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The National Transportation Safety Board (NTSB) issued five recommendations to the FAA on August 28, 2006, related to safety concerns with the General Electric CF6-80A jet engine. Two of the five issues are considered serious enough that the NTSB has classified them as "urgent."
Questions about the GE CF6-80A first arose during an uncontained engine failure experienced by an American Airlines Boeing B-767 while running up on a test stand at Los Angeles International Airport (LAX) on June 2, 2006.
According to the NTSB, the high-pressure turbine (HPT) stage 1 disk in the left engine failed. "Debris from the failed engine punctured the airplane's left and right wing fuel tanks; leaking fuel ignited and damaged the wing and fuselage. Pieces of the ruptured disk also penetrated the fuselage and the right engine, and another, found about 2,500 feet from the airplane against an airport perimeter fence, had crossed two active runways and taxiways."
The safety agency says that metallurgical tests revealed that the disk rupture was the result of a rim-to-bore radial fracture that originated at a small dent found at the bottom of a blade slot. The examination also revealed two other similar cracks on the disk. The disk had accumulated 9,186 cycles in service (48,429 hours), with 5,814 cycles remaining for the disk's life limit of 15,000 cycles.
While the NTSB acknowledged the Federal Aviation Administration's (FAA) effort to issue an airworthiness directive (AD) effecting removal, inspection and reworking of the HPT stage 1 disk at 6,900 cycles, it does not believe these steps are sufficient safety measures.
So according to a statement issued by the NTSB, the Safety Board proposes an urgent FAA requirement that the disks be immediately removed for maintenance if they have been in service for more than 3,000 cycles since new or since the last inspection. This significantly more stringent standard would not permit disks to remain in service without inspection beyond the earliest known number of cycles at which cracks have been detected or failure has occurred, according to the NTSB.
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