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Safety Recommendation Details

Safety Recommendation A-90-090
Details
Synopsis: 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.
Recommendation: THE NTSB RECOMMENDS THAT THE FEDERAL AVIATION ADMINISTRATION: AMEND 14 CFR PART 33 TO REQUIRE THAT TURBINE ENGINES CERTIFICATED UNDER THIS RULE ARE EVALUATED TO IDENTIFY THOSE ENGINE COMPONENTS THAT, IF THEY SHOULD FRACTURE AND SEPARATE, COULD POSE A SIGNIFICANT THREAT TO THE STRUCTURE OR SYSTEMS OF AN AIRPLANE; AND REQUIRE THAT A DAMAGE TOLERANCE EVALUATION OF THESE COMPONENTS BE PERFORMED. BASED ON THIS EVALUATION, REQUIRE THAT THE MAINTENANCE PROGRAMS FOR THESE ENGINES INCLUDE INSPECTIONS OF THE CRITICAL COMPONENTS AT INTERVALS BASED UPON THE CRACK SIZE DETECTABLE BY THE INSPECTION METHOD USED, THE STRESS LEVEL AT VARIOUS LOCATIONS IN THE COMPONENT, AND THE CRACK PROPAGATION CHARACTERISTICS OF THE COMPONENT MATERIAL.
Original recommendation transmittal letter: PDF
Overall Status: Closed - Acceptable Alternate Action
Mode: Aviation
Location: SIOUX CITY, IA, United States
Is Reiterated: No
Is Hazmat: No
Is NPRM: No
Accident #: DCA89MA063
Accident Reports: United Airlines Flight 232 McDonnell Douglas DC-10-10‚Äč
Report #: AAR-90-06
Accident Date: 7/19/1989
Issue Date: 6/18/1990
Date Closed: 5/28/1993
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Alternate Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 5/28/1993
Response: ALTHOUGH THE FAA HAS NOT SPECIFICALLY COMPLIED WITH RECOMMENDATIONS A-90-89 AND -90, THE BOARD BELIEVES THAT THE FAA'S ACTIONS DEMONSTRATE THAT THE FAA IS SERIOUSLY CONSIDERING APPLICATION OF DAMAGE TOLERANCE CONCEPTS TO CRITICAL ROTATING COMPONENTS IN EXISTING & FUTURE ENGINES. BECAUSE THESE ACTIONS, TAKEN AS A WHOLE, ARE AN ACCEPTABLE ALTERNATE APPROACH TO ACHIEVING THE INTENT OF THESE RECOMMENDATIONS, THE BOARD IS CLASSIFYING A-90-89 & -90 AS "CLOSED--ACCEPTABLE ALTERNATE ACTION."

From: FAA
To: NTSB
Date: 4/6/1993
Response: THE RESPONSE LISTS NUMEROUS EXAMPLES OF ACTIVITIES THAT THE FAA HAS SPONSORED OR BEEN A PART OF THAT ARE RESPONSIVE TO THESE TWO RECOMMENDATIONS. THE FAA INDICATES THAT THE REQUIREMENTS OF 14 CFR PART 33 DO NOT NEED TO BE REVISED, BUT THAT POLICY AND ADVISORY MATERIAL WILL BE REVISED OR DEVELOPED AS NECESSARY BASED ON THE RESULTS OF ONGOING EFFORTS THAT ADDRESS THESE RECOMMENDATIONS.

