- United DC-10 at Sioux City
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- United DC-10 at Sioux City
Resulting Safety Initiatives
14 CFR Part 33: No rule changes
14 CFR Part 25: No rule changes
AC 20-128A (and its appendix 1) defines a method of compliance with the CFR that requires design precautions to minimize the hazards to an aircraft in the event of the uncontained engine or auxiliary power unit (APU) failure. AC 20-128 was issued in 1988 following a change to 14 CFR 25.903(d) in Amendment 23 and incorporated much of the same information contained in FAA Order No. 8110.11, dated November 19, 1975. The FAA Order and AC were not available at the time of the DC-10-10 certification. The certification requirements for the DC-10-10 included Part 25 Amendment 1-22 and Special Condition (SC) 25-18-WE-7, dated January 7, 1970.
When AC 20-128 was initially published, it contained the same information that was in FAA Order No. 8110.11. Based on the Sioux City accident, other service events, and a better understanding of the hazards associated with uncontained engine and APU failures, the FAA subsequently revised AC 20-128. The adjacent graphic is a diagram of horizontal stabilizer damage caused by the No. 2 engine debris and the estimated fragment spread angle of the rotor that is described in AC 20-128.
Illustration of Debris Fragment Spread Angle (from NTSB Accident Report, Appendix C)
The fan disk that failed on UAL 232 was manufactured using a double VAR process, which was the process used by the majority of engine manufacturers during the timeframe the disk was manufactured. In the early 1980's timeframe, the FAA and industry began efforts to improve the manufacturing processes by transitioning to enhance manufacturing processes for titanium rotors (e.g., triple VAR). The follow-on activities from the Sioux City accident resulted in additional research and studies of the manufacturing and inspection processes and higher industry standards to further reduce the likelihood of manufacturing quality defects during the manufacturing process. These improved processes and standards are being applied today.
The fan disk that failed on UAL 232 had a metallurgical defect that was introduced at the time of manufacture but was not detected by the manufacturing inspections. The NTSB investigation concluded that the size and location of the defect might not have been detectable using the type of inspections performed during the manufacturing process. The defect resulted in a crack that continued to grow in revenue service and was not detected in the six subsequent fluorescent penetrant inspections performed during the life of the part.
The accident investigation determined that the inspection techniques used as part of the manufacturing process for titanium rotating components require further enhancements to provide a higher probability of detecting metallurgical defects. The investigation also determined that the fluorescent penetrant inspection process has significant potential limitations that must be considered if it is to be applied as the primary inspection technique for critical rotating parts such as fan disks.
The follow-on activities from the Sioux City accident include FAA and industry research and studies on improving the inspection processes during manufacture and overhaul of titanium alloy as well as other material rotors. An example is the blue etch surface inspection to detect surface breaking defects that has been included in the manufacturing process. These enhancements and higher industry standards are being applied today.
Before the Sioux City accident, conventional rotor life management methodologies used by the majority of engine manufacturers did not explicitly address the occurrence of material and manufacturing anomalies. Parts were lifed assuming no defects. Following the Sioux City accident, the FAA recognized the possibility that even with the very best manufacturing and inspection processes, material and manufacturing defects could occur and that these defects could potentially degrade the structural integrity of high-energy rotors. The FAA has worked with industry to develop guidelines and criteria that incorporate a damage tolerance approach in the design and life management of high-energy rotors. This information was published in AC 33.14-1 and has resulted in a titanium alloy rotor design that has greater robustness from catastrophic failure if it were to have a material defect introduced at manufacture.
FAA /Industry Teams and Activities
As a result of this accident, there were a number of follow-on initiatives involving FAA, engine and aircraft manufacturers, and foreign airworthiness authorities to address many of the deficiencies identified in the NTSB safety recommendations and lessons learned. These initiatives resulted in revisions to existing AC material, publication of new ACs, and the implementation of new industry standards. These initiatives did not focus on the DC-10 aircraft and CF6 engine. They were global in nature and applied to large transport aircraft and engines throughout the commercial aviation industry. The following is a partial list of the initiatives and policy changes that resulted from the UAL 232 Sioux City accident.
- Engine Hazard Working Group (EHWG)
a) FAA/Industry/Foreign Airworthiness Authority team was formed to address the question of transport engine non-containment as it relates to aircraft survivability. Assessed blade, disk fan blade containment.
b) Final report dated April 1, 1991.
- FAA Titanium Rotating Components Review Team (TRCRT)
a) Initiative dedicated to data collection, review, and analysis concerning the design, manufacture and inspection of turbine engine titanium rotor disks, hubs, and spools.
b) Report to industry in May 1991.
- SAE Committee on Uncontained Turbine Engine Rotor Events
a) Committee issued report #SP1270.
b) The report is the result of FAA AIR letter following UAL 232 accident and addressed NTSB safety recommendations A-90-172 and A-90-170.
- ARAC Powerplant Installation Harmonization Working Group (PPIHWG)
a) Addressed NTSB safety recommendation A-90-170 and worked on AC 20-128A/B.
- Jet Engine Titanium Quality Committee (JETQC)
a) FAA-sponsored committee to implement a system to report and develop a database of all inclusions found in titanium alloy billet, bar and forgings.
b) Comprised of FAA and engine manufacturers.
- Aerospace Industries of America (AIA) Rotor Integrity Subcommittee (RISC)
a) Committee of industry specialists to address FAA TRCRT recommendations.
b) Standing committee to address rotor integrity issues.
c) Committee helped the FAA develop the damage tolerance framework for engine rotors.
- Transport Aircraft Safety Subcommittee (TASS)
a) In 1989, the FAA Administrator established TASS, which reports to FAA Research, Engineering and Development Advisory Committee.
b) Reporting to TASS was:
• Airworthiness Assurance Task Force (AATF)
• Systems Review Task Force (SRTF)
- Engine Titanium Consortium
a) Chartered to review and improve manufacturing and in-service inspection processes for all rotating rotors.
- AIA Materials and Structures Committee (MSC)
a) FAA/Industry team was founded n 1992 to establish and implement the best manufacturing practices of titanium rotors.
- AC 20-128 (no longer available) addresses design precautions for minimizing hazards to aircraft from uncontained turbine engine and auxiliary power unit rotor failures. This AC existed at the time of the Sioux City accident and was subsequently revised.
- AC 33.15-1 addresses the best manufacturing procedures for titanium alloy rotating engine components. This AC was issued subsequent to the Sioux City accident and was one of the products of the post-Sioux City accident FAA and industry initiatives.