ANCHORAGE, AK, USA
N707CK
Boeing 747-269B
During climbout, the Boeing 747 crew noted a loud 'thump,' followed by a change in the number three engine instrument indications. One crewmember noted there was a fist-sized hole in the side of the engine cowling. The captain shut down the number three engine, returned to the departure airport, and landed without further incident. A postlanding inspection revealed that the inboard aileron sustained substantial damage. Disassembly of the engine revealed that there were eleven 2nd stage turbine blades that were fractured in the midspan area, and one other 2nd stage turbine blade that was elongated and necked down in the midspan area with most of the shroud tip rubbed off. The turbine exhaust case had a 16 1/2-inch long hole between the case front flange and the front mount rail. Engine service records revealed that when the turbine exhaust case was overhauled, Airworthiness Directive (AD) 96-25-10 was complied with by incorporating Chromalloy Anniston Division's, Supplemental Type Certificate (STC) SE00047AT-D. The intent of STC SE00047AT-D is to comply with FAA Airworthiness Directive (AD) 96-25-10 by increasing the wall thickness of the turbine exhaust case, thus providing enhanced containment capabilities in the event of an internal engine failure. A portion of the turbine exhaust case was sent to the NTSB Materials Laboratory for examination. A Safety Board metallurgist confirmed that both the hardness and microstructure were consistent with design specifications stipulated in Chromally Anniston Division's STC.
On July 29, 1999, about 1500 Alaska daylight time, a Boeing 747-269B airplane, N707CK, had an uncontained engine failure during climb-out from the Anchorage International Airport, Anchorage, Alaska. The flight was being conducted under Title 14, CFR Part 121, as a nonscheduled international cargo flight, operated by Kitty Hawk International, Inc., as Flight 11281. There were no injuries to the five crewmembers aboard. Visual meteorological conditions prevailed at the time of departure from Anchorage, and an instrument flight plan had been filed for the flight to Bangkok, Thailand. During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge on July 30, a company quality assurance official stated that about 20 minutes after departure, the crew noted a loud "thump," followed by a change in the number three engine instrument indications. He said that one of the crewmembers went to the back of the airplane to visually check the status of the engine, and noted there was a "fist-sized hole in the side of the engine cowling." The quality assurance official stated that the captain shut down the number three engine, returned to Anchorage International Airport, and landed without further incident. A postaccident inspection revealed that the inboard aileron sustained substantial damage. On July 30, 1999, the airplane was three-engine ferried from Anchorage to the operator's maintenance base in Oscoda, Michigan, where the engine was removed. The engine was subsequently shipped to a maintenance facility in Tel Avia, Israel, for disassembly and examination. On September 7, 1999, the engine was disassembled under the direction of an NTSB Powerplant Group Chairman, and in the presence of the parties to the investigation. The disassembly revealed that there were eleven 2nd stage turbine blades that were fractured in the midspan area, and one other 2nd stage turbine blade that was elongated and necked down in the midspan area, with most of the shroud tip rubbed off. The turbine exhaust case had a 16 1/2-inch long hole between the case front flange and the front mount rail. The Powerplants Group completed its examination of the engine on September 8. A copy of the Group Chairman's report is included in this report. An examination of the engine maintenance records revealed the engine was removed from service due to a cracked diffuser case on August 1, 1998, at 43,946 hours total time, and 11,629 cycles. During this maintenance procedure, an overhauled turbine exhaust case was installed. The engine records revealed that when the turbine exhaust case was overhauled, FAA Airworthiness Directive (AD) 96-25-10 was complied with by incorporating Chromalloy Anniston Division's, Supplemental Type Certificate (STC) SE00047AT-D. The intent of STC SE00047AT-D is to comply with FAA Airworthiness Directive (AD) 96-25-10 by increasing the wall thickness of the turbine exhaust case, thus providing enhanced containment capabilities in the event of an internal engine failure. A portion of the turbine exhaust case was sent to the National Transportation Safety Board's Materials Laboratory for examination. A Safety Board metallurgist confirmed that both the hardness and microstructure were consistent with design specifications stipulated in Chromally Anniston Division's STC. A copy of the NTSB materials laboratory factual report is included in this report.
The failure of a turbine blade, and subsequent penetration of the shroud (containment ring). A factor associated with the accident was the FAA's insufficient design standards/requirements addressed in an FAA Air Worthiness Directive, which called for a strengthened containment ring. The failed containment ring was in compliance with the Air Worthiness Directive.
Source: NTSB Aviation Accident Database
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