Aircraft Accident Report

Loss of Control and Impact with Pacific

Ocean Alaska Airlines Flight 261

McDonnell Douglas MD-83, N963AS

About 2.7 Miles North of

Anacapa Island, California

January 31, 2000

National 

Transportation 

Safety Board

Washington, D.C.

A

A

N TION

L TRA SPOR

ATIO

N

B OARD

SAF E T Y

N

T

E PLURIBUS UNUM 

NTSB/AAR-02/01

PB2002-910402

Aircraft Accident Report

Loss of Control and Impact with Pacific Ocean 

Alaska Airlines Flight 261

McDonnell Douglas MD-83, N963AS

About 2.7 Miles North of

Anacapa Island, California

January 31, 2000

NTSB/AAR-02/01

PB2002-910402 National Transportation Safety Board

Notation 7263E 490 LíEnfant Plaza, S.W.

Adopted December 30, 2002 Washington, D.C. 20594

E PLUR IBUS UNUM

NATIONAL TRA S PORTATION

B OAR D

SAFE T Y

N
National Transportation Safety Board. 2003. Loss of Control and Impact with Pacific Ocean, Alaska

Airlines Flight 261, McDonnell Douglas MD-83, N963AS, About 2.7 Miles North of Anacapa Island,

California, January 31, 2000. Aircraft Accident Report NTSB/AAR-02/01. Washington, DC.

Abstract: This report explains the accident involving Alaska Airlines flight 261, a McDonnell Douglas

MD-83, which crashed into the Pacific Ocean about 2.7 miles north of Anacapa Island, California.

Safety issues discussed in this report include lubrication and inspection of the jackscrew

assembly, extension of lubrication and end play check intervals, jackscrew assembly overhaul

procedures, the design and certification of the MD-80 horizontal stabilizer trim control system,

Alaska Airlinesí maintenance program, and Federal Aviation Administration (FAA) oversight of

Alaska Airlines. Safety recommendations are addressed to the FAA.

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 LíEnfant 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 PB2002-910402 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

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii

1. Factual Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 History of Flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Injuries to Persons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3 Damage to Airplane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4 Other Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5 Personnel Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5.1 The Captain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5.2 The First Officer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.6 Airplane Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.6.1 MD-80 Longitudinal Trim Control System Information . . . . . . . . . . . . . . . . . . . . . 13

1.6.1.1 Primary Trim Control System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.6.1.2 Alternate Trim Control System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.6.1.3 Autopilot Pitch Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.6.2 Design and Certification of MD-80 Series Airplanes . . . . . . . . . . . . . . . . . . . . . . . 19

1.6.2.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.6.2.2 Longitudinal Trim Control System Certification. . . . . . . . . . . . . . . . . . . . . . . . . 19

1.6.3 Maintenance Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.6.3.1 Alaska Airlinesí MD-80 Maintenance Program . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.6.3.1.1 Maintenance Program Development Guidance. . . . . . . . . . . . . . . . . . . . . . . 24

1.6.3.1.2 Alaska Airlinesí Continuous Airworthiness Maintenance Program . . . . . . . 26

1.6.3.1.3 Alaska Airlinesí Reliability Analysis Program. . . . . . . . . . . . . . . . . . . . . . . 26

1.6.3.2 Jackscrew Assembly Lubrication Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.6.3.2.1 Jackscrew Assembly Lubrication Intervals. . . . . . . . . . . . . . . . . . . . . . . . . . 31

1.6.3.2.1.1 Manufacturer-Recommended Lubrication Intervals. . . . . . . . . . . . . . 31

1.6.3.2.1.2 Alaska Airlinesí Lubrication Intervals . . . . . . . . . . . . . . . . . . . . . . . . 32

1.6.3.2.2 Chronology of Grease Type Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

1.6.3.2.3 July 19, 2002, Alaska Airlines Maintenance Information Letter . . . . . . . . . 36

1.6.3.3 Procedures for Monitoring Acme Nut Thread Wear . . . . . . . . . . . . . . . . . . . . . . 37

1.6.3.3.1 Defining and Calculating Wear Rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

1.6.3.3.2 Development of End Play Check Procedures . . . . . . . . . . . . . . . . . . . . . . . . 38

1.6.3.3.2.1 Alaska Airlinesí End Play Check Procedure . . . . . . . . . . . . . . . . . . . 43

1.6.3.3.3 Horizontal Stabilizer Restraining Fixture . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

1.6.3.3.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

1.6.3.3.3.2 Alaska Airlines-Manufactured Restraining Fixtures . . . . . . . . . . . . . 44

1.6.3.3.4 End Play Check Intervals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

1.6.3.3.4.1 Manufacturer-Recommended End Play Check Intervals . . . . . . . . . . 47

1.6.3.3.4.2 Alaska Airlinesí End Play Check Intervals . . . . . . . . . . . . . . . . . . . . 47

1.6.3.4 Accident Airplaneís Maintenance Information . . . . . . . . . . . . . . . . . . . . . . . . . . 49

1.6.3.4.1 Accident Airplaneís Last C Check Requiring an End Play Check . . . . . . . . 50
Contents iv Aircraft Accident Report

1.6.3.4.2 Maintenance Personnel Statements Regarding the Accident

Airplaneís Last End Play Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

1.7 Meteorological Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1.8 Aids to Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1.9 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1.10 Airport Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

1.11 Flight Recorders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

1.11.1 Cockpit Voice Recorder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

1.11.1.1 CVR Sound Spectrum Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

1.11.2 Flight Data Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

1.12 Wreckage and Impact Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

1.12.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

1.12.2 Horizontal Stabilizer Jackscrew Assembly Components. . . . . . . . . . . . . . . . . . . . . 58

1.13 Medical and Pathological Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

1.14 Fire. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

1.15 Survival Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

1.16 Tests and Research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

1.16.1 Airplane Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

1.16.1.1 Airplane Performance Simulation Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

1.16.2 Metallurgical Examinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

1.16.2.1 Horizontal Stabilizer Forward Spar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

1.16.2.2 Acme Screw. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

1.16.2.3 Acme Screw Torque Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

1.16.2.4 Lower Mechanical Stop and Corresponding Spline Area on the Acme Screw . . 71

1.16.2.5 Acme Nut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

1.16.2.6 Acme Nut Thread Remnant Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

1.16.3 Shear Load Capability of the Acme Nut Threads . . . . . . . . . . . . . . . . . . . . . . . . . . 77

1.16.4 Studies of Thread Stress and Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

1.16.5 Torque Tube Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

1.16.6 Additional Safety Board Jackscrew Assembly Examinations . . . . . . . . . . . . . . . . . 80

1.16.6.1 February 2000 Post-AD 2000-03-51 Jackscrew Assembly Examinations . . . . . 80

1.16.6.2 Examinations of Hawaiian Airlinesí Jackscrew Assemblies. . . . . . . . . . . . . . . . 81

1.16.6.3 Examinations of Alaska Airlines Reports of Acme Screw ìWobbleî. . . . . . . . . 83

1.16.7 Chemical and Microscopic Analyses of Grease Residues from the Accident

Jackscrew Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

1.16.8 Additional Grease Testing, Experiments, and Analysis . . . . . . . . . . . . . . . . . . . . . 85

1.16.8.1 Standardized Grease Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

1.16.8.2 Experiments and Analysis on the Surface Chemistry of Acme

Nut Material When Exposed to Various Greases or Grease Mixtures . . . . . . . . 85

1.16.8.3 Wear Testing Under Various Grease Conditions. . . . . . . . . . . . . . . . . . . . . . . . . 86

1.17 Organizational and Management Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

1.17.1 Alaska Airlines, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

1.17.2 Alaska Airlines Flight Crew Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

1.17.2.1 Stabilizer Trim Check Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

1.17.2.2 Runaway Stabilizer Checklist Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

1.17.2.3 Stabilizer Inoperative Checklist Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

1.17.2.3.1 Postaccident Boeing Flight Operations Bulletin on Stabilizer

Trim Inoperative/Malfunction Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 92

1.17.3 FAA Oversight of Alaska Airlines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

1.17.3.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Contents v Aircraft Accident Report

1.17.3.2 Preaccident FAA National Aviation Safety Inspection Program Inspection. . . . 93

1.17.3.3 FAA Air Transportation Oversight System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

1.17.3.3.1 Seattle CMO Memorandum on Staff Shortages for Surveillance

of Alaska Airlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

1.17.3.4 FAA Postaccident Special Inspection of Alaska Airlines . . . . . . . . . . . . . . . . . . 95

1.17.3.4.1 FAA Proposed Suspension of Alaska Airlinesí Heavy Maintenance

Authority . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

1.17.3.4.2 Alaska Airlines Airworthiness and Operations Action Plan. . . . . . . . . . . . . 99

1.17.3.4.3 FAA Followup Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

1.17.3.4.4 DOT Office of the Inspector General Report on FAA Oversight

of Continuing Analysis and Surveillance Programs . . . . . . . . . . . . . . . . . . 102

1.17.4 Other Safety Evaluations of Alaska Airlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

1.17.4.1 DoD Capability Survey of Alaska Airlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

1.17.4.2 Criminal Investigation of Alaska Airlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

1.17.4.3 Postaccident Independent Safety Assessment of Alaska Airlines . . . . . . . . . . . 104

1.18 Additional Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

1.18.1 Postaccident Airworthiness Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

1.18.1.1 Safety Board and FAA Correspondence Regarding the 2,000-Flight-Hour

End Play Check Interval Specified in ADs 2000-03-51 and 2000-15-15 . . . . . 106

1.18.2 Safety Board Statistical End Play Data Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

1.18.3 Previous Safety Recommendations Resulting from the Alaska Airlines

Flight 261 Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

1.18.4 Alaska Airlines Fleetwide MD-80 Jackscrew Assembly Data and Tracking

History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

1.18.5 MD-11 Jackscrew Assembly Acme Nut Wear History . . . . . . . . . . . . . . . . . . . . . 116

1.18.6 Safety Board Observations of Jackscrew Assembly Lubrications. . . . . . . . . . . . . 116

1.18.7 End Play Check Anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

1.18.7.1 Safety Board End Play Check Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

1.18.7.2 Postaccident Alaska Airlines-Reported Near-Zero End Play Measurements . . 118

1.18.8 Jackscrew Assembly Overhaul Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

1.18.8.1 Jackscrew Overhaul Specifications and Authority . . . . . . . . . . . . . . . . . . . . . . 119

1.18.8.2 Review of the Boeing DC-9 Overhaul Maintenance Manual . . . . . . . . . . . . . . 120

1.18.8.3 Safety Board Maintenance Facility Observations . . . . . . . . . . . . . . . . . . . . . . . 121

1.18.8.4 Overhauled Jackscrew Assembly Data From 1996 to 1999 . . . . . . . . . . . . . . . 122

1.18.9 Industrywide Jackscrew Assembly Maintenance Procedures . . . . . . . . . . . . . . . . 122

1.18.10 FAA Commercial Airplane Certification Process Study . . . . . . . . . . . . . . . . . . . . 123

1.18.11 Fail-Safe Jackscrew Assembly Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

2. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

2.2 Accident Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

2.2.1 Takeoff and Climbout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

2.2.2 Jamming of the Horizontal Stabilizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

2.2.2.1 Cause of the Jam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

2.2.3 Release of the Jam and the Initial Dive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

2.2.4 The Second and Final Dive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

2.2.5 Flight Crew Decision-Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

2.2.5.1 Decision to Continue Flying Rather than Return to PVR . . . . . . . . . . . . . . . . . 135

2.2.5.2 Use of the Autopilot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

2.2.5.3 Configuration Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Contents vi Aircraft Accident Report

2.2.5.4 Activation of the Primary Trim Motor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

2.2.5.5 Adequacy of Current Guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

2.3 Evaluation of Potential Reasons for Excessive Acme Nut Thread Wear . . . . . . . . . . . 140

2.3.1 Use of Aeroshell 33 for Lubrication of the Jackscrew Assembly . . . . . . . . . . . . . 141

2.3.2 Acme Screw Thread Surface Finish. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

2.3.3 Foreign Debris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

2.3.4 Abnormal Loading of the Acme Nut Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

2.3.5 Summary of Possibilities Ruled Out as Reasons for the Excessive Acme

Nut Thread Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

2.3.6 Insufficient Lubrication of the Jackscrew Assembly. . . . . . . . . . . . . . . . . . . . . . . 144

2.3.6.1 Analysis of How Many Recently Scheduled Lubrications Might Have

Been Missed or Inadequately Performed Before the Accident . . . . . . . . . . . . . 146

