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General Aviation Safety
THE NATIONAL TRANSPORTATION SAFETY BOARD'S INVESTIGATION OF THE UNITED AIRLINES DC-10 ACCIDENT AT SIOUX CITY, IOWA, ON JULY 19, 1989, IS CONTINUING. THE INVESTIGATION THUS FAR INDICATES THAT MOST OF THE FAN ROTOR ASSEMBLY SEPARATED FROM THE NO. 2 ENGINE IN FLIGHT. THE SEPARATION, FRAGMENTATION, AND FORCEFUL DISCHARGE OF FAN ROTOR PARTS SEVERED OR LOOSENED HYDRAULIC LINES ASSOCIATED WITH ALL THREE HYDRAULICS SYSTEMS, WHICH RESULTED IN THE LOSS OF ALL HYDAULIC SERVICES INCLUDING THOSE TO THE FLIGHT CONTROLS. FOLLOWING THE LOSS OF THE HYDRAULIC SERVICES, THE AIRPLANE COULD BE CONTROLLED ONLY BY THE FLIGHTCREW'S USE OF DIFFERENTIAL THRUST FROM THE NOS. 1 AND 3 ENGINES. THE AIRPLANE CRASHED DURING AN ATTEMPTED EMERGENCY LANDING AT SIOUX CITY. OF THE 296 PERSONS ON BOARD, 111 DIED FROM INJURIES RECEIVED IN THE CRASH, AND 185 PERSONS SURVIVED.
TO THE FEDERAL AVIATION ADMINISTRATION: Conduct a directed safety investigation (DSI) of the general electric cf6-6 turbine engine to establish a cyclic threshold at which the fan forward shaft and the fan disks should be separated and inspected for defects in the components. The DSI should include a review and analysis of: (a) the certification, testing, and stress analysis data that were used to establish the life limits of the fan disks and fan shaft components and the recommended inspection frequencies for these components; (b) the manufacturing processes associated with the production of the fan assembly and fan forward shaft; (c) metallurgical analysis of the front flange of the fan forward shaft in which cracks were recently discovered; (d) the maintenance practices involved in the assembly and disassembly of the fan disks and the fan forward shaft for the potential to damage the components during these processes; (e) nondestructive inspection of spare fan disks and fan forward shafts beginning with those components with the highest number of cycles in service; and (f) nondestructive inspections of fan disks on installed engines that may be performed by an approved inspection procedure. (Superseded by A-90-088) (Urgent)
Original recommendation transmittal letter:
Closed - Superseded
SIOUX CITY, IA, United States
United Airlines Flight 232 McDonnell Douglas DC-10-10
Addressee(s) and Addressee Status:
FAA (Closed - Superseded)
Safety Recommendation History
Per Green Sheet A-90-88 through -91: On August 17, 1989, the Safety Board issued Recommendations A-89-95, A-89-96, and A-89-97. These recommendations requested a directed safety study of the CF6-6 engine, issuance of an Airworthiness Directive to require inspections identified by the directed safety study, and an evaluation of the need for a directed safety study of all GEAE CF6 series engines. The Safety Board believes that Recommendations A-90-88, A-90-89, and A-90-90 (the first three recommendations in this letter) encompass these earlier recommendations, and the Safety Board is therefore classifying Recommendations A-89-95, A-89-96, and A-89-97 as "Closed--Superseded."
