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General Aviation Safety
The National Transportation Safety Board's investigation of the United Airlines DC-IO accident in Sioux City, Iowa, on July 19, 1989, has revealed that the separation of the fan Stage I rotor disk of the No. 2 engine (a General Electric CF6-6D engine) initiated from a metallurgical anomaly (hard alpha) on the inside diameter surface of the disk bore near its forward corner. As a result of the accident, the Federal Aviation Administration issued Airworthiness Directive (AD) 89-20-01, effective October 7 , 1989.This AD required that certain CF6-6 engine fan Stage I rotor disks be inspected in accordance with General Electric Aircraft Engines (GEAE) Service Bulletin 72-947, dated September 15, 1989. Both the service bulletin and the AD were issued before the pieces of the separated disk were recovered in October, 1989. Titanium metal is processed in batches, referred to as "heats", that have the composition of the final alloy. The raw material that goes into a given heat can consist of processed ore (primarily pure titanium), alloying elements, scrap, and portions of other heats. Each heat of metal is homogenized and purified by repeatedly arc melting the heat in a vacuum furnace. Many of the CF6-6 engine fan disks were made from heats that had been melted twice (the double vacuum melting process). After 1971, General Electric changed their specifications to require three vacuum meltings (the triple vacuum melting process). The AD and service bulletin identify those CF6-6 engine fan Stage I rotor disks that records indicate are from the same heat of metal as the separated disk (Category I disks), those that have raw material in common with the separated disk (Category I1 disks), and those that were made with the double vacuum melting process instead of the triple vacuum melting process (Category I11 disks). The service bulletin also describes a contact ultrasonic inspection, which can be accomplished on-wing, and an immersion ultrasonic inspection, which requires that the disk be removed from the engine. The disks listed in AD 89-20-01 as being in Category I have been removed from service.1 In accordance with AD 89-20-01, all Category I1 and 111 disks should have been subjected to an initial contact ultrasonic inspection, and all Category II disks should have received an immersion ultrasonic inspection by April 1, 1990. The immersion ultrasonic inspection of Category 111 disks can be postponed until December 31, 1990, as long as the disks are subjected to the contact ultrasonic inspection at intervals of 500 cycles or less.
THE NTSB RECOMMENDS THAT THE FEDERAL AVIATION ADMINISTRATION: DEVELOP, WITH THE ASSISTANCE OF GENERAL ELECTRIC AIRCRAFT ENGINES, AN ALTERNATE METHOD OF INSPECTING THE BORE AREA OF THE CF6-6 ENGINE FAN STAGE I ROTOR DISKS FOR THE PRESENCE OF SURFACE CRACKS; ISSUE AN AIRWORTHINESS DIRECTIVE TO REQUIRE THAT THESE DISKS BE INSPECTED WITH THIS METHOD ON AN EXPEDITED BASIS, THAT DISKS FOUND TO HAVE CRACKS BE REMOVED FROM SERVICE, AND THAT THE INSPECTION BE REPEATED AT A CYCLIC INTERVAL BASED UPON THE CRACK SIZE DETECTABLE BY THE INSPECTION METHOD, THE STRESS LEVEL IN THE APPLICABLE AREA OF THE DISK, AND THE CRACK PROPAGATION CHARACTERISTICS OF THE DISK MATERIAL. (URGENT)
Original recommendation transmittal letter:
Closed--No Longer Applicable
SIOUX CITY, IA, United States
United Airlines Flight 232 McDonnell Douglas DC-10-10
Addressee(s) and Addressee Status:
FAA (Closed--No Longer Applicable)
Safety Recommendation History
Safety Recommendation A-90-88 states that the FAA and General Electric Aircraft Engines should develop an alternate method of inspecting the bore area of the CF6-6 engine fan Stage 1 rotor disks and develop corrective action. The Safety Board is aware that General Electric has unilaterally revised the CF6-6 engine shop manual to include an eddy current inspection of the bore of the disk. Because the shop manual is a part of the FAA-approved maintenance program, it will be mandatory for operators to perform this inspection, as well as an FPI inspection, each time the disk is disassembled from the fan module. The Safety Board believes that these actions largely satisfy the intent of Safety Recommendation A-90-88. Therefore, this recommendation is classified as "Closed--No Longer Applicable."