2.3.6.2 Alaska Airlinesí Lubrication Interval Extension . . . . . . . . . . . . . . . . . . . . . . . . 146

2.3.6.2.1 Safety Implications of Lubrication Interval Extension . . . . . . . . . . . . . . . . 147

2.3.6.3 Adequacy of Lubrication Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

2.4 Monitoring Acme Nut Thread Wear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

2.4.1 Alaska Airlinesí Preaccident End Play Check Intervals . . . . . . . . . . . . . . . . . . . . 151

2.4.1.1 Adequacy of Existing Process for Establishing Maintenance Task Intervals . . 153

2.4.2 Adequacy of Current End Play Check Intervals . . . . . . . . . . . . . . . . . . . . . . . . . . 154

2.4.3 Adequacy of End Play Check Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

2.5 Deficiencies of Jackscrew Assembly Overhaul Procedures and Practices. . . . . . . . . . 159

2.6 Horizontal Stabilizer Trim System Design and Certification Issues . . . . . . . . . . . . . . 162

2.6.1 Acme Nut Thread Loss as a Catastrophic Single-Point Failure Mode . . . . . . . . . 162

2.6.2 Prevention of Acme Nut Thread Loss Through Maintenance and Inspection. . . . 165

2.6.3 Elimination of Catastrophic Effects of Acme Nut Thread Loss Through Design . 166

2.6.4 Consideration of Wear-Related Failures During Design and Certification . . . . . . 167

2.7 Deficiencies in Alaska Airlinesí Maintenance Program. . . . . . . . . . . . . . . . . . . . . . . . 167

2.7.1 April 2000 FAA Special Inspection Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

2.7.2 Maintenance-Related Deficiencies Identified During This Accident

Investigation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

2.7.2.1 General Policy Decisions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

2.7.2.2 Specific Maintenance Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

2.7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

2.8 FAA Oversight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

3.1 Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

3.2 Probable Cause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

4. Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.1 New Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.2 Previously Issued Recommendations Resulting From This Accident Investigation . . 184

Board Member Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

5. Appendixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
vii Aircraft Accident Report

Figures

1. The accident airplaneís flightpath, starting about 1609 (about the time of the initial dive)

and ending about 1620 (about the time of the second and final dive). . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. Radar altitude data and selected ATC transmissions from about 1609 to 1620.. . . . . . . . . . . . . . . . . . 6

3. Installation of jackscrew assembly within the horizontal and vertical stabilizers. . . . . . . . . . . . . . . . 13

4. The MD-80 horizontal and vertical stabilizer tail structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5. Acme screw and nut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6. Detailed schematic of the longitudinal trim actuating mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7. Cockpit switches, handles, and indicators for the longitudinal control system. . . . . . . . . . . . . . . . . . 17

8. Depiction of how restraining fixture should be placed on the horizontal stabilizer during an

end play check. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

9. View of a typical dial indicator set up for end play checks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

10. Two Alaska Airlines-fabricated restraining fixtures and three Boeing- manufactured fixtures. . . . . 46

11. The September 27, 1997, MIG-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

12a. The acme screw immediately after it was brought on board the recovery ship . . . . . . . . . . . . . . . . . 59

12b. The acme screw immediately after it was brought to shore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

12c. The acme screw during initial inspection.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

13. A photograph of the acme screw thread remnants wrapped around the screw, the red rust

areas, and the white deposits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

14. A photograph of the sand/grease mixture packed between the acme screwís lower threads. . . . . . . 69

15. A photograph of an overall view of the recovered acme nut assembly. . . . . . . . . . . . . . . . . . . . . . . . 73

16. A photograph of a closer view of the interior of the acme nut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

17. A diagram of a cross-section through the acme nutís grease passageway and fitting. Red areas

denote the locations where red grease was found, and gray areas denote the location where the

dry residue was found. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

18. A photograph of the grease passageway and counterbore after the acme nut was sectioned. . . . . . . 75
Figures viii Aircraft Accident Report

19. A summary of the results of the high-load wear tests, including the contamination

and low-temperature tests.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

20. A graphical depiction of the stages of acme nut thread wear to the point of fracture. . . . . . . . . . . 131
ix Aircraft Accident Report

Abbreviations

AC advisory circular

AD airworthiness directive

ALPA Air Line Pilots Association

AMFA Aircraft Mechanics Fraternal Association

AOL all operators letter

ARTCC Air Route Traffic Control Center

ASB alert service bulletin

ATC air traffic control

ATOS Air Transportation Oversight System

ATP airline transport pilot

CAM cockpit area microphone

CAR Civil Aeronautics Regulations

CFR Code of Federal Regulations

CG center of gravity

CMO Certificate Management Office

CMR certification maintenance requirement

CSP comprehensive surveillance plan

CVR cockpit voice recorder

DFGC digital flight guidance computer

DoD Department of Defense

DOT Department of Transportation

EDS (x-ray) energy dispersive spectroscopy

E.O. engineering order

FAA Federal Aviation Administration

FARs Federal Aviation Regulations

FDR flight data recorder
Abbreviations x Aircraft Accident Report

FEA finite element analysis

FL flight level

FSAW Flight Standards Information Bulletin for Airworthiness

FSDO Flight Standards District Office

Ftu minimum ultimate strength

GMM General Maintenance Manual

KIAS knots indicated airspeed

LAX Los Angeles International Airport, Los Angeles,

California

MAC mean aerodynamic chord

MRB Maintenance Review Board

MSG Maintenance Steering Group 

MSI maintenance significant item

MTBR mean time between removal

MTBUR mean time between unscheduled removal

NASIP National Aviation Safety Inspection Program

OAK Oakland International Airport, Oakland, California

OAMP on-aircraft maintenance planning

OJT on-the-job training

P/N part number

PMI principal maintenance inspector

psi pounds per square inch

PST Pacific standard time

PTRS Program Tracking and Reporting System

PVR Lic Gustavo Diaz Ordaz International Airport, Puerto

Vallarta, Mexico

QRH Quick Reference Handbook

RMS root mean squared

S/N serial number

SAE Society of Automotive Engineers
Abbreviations xi Aircraft Accident Report

SB service bulletin

SEA Seattle-Tacoma International Airport, Seattle,

Washington

SEM scanning electron microscopy

SFO San Francisco International Airport, San Francisco,

California

UNS unified numbering system
xii Aircraft Accident Report

Executive Summary

On January 31, 2000, about 1621 Pacific standard time, Alaska Airlines, Inc.,

flight 261, a McDonnell Douglas MD-83, N963AS, crashed into the Pacific Ocean about

2.7 miles north of Anacapa Island, California. The 2 pilots, 3 cabin crewmembers, and

83 passengers on board were killed, and the airplane was destroyed by impact forces.

Flight 261 was operating as a scheduled international passenger flight under the

provisions of 14 Code of Federal Regulations Part 121 from Lic Gustavo Diaz Ordaz

International Airport, Puerto Vallarta, Mexico, to Seattle-Tacoma International Airport,

Seattle, Washington, with an intermediate stop planned at San Francisco International

Airport, San Francisco, California. Visual meteorological conditions prevailed for the

flight, which operated on an instrument flight rules flight plan.

The National Transportation Safety Board determines that the probable cause of

this accident was a loss of airplane pitch control resulting from the in-flight failure of the

horizontal stabilizer trim system jackscrew assemblyís acme nut threads. The thread

failure was caused by excessive wear resulting from Alaska Airlinesí insufficient

lubrication of the jackscrew assembly. 

Contributing to the accident were Alaska Airlinesí extended lubrication interval

and the Federal Aviation Administrationís (FAA) approval of that extension, which

increased the likelihood that a missed or inadequate lubrication would result in excessive

wear of the acme nut threads, and Alaska Airlinesí extended end play check interval and

the FAAís approval of that extension, which allowed the excessive wear of the acme nut

threads to progress to failure without the opportunity for detection. Also contributing to

the accident was the absence on the McDonnell Douglas MD-80 of a fail-safe mechanism

to prevent the catastrophic effects of total acme nut thread loss. 

The safety issues discussed in this report include lubrication and inspection of the

jackscrew assembly, extension of lubrication and end play check intervals, jackscrew

assembly overhaul procedures, the design and certification of the MD-80 horizontal

stabilizer trim control system, Alaska Airlinesí maintenance program, and FAA oversight

of Alaska Airlines. Safety recommendations are addressed to the FAA.
Factual Information 1 Aircraft Accident Report

1. Factual Information

1.1 History of Flight

On January 31, 2000, about 1621 Pacific standard time (PST),1 Alaska Airlines,

Inc., flight 261, a McDonnell Douglas MD-83 (MD-83), N963AS, crashed into the Pacific

Ocean about 2.7 miles north of Anacapa Island, California. The 2 pilots, 3 cabin

crewmembers, and 83 passengers2 on board were killed, and the airplane was destroyed by

impact forces. Flight 261 was operating as a scheduled international passenger flight

under the provisions of 14 Code of Federal Regulations (CFR) Part 121 from Lic Gustavo

Diaz Ordaz International Airport (PVR), Puerto Vallarta, Mexico, to Seattle-Tacoma

International Airport (SEA), Seattle, Washington, with an intermediate stop planned at

San Francisco International Airport (SFO), San Francisco, California. Visual

meteorological conditions prevailed for the flight, which operated on an instrument flight

rules flight plan.

The accident airplane arrived at PVR about 1239 on the day of the accident.3 The

inbound pilots stated that they met the accident pilots outside the airplane and briefly

discussed its status.4

FDR information from the accident flight indicated that during taxi for takeoff,

when the FDR started recording, the horizontal stabilizer5

 was at the 7° airplane-nose-up

position, which was the takeoff pitch trim6 setting. About 1337, the accident airplane

departed PVR as flight 261. A National Transportation Safety Board review of FAA air

traffic control (ATC) tapes and FDR data from the accident flight indicated that the first

1 Unless otherwise indicated, all times are PST, based on a 24-hour clock. The times reported in the

cockpit voice recorder (CVR) transcript were based on a coordinated universal time reference clock used by

the Federal Aviation Administrationís (FAA) Southern California Terminal Radar Approach Control facility

for recorded radar data. CVR times were also correlated to flight data recorder (FDR) data. After the

correlation was accomplished, all times from both recorders were converted to PST.

2 The 83 passengers included 3 children who were less than 2 years old. These children were lap-held

passengers.

3 FDR data indicated that the accident airplane landed at PVR with a horizontal stabilizer angle of

6o airplane nose up.

4 The inbound captain described the accident flight crew as rested, relaxed, and in good spirits. The

inbound first officer stated that the accident captain was happy to see them, upbeat, rested, and ready to go to

work.

5 The MD-80ís horizontal stabilizer is a critical flight control located at the top of the vertical stabilizer.

The horizontal stabilizer is hinged near its trailing edge so that the leading edge can traverse up and down to

provide trim for the airplane in the pitch axis. For a description of the airplaneís horizontal stabilizer and

other components of the longitudinal trim control system, see section 1.6.1. 

6 Pitch trim (also known as longitudinal trim) is the adjustment of the airplaneís horizontal stabilizer to

achieve a balanced, or stable, flight condition. The longitudinal trim control system is designed to minimize

or eliminate the pilot control forces needed to hold the flight controls in the proper position to achieve the

desired pitch for a particular phase of flight.
Factual Information 2 Aircraft Accident Report

officer was the pilot flying immediately after the airplaneís departure.7 The flight plan

indicated that flight 261ís cruising altitude would be flight level (FL) 310.8

FDR data indicated that during the initial portion of the climb, the horizontal

stabilizer moved at the primary trim motor rate of 1/3o per second from 7o to 2o airplane

nose up. According to FDR data, the autopilot was engaged at 1340:12, as the airplane

climbed through an altitude of approximately 6,200 feet. Thereafter, the FDR recorded

horizontal stabilizer movement at the alternate trim motor rate of 1/10o per second from 2°

airplane nose up to 0.4° airplane nose down.9

 At 1349:51, as the airplane continued to

climb through approximately 23,400 feet at 331 knots indicated airspeed (KIAS), the

CVR recorded the horizontal stabilizer move from 0.25° to 0.4° airplane nose down.10

This was the last horizontal stabilizer movement recorded until the airplaneís initial dive

about 2 hours and 20 minutes later. At 1353:12, when the airplane was climbing through

28,557 feet at 296 KIAS, the autopilot disengaged. 

FDR information and airplane performance calculations indicated that, during the

next 7 minutes, the airplane continued to climb at a much slower rate. During this part of

the ascent, the elevators were deflected between -1° and -3°, and the airplane was flown

manually using up to as much as 50 pounds of control column pulling force.11 After

reaching level flight, the airplane was flown for about 24 minutes using approximately

30 pounds of pulling force at approximately 31,050 feet and 280 KIAS. The airspeed was

then increased to 301 KIAS, and the airplane was flown for almost another 1 hour

22 minutes using about 10 pounds of pulling force. At 1546:59, the autopilot was

re-engaged.

According to Alaska Airlines documents, ATC and CVR information, and

postaccident interviews with Alaska Airlines dispatch and maintenance personnel, the

flight crew contacted the airlineís dispatch and maintenance control facilities in SEA some

time before the beginning of the CVR transcript at 1549:4912 to discuss a jammed

horizontal stabilizer and a possible diversion to Los Angeles International Airport (LAX),

Los Angeles, California.13 These discussions were conducted on a shared company radio

7 According to FDR data, the No. 2 autopilot was selected during the initial part of the flight. Selection

of the No. 2 autopilot commands the No. 2 digital flight guidance computer (DFGC) to provide steering

guidance to the first officerís instruments. 

8 FL 310 is 31,000 feet mean sea level, based on an altimeter setting of 29.92 inches of mercury.

9 Operation of the alternate trim motor during this period is consistent with pitch control being

commanded by the autopilot, which was engaged at 1340:12. 

10 KIAS is the speed of the airplane as shown on the airspeed indicator on the cockpit control panel. All

airspeed information from the FDR is given in KIAS.

11 All estimates of pulling force in this report refer to the combined control column forces from both the

captainís and the first officerís control columns. It is not known whether these forces were applied by the

captain, the first officer, or both.

