could have been detected, “ said NTSB Chairman James L. Kolstad. “United Airlines is the primary cause of the accident, but it seems to me that General Electric was involved to a lesser extent.”
Bob Doll, vice president of engineering and quality assurance at United, said he was amazed at the board’s conclusion.
“There was not a detectable crack in the . . . disk,” Doll said. “Our inspectors do their job well. No one has ever criticized our process.”
Doll said that United received “a defective disk that was delivered by the supplier.” He would not blame General Electric for the defect, however, characterizing the company as “a good supplier” and declining further comment on its role.
The board said that General Electric records on the manufacture of the fan disk were incomplete and did not specify the source of the flawed metal used in the engine part.
The plane, which was en route to Chicago from Denver, was marginally flyable after the engine disintegrated, the board stated, but a safe landing “was virtually impossible” because of the hydraulic system damage. The crash occurred as the pilots attempted to land the aircraft.
United personnel inspected the flawed fan disk on six occasions the last 17 months before the crash. The inspection process, known as fluorescent penetrant inspection, uses a fluorescent dye that permeates cracks and becomes visible in ultraviolet light. Both General Electric and the Federal Aviation Administration had approved the inspection procedure.
But an NTSB metallurgist described the process as “fraught with human errors,” noting that unless a crack was large enough to be visible to the human eye during the ultraviolet inspection, it would go undetected.
Since the crash, the FAA has mandated the installation of shut-off valves on the hydraulic systems of all DC-10 aircraft by next summer. The valves close automatically if hydraulic fluid levels drop below a certain point, avoiding the loss of all hydraulic fluid and helping the flight crew to maintain control in an emergency.
In addition, United has changed its inspection procedure as a result of the crash. The airline now uses a scanning process involving more-advanced technology to detect flaws by passing an electric current through engine parts, Doll said.
The design of the DC-10 also was criticized by the board, which noted that the crucial hydraulic systems were vulnerable to flying debris such as the broken fan disk.