From: NTSB
To: FAA
Date: 10/6/1992
Response: THE BOARD BELIEVES THAT THE FAA ACTIONS, INCLUDING THE RECOMMENDATIONS OF THE TRCRT, INDICATE THAT THE FAA IS SERIOUSLY CONSIDERING DAMAGE TOLERANCE EVALUATIONS & INSPECTIONS OF CRITICAL ROTATING COMPONENTS IN EXISTING & FUTURE TURBINE ENGINES. ALSO, THE BOARD RECOGNIZES THAT THE INTENT OF RECOMMENDATIONS A-90-89 & -90 COULD BE SATISFIED WITHOUT ISSUING ADS OR AMENDING 14 CFR PART 33. FOR EXAMPLE, THE NEW ADVISORY MATERIAL ON LIFING ANALYSIS & LIFE MANAGEMENT PROCEDURES FOR ENGINE LIFE-LIMITED PARTS COULD DEMONSTRATE HOW DAMAGE TOLERANCE EVALUATIONS & INSPECTIONS COULD BE INTEGRATED INTO THE MAINTENANCE SCHEDULE FOR TURBINE ENGINES. ONE OF THE TRCRT'S RECOMMENDATIONS WAS TO INSPECT ALL CRITICAL, LIFE-LIMITED, IN-SERVICE PARTS AT INTERVALS ESTABLISHED BY FRACTURE MECHANICS TECHNOLOGY. YOUR LETTER ALSO INDICATED THAT THE SRTF PROBABLY WILL ISSUE A SIMILAR RECOMMENDATION APPLICABLE TO CRITICAL ENGINE COMPONENTS OF ANY ALLOY TYPE. HOWEVER, YOUR LETTER DID NOT INDICATE HOW THE RECOMMENDATIONS FOR INSPECTION OF ALL CRITICAL, LIFE-LIMITED PARTS AT INTERVALS ESTABLISHED BY FRACTURE MECHANICS TECHNOLOGY WOULD BE IMPLEMENTED. IF SUCH INSPECTIONS BECOME A PART OF THE MAINTENANCE SCHEDULE FOR TURBINE ENGINES, THE BOARD BELIEVES THAT THE INTENT OF RECOMMENDATIONS A-90-89 & -90 MAY BE SATISFIED. PENDING REVIEW OF THE RECOMMENDATIONS OF THE SRTF & RECEIPT OF INFO ON HOW THESE RECOMMENDATIONS ARE TO BE IMPLEMENTED BY THE FAA, RECOMMENDATIONS A-90-89 & -90 ARE CLASSIFIED AS "OPEN--ACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 5/21/1992
Response: THE FAA DOES NOT AGREE THAT 14 CFR PART 33 SHOULD BE AMENDED. THE LETTER NOTES THAT THE TITANIUM ROTATING COMPONENTS REVIEW TEAM (TRCRT) RECOMMENDED (1) THAT LIFE MANAGEMENT METHODOLOGIES BE REQUIRED TO CONSIDER THE EFFECT OF UNDETECTED METALLURGICAL DEFECTS ON PART LIFE AND (2) THAT NEW ADVISORY MATERIAL ON LIFING ANALYSIS AND LIFE MANAGEMENT PROCEDURES BE DEVELOPED FOR LIFE LIMITED ENGINE PARTS. THE FAA IS ALSO PLANNING TO REVISE ADVISORY CIRCULAR 20-128, DESIGN CONSIDERATIONS FOR MINIMIZING HAZARDS CAUSED BY UNCONTAINED TURBINE ENGINE AND AUXILIARY POWER UNIT ROTOR AND FAN BLADE FAILURES.

From: NTSB
To: FAA
Date: 3/4/1991
Response: The Safety Board recognizes that it may be necessary to develop improved or automated methods of inspection before damage tolerance concepts can be implemented for engine components, and the Safety Board encourages the FAA to foster technologies of this type. Because of this, the Safety Board believes that to properly address Safety Recommendations A-90-89 and A-90-90, an overall review of the philosophy of turbine engine certification and the technologies needed for damage tolerance evaluations must be conducted. The FAA response to these recommendations does not indicate to the Safety Board if a comprehensive reevaluation of the philosophy of the certification of turbine engines will be Data Source: NTSB Recommendations to FAA and FAA Responses taken. The Safety Board reiterates its desire that this central point of these recommendations be thoughtfully and thoroughly considered. Because the Safety Board is not convinced that this central point of these recommendations has been thoroughly evaluated, Safety Recommendations A-90-89 and A-90-90 are classified as "Open--Await Response."

From: FAA
To: NTSB
Date: 7/31/1990
Response: As stated during testimony last November before the Subcommittee on Transportation, Aviation, and Materials of the Committee on Science, Space, and Technology, U.S. House of Representatives, the FAA has formed a Titanium Rotating Components Review Team. The team is comprised of FAA technical specialists to review all pertinent design, manufacturing, quality control, and inspection procedures and techniques used in the past 20 years and in use today in the design and production of critical titanium rotating engine components. The substance of this safety recommendation, as well as other issues, is being considered by this review team. The team has completed its data review and field visits and is developing its first draft report. This report is expected to be available for review and comment by September 1990. The FAA is planning to conduct a public conference after the report has been circulated to provide industry and any other interested persons an opportunity to contribute to the final analysis. I look forward to the Board's participation at this conference. I will provide the Board with a copy of the draft report as soon as it becomes available.