12 For a complete transcript of the 31-minute CVR recording, see appendix B.

13 FDR data indicated that a continuing series of radio transmissions began over the very-high

frequency 2 channel (used for all non-ATC radio transmissions) about 1521 and continued until the end of

the FDR recording about 1620:56. 
Factual Information 3 Aircraft Accident Report

frequency between Alaska Airlinesí dispatch and maintenance facilities at SEA and its

operations and maintenance facilities at LAX. 

At 1549:56, the autopilot was disengaged; it was re-engaged at 1550:15.

According to the CVR transcript, at 1550:44, SEA maintenance asked the flight crew,

ìunderstand youíre requestingÖdiversion to LAÖis there a specific reason you prefer LA

over San Francisco?î14 The captain replied, ìwell a lotta times its windy and rainy and wet

in San Francisco and uh, it seemed to me that a dry runwayÖwhere the wind is usually

right down the runway seemed a little more reasonable.î

At 1552:02, an SEA dispatcher provided the flight crew with the current SFO

weather (wind was 180° at 6 knots; visibility was 9 miles). The SEA dispatcher added, ìif

uh you want to land at LA of course for safety reasons we will do thatÖweíllÖtell you

though that if we land in LAÖweíll be looking at probably an hour to an hour and a half

we have a major flow program going right now.î15 At 1552:41, the captain replied, ìI

really didnít want to hear about the flow being the reason youíre calling us cause Iím

concerned about overflying suitable airports.î At 1553:28, the captain discussed with the

first officer potential landing runways at SFO, stating, ìone eight zero at sixÖso thatís

runway one six what we need is runway one nine, and theyíre not landing runway one

nine.î The first officer replied, ìI donít think so.î At 1553:46, the captain asked SEA

dispatch if they could ìget some supportî or ìany ideasî from an instructor to troubleshoot

the problem; he received no response. At 1555:00, the captain commented, ìit just blows

me away they think weíre gonna land, theyíre gonna fix it, now theyíre worried about the

flow, Iím sorry this airplaneís [not] gonna go anywhere for a whileÖso you know.î A

flight attendant replied, ìso theyíre trying to put the pressure on you,î the captain stated,

ìwell, no, yea.î

At 1556:08, the SEA dispatcher informed the flight crew that, according to the

SFO automatic terminal information service, the landing runways in use at SFO were 28R

and 28L and that ìit hasnít rained there in hours so Iím looking atÖprobably a dry

runway.î At 1556:26, the captain stated that he was waiting for a requested center of

gravity (CG) update (for landing), and then he requested information on wind conditions

at LAX. At 1556:50, the SEA dispatcher replied that the wind at LAX was 260° at 9 knots.

Nine seconds later, the captain, comparing SFO and LAX wind conditions, told the

SEA dispatcher, ìversus a direct crosswind which is effectively no change in

groundspeedÖI gotta tell you, when I look at it from a safety point I think that something

that lowers my groundspeed makes sense.î16 The SEA dispatcher replied, ìthatíll mean

LAX then for you.î He then asked the captain to provide LAX operations with the

information needed to recompute the airplaneís CG because ìthey can probably whip out

that CG for you real quick.î At 1558:15, the captain told the SEA dispatcher, ìweíre goin

14 This is the first reference on the CVR recording to the accident flight crewís request to divert to LAX.

15 The SEA dispatcherís comment refers to ongoing departure delays at LAX caused by heavy air traffic

congestion.

16 If the airplane had landed at SFO, it would have encountered a direct crosswind from the south. If the

airplane had landed at LAX, it would have encountered a minimal crosswind from the southwest. 
Factual Information 4 Aircraft Accident Report

to LAX weíre gonna stay up here and burn a little more gas[17] get all our ducks in a row,

and then weíll uh be talking to LAX when we start down to go in there.î At 1558:45, the

captain asked LAX operations if it could ìcompute [the airplaneís] current CG based on

the information we had at takeoff.î

At 1602:33, the captain asked LAX operations for wind information at SFO. LAX

operations replied that the winds at SFO were 170° at 6 knots. The captain replied, ìthatís

what I needed. We are comin in to see you.î At 1603:56, the first officer began giving

LAX operations the information it needed to recompute the airplaneís CG for landing. 

At 1607:54, a mechanic at Alaska Airlinesí LAX maintenance facility contacted

the flight crew on the company radio frequency and asked, ìare you [the] guys with the

uh, horizontal [stabilizer] situation?î The captain replied, ìaffirmative,î and the mechanic,

referring to the stabilizerís primary trim system, asked, ìdid you try the suitcase handles

and the pickle switches?î18 At 1608:03, the captain replied, ìyea we tried everything

together.î At 1608:08, the captain added, ìweíve run just about everything if youíve got

any hidden circuit breakers weíd love to know about ëem.î19 The mechanic stated that he

would ìlook at the uh circuit breaker uh guide just as a double check.î The LAX mechanic

then asked the flight crew about the status of the alternate trim system, and, at 1608:35, the

captain replied that ìit appears to be jammedÖthe whole thing, it [the AC load meter]

spikes out when we use the primary, we get AC [electrical] load that tells me the motorís

tryin to run but the brake wonít move it. when we use the alternate, nothing happens.î20

At 1608:50, the LAX mechanic asked, ìyou say you get a spikeÖon the meter up

there in the cockpit when you uh try to move it with theÖprimary right?î According to

the CVR transcript, at 1608:59, the captain addressed the first officer before responding to

the mechanic, stating, ìIím gonna click it off you got it.î One second later, the first officer

replied, ìok.î At 1609:01, the captain reiterated to the LAX mechanic that the spike

occurred ìwhen we do the primary trim but thereís no appreciable uh change in the uh

electrical uh when we do the alternate.î The LAX mechanic replied that he would see

them when they arrived at the LAX maintenance facility.

17 The airplane did not have an in-flight fuel dumping system.

18 ìSuitcase handlesî is a colloquial term for the longitudinal trim handles located on the center control

pedestal. ìPickle switchesî is a colloquial term for the trim switches located on the outboard side of each of

the control wheels. 

19 According to the CVR transcript, at 1550:14, the first officer had asked the captain to move his seat

forward so that he could check a panel behind the captainís seat that contained circuit breakers for the

horizontal stabilizer trim system. The first officer then stated to the captain, ìI donít think thereís anything

beyond that we havenít checked.î

20 According to postaccident testing, the indicator needle on the AC load meter in the cockpit normally

ìspikes,î or rises, to a maximum indication when the primary trim motor is engaged. Once the acme screw

begins to rotate, the amount of electrical current required to sustain the rotation drops significantly, and the

indicator needle will quickly drop to near zero. However, if the acme screw is held fixed or jammed while

the primary trim motor is engaged, the indicator needle will remain at the maximum indication. Engagement

of the alternate trim motor, regardless of the acme screwís condition, does not cause the indicator needle to

spike because of its significantly lower electrical current requirements. For more information about the

primary and alternate trim systems, see sections 1.6.1.1 and 1.6.1.2.
Factual Information 5 Aircraft Accident Report

At 1609:13, the captain stated, ìlets do that.î At 1609:14.8, the CVR recorded the

sound of a click and, at the same time, the captain stating, ìthisíll click it off.î According

to FDR data, the autopilot was disengaged at 1609:16. At the same time, the CVR

recorded the sound of a clunk, followed by two faint thumps in short succession at

1609:16.9. The CVR recorded a sound similar to the horizontal stabilizer-in-motion tone21

at 1609:17. At 1609:19.6, the CVR again recorded a sound similar to the horizontal

stabilizer-in-motion tone, followed by the captainís comment, ìyou got it?î (FDR data

indicated that during the 3 to 4 seconds after the autopilot was disengaged, the horizontal

stabilizer moved from 0.4° to a recorded position of 2.5° airplane nose down, and the

airplane began to pitch nose down, starting a dive that lasted about 80 seconds as the

airplane went from about 31,050 to between 23,000 and 24,000 feet.)22 Figure 1 shows the

accident airplaneís flightpath, starting about 1609 (about the time of the initial dive) and

ending about 1620 (about the time of the second and final dive). Figure 2 shows the radar

altitude data and selected ATC transmissions for about the same time period.

Figure 1. The accident airplaneís flightpath, starting about 1609 (about the time of the 

initial dive) and ending about 1620 (about the time of the second and final dive).

21 The MD-80 is equipped with a sensor to detect movement of a cable loop that is connected to the

horizontal stabilizer. If the horizontal stabilizer continuously moves through about 1.1°, a 1-second audible

signal is produced in the cockpit by the sensor. This audible signal is produced about every 0.55° of

continuous movement thereafter. The sensor resets when the horizontal stabilizer stops moving.

22 During the initial dive, the airplane accelerated to a maximum speed of 353 KIAS, and this value

remained nearly constant until 1610:23 as the airplane was descending through 24,200 feet.
Factual Information 6 Aircraft Accident Report

Figure 2. Radar altitude data and selected ATC transmissions from about 1609 to 1620.

At 1609:26, the captain stated, ìit got worse,î and, 5 seconds later, he stated

ìyouíre stalled.î One second later, the CVR recorded a sound similar to airframe vibration

getting louder. At 1609:33, the captain stated, ìno no you gotta release it ya gotta release

it.î This statement was followed by the sound of a click 1 second later. At 1609:52, the

captain stated, ìhelp me back help me back.î Two seconds later, the first officer

responded, ìok.î 

One second later, at 1609:55, the captain contacted the Los Angeles Air Route

Traffic Control Center (ARTCC)23 and stated, ìcenter Alaska two sixty one we are uh in a

dive here.î At 1610:01.6, the captain added, ìand Iíve lost control, vertical pitch.î

At 1610:01.9, the CVR recorded the sound of the overspeed warning (which continued for

the next 33 seconds). At 1610:05, the controller asked flight 261 to repeat the

transmission, and, at 1610:06, the captain responded, ìyea weíre out of twenty six

23 According to the CVR and ATC transcripts, flight 261 had established contact with the Los Angeles

ARTCC controller about 1551:21.
Factual Information 7 Aircraft Accident Report

thousand feet, we are in a vertical diveÖnot a dive yetÖbut uh weíve lost vertical control

of our airplane.î At 1610:20, the captain stated, ìjust help me.î 

At 1610:28.2, the captain informed the Los Angeles ARTCC controller, ìweíre at

twenty three seven request uh.î At 1610:33, the captain added, ìyea we got it back under

control here.î One second later, the first officer transmitted, ìno we donít.î At 1610:45,

the first officer stated, ìletís take the speedbrakes off.î One second later, the captain

responded, ìno no leave them there. it seems to be helping.î At 1610:55, the captain

stated, ìok it really wants to pitch down.î At 1611:06.6, the captain stated that they were

at ìtwenty four thousand feet, kinda stabilized.î Three seconds later he added, ìweíre

slowiní here, and uh, weíre gonna uh do a little troubleshooting, can you gimme a block

[altitude] between uh, twenty and twenty five?î24 FDR data indicated that, by 1611:13, the

airplaneís airspeed had decreased to 262 KIAS, and the airplane was maintaining an

altitude of approximately 24,400 feet with a pitch angle of 4.4°. At 1611:21, the controller

assigned flight 261 a block altitude of between FL 200 and 250. Airplane performance

calculations indicated that between about 130 and 140 pounds of pulling force was

required to recover from the dive.

At 1611:43, the first officer stated, ìwhatever we did is no good, donít do that

again.î One second later, the captain responded, ìyea, no it went down it went to full nose

down.î Four seconds later, the first officer asked, ìuh itís a lot worse than it was?î At

1611:50, the captain replied, ìyea yea weíre in much worse shape now,î adding, at

1611:59, ìI think itís at the stop, full stopÖand Iím thinkingÖcan it go any worseÖbut it

probably canÖbut when we slowed down, lets slow it lets get down to two hundred knots

and see what happens.î

At 1612:33, the captain told LAX maintenance, ìwe did both the pickle switch and

the suitcase handles and it ran away full nose trim down.î At 1612:42, the captain added,

ìand now weíre in a pinch so weíre holding uh weíre worse than we were.î At 1613:04,

the captain indicated to LAX maintenance that he was reluctant to try troubleshooting the

trim system again because the trim might ìgo in the other direction.î At 1613:10, the LAX

mechanic responded, ìok well your discretion uh if you want to try it, thatís ok with me if

not thatís fine. um weíll see you at the gate.î At 1613:22, the captain stated, ìI went tab

downÖright, and it should have come back instead it went the other way.î At 1613:32, the

captain asked the first officer, ìyou wanna try it or not?î The first officer replied, ìuhh no.

boy I donít know.î Airplane performance calculations indicated that about 120 pounds of

pulling force was being applied to the pilotsí control columns at this point.