On July 3, 1990, the FAA convened a Nondestructive Inspection (NDI) Review Team consisting of the Fracture Mechanics/Metallurgy National Resource Specialist, the Nondestructive Evaluation National Resource Specialist, and an engine certification specialist. The team was charged with determining the continued adequacy of the current surface and subsurface inspection methods and inspection intervals specified in Airworthiness Directive (AD) 89-20-01 in detecting defects to ensure the safety of the CF6-6 titanium Stage I fan disk. The NDI review team concluded and recommended that: The contact ultrasonic inspection method prescribed by AD 89-20-01 provided adequate probability of detecting a worst case 0.2-inch corner flaw located in the bore forward corner, and the AD continues to be an adequate screening process for detecting defects in the CF6-6 Stage I fan disk. The 500-cycle reinspection interval for the contact ultrasonic inspection is technically supported by General Electric's fracture mechanics analysis and is an adequate interval for reinspection. Fluorescent penetrant inspection methods, when properly applied, remain a viable inspection method for detecting surface cracks on shot-peened surfaces. The eddy current inspection is an enhancement to the fluorescent penetrant inspection required during normal shop visits. The team did not recommend regulatory implementation. Based on the team's findings, the FAA has concluded that the on-wing (contact) ultrasonic inspection intervals prescribed by AD 89-20-01 are adequate and that, as an enhancement to the current surface inspection requirements, an eddy current inspection of the bore surface in the most critical area (highly stressed) should be accomplished as part of the normal inspection requirements for the disk at the piece part level. General Electric has included an eddy current inspection as part of the shop manual requirements for all CF6 series turbofan engines. Subsequent to the conclusions and recommendations of the NDI Review Team, General Electric proposed a conservative program for the removal of the Stage I fan disks produced from suspect material, Category 2 and Category 3 disks identified in AD 89- 20-01, prior to accumulating 1,250 cycles in service after the accomplishment of the immersion ultrasonic inspection or at the next engine shop visit, whichever occurs first, or prior to accumulating 1,500 cycles since the immersion ultrasonic inspection if an eddy current bore inspection has been accomplished. The FAA concurs with General Electric and is considering amending AD 89-20-01 proposing to make this procedure mandatory. I will provide the Board with a copy of any document that may be issued. Although the FAA disagrees with the need for an alternate means of inspecting the bore area of the CF6-6 Stage I fan disk, it believes that the proposed action meets the intent of the safety recommendation.
The FAA's aircraft certification specialists have significant disagreement with several of the key technical statements and conclusions made by the Safety Board in the preamble to this safety recommendation. The FAA continues to believe that the inspections required by Airworthiness Directive (AD) 89-20-01 are adequate to ensure the safety of the Model DC-10 aircraft having General Electric CF6-6 engines installed. Therefore, I do not believe that there is a need to amend immediately the AD to add an alternative method of inspecting the bore area of the engine fan Stage I rotor disc. Public confidence in the continued operational safety of these engine does, however, warrant that priority be given to a further investigation on the part of the FAA. The adequacies of the various nondestructive inspection methods available is the key issue in question. Therefore, the FAA's Aircraft Certification Service has formed a three-member team comprised of the Nondestructive Evaluation National Resource Specialist, the Fracture Mechanics National Resource Specialist, and a senior engine certification engineer to reassess the longer term adequacy of the inspections required by AD 89-20-01. The team will want to meet with the technical staffs of the Board to ensure it fully understands their point of view. The team will also meet with the technical staffs from United Airlines and General Electric individually to understand their points of view. The team will report its findings to me by August 3, 1990. As soon as I have reviewed the team's findings, I will make a decision concerning the need to amend the inspections required by AD 89-20-01.
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