Uncontrolled Descent and Collision with Terrain, United Airlines 585
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PB2001-910401 NTSB/AAR-01/01 DCA91MA023 NATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C. 20594 AIRCRAFT ACCIDENT REPORT Uncontrolled Descent and Collision With Terrain United Airlines Flight 585 Boeing 737-200, N999UA 4 Miles South of Colorado Springs Municipal Airport Colorado Springs, Colorado March 3, 1991 5498C Aircraft Accident Report Uncontrolled Descent and Collision With Terrain United Airlines Flight 585 Boeing 737-200, N999UA 4 Miles South of Colorado Springs Municipal Airport Colorado Springs, Colorado March 3, 1991 RAN S P T O L R A T LURIBUS N P UNUM A E O T I I O T N A N S A D FE R NTSB/AAR-01/01 T Y B OA PB2001-910401 National Transportation Safety Board Notation 5498C 490 L’Enfant Plaza, S.W. Adopted March 27, 2001 Washington, D.C. 20594 National Transportation Safety Board. 2001. Uncontrolled Descent and Collision With Terrain, United Airlines Flight 585, Boeing 737-200, N999UA, 4 Miles South of Colorado Springs Municipal Airport, Colorado, Springs, Colorado, March 3, 1991. Aircraft Accident Report NTSB/AAR-01/01. Washington, DC. Abstract: This amended report explains the accident involving United Airlines flight 585, a Boeing 737-200, which entered an uncontrolled descent and impacted terrain 4 miles south of Colorado Springs Municipal Airport, Colorado Springs, Colorado, on March 3, 1991. Safety issues discussed in the report are the potential meterological hazards to airplanes in the area of Colorado Springs; 737 rudder malfunctions, including rudder reversals; and the design of the main rudder power control unit servo valve. The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine, pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and statistical reviews. Recent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also may be obtained from the Web site or by contacting: National Transportation Safety Board Public Inquiries Section, RE-51 490 L'Enfant Plaza, S.W. Washington, D.C. 20594 (800) 877-6799 or (202) 314-6551 Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To purchase this publication, order report number PB2001-910401 from: National Technical Information Service 5285 Port Royal Road Springfield, Virginia 22161 (800) 553-6847 or (703) 605-6000 The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report. iii Aircraft Accident Report Contents Introduction to Amended Report . ix Abbreviations . xi Executive Summary . xv 1. Factual Information . 1 1.1 History of Flight . 1 1.2 Injuries to Persons. 4 1.3 Damage to Aircraft . 5 1.4 Other Damage. 5 1.5 Personnel Information. 5 1.5.1 The Captain . 5 1.5.2 The First Officer. 6 1.5.3 Flight Crew Activities . 6 1.5.4 Air Traffic Control Personnel . 7 1.6 Airplane Information . 7 1.6.1 General. 7 1.6.2 Maintenance History . 8 1.6.3 Boeing 737 Hydraulic System Information . 9 1.6.4 Boeing 737 Flight Control Systems. 11 1.6.4.1 Rudder Control System . 13 1.6.4.1.1 Main Rudder PCU and Servo Valve . 19 1.7 Meteorological Information . 23 1.7.1 Observations and Forecasts . 23 1.7.2 Topics from Meteorological Meeting . 32 1.7.3 Witness Information and Satellite Data on Vortices . 34 1.7.4 Previous Accidents/Incidents Attributed to Vortices. 35 1.7.5 Review of Information Obtained from A.J. Bedard, Jr., NOAA . 36 1.8 Aids To Navigation. 37 1.9 Communications . 37 1.10 Aerodrome Information . 37 1.10.1 General. 37 1.10.2 Weather Related Accidents/Incident Data . 38 1.11 Flight Recorders . 38 1.12 Wreckage and Impact Information . 40 1.12.1 Engine Mount Examinations . 44 1.12.2 Examination of Roll and Pitch Flight Controls Components and Other Systems . 44 1.12.2.1 Hydraulic System A and B Pressure Modules. 45 1.12.2.2 Roll Control Components . 47 1.12.2.3 Pitch Control Components. 49 1.12.3 Examination of Yaw Control Components . 49 1.12.3.1 System Main Rudder Power Control Unit. 49 Contents iv Aircraft Accident Report 1.12.3.2 Standby Rudder Actuator. 50 1.13 Medical And Pathological Information . 50 1.14 Fire . 52 1.15 Survival Aspects . 52 1.16 Tests and Research . 52 1.16.1 Recorded Radar Data . 52 1.16.2 Modeling and Simulations of Atmospheric Disturbances and Airplane Flight Dynamics Conducted During the Initial Investigation (1991/1992). 53 1.16.2.1 Modeling of Atmospheric Disturbances (1991/1992): NCAR Weather Study. 53 1.16.2.2 Safety Board Simulations (1991/1992) . 56 1.16.2.3 Boeing Simulations (1991/1992) . 57 1.16.3 Detail Examination and Tests of Standby Rudder Actuator Input Shaft and Bearing . 62 1.16.3.1 Additional Standby System Actuator Binding/Jam Tests . 66 1.16.4 Other Documented Rudder Control Incidents . 68 1.16.5 Additional Examination and Testing of 737 Flight Control Systems/Components . 71 1.16.5.1 Detailed Examinations of Main Rudder PCU Servo Valves . 72 1.16.5.2 PCU Servo Valve Chip Shear Tests . 74 1.16.5.3 PCU Thermal Testing . 75 1.16.5.3.1 Baseline Test Condition. 76 1.16.5.3.2 Simulated Hydraulic System Failure Condition . 76 1.16.5.3.3 Extreme Temperature Differential Test Condition . 76 1.16.5.3.4 Additional Testing . 78 1.16.5.4 Rudder Actuator Reversals During Servo Valve Secondary Slide Jams . 79 1.16.5.5 Ground Demonstration of Rudder PCU Servo Valve Jam. 83 1.16.6 Flight Performance Simulation Studies Conducted During the USAir Flight 427 Investigation (1994-1999) . 85 1.16.6.1 United Flight 585 Simulation Studies . ..