At 1614:54, the Los Angeles ARTCC controller instructed the flight crew to

contact another ARTCC controller on frequency 126.52, which the flight crew

acknowledged. At 1615:19, the first officer contacted another ARTCC controller on

126.52 and stated, ìweíre with you weíre at twenty two five, we have a jammed stabilizer

and weíre maintaining altitude with difficulty. uh but uh we can maintain altitude we

thinkÖour intention is to land at Los Angeles.î The controller cleared the airplane direct

24 A block altitude assignment allows an airplane to operate between the assigned upper and lower

altitude limits. 
Factual Information 8 Aircraft Accident Report

to LAX and then asked, ìyou want lower [altitude] now or what do you want to do sir?î At

1615:56, the captain replied, ìI need to get down about ten, change my configuration,

make sure I can control the jet and Iíd like to do that out here over the bay if I may.î 

At 1616:32, the Los Angeles ARTCC controller issued flight 261 a heading of

280° and cleared the flight to descend to 17,000 feet. At 1616:39, the captain

acknowledged, ìtwo eight zero and one seven seventeen thousand Alaska two sixty one.

and we generally need a block altitude.î At 1616:45, the controller responded, ìok and

just um I tell you what do that for now sir, and contact LA center on one three five point

five theyíll have further uh instructions for you sir.î At 1616:56.9, the first officer

acknowledged, ìok thirty five five say the altimeter setting.î The controller responded,

ìthe LA altimeter is three zero one eight.î At 1617:02, the first officer responded, ìthank

you.î According to the CVR and ATC recordings, this was the last radio transmission

made from flight 261.

 After the radio transmission, the captain told a flight attendant that he needed

ìeverything picked upî and ìeverybody strapped down.î At 1617:04, the captain added,

ìIím gonna unload the airplane and see if we canÖwe can regain control of it that way.î

At 1617:09, the flight attendant stated, ìok we had like a big bang back there,î and, the

captain replied, ìyea I heard it.î At 1617:15, the captain stated, ìI think the stab trim thing

is broke.î At 1617:21, the captain again told the flight attendant to make sure the

passengers were ìstrapped in now,î adding 3 seconds later, ìcause Iím gonna Iím going to

release the back pressure and see if I can get itÖback.î

At 1617:54, the captain stated, ìgimme slats extend,î and, at 1617:56.6, a sound

similar to slat/flap handle movement was recorded by the CVR. At 1617:58, the captain

added, ìIím test flyin now.î At 1618:05, the captain commanded an 11° flap deployment,

and, at 1618:07, a sound similar to slat/flap handle movement was recorded. At 1618:17,

the captain stated, ìits pretty stable right hereÖsee but we got to get down to a hundred

an[d] eighty [knots].î At 1618:26, the captain stated, ìOKÖbring bring the flaps and slats

back up for me,î and, at 1618:36.8, sounds similar to slat/flap handle movement were

recorded. At 1618:47, the captain stated, ìwhat I wanna doÖis get the nose upÖand then

let the nose fall through and see if we can stab it when itís unloaded.î

At 1618:56, the first officer responded, ìyou mean use this again? I donít think we

shouldÖif it can fly.î At 1619:01, the captain replied, ìitís on the stop now, itís on the

stop.î At 1619:04, the first officer replied, ìwell not according to that itís not.î At this

time, FDR data indicated a horizontal stabilizer angle of 2.5° airplane nose down. Three

seconds later, the first officer added, ìthe trim might be, and then it might be uh, if

somethingís popped back thereÖ it might be mechanical damage too.î At 1619:14, the

first officer stated, ìI think if itís controllable, we oughta just try to land it.î Two seconds

later, the captain replied, ìyou think so? ok lets head for LA.î

About 5 seconds later, the CVR recorded the sound of a series of at least four

distinct ìthumps.î25 At 1619:24, the first officer asked, ìyou feel that?î and the captain

25 The CVR transcript describes this sound as a ìfaint thump.î
Factual Information 9 Aircraft Accident Report

replied, ìyea.î At 1619:29, the captain stated, ìok gimme sl---.î At 1619:32.8, the CVR

recorded the sound of two clicks similar to the sound of slat/flap movement. At 1619:36.6,

the CVR recorded the sound of an ìextremely loud noiseî and the sound of background

noise increasing, which continued until the end of the recording. At the same time, the

CVR also recorded sounds similar to loose articles moving around the cockpit. FDR data

indicated that at 1619:36.6, the flaps were extending and the slats were moving to the mid

position. The next few seconds of FDR data indicated a maximum airplane-nose-down

pitch rate of nearly 25° per second. The FDR recorded a significant decrease in vertical

acceleration values (negative Gs),26 a nose-down pitch angle, and a significant decrease in

lateral acceleration values. By 1619:40, the airplane was rolling left wing down, and the

rudder was deflected 3° to the right.

FDR data indicated that, by 1619:42, the airplane had reached its maximum valid

recorded airplane-nose-down pitch angle of -70°. At this time, the roll angle was passing

through -76° left wing down. At 1619:43, the first officer stated, ìmayday,î but did not

make a radio transmission. Six seconds later, the captain stated, ìpush and roll, push and

roll.î FDR data indicated that, by 1619:45, the pitch angle had increased to -28°, and the

airplane had rolled to -180° (inverted). Further, the airplane had descended to 16,420 feet,

and the indicated airspeed had decreased to 208 knots.

At 1619:54, the captain stated, ìok, we are invertedÖand now we gotta get it.î

FDR data indicated that at this time, the left aileron moved to more than 16° (to command

right wing down), then, during the next 6 seconds, it moved in the opposite direction to

-13° (to command left wing down). At 1619:57, the rudder returned to the near 0°

position, the flaps were retracted, and the airplane was rolling through -150° with an

airplane-nose-down pitch angle of -9°. After 1619:57, the airplane remained near inverted

and its pitch oscillated in the nose-down position. 

At 1620:04, the captain stated, ìpush push pushÖpush the blue side up.î At

1620:16, the captain stated, ìok now lets kick rudderÖleft rudder left rudder.î Two

seconds later, the first officer replied, ìI canít reach it.î At 1620:20, the captain replied,

ìok right rudderÖright rudder.î At 1620:38, the captain stated, ìgotta get it over

againÖat least upside down weíre flyin.î At 1620:49, the CVR recorded sounds similar to

engine compressor stalls and engine spooldown.27 At 1620:54, the captain commanded

deployment of the speedbrakes, and, about 1 second later, the first officer replied, ìgot it.î

At 1620:56.2, the captain stated, ìah here we go.î The FDR recording ended at 1620:56.3,

and the CVR recording ended at 1620:57.1. 

The airplane impacted the Pacific Ocean near Port Hueneme, California. Pieces of

the airplane wreckage were found floating on and beneath the surface of the ocean. The

main wreckage was found at 34° 03.5' north latitude and 119° 20.8' west longitude.

26 A G is a unit of measurement of force on a body undergoing acceleration as a multiple of its weight.

The normal load factor for an airplane in straight and level flight is about 1 G. As the load factor decreases

from 1 G, objects become increasingly weightless, and at 0 G, those objects float. 

27 An engine compressor stall, or surge, results from interruption of normal airflow through the engine,

which can be caused by an engine malfunction or disturbance of inlet airflow at high angles-of-attack.
Factual Information 10 Aircraft Accident Report

1.2 Injuries to Persons

Table 1. Injury chart.

1.3 Damage to Airplane

The airplane was destroyed by impact forces. 

1.4 Other Damage

No other damage resulted from this accident.

1.5 Personnel Information

The accident flight crew spent the night before the accident at Alaska Airlinesí

layover hotel, which was in a resort located outside of Puerto Vallarta. Witness statements

and hotel records indicated that the captain and first officer watched the Super Bowl in the

hotel lounge and ate both breakfast and dinner in the hotel restaurant.28 The flight crew

that flew the accident airplane into PVR spoke to the accident flight crewmembers, and

they indicated that the flight crew appeared rested before the accident flight.

1.5.1 The Captain

The captain, age 53, was hired by JetAmerica29 on August 16, 1982. The captain

held an airline transport pilot (ATP) certificate, issued March 16, 1984, with a type rating

Injuries Flight Crew Cabin Crew Passengers Other Total

Fatal 2 3 83 0 88

Serious 0 0 00 0

Minor 0 0 00 0

None 0 0 00 0

Total 2 3 83 0 88

28 For more information about the flight crewís preaccident activities, see the Operations/Human

Performance Group Chairmanís report and addendums in the Safety Boardís public docket for this accident.

29 JetAmerica merged with Alaska Airlines in 1987. According to Alaska Airlines records, the captain

began JetAmerica merger training on September 8, 1987.
Factual Information 11 Aircraft Accident Report

in the Douglas DC-9 (DC-9)/McDonnell Douglas MD-80 (MD-80). Additionally, he held

a turbojet flight engineer certificate and ground and flight instructor certificates. The

captainís most recent FAA first-class medical certificate was issued on November 15,

1999, and contained the limitation that he must wear corrective lenses. 

According to Alaska Airlines records, the captain had flown approximately

17,750 total flight hours, including 10,460 hours as pilot-in-command, of which about

4,150 hours were as pilot-in-command in the MD-80. He had flown approximately 133,

52, and 24 hours in the last 90, 30, and 7 days, respectively, before the accident.

According to company records, the captain satisfactorily completed his last line check on

July 15, 1999; his last recurrent training on November 19, 1999; and his last proficiency

check on November 23, 1999. Company records also indicated that the captain was

off-duty for 2 days before the day of the accident flight. A search of FAA and company

records showed no accident, incident, or enforcement or disciplinary actions, and a search

of records at the National Driver Register found no history of driverís license revocation

or suspension.

1.5.2 The First Officer

The first officer, age 57, was hired by Alaska Airlines on July 17, 1985.30 The first

officer held an ATP certificate, issued April 3, 1985, with type ratings in the DC-9/MD-80,

DC-6, and DC-7. The flight officerís most recent FAA second-class medical certificate

was issued on April 7, 1999, and contained the limitation that he must wear corrective

lenses. 

According to Alaska Airlines records, the first officer had flown approximately

8,140 total flight hours, including about 8,060 hours as first officer in the MD-80. He had

flown approximately 142, 78, and 15 hours in the past 90, 30, and 7 days, respectively,

before the accident. According to company records, the first officer satisfactorily completed

his last line check on March 16, 1997; his last recurrent training on April 23, 1999; and his

last proficiency check on April 25, 1999. Company records also indicated that the first

officer was off-duty for 3 days before the day of the accident flight. A search of FAA and

company records showed no accident, incident, or enforcement or disciplinary actions, and

a search of records at the National Driver Register found no history of driverís license

revocation or suspension.

1.6 Airplane Information

The MD-80 is a low-wing, twin engine, transport-category airplane. The MD-80,

which the FAA certified in August 1980, was derived from earlier DC-9 series model

airplanes. As a result, much of the MD-80ís structure and many of its systems,

components, and installations are similar to the earlier DC-9 model. According to

30 According to Alaska Airlines records, the first officer also began JetAmerica merger training on

September 8, 1987.
Factual Information 12 Aircraft Accident Report

Boeing,31 the first DC-9 airplane entered service in December 1965; the final DC-9

airplane entered service in October 1982. MD-80 series model airplanesóthe MD-81,

-82, -83, and -88ówere in production through 1999.32 The DC-9 family of airplanes also

includes McDonnell Douglas MD-90 (MD-90) and Boeing 717 (717) series airplanes.33

Alaska Airlines began operating MD-80 airplanes on June 30, 1984. The accident

airplane, N963AS, an MD-83, serial number (S/N) 53077, was manufactured in 1992 and

added to Alaska Airlinesí operating certificate on May 27, 1992.34 According to Alaska

Airlines records, the airplane had accumulated about 26,584 total hours of operation

(14,315 flight cycles)35 at the time of the accident. The airplane was configured to seat a

maximum of 12 first-class and 128 economy-class passengers and to carry cargo.

The accident airplane was equipped with two Pratt & Whitney JT8D-217C

turbofan engines. Company records indicated that the No. 1 (left) engine, S/N 728068,

was manufactured in May 1995 and installed on the accident airplane on April 20, 1999,

and had operated about 16,970 hours (8,994 flight cycles) since new. At the time of the

accident, the No. 1 engine had 3,006 flight cycles remaining until a limited inspection of

its low-pressure turbine shaft was required. The No. 2 (right) engine, S/N 726852, was

manufactured in April 1992 and installed on the accident airplane on May 24, 1999, and

had operated about 24,034 hours (13,266 flight cycles) since new. At the time of the

accident, the No. 2 engine had 6,305 flight cycles remaining until a limited inspection of

its low-pressure turbine shaft was required.

According to Alaska Airlines dispatch documents for the accident flight, the accident

airplaneís takeoff weight was calculated to be 136,513 pounds,36 including 34,902 pounds

of fuel. Alaska Airlinesí load sheet for the accident flight indicated that there were 10

passengers in the first-class cabin, 70 passengers (not including 3 children)37 in the coach

cabin, and 5 crewmembers (2 flight crewmembers and 3 cabin crewmembers) on board the

airplane. The dispatch documents indicated that the airplaneís takeoff CG was calculated to

be 12.8 percent mean aerodynamic chord (MAC).38

31 The Boeing Commercial Airplane Group and the McDonnell Douglas Corporation merged in

August 1997. The Douglas Aircraft Company became the McDonnell Douglas Corporation in April 1967

when it merged with the McDonnell Aircraft Company.

32 A total of 1,191 MD-80 series airplanes were produced.

33 A total of 117 MD-90 series airplanes were produced from February 13, 1993, to October 23, 2000. A

total of 104 717s have been produced, starting on September 23, 1999, and 717s are still in production.