As stated in the FAA's letter to the Safety Board dated September 5, 1989, all of the elements of this safety recommendation have been addressed by both the FAA and General Electric (GE). The FAA continues to believe that a further formal directed safety investigation is not necessary. The following actions were initiated: (a) Reevaluation of the fan module rotating component life limits was completed utilizing current state-of-the-art technology. The life limits originally calculated by GE were both conservative and within engineering design practice guidelines. No need has been identified to reduce the published life limit of either the fan disk or the driveshaft. The Sioux City fan disk failure has been attributed to a quality/manufacturing defect. The life limits for critical rotating components are established with no requirement for maintenance inspections throughout the life of the component. Soft inspection times are developed by the operator/manufacturer for durability assurance. Based on the findings of the investigation to date, no changes to fan disk soft inspection times are indicated. (b) As a consequence of the Sioux City accident on July 19, 1989, the FAA established a titanium rotating components review team. This team is charged with the review of all pertinent design manufacturing quality control life limiting methodology and inspection procedures used in the production of these life-limited components. Specifically, Type 1 hard alpha metallurgical defects are of concern; however, any other defects that could substantially jeopardize the integrity of titanium rotating components will be considered. This activity is to be completed by June 1990, at which time the FAA will make the necessary recommendations to all aspects of industry involved in titanium production. A review of the manufacturing billet records for the billet from which the accident disk was manufactured was completed. Fan disks produced from the same billet have been returned to GE for inspection and metallurgical analysis. The following disk Serial Number 388 was found to have nitrogen stabilized alpha. No further analysis of this disk has been accomplished. General Electric is awaiting the Safety Board's instructions before further analysis is performed. Additionally, in April 1990, the FAA, together with the Society of Automotive Engineers, will review all titanium material specifications. (c) The engineering analysis of the fan forward drive shaft found with flange bolt hole radial cracks was completed. The cause of this defect was attributed to classical stress corrosion cracking induced by the presence of chlorine. No evidence of low- or high-cycle fatigue was found. This failure mode and that of the component involved in the Sioux City accident in no way contributed to the resultant fan disk separation. This problem will be addressed as a separate FAA airworthiness action. (d) Maintenance assembly and disassembly practices for the fan disks and the fan forward shaft have been reviewed at the "engine manual" level and at the operators' maintenance facilities. No abnormalities were found that could be a potential cause for catastrophic failure of the fan module or any individual components. Also, maintenance records for the accident disk have been reviewed and no anomalies found. (e) The nondestructive inspection technique developed for off-wing fan disks requires an immersion ultrasonic inspection. This method has been thoroughly tested and found to have excellent probability of detecting flaws in fan disks. Inspection was completed on all companion disks manufactured from the same billet as the accident disk. This ultrasonic inspection is the subject of FAA Airworthiness Directive (AD) 89-20-01, which specifies three categories of fan disks for scheduled removal and inspection based on operating cycles and manufacturing methodology. This AD was also amended to include 52 additional disks--2 in Category 2 and 50 in Category 3. (f) An on-wing ultrasonic inspection procedure was developed by GE in conjunction with all affected operators. This procedure utilizes contact ultrasonic probes with access limited to the forward side of the disk and bore. The sensitivity of this inspection is adjusted to provide a minimum of 1,000 cycles residual life. This inspection is the subject of AD 89-20-01 which allows a 500- cycle on-wing reinspection interval for Category 2 disks, and two 500-cycle reinspections for Category 3 disks before removal of fan disks for a full immersion ultrasonic inspection (this is conservative by a factor of two). As stated in our previous response, I consider the FAA's action to be completed on this safety recommendation. I urge the Safety Board to classify this safety recommendation as "closed acceptable action."
As you are aware, the Safety Board has just completed a 4-day hearing into the circumstances surrounding this accident, and the investigation is ongoing. For the present, therefore, we acknowledge the FAA's and GE's ongoing engineering evaluation and classify these safety recommendations as "Open--Acceptable Action."
The FAA and General Electric have addressed all of the elements of this safety recommendation. The FAA does not believe that a further formal directed safety investigation is necessary. Specifically, the following actions have been initiated: (a) Reevaluation of the fan module rotating component life limits is underway utilizing current state-of-the-art technology. The initial certification data have been reviewed and no inconsistencies have been found. However, any information learned from the accident investigation will be incorporated in the analysis procedures. Life limitations are established for critical rotating components with no requirements for maintenance inspections throughout the life of the components. Soft inspection times are developed by the operator/manufacturer for durability assurance, and these inspection recommendations are presently being evaluated. (b) A review of the billet records for the billet from which the accident disk was manufactured has been completed. Fan disks produced from the same billet have been returned to General Electric for inspection and metallurgical analysis, if necessary. A review of the manufacturing processes and procedures is currently in process for the manufacture of the fan disks and shafts. In addition, a review of all titanium material specifications has been implemented. Metallurgical analysis of one returned fan disk indicates a grain structure anomaly which is currently being evaluated on an expedited basis. (c) Metallurgical examination of the fan forward shaft is in process. Initial scanning electron microscope inspection of the flange cracking in the inner diameter of the bolt holes confirms no presence of either low- or high-cycle fatigue. There is no evidence of chrome or other plating material in or adjacent to the fracture surface of the initial bolt hole inspected. Metallurgical examination of the fan forward shaft is continuing. Structural analysis of the fan forward shaft to determine its load carrying margin in a cracked state is underway. (d) Maintenance assembly and disassembly practices for the fan disks and the fan forward shaft are currently being reviewed at the "engine manual" level and at the operators' maintenance facilities. Maintenance records for the accident disk have been reviewed and no anomalies found. (e) The nondestructive inspections of disks, including spares and companion disks, to the accident disk are currently in process. Spare disks are to be inspected to establish a rotatable pool for replacement of disks in service. Schedules for prioritizing disk removal will be developed based on operating cycles, manufacturing data, and the results of the investigation. (f) Inspection requirements and methods are currently being developed and demonstrated to determine viability of an installed engine inspection program and will be implemented when available. I believe that the FAA has met fully the intent of this safety recommendation, and I consider the FAA's action to be completed.
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