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General Aviation Safety
On 9/8/94, about 1903:23 eastern daylight time, USAir (now US Airways) flight 427, a Boeing 737-3B7 (737-300), N513AU, crashed while maneuvering to land at Pittsburgh Int'l. Airport, Pittsburgh, PA. Flight 427 was operating under the provisions of 14 code of federal regulations (CFR) part 121 as a scheduled domestic passenger flight from Chicago-O'Hare Int'l. Airport, Chicago, Il, to Pittsburgh. The flight departed about 1810, with 2 pilots, 3 flight attendants, and 127 passengers on board. The airplane entered an uncontrolled descent and impacted terrain near Aliquippa, PA. All 132 people on board were killed, and the airplane was destroyed by impact forces and fire. Visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules flight plan.
TO THE FEDERAL AVIATION ADMINISTRATION: Convene an engineering test and evaluation board to conduct a failure analysis to identify potential failure modes, a component and subsystem test to isolate particular failure modes found during the failure analysis, and a full-scale integrated systems test of the Boeing 737 rudder actuation and control system to identify potential latent failures and validate operation of the system without regard to minimum certification standards and requirements in 14 Code of Federal Regulations part 25. Participants in the engineering test and evaluation board should include the FAA; NTSB technical advisors; the Boeing company; other appropriate manufacturers; and experts from other government agencies, the aviation industry, and academia. A test plan should be prepared that includes installation of original and redesigned Boeing 737 main rudder power control units and related equipment and exercises all potential factors that could initiate anomalous behavior (such as thermal effects, fluid contamination, maintenance errors, mechanical failure, system compliance, and structural flexure). The engineering board's work should be completed by March 31, 2000, and published by the FAA.
Original recommendation transmittal letter:
Closed - Acceptable Action
ALIQUIPPA, PA, United States
Uncontrolled Descent and Collision With Terrain, USAir Flight 427, Boeing 737-300, N513AU
Addressee(s) and Addressee Status:
FAA (Closed - Acceptable Action)
Safety Recommendation History
The Safety Board reviewed a copy of the report and found it to be thorough and comprehensive, and the Board is pleased that the ETEB identified many of the same problems and findings identified by the Board. However, on the basis of a briefing by the FAA, the Board is concerned that certain ETEB recommendations, particularly those regarding an independent switch to stop the hydraulic flow to the rudder and a rudder position indicator in 737 cockpits, will not be adopted. Despite this concern, the Board notes that the FAA satisfied the intent of this recommendation. Accordingly, Safety Recommendation A-99-21 is classified "Closed-Acceptable Action."
Letter Mail Controlled 10/26/2000 4:59:28 PM MC# 2001592 The Boeing 737 engineering test and evaluation board defined a number of ground, flight, and laboratory tests to support the failure modes and effects analysis. These tests require the use of Boeing facilities and the direct participation of The Boeing Company. In January 2000, the Society of Professional Engineering Employees Association's labor union went on strike for over 1 month. Five Boeing members of the engineering test and evaluation board and virtually all of the Boeing test personnel participated in this labor strike. The engineering test and evaluation board, while having made progress before the strike, was hindered by Boeing's temporary absence. Further, one of the test facilities used by the engineering test and evaluation board had been partially disassembled and stored prior to the strike and required time to reassemble before work could resume. In addition to the impact of the strike, there were technical challenges to be overcome in the course of executing the engineering test and evaluation board testing. Many of the engineering test and evaluation board tests involve specialized, one-of-a-kind equipment fabricated specifically for the engineering test and evaluation board. There were many unknowns in using this equipment, and it has taken more time to resolve these unforeseen technical issues than was previously anticipated. A third factor that impacted the completion of the engineering test and evaluation board activity by March 31, 2000, is that the engineering test and evaluation board test data have provided new information about the performance of the rudder system. As the test data were analyzed, additional test requirements were added to resolve issues related to this new information. The engineering test and evaluation board has completed its final report, and a copy of the report was provided to your staff. I consider the FAA's action to be completed on this safety recommendation.
In response to each of these three Safety Recommendations (A-99-20, A-99-21, and A-99-22), the FAA notes that it has convened an engineering test and evaluation board to conduct an in-depth failure analysis of the 737 rudder system. The FAA states that the engineering board will examine all of the issues specified in A-99-21 and, in addition, will also focus on any malfunction that could affect lateral/directional control. The FAA states that the engineering board consists of representatives from the FAA, Boeing, the Safety Board, the National Aeronautics and Space Administration, the Department of Defense, the Air Line Pilots Association, the Air Transport Association of America, and also includes engineers from the Ford Motor Company and from Iluyshin Aviation Complex (a Russian aircraft manufacturing company). The Safety Board supports the establishment of the engineering board and has been impressed with the results of its work to date. The engineering board has developed a substantial amount of additional information about failure modes in the 737 rudder system that were previously identified by the Safety Board, and has identified numerous additional potential failure modes. The engineering board's identification of these additional potential failure modes confirms the validity of the concern, expressed by the Safety Board in its report on the USAir flight 427 accident, that although two identified failure modes associated with rudder reversal had been eliminated by design changes to the 737 rudder system, additional unidentified failure modes might still exist. Therefore, pending receipt of the engineering board's final report, Safety Recommendation A-99-21 is classified "Open--Acceptable Response." The engineering board's findings have reinforced the fact that the current 737 rudder system, even with its recent modifications, is not reliably redundant and that design changes are necessary. With regard to Safety Recommendation A-99-20, the FAA states that the engineering board will provide valuable insights, information, and data to determine an appropriate course of action. The Safety Board recognizes that some of the information being developed by the engineering board may be useful in connection with redesigning the 737 rudder system. However, the engineering board was not charged with and has not been, developing a reliably redundant rudder system for the 737 and, therefore, the establishment of the engineering board does not meet the intent of this recommendation. Waiting until the engineering board completes its work and publishes a report will only unnecessarily delay action that has already been demonstrated to be needed and that should have been undertaken as a parallel effort: the provision of a reliably redundant rudder actuation system on 737s. The Safety Board indicated in its report on the USAir flight 427 accident that reliable redundancy could be achieved by developing a multiple-panel rudder surface or providing multiple actuators for a single rudder surface. The Board also suggested two other possible methods by which redundancy might be achieved through use of the standby rudder system. Although the FAA states that it is working closely with Boeing to explore various design options for existing and future 737s, the FAA has not provided any information about progress on the design of a reliably redundant rudder system for the 737. Pending receipt of information about the design options being explored with Boeing and an approximate date when these options could be made available for existing and future 737s, Safety Recommendation A-99-20 is classified "Open Unacceptable Response." Regarding Safety Recommendation A-99-22, the FAA states that it expects that information developed by the engineering board will determine an appropriate course of action for certification of future transport-category airplanes. However, it is unclear how information about the specific failure modes of the rudder system on the 737 (which is the only transport-category airplane with wing-mounted engines that has a single actuator/single rudder surface design) will assist the FAA in ensuring that transport-category airplanes certificated in the future will provide a reliably redundant rudder actuation system, as specified in Safety Recommendation A-99-22. The establishment of the engineering board does not meet the intent of this recommendation. Therefore, because the FAA has not yet taken action to directly address Safety Recommendation A-99-22, pending further appropriate action, it is classified "Open--Unacceptable Response."
Letter Mail Controlled 02/29/2000 11:50:04 AM MC# 2000322
Letter Mail Controlled 6/30/99 3:02:35 PM MC# 990699: The FAA agrees with the intent of this safety recommendation and has convened an engineering test and evaluation board to conduct a failure analysis of the rudder system. The engineering board consists of representatives from the FAA (including two national resource specialists), the Boeing Company, the Safety Board, the National Aeronautics and Space Administration, the Department of Defense, the Air Line Pilots Association, and the Air Transport Association of America. The engineering board will also include an engineer from the Ford Motor Company and an engineer from Iluyshin Aviation Complex (a Russian aircraft manufacturing company). The engineer from Ford is included on the engineering board to obtain an industry perspective that is different from the aviation industry's perspective. The group will identify potential failure modes, component and subsystem tests to isolate particular failure modes found during the failure analysis, and a full-scale integrated system test of the Boeing 737 rudder actuation and control systems to identify potential latent failures and to validate operation of the system without regard to minimum certification standards and requirements in 14 CFR Part 25. The engineering board will also focus on any malfunction that could affect lateral/directional control of the Boeing 737 airplane. The engineering board will prepare a test plan to include installation of original and redesigned Boeing 737 main rudder power control units and related equipment and exercise all potential factors that could initiate anomalous behavior (like thermal effects, fluid contamination, maintenance errors, mechanical failure, system compliance, and structural flexure). The engineering board is tasked to complete its action and publish a report by March 31, 2000. I will keep the Board informed of the FAA's progress on this safety recommendation.
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