34 According to Alaska Airlines, the registered airplane owner was the First Security Bank of Utah.

35 A flight cycle is one complete takeoff and landing sequence.

36 The accident airplaneís weight and balance were calculated using the January 10, 2000, revision of

the Alaska Airlines MD-80 Loading Handbook. The maximum certificated takeoff gross weight for the

accident airplane was 160,000 pounds, the maximum climb weight for takeoff was 153,900 pounds, and the

maximum landing weight was 130,000 pounds. 

37 According to the Alaska Airlines MD-80 Loading Handbook, passengers under 2 years of age are not

considered for weight and balance purposes during normal operations.

38 The maximum forward CG limit for the accident airplane was 8.0 percent MAC, and the maximum

aft CG limit was 26.0 percent MAC.
Factual Information 13 Aircraft Accident Report

1.6.1 MD-80 Longitudinal Trim Control System Information

Longitudinal control for the MD-80 (and for all DC-9, MD-90, and 717 series

airplanes), that is, control of the airplaneís pitch movements, is provided by the horizontal

stabilizer and the elevators. The horizontal stabilizer is mounted on top of the 18-foot-high

vertical stabilizer; they are connected by two hinges at the aft spar of the horizontal

stabilizer and with a single jackscrew assembly at the front spar of the stabilizer in a T-tail

configuration (see figure 3). The horizontal stabilizer is about 40 feet long and comprises a

center box and a left and a right outboard section. Each outboard section of the horizontal

stabilizer has an elevator hinged to its trailing edge. Coarse pitch movements are achieved

with elevator movement via mechanical linkage from the elevator control tabs to the

cockpit control columns (see figure 4). Finer adjustments to pitch are achieved by

changing the angle of the entire horizontal stabilizer. The leading edge of the horizontal

stabilizer is raised or lowered by the jackscrew assembly as the stabilizerís trailing edge

pivots (rotates) about its hinge points.

Figure 3. Installation of jackscrew assembly within the horizontal and vertical stabilizers.
Factual Information 14 Aircraft Accident Report

Figure 4. The MD-80 horizontal and vertical stabilizer tail structure.

Movement of the horizontal stabilizer is provided by the jackscrew assembly,

which consists of an acme screw and nut, a torque tube inside the acme screw, two

gearboxes, two trim motors (an alternate and a primary), and associated components and

supports. (See figures 3 through 6 for details of the longitudinal trim system actuating

mechanism and the jackscrew assembly.) The upper end of the jackscrew assembly is

attached to the front spar of the horizontal stabilizer, and the lower end is threaded through

the acme nut, which is attached to the vertical stabilizer with a gimbal ring and retaining

pins. The acme screw and nut each have two threads that rotate in a spiral along their

length. Figure 6 shows the longitudinal trim system and the jackscrew assembly.
Factual Information 15 Aircraft Accident Report

Figure 5. Acme screw and nut.

Movement of the horizontal stabilizer is commanded either automatically by the

autopilot when it is engaged, or manually by the flight crew by depressing either set of

dual trim switches (located on each control wheel), moving the dual longitudinal trim

handles on the center control pedestal, or moving the dual alternate trim control switches

on the center pedestal (see figure 7). Any of these commands activates one of the two

electric motors that rotate the acme screw by applying torque to the titanium torque tube

that is held fixed inside the acme screw. The motors are deenergized whenever either the

autopilot senses that the horizontal stabilizer has reached the desired pitch trim condition,

when pilot commands are terminated, or when the horizontal stabilizer reaches its

maximum travel limits. Electrical travel limit shutoff switches (also known as the

electrical stops) stop the motors at the maximum limits of travel. The MD-80 horizontal

stabilizerís design limits are 12.2° leading edge down, which results in airplane-nose-up

trim, and 2.1° leading edge up, which results in airplane-nose-down trim, as set by the

electrical stops.
Factual Information 16 Aircraft Accident Report

Figure 6. Detailed schematic of the longitudinal trim actuating mechanism.
Factual Information 17 Aircraft Accident Report

Figure 7. Cockpit switches, handles, and indicators for the longitudinal control system.

The jackscrew assembly also has upper and lower mechanical stops splined to the

acme screw to stop screw rotation if the travel limit shutoff switches malfunction. The

mechanical stops rotate with the acme screw as it turns and act as the mechanical limits for
Factual Information 18 Aircraft Accident Report

its range of travel.39 Stopping action is provided by contact between the head of a steel

stop bolt in the mechanical stop and the head of a similar bolt in the acme nut. 

1.6.1.1 Primary Trim Control System

As previously stated, the primary trim control system is activated by depressing

either set of control wheel trim switches or moving the dual longitudinal trim handles. The

pair of switches located on each control wheel is designed to be depressed simultaneously.

Both switches on the same control wheel must be moved simultaneously in the same

direction to command up or down movement of the horizontal stabilizer. One switch is a

motor control switch; the other switch is a brake control switch. Moving the motor control

switch energizes the airplane-nose-up or nose-down circuits. Moving the brake control

switch energizes the corresponding brake control relay and releases the brake in the trim

motor. Energizing the brake control relay automatically disengages the autopilot. 

The pair of longitudinal trim handles on the left side of the control pedestal also

serve as a mechanical means of activating the trim control system in the event of an

electrical circuit failure in the control wheel switches. The pair of handles is designed to

be gripped with one hand and moved simultaneously in the same direction. Operation of

the handles in a trim direction opposite that being commanded by the control wheel trim

switches will override the control wheel trim switch command. A primary trim brake

switch is also installed on the center pedestal; it is designed to stop horizontal stabilizer

movement, when activated, if a malfunction occurs in the primary trim control system. 

The primary trim control system uses the primary trim motor, which is mounted on

top of the jackscrew assembly. When powered by the primary trim motor, the horizontal

stabilizer moves about 1/3° per second. The primary trim motor rotates the acme screw at

speeds of about 35 rpm and has a maximum torque capability (stall torque) of 18,850

inch-pounds. 

1.6.1.2 Alternate Trim Control System

As previously mentioned, the alternate trim control system is activated by

autopilot commands or by activation of a pair of alternate trim control switches, which are

located in the center of the control pedestal, aft of the throttles. These alternate trim

control switches must be moved simultaneously in the same direction to activate the

alternate trim system. As with the primary trim system, one switch controls the motor, and

the other switch controls the brake.

The alternate trim control system uses the alternate trim motor, which is mounted

on the jackscrew assembly next to the primary trim motor. When powered by the alternate

trim motor, the horizontal stabilizer moves about 1/10° per second. The alternate trim

39 Boeing engineering data, computer modeling, and ground demonstrations on an actual airplane

indicated that the maximum possible horizontal stabilizer position when the acme screw lower mechanical

stop is pulling up on, but is restrained by, the acme nut is about 3.1o airplane nose down.
Factual Information 19 Aircraft Accident Report

motor rotates the acme screw at speeds of about 10 rpm and has a maximum torque

capability of 4,400 inch-pounds. 

1.6.1.3 Autopilot Pitch Control 

When the autopilot is engaged and the flight crew selects a desired flight mode (for

example, maintaining a constant altitude, pitch angle, or rate of climb), the autopilot will

transmit pitch control signals to the DFGC. The DFGC processes the signals and sends

them to the elevator control tab servos, which move the elevator control tabs the proper

amount to execute the commanded pitch maneuver. If the signals to the servos are

sustained for more than 5 seconds, the DFGC will then command the alternate trim motor

to move the horizontal stabilizer in a direction that will relieve the elevator deflection.

Autopilot trim annunciator warning lights in the cockpit illuminate to alert the

flight crew if a horizontal stabilizer out-of-trim condition exists while the autopilot is

engaged. Specifically, after 10 seconds if there is no horizontal stabilizer movement in

response to autopilot commands, the autopilot trim annunciator warning lights illuminate.

If such a condition exists, the autopilot will not automatically disengage. The light will

remain on until the autopilot is manually disengaged. According to Boeing, both the

captainís and the first officerís control panels have flight mode annunciators to annunciate

ìA/P [autopilot] TRIM.î

1.6.2 Design and Certification of MD-80 Series Airplanes

1.6.2.1 General

The original DC-9 was certified in 1965 under Civil Aeronautics Regulations

(CAR) 4b, as revised through Amendment 12, March 30, 1962, as were other early models

in the DC-9 series. The MD-80 was certified in 1980. More recent models of the DC-9,

MD-80, MD-90, and 717 were certified under 14 CFR Part 25 and applicable

amendments. However, systems that were similar to or that did not change significantly

from the earlier DC-9 models, such as the longitudinal trim control system, were not

required to be recertified under 14 CFR Part 25. Therefore, CAR 4b remained the

certification basis for those parts of the MD-80, MD-90, and 717. 

1.6.2.2 Longitudinal Trim Control System Certification

Pertinent sections of CAR 4b that were applicable to the original certification of

the DC-9ís longitudinal trim control system are outlined below.

CAR Section 4b.201(a), ìStrength and Deformation,î stated that structure ìshall

be capable of supporting limit loads without suffering detrimental permanent

deformation.î40 CAR Section 4b.270, ìFatigue Evaluation of Flight Structure,î stated the

following:41

40 Substantially similar requirements are currently contained in 14 CFR 25.305.

41 Substantially similar requirements are currently contained in 14 CFR 25.571.
Factual Information 20 Aircraft Accident Report

The strength, detail design, and fabrication of those portions of the airplaneís

flight structure in which fatigue may be critical shall be evaluated in accordance

with the provisions of either paragraph (a) or (b) of this section.

(a) Fatigue strength. The structure shall be shown by analysis and/or tests to be

capable of withstanding the repeated loads of variable magnitude expected in

serviceÖ. 

(b) Fail safe strength. It shall be shown by analysis and/or tests that catastrophic

failure or excessive structural deformation, which could adversely affect the

flight characteristics of the airplane, are not probable after fatigue failure or

obvious partial failure of a single principal structural element. After such

failure, the remaining structure shall be capable of withstanding static loads

corresponding with the flight loading condition specified [in CAR 4b]. These

loads shall be multiplied by a factor of 1.15 unless the dynamic effects of

failure under static load are otherwise taken into consideration.

CAR Section 4b.320(d), ìControl Systems,î stated the following:42 ìEach

adjustable stabilizer must have a means to allow any adjustment necessary for continued

safety of flight after the occurrence of any reasonably probable single failure of the

actuating system.î 

CAR Section 4b.322, ìTrim Controls and Systems,î described the trim control

systems requirements as follows:43

(a) Trim controls shall be designed to safe guard against inadvertent or abrupt

operationÖ.

(e) Trim devices shall be capable of continued normal operation in the event of

failure of any one connecting or transmitting element of the primary flight

control system.

CAR Section 4b.325, ìControl System Stops,î stated the following:44

(a) All control systems shall be provided with stops which positively limit the

range of motion of the control surfaces.

(b) Control system stops shall be so located in the systems that wear, slackness, or

take-up adjustments will not affect adversely the control characteristics of the

airplane because of a change in the range of surface travel.

(c) Control system stops shall be capable of withstanding the loads corresponding

with the design conditions for the control systems.

42 Substantially similar requirements are currently contained in 14 CFR 25.671.

43 Substantially similar requirements are currently contained in 14 CFR 25.677.

44 Substantially similar requirements are currently contained in 14 CFR 25.675.
Factual Information 21 Aircraft Accident Report

CAR Section 4b.606, ìEquipment, Systems, and Installations,î Subsection (b),

ìHazards,î stated, ìAll equipment, systems, and installations shall be designed to

safeguard against hazards to the airplane in the event of their malfunctioning or failure.î

Similar requirements are currently contained in 14 CFR 25.1309. Specifically, 14 CFR

25.1309 requires that airplane systems and associated components be designed so that ìthe

occurrence of any failure condition which would prevent the continued safe flight and

landing of the airplane[45] is extremely improbable.î Further, FAA AC 25.1309-1A,

ìSystem Design and Analysis,î defines ìextremely improbableî failure conditions as

ìthose so unlikely that they are not anticipated to occur during the entire operational life of

all airplanes of one typeî and ìhaving a probability on the order of 1 x 10-9 or less each

flight hour, based on a flight of mean duration for the airplane type.î AC 25.1309-1A

specifies that in demonstrating compliance with 14 CFR 25.1309, ìthe failure of any

single element, component, or connection during any one flightÖshould be assumed,

regardless of its probability,î and ìsuch single failures should not prevent continued safe

flight and landing, or significantly reduce the capability of the airplane or the ability of the

crew to cope with the resulting failure condition.î

According to Boeing, the longitudinal trim control system on the DC-9 was similar

to the one used on earlier Douglas DC-8 (DC-8) airplanes. However, the DC-8 was

equipped with a dual jackscrew design that provided structural and operational

redundancy. According to Boeing, even though the DC-9 had a single jackscrew assembly,

structural redundancy was accomplished by inserting a torque tube within the acme screw.

A Safety Board review of DC-9, MD-80/90, and 717 series airplane certification

documents46 indicated that the manufacturer provided material strength and loads

information, stress calculations, and analyses to support assertions that the acme screw

and torque tube were designed so that each of these two components could withstand

tensile and compressive loads well beyond that which could be generated by aerodynamic

forces acting on the horizontal stabilizer.47 The documents also indicated that these

assertions were based on the assumption of a new, intact acme screw and nut that met

design specifications and that the acme screw and nut threads were intact and engaged to

act as a load path. 

Certification documents also included calculations and analyses to support the

assertions that the jackscrew assembly was capable of carrying limit loads, with a margin

45 A failure that would prevent continued safe flight and landing is referred to in Advisory Circular (AC)

25.1309-1A as a ìcatastrophicî failure condition.

46 The Safety Board requested all available documentation addressing the certification of these airplane

designs. Documentation provided by Boeing included the April 15, 1965, DC-9 Flight Controls System

Fault Analysis Report (revised July 14, 1997); the 1964 DC-9 Control System Design Criteria; and the April

1998 MD-95-30 Control Systems Loads and Criteria Report. (The MD-95 was renamed the Boeing 717

when McDonnell Douglas and Boeing merged.) Internal design memorandums that addressed the horizontal

stabilizer trim system were also reviewed.

47 According to certification documents provided by Boeing, the DC-9/MD-80 series horizontal

stabilizer acme screw was designed to carry an ultimate tension load of about 53,000 pounds. This load was

calculated by assuming the highest aerodynamic loads that could be safely carried by the horizontal

stabilizer structure in flight and then multiplying that load by 150 percent.
Factual Information 22 Aircraft Accident Report

of safety, in either pure tension or compression, without failing, for each of the following

scenarios:

ï A fractured acme screw. According to the certification documents, this was

accomplished by designing the inner torque tube with the capability to support

limit operational tension and compression loads (in addition to the normal

operational torsion loads) generated by the horizontal stabilizer. 

ï A fractured torque tube. According to the certification documents, this was

accomplished by designing the jackscrew assembly so that if a torque tube

fractured, trim operation would immediately cease. Under this failure scenario,

the loads generated by the horizontal stabilizer would continue to be carried by

the acme screw and nut, and pitch control would still be possible through the

elevators.

ï The loss of 90 percent of acme screw and nut threads. According to the

certification documents, this was accomplished by designing the acme screw

and nut with more threads than were structurally necessary.

ï The failure of one entire thread spiral in the acme screw or nut. According

to the certification documents, this was accomplished by incorporating two

independent thread spirals along the acme screwís length. A Boeing Airplane

Services structures engineering manager who testified at the Safety Boardís

public hearing on the flight 261 accident confirmed that ìthe reason behind

having two independent threads is that if one of the threads were to fail, the

other thread will carry the load.î

A Safety Board review of the April 15, 1965, DC-9 Flight Controls System Fault

Analysis Report (revised July 14, 1997) indicated that no contingency for the complete

loss of the acme nut threads was incorporated into the design for the longitudinal trim

control system. According to the fault analysis, the acme nut was designed with a softer

material than the acme screw and its threads were designed to wear. Acme nut threads are

made of an aluminum-bronze alloy and are about 0.15 inch thick at the minor diameter

when new. The acme screw threads are made of case-hardened steel.

The DC-9 jackscrew assembly was originally designed for a service life of

30,000 flight hours and was not originally subject to periodic inspections for wear.

In 1966, 1 year after the DC-9 went into service, the discovery of several assemblies with

excessive wear resulted in the development and implementation of an on-wing end play

check procedure to measure the gap between the acme screw and nut threads as an

indicator of wear.48 Thereafter, Douglas guidance specified that acme nut thread wear

periodically be measured using an end play check procedure, and the acme nut was to be

replaced when the specified end play measurement (0.040 inch) was exceeded.49 

An FAA senior Aircraft Certification Office aerospace engineer testified at the

public hearing that the jackscrew assembly was considered ìprimary structure because it

48 End play is an indirect measurement of acme nut thread wear. For more information, see sections

1.6.3.3.1 and 1.6.3.3.2.
Factual Information 23 Aircraft Accident Report

is the primary load-carrying load path from the horizontal [stabilizer] to the vertical

[stabilizer], other than the pivot attaches at the rear spar.î The FAA certification engineer

also testified that it was not considered a principal structural element and added that ìnot

all primary structure is a principal structural element.î He defined a principal structural

element, ìin damage tolerance philosophy,î as an element ìwhose failure, if it remained

undetected, could lead to loss of the aircraft.î

The FAA senior certification engineer further testified that the ìjackscrew

assembly and acme nut in particularÖhave to comply with the ultimate strength and limit

load deflection criteria which provides for their strength.î However, according to the FAA

certification engineer, ìbecause of the evaluation which indicates that the fatigue stresses

are so low as to preclude the initiation or a propagation of a crack in the thread, they need

not be considered under Part A or Part B of [CAR] 4b.270 [Fatigue Evaluation of Flight

Structure].î According to the FAA certification engineer, who was referring to Boeing

information, the ìnormal flight loadÖin the jackscrew to be transmitted from the screw

through the nut to the vertical stabilizer is in the neighborhood of 4,000 or 5,000 pounds.î

A Boeing structures engineering manager testified at the public hearing that ìif the

torque tube fails, the acme screw will carry the load, and if the acme screw fails, the torque

tube will carry the load.î The Boeing manager also stated that ìwe have no reported cases

of fatigue failures or fatigue damage on this torque tube design for the four basic models,

DC-9, MD-80, MD-90 and 717, and that is over 95 million accumulated flight hours,

andÖover 2,300 airplanes [were] delivered.î

The FAA senior certification engineer testified at the public hearing that the

jackscrew assembly was a ìcombination structural element and systems elementÖand

[that] as such, the systems portion would fall under the systems requirements, and the

structures portion would be required to address the structural requirements.î The FAA

certification engineer also indicated that the acme nut was not considered a system and,

therefore, was not required to comply with airplane systemsí certification requirements.

The Boeing structures engineering manager testified at the public hearing that ìthe

acme nut thread is a wear component. Since the two independent threads in the acme nut

are both subject to wear due to jackscrew rotation, we took this into account when we

designed the system and defined the maintenance procedures to manage and inspect the

wear.ÖWhen the acme nut is new, the strength of the threads is over 15 times in excess of

design loads.î The Boeing manager further testified that an acme screw and nut ìunit

worn to an end play reading of 0.040 of an inch remains 10 times stronger [than the

maximum design loads].î He added, ìTaking the end play reading to 0.080 of an inch, or

twice the removal threshold, still leaves us with an installation five times stronger than the

maximum design loads. [The] current in-service end play limit 0.040 of an inch is

conservative from both the strength and the design standpoint. This assembly is

49 Before the accident, there was no requirement to record or track end play measurements obtained

during end play check procedures. After the accident, Airworthiness Directives (AD) 2000-03-51 and

2000-15-15 required that end play measurements be reported to Boeing. For more information about these

ADs, see section 1.18.1.
Factual Information 24 Aircraft Accident Report

recognized as a wear component which is addressed initially by its robust design and later

by in-service maintenance action to control and monitor that wear.î 

The FAA senior certification engineer testified that wear is ìnot considered as a

mode of failure for either a systems safety analysis or for structural considerations.î The

FAA certification engineer further stated that the ìDouglas design of the acme nut and

acme screw provided enough over-strength so that the regulatory requirements could be

met with a significant amount of wear.î He indicated that even if 14 CFR 25.1309 had

been applicable to the entire jackscrew assembly, acme nut thread wear would not have

been considered in the required systems safety analysis. He explained, ìif you refer to a

[section 25.1309] type safety analysis to try and determine a failure rate for a wear item,

there are not, and thereís really no such thing as a wear-critical item, never has been. The

question of wear being a quantifiable element so that one could do a safety-type analysis

for structureóitís not feasible. The data to do such an evaluation is not available. It

doesnít exist.î50

The FAA senior certification engineer stated that regulatory definitions of type

design safety assume that the ìsafety of that type design is maintainedÖ[because] the type

certificate holder is required to recommend a maintenance program for [operators] such

that the level of safety is maintainedÖ.We presume that those maintenance requirements

are complied with.î The FAA certification engineer added, ìwear is considered as part of a

designÖand, as with any mechanism, maintenance is necessary.î He concluded by

stating, ìI believe that the current design of the MD-80, DC-9, 717 and MD-90 pitch trim

system demonstrates compliance to the applicable regulations and provides the level of

safety associated with those certification requirements.î

1.6.3 Maintenance Information

1.6.3.1 Alaska Airlinesí MD-80 Maintenance Program 

1.6.3.1.1 Maintenance Program Development Guidance

In 1968, an aviation industry team Maintenance Steering Group (MSG) developed

a maintenance program development document for the then-new Boeing 747 (747). This

guidance document contained decision logic and procedures for use in developing initial

minimum maintenance and inspection requirements. Using the experience gained through

the 747 project, in 1970, another MSG developed an updated guidance document, known

as the MSG-2, which was designed to be applicable to all future newly type-certificated

aircraft and/or powerplants, not just the 747. According to the MSG-2 guidance document,

it was intended ìto present a means for developing a maintenance programî that will be

acceptable to the FAA, operators, and manufacturers.

In September 1980, through the combined efforts of the FAA, the Air Transport

Association of America, U.S. and European aircraft and engine manufacturers, and U.S.

50 The Boeing manager confirmed that ìthe condition of wear or wear-out was not included in the

original DC-9 fault analysis.î 
Factual Information 25 Aircraft Accident Report

and foreign airlines, a third guidance document, known as MSG-3, was issued. The

MSG-3 guidance document, which incorporated data from more than 10 years of airline

maintenance programs analysis and service experience, was developed, in part, to reduce

confusion about how to interpret maintenance processes defined in the MSG-2

methodology, including hard-time, on-condition, condition-monitoring, and overhaul.51

The MSG-3 task-oriented logic methodology used a ìconsequence of failure approach.î

For example, servicing and lubrication were included in the MSG-3 guidance document as

part of the logic diagram to ensure that this category of task was considered each time an

item was analyzed. After using MSG-3 analysis procedures on a number of new aircraft

and powerplants in the first half of the 1980s, the airline industry decided that the

experience should be used to improve the document for future applications, and, in 1988,

Revision 1 to MSG-3 was issued; Revision 2 was issued in 1993. 

The MSG-2 and MSG-3 guidance documents are accepted by the FAA and are

used by Maintenance Review Boards (MRB) that are convened to develop reports

containing guidance specific to an airplane type or model. MRB reports outline the

recommended initial minimum scheduled maintenance and inspection requirements for a

particular transport-category airplane and on-wing engine program. MRB reports are

initially developed by an industry steering committee that is headed by an FAA

chairperson. The steering committee directs working groups that include representatives

from aircraft manufacturers and operators. The working groups are responsible for the

development of initial minimum scheduled maintenance and inspection requirements for

new or derivative aircraft to be used in the development of an operatorís continuing

airworthiness maintenance program. MRB reports are published and distributed by the

manufacturer, but they contain FAA requirements. According to FAA AC 121-22A, the

MRB report is ìa means, in part, of developing instructions for continuing airworthiness,

as required by [14 CFR section] 25.1529.î The MRB report is submitted to the FAA for

approval as part of these instructions. 

On the basis of the guidance in the applicable MRB report, manufacturers issue

on-aircraft maintenance planning (OAMP) documents and generic task cards outlining

specific maintenance tasks. The tasks and intervals in an OAMP document are typically

the same as those contained in the associated MRB report. Two MRB reports and

associated OAMP documents are currently used as the basis for the maintenance of the

MD-80: those derived from MSG-2, issued in 1993, and those derived from MSG-3,

issued in 1996. Carriers can use the guidance in the MRB report and OAMP documents to

51 Hard-time is a maintenance process that requires an item to be removed from service or overhauled at 

or before a previously specified time. On-condition is a maintenance process that requires periodic

inspections or checks of a unit against an appropriate physical standard to determine whether it can continue

in service until the next scheduled check and to remove the unit from service before failure occurs during

normal operations. Condition-monitoring is a maintenance process for items that do not have hard-time or

on-condition as the primary maintenance process and is accomplished through continuous data collection

and analysis of components or systems. Analysis of failures or other indications of deterioration are used to

evaluate the continuing airworthiness of the airplane. Overhaul is the disassembly, cleaning, inspection,

repair, and testing of a component to the extent necessary to ensure (substantiated by service experience and

accepted practices) that it is in satisfactory condition to operate for a given period. It includes the

replacement, repair, adjustment, or refinishing of such parts as required, which, if not properly

accomplished, would adversely affect the airworthiness of the airplane.
Factual Information 26 Aircraft Accident Report

develop their maintenance programs.52 Carriers with maintenance programs based on the

MSG-2 MRB report are permitted to use the MSG-3 MRB report and its listed intervals to

adjust existing intervals in coordination with the airlineís FAA principal maintenance

inspector (PMI).53

1.6.3.1.2 Alaska Airlinesí Continuous Airworthiness Maintenance Program

Alaska Airlinesí continuous airworthiness maintenance program for its MD-80

fleet received initial FAA approval in March 1985 and was based on the FAA-accepted

MRB report for DC-9/MD-80 airplanes derived from the MSG-2 maintenance program

development document. The MRB report and the resulting OAMP document

recommended C-check intervals of 3,500 flight hours or 15 months, whichever came first.

Alaska Airlinesí initial C-check interval was 2,500 flight hours. In 1988, it was extended

to every 13 months (which, based on the average airplane utilization rate at Alaska

Airlines at the time, was about 3,200 flight hours). In 1996, the C-check interval was

extended to 15 months (which, based on the average airplane utilization rate at Alaska

Airlines at the time, was about 4,775 flight hours).

In addition to the 15-month C-check interval, Alaska Airlinesí General

Maintenance Manual (GMM), dated February 5, 1998, revealed that Alaska Airlines had

the following maintenance inspections and intervals for its MD-80 fleet at the time of the

accident: 

ï Walk-around checkóaccomplished by Alaska Airlines maintenance personnel

after each arrival. 

ï Service checkóaccomplished when the airplane remains on the ground for 8

or more hours at a maintenance facility capable of conducting at least A check

maintenance.

ï A checkóaccomplished at intervals not to exceed 250 flight hours.

1.6.3.1.3 Alaska Airlinesí Reliability Analysis Program

CFR Section 121.373(a), ìContinuing Analysis and Surveillance,î requires

operators to establish and maintain a system for the continuing analysis and surveillance

of the performance and effectiveness of their maintenance and inspection programs and

for the correction of any deficiency found in those programs. According to FAA

Order 8300.10, chapter 37, continuing analysis and surveillance programs should have, as

a minimum, procedures for monitoring mechanical performance, which should be done

52 FAA AC 121-22A states that an MRB report ìcontains the initial minimum scheduled

maintenance/inspection requirements for a particularÖaircraftÖ[it should] not to be confused with, or

thought of, as a maintenance program. After approval by the FAA, the requirements become a base or

framework around which each air carrier develops its own individual maintenance program.î 

53 Alaska Airlines used the MSG-2 MRB report as the basis for its maintenance program; however, it

used the MSG-3 MRB report to extend some maintenance task intervals by replacing them with the newer

intervals specified in the MSG-3 MRB report. The FAAís MD-80 MRB Chairman confirmed that existing

MSG-2 operators, such as Alaska Airlines, could use intervals from MSG-3. Alaska Airlinesí manager of

reliability and maintenance programs testified that the airline had done so in some areas.
Factual Information 27 Aircraft Accident Report

through emergency response, day-to-day and long-term monitoring, and auditing

functions. The handbook states that ìsome operators with approved reliability programs

use the reliability program to fulfill the monitoring mechanical performance functions

requirement of its [continuing analysis and surveillance] program.î Typical sources of

data collection for reliability programs are unscheduled removals, mechanical delays and

cancellations, pilot reports, sampling inspections, shop findings, functional checks, bench

checks, service difficulty reports, and mechanical interruption summaries. The objective

of data analysis is to recognize the need for corrective action, establish what corrective

action is needed, and determine the effectiveness of that action. Other indicators of

component reliability used to detect unfavorable trends are operational incident reports

and direct information from maintenance control, component shops, or repair control

about component discrepancies.

Alaska Airlinesí reliability analysis program was authorized on April 3, 1995, as

part of the airlineís FAA-approved operations specifications. According to reliability

analysis program guidelines, Alaska Airlines managed its maintenance program by

implementing decisions based on continuing analysis of operational data. All scheduled

maintenance/inspection tasks, checks, and intervals were included in the reliability

analysis program. Reliability analysis program findings could result in changes, additions,

or deletions of maintenance tasks, intervals, or operating limits. 

Alaska Airlines has made changes to its reliability analysis program since the

flight 261 accident. This discussion describes Alaska Airlinesí reliability analysis program

up until the time of the accident. The Reliability Analysis Program Control Board, which

reported to Alaska Airlinesí vice president of maintenance and engineering, was

responsible for the overall program.54 The Reliability Analysis Program Control Board

was authorized to determine what course of remedial action was necessary and to order

implementation of its decisions. The Reliability Department, which reported to the

Reliability Analysis Program Control Board, administered the program and was

responsible for reviewing and analyzing collected data, issuing alert notices, and

providing notification of reliability issues. 

The Reliability Analysis Program Control Board also included a smaller

Reliability Action Board that addressed problems considered critical or urgent and that

had to be considered before the next Reliability Analysis Program Control Board

meeting.55 Reliability Action Board decisions that required changes to the activities

controlled by the reliability analysis program required the preparation and approval of a

Reliability Analysis Program Control Board directive. 

54 Alaska Airlines Reliability Analysis Program Control Board, which met once per month, comprised

the airlineís reliability manager (who served as board chairman), the director of base maintenance, the

director of line maintenance, the director of quality control, the director of engineering, the manager of

maintenance control/technical services, the manager of maintenance programs and publications, and the

director of maintenance planning and production control.

55 A Reliability Action Board meeting did not require the attendance of the full Reliability Analysis

Program Control Board but had to be attended by the Reliability Analysis Program Control Board Chairman

(or designee) and other members, as appropriate to the issue being discussed.
Factual Information 28 Aircraft Accident Report

In addition, a company MRB at Alaska Airlines met biweekly to review and

approve changes to the maintenance program.56 The MRB also reviewed ME-01 change

requests57 that required MRB approval. Under Alaska Airlinesí MRB program, the

manager of maintenance programs and publications reviewed ME-01 change requests to

determine whether the change required MRB approval. If the ME-01 did not require

MRB/Reliability Analysis Program Control Board approval, the request was routed to

management personnel in the following order: reliability, director of engineering, director

of line maintenance, director of base maintenance, manager of maintenance control,

director of quality control, director of maintenance planning and production control. The

MRB/Reliability Analysis Program Control Board voting member representing the

department that requested the change reviewed the ME-01 and then decided whether to

sign it. If signed, the ME-01 is forwarded to the manager of the maintenance programs and

technical publications department for processing.

MRB approval was required for any change to Alaska Airlinesí GMM. Reliability

analysis program approval was required for any maintenance program change, some of

which required FAA approval. According to Alaska Airlinesí reliability analysis program,

maintenance program changes that required FAA approval included the following:

ï Addition or deletion of an aircraft manufacturerís MSG-2 OAMP

recommended maintenance significant item (MSI),58 or an aircraft

manufacturerís MSG-3 OAMP document recommended systems maintenance,

structural, or zonal task.59

ï Addition or deletion of an MSG-2 MRB recommended MSI or an MSG-3

MRB recommended systems maintenance, structural, or zonal task.

ï Escalation of hard-time component overhaul intervals, periodic maintenance

check intervals, or structural inspection intervals to greater than 10 percent of

the present interval. 

ï Escalation of an individual maintenance task interval (currently performed at

or in excess of the interval recommended in the applicable aircraftís OAMP

document) to greater than 10 percent of the current interval.

Changes that did not require FAA approval included the following:

56 Alaska Airlinesí company MRB members were the same as those who served on the Reliability

Analysis Program Control Board.

57 The ME-01 form is used by Alaska Airlines to process requests and recommendations to change

existing maintenance program procedures, intervals, and documents (task cards, maintenance manuals, and

policy and procedures manuals), and other documents under the control of the airlineís engineering and

quality departments. The ME-01 is not required for typographical errors, P/N changes, clarification of

existing procedures, changes to torques, clearances and limits required to reflect manual revisions, and

changes generated by previously authorized airplane modifications or actions to meet AD or Federal

Aviation Regulations (FAR) requirements.

58 An MSI is an item identified by the manufacturer, the failure of which is undetectable during

operations and could affect safety (on ground or in flight) or could have significant operational economic

impact. 

59 Zonal tasks are tasks associated with specific areas of the airplane, such as the wing and fuselage.
Factual Information 29 Aircraft Accident Report

ï Escalation of hard-time component overhaul intervals, periodic maintenance

check intervals, or structural inspection intervals that did not exceed 10 percent

of the current interval. (Although FAA approval is not required for these

escalations, data justifying the change must be available to the FAA on

request.)

ï Escalation of an individual maintenance task interval (which is not listed on

the airplaneís MRB) up to the interval recommended in the applicable

airplaneís OAMP document, regardless of the increase of the current interval.

ï Periodic maintenance intervals of MSIs identified as condition-monitored

items, which have a periodic maintenance activity or task at some determined

interval.

Items that cannot be changed or that required approval beyond the local FAA

Flight Standards District Office (FSDO) included the following:

ï Those tasks listed as certification maintenance requirements (CMR),60

airworthiness limitation items,61 or fixed maintenance intervals unless

approval is granted from the FAA Aircraft Certification Office.

ï Life limits mandated by the manufacturer.

ï Time limits mandated by an AD, except in accordance with the AD.

ï Time limits mandated by an aircraftís type certificate data sheet.

1.6.3.2 Jackscrew Assembly Lubrication Procedures

The Alaska Airlines maintenance task card for elevator and horizontal stabilizer

lubrication (No. 28312000) specified the following steps for lubricating the jackscrew

assembly:62

A. Open access doors 6307, 6308, 6306 and 6309.

B. Lube per the following

60 According to AC 25-19, ìCertification Maintenance Requirements,î dated November 28, 1994, ìa

CMR is a required periodic task, established during the design certification of the airplane as an operating

limitation of the type certificateÖ[and is] intended to detect safety-significant latent failures that would, in

combination with one or more other specific failures or events, result in a hazardous or catastrophic failure

condition.î According to the AC, ìCMRs are designed to verify that a certain failure has or has not occurred,

and does not provide any preventative maintenance function.î (Emphasis in original.)

61 Airworthiness limitation items are documents that contain information that defines mandatory

replacement times for safe-life structure and inspection requirements for principal structural elements and

provide specific nondestructive inspection techniques and procedures for each principal structural element. 

62 Illustrations were attached to the task card to show the areas to be lubricated.
Factual Information 30 Aircraft Accident Report

C. Close doors 6307, 6308, 6306 and 6309. 

General lubrication guidance contained in Boeingís Maintenance Manual states

that, when lubricating vented bearings, ìforce grease into fittings until all old grease is

extruded.î According to a February 25, 2002, letter from Boeing to the Safety Board, this

lubrication task requires about 4.5 person hours to accomplish, including removal and

closure of the access panels. 

During the investigation, Safety Board investigators conducted interviews with

Alaska Airlines mechanics who had lubricated the accident airplaneís elevators and

horizontal stabilizer components to determine how they understood and practiced the

lubrication procedures. The Alaska Airlines mechanic who was responsible for lubricating

the accident airplaneís elevators and horizontal stabilizer components during a C check at

the airlineís maintenance facility at Oakland International Airport (OAK), Oakland,

California, in September 199763 told Safety Board investigators that lubrication of the

elevators and horizontal stabilizer components ìtakes a couple of hours.î The mechanic

stated that if the lubrication of the elevators and horizontal stabilizer components is

accomplished during a C check, the task is accomplished by a mechanic standing on a

ITEM

NO.

ITEM DESCRIPTION NO. OF FITTINGS

OR AREAS

1. MAIN GEAR BOX 1

Check oil level and fill to

fill plug with approved jet

engine oil. Oil capacity is

approximately 0.6 pint.

2. ACTUATOR ASSEMBLY 1

3. JACKSCREW 0

Apply light coat of grease to

threads, then operate

mechanism through full

range of travel to distribute

lubricant over length of

jackscrew.

4. SCREW GIMBAL 3

5. HORIZONTAL

STABILIZER HINGE

2

6. JACKSCREW STOP 0

Fill cavity between screw and

stop with Parker-O-Lube.
Factual Information 31 Aircraft Accident Report

tail-stand platform in the hangar. He stated that he would apply grease to acme nut grease

fittings until ìthe grease starts to come outÖthe sides where youíre greasing, where it

starts to bulge out.î The mechanic stated that he would also use a ìpaint brushî to apply

grease to the acme screw threads. He added, ìWhen I do it, I do the brush, too, but I donít

feel itís enough. I always do an over-kill. I always like to put more onÖ[to] put a big glob

on my hand and make sure itís on. Moreís better. Itís not going to hurt anything.î

The Alaska Airlines mechanic who was responsible for performing the accident

airplaneís last elevator and horizontal stabilizer components lubrication at SFO in

September 1999 told Safety Board investigators that he would know if the grease fitting

for the acme nut was clogged ìbecause youíd feel it in the [grease] gun as you try to put it

in. If it wasnít going in, you could feel it.î When asked how he determined whether the

lubrication was being accomplished properly and when to stop pumping the grease gun,

the mechanic responded, ìI donít.î When asked whether he would be able to see grease

coming out of the top of the acme nut during lubrication, the mechanic responded, ìYou

know, I canít remember looking to see if there was.î He added that lubrications at SFO

were accomplished most often during the evening, outside of the hangar (even if it was

raining), and in the basket of a lift-truck. He added that he used a battery-operated head

lamp during night lubrication tasks. He stated that the lubrication task took

ìroughlyÖprobably an hourî to accomplish. It was not entirely clear from his testimony

whether he was including removal of the access panels in his estimate. When asked

whether his 1-hour estimate included gaining access to the area, he replied, ìNo, that

would probably take a littleówell, youíve got probably a dozen screws to take out of the

one panel, so thatísóI wouldnít think any more than an hour.î The questioner then stated,

ìincluding access?î and the mechanic replied, ìYeah.î The mechanic also stated that he

applied grease to the acme screw with a brush, adding, ìYou basically take out a little bit

of grease with you, or you pump it out of the grease gun onto a paint brush and just paint a

real light coat on there.î64

1.6.3.2.1 Jackscrew Assembly Lubrication Intervals 

1.6.3.2.1.1 Manufacturer-Recommended Lubrication Intervals

Original DC-9 certification documents, including Douglas Process

Standard 3.17-49 (issued August 1, 1964), specified a lubrication interval for the

jackscrew assembly of 300 to 350 flight hours. The manufacturerís initial OAMP

document for the DC-9 and the MD-80 specified lubrication intervals of 600 to 900 flight

hours. In 1996, McDonnell Douglas issued another OAMP document, which was

developed to reflect the guidance and philosophy in the MSG-3 MSG-3 MRB report for

the MD-80. The MSG-3 MRB called for lubrication of the jackscrew assembly at C-check

63 There were three scheduled lubrications after the September 1997 C check. The C check performed in

September 1997 was the last C check accomplished on the accident airplane that required an end play check

of the acme screw and nut. For more information about the accident airplaneís maintenance history, see

section 1.6.3.5.

64 For information about Safety Board observations of jackscrew assembly lubrications performed by

maintenance personnel from two MD-80 operators, see section 1.18.6.
Factual Information 32 Aircraft Accident Report

intervals (every 3,600 flight hours or 15 months, whichever comes first). The MSG-3

OAMP also called for lubrication of the jackscrew assembly at C-check intervals (every

3,600 flight hours).65

Testimony at the Safety Boardís public hearing and Boeing documents indicated

that Douglasí earlier recommended lubrication interval of 600 to 900 flight hours was not

considered during the MSG-3 decision-making process to extend the recommended

interval. Further, Boeing design engineers were not consulted about nor aware of the

extended lubrication interval specified in the MSG-3 documents. The FAAís MD-80 MRB

chairman testified at the public hearing that the longer interval in the MSG-3 documents

was supported by reliability data from the carriers and the manufacturer. He explained that

the MSG-3 MD-80 MRB considered the jackscrew lubrication task change as part of a

larger C-check ìpackageî and that the extension of C-check intervals in the MSG-3

MD-80 MRB did not involve a task-by-task analysis of each task that would be affected

by the changed interval.

1.6.3.2.1.2 Alaska Airlinesí Lubrication Intervals

Alaska Airlines maintenance records chronicled the following changes in

lubrication intervals from 1985 to April 2000:

ï In March 1985, the B-check interval was 350 flight hours, and the jackscrew

assembly lubrication was to be accomplished at every other B check, or every

700 flight hours.

ï In March 1987, the B-check interval was increased to 500 flight hours, and the

jackscrew assembly lubrication was changed to be accomplished at every

B check, or every 500 flight hours.

ï In July 1988, B checks were eliminated from Alaska Airlinesí MD-80

maintenance program, and the tasks accomplished during B checks were

incorporated into the A and C checks. A checks were to be accomplished every

125 flight hours. The jackscrew assembly lubrication was to be accomplished

at every eighth A check, or every 1,000 flight hours. 

ï In February 1991, A-check intervals were increased to 150 flight hours.

Jackscrew assembly lubrication was still to be accomplished at every eighth

A check, or every 1,200 flight hours.

ï In December 1994, the A-check interval was increased to 200 flight hours.

Jackscrew assembly lubrication was still to be accomplished at every eighth

A check, or every 1,600 flight hours.

ï In July 1996, the jackscrew assembly lubrication task was removed from the A

check and placed on a time-controlled task card with a maximum interval of

8 months.66 There was no accompanying flight-hour limit; however, based on

airplane utilization rates at that time, 8 months was about 2,550 flight hours.

65 The FAAís MD-80 MRB Chairman testified at the public hearing that he assumed the absence of any

calendar-time interval in the MSG-3 OAMP document was a typographical error.
Factual Information 33 Aircraft Accident Report

Jackscrew assembly lubrication was also to be performed at each C check

using time-controlled task card No. 28312000. According to the FAA PMI for

Alaska Airlines, who reviewed and accepted the 1996 lubrication interval

extension, the extended MSG-3 interval (3,600 flight hours or 15 months,

whichever comes first) was presented as justification for the interval increase. 

ï In October 1996, jackscrew assembly lubrication was combined with the

lubrication of the elevators and elevator tabs. The interval remained at

8 months, or about 2,550 flight hours, and this was the lubrication interval in

effect at the time of the accident.

ï On April 6, 2000, after the accident, the jackscrew assembly lubrication

interval was changed from 8 months to 650 flight hours, in accordance with

ADs 2000-03-51 and 2000-15-15.67 Table 2 shows a comparison of

manufacturer-recommended jackscrew assembly lubrication intervals with

Alaska Airlinesí intervals.

Table 2. Comparison of manufacturer-recommended jackscrew assembly lubrication 

intervals with Alaska Airlinesí intervals.

66 A time-controlled task card is a stand-alone maintenance task card that is tracked individually by the

operator.

67 For more information about ADs 2000-03-51 and 2000-15-15, see section 1.18.1.

MSG-2 MRB MSG-2 OAMP MSG-3 MRB MSG-3 OAMP 

Not included in logic 

diagram

600 to 900 flight 

hours

C check

(3,600 flight hours or 15 

months,

whichever comes first)

C check

(3,600 flight hours)

Alaska 

Airlines 

1985

Alaska 

Airlines 

1987

Alaska 

Airlines 

1988

Alaska 

Airlines 

1991

Alaska 

Airlines 

1994

Alaska 

Airlines 

1996 to April 

2000

Alaska 

Airlines 

April 

2000 to 

Presenta

a. All carriers currently meet this requirement.

Every other 

B check

(700 flight 

hours)

B check 

(500 flight 

hours)

Every 

eighth A 

check

(1,000 

flight 

hours)

Every 

eighth A 

check

(1,200 flight 

hours)

Every 

eighth A 

check

(1,600 

flight 

hours)

Time-controll

ed task 

cardó

8 months 

maximum

(About 2,550 

flight hours)

650 flight 

hours
Factual Information 34 Aircraft Accident Report

1.6.3.2.2 Chronology of Grease Type Changes

Boeingís Airplane Maintenance Manual for DC-9, MD-80/90, and 717 series

airplanes specifies the use of MIL-G-8132268 grease to lubricate the jackscrew assembly.

Initially, Alaska Airlines used Mobilgrease 28, which meets the requirements of

MIL-G-81322. On January 16, 1996, Alaska Airlines asked its McDonnell Douglas field

representative if Aeroshell 33 could be used in place of Mobilgrease 28 on suitable areas

of its MD-80 fleet airplanes, including the jackscrew assembly. Aeroshell 33 was a

relatively new type of grease that was developed to meet Boeing Material

Specifications 3-33 and subsequently qualified under MIL-G-23827.69

According to Alaska Airlines, this change was part of an effort to standardize and

reduce the number of greases used by the airline in its mixed fleet of 737s and MD-80s.

On February 23, 1996, McDonnell Douglas engineers replied that laboratory testing

would be required to determine whether Aeroshell 33 could be approved for use on

Douglas airplanes. Alaska Airlines was informed on January 24, 1997, that laboratory

testing of Aeroshell had begun and that the testing could take about a year.

On June 19, 1997, Douglas informed its Alaska Airlines field service

representative that it was pursuing the possibility of a ìno technical objectionî for Alaska

Airlines to use Aeroshell 33 on Douglas airplanes. However, the message stated, ìthis is

not a sure thing, considering the liability of providing consent for the use of such an

important substanceÖbefore the substance has been fully evaluated.î The message stated

that another possibility was an in-service evaluation of the grease by Alaska Airlines. On

June 23, 1997, the Douglas field service representative sent a message to Douglas

headquarters noting that Alaska Airlines welcomed the offer to conduct an in-service

evaluation of Aeroshell 33 and was awaiting guidelines regarding its use.

On July 23, 1997, Alaska Airlines initiated an internal form titled, ìMaintenance

Programs/Technical Publications Change Requestî (ME-01 No. 97-002974), to revise its

maintenance lubrication task cards by replacing Mobilgrease 28 with Aeroshell 33 for use

on flight controls, including the jackscrew assembly, doors, and landing gear (except

wheel bearings), on MD-80 series airplanes in its fleet. This form was signed by the

initiator of the request, the manager of maintenance programs and technical publications

(dated September 17, 1997), and the director of engineering (dated July 25, 1997).

Signature blocks for the director of base maintenance and the director of maintenance

planning and production control were blank but struck out by hand. The ìspecial action

68 The ìMILî specification is a U.S. military material specification that defines the requirements for a

material. Requirements that are defined for grease MIL specification categories include, in part, viscosity,

temperature range, and corrosion protection. A partís function and exposure to such factors as loads, friction,

extreme temperatures, and water and other contaminants determine which grease MIL specification category

is appropriate. In 1999, the MIL-G nomenclature (in which G stands for grease) was replaced with MIL-PRF

(in which PRF stands for performance); no change in specifications was made. Several products from

different manufacturers may meet a specific MIL specification. After testing and qualification, these greases

are listed on the Qualified Products List for each MIL specification. For example, greases that meet the

specifications for MIL-G-81322 include Mobilgrease 28, Aeroshell Grease 22, and Royco Grease 22.

69 MIL-PRF-23827 superseded MIL-G-23827 in 1998, with no change in specifications. 
Factual Information 35 Aircraft Accident Report

requiredî70 section was blank, as was the signature block at the bottom of the form to

indicate ìChange Request accomplished.î At the Safety Boardís public hearing, Alaska

Airlines former director of engineering (who at the time that the grease change was

initiated was Alaska Airlines manager of maintenance programs and technical

publications) and the current director of engineering indicated that Reliability Analysis

Program Control Board approval was required to implement the change. However, neither

individual could explain how the change was implemented without the required signatures

or Reliability Analysis Program Control Board approval.

On September 26, 1997, McDonnell Douglas informed Alaska Airlines in Douglas

Communication SVC-SEA-0122/MRL that it had ìno technical objection to the use of

[Aeroshell 33] grease in place of MIL-G-81322 grease on Alaska Airlines MD-80 aircraft

with one known restriction. Aeroshell 33 grease may not be used in areas subjected to

temperatures in excess of 250° F including landing gear wheel bearings.î The McDonnell

Douglas communication added the following:

It should also be noted that the initial results of laboratory testing that compared

Aeroshell 33 grease with Mobile grease 28 (MIL-G-81322) indicated that

Aeroshell 33 grease is somewhat less resistant to water wash-out than Mobile

grease 28. It is not known whether the difference in water wash-out resistance

noted in the laboratory will result in any notable difference of the in-service

grease performance. However, the potential exists that the required frequency of

lubrication could be affected in areas of the aircraft exposed to outside ambient

conditions or aircraft washing and cleaning.

This no technical objection is provided prior to the completion of a Douglas study

intended to determine the acceptability of Aeroshell 33 grease for use in

Douglas-built aircraft.[71] As such, Douglas cannot yet verify the performance of

this grease. It will be the responsibility of Alaska Airlines to monitor the areas

where Aeroshell 33 grease is used for any adverse reactions. Further, it will be the

responsibility of Alaska Airlines to obtain any FAA approval required by their

principal maintenance inspector for the use of Aeroshell 33 grease in their MD-80

aircraft.

The McDonnell Douglas communication concluded with the following:

Because the Douglas study of Aeroshell 33 grease will be based on laboratory

tests rather than an in-service test, any data we can obtain from in-service aircraft

would be valuable to help confirm or refute the laboratory results. Any

information Alaska Airlines is able to provide to Douglas, positive or negative,

regarding the in-service performance of Aeroshell 33 grease on the MD-80 will be

greatly appreciated.

70 The possible choices in this section, each preceded by a block to check off, were ìMRB action

required,î ì[Reliability Analysis Program] Control Board action required,î or ìRoutine.î

71 According to Boeing, McDonnell Douglas never completed this testing.
Factual Information 36 Aircraft Accident Report

On December 18, 1997, Alaska Airlines task card No. 24312000 for lubrication of

the horizontal stabilizer components was revised (based on ME-01 No. 97-002974) to

reflect the change from Mobilgrease 28 to Aeroshell 33.72 On January 6, 1998, task card

No. 28312000 for lubrication of the elevators and horizontal stabilizer components was

also revised to reflect the change from Mobilgrease 28 to Aeroshell 33.73

According to Alaska Airlines records, it sent the monthly maintenance task card

audit report for December 1997, which included all task card changes since its last report,

including revision dates, to the FAA. Although Alaska Airlines notified the FAA of the

change in the grease type specified in its maintenance instructions, as required, it did not

provide any substantiating justification at that time nor was such action required. The FAA

PMI for Alaska Airlines at the time of the lubrication change stated during postaccident

interviews that such changes could be made without prior FAA approval under provisions

of the airlineís FAA-approved maintenance program.74

As a result of the flight 261 accident, in March 2000, the FAA requested

documentation from Alaska Airlines that supported the grease change to lubricate the

jackscrew assembly. Alaska Airlines subsequently submitted the requested

documentation.75 In an April 5, 2000, response letter, the FAA informed Alaska Airlines

that the substantiating documents it had submitted did not support the change, and it

disapproved the use of Aeroshell 33 as a substitute for Mobilgrease 28. After receiving

this disapproval notification, Alaska Airlines modified its maintenance procedures to

again specify the use of Mobilgrease 28 to lubricate the jackscrew assembly.

1.6.3.2.3 July 19, 2002, Alaska Airlines Maintenance Information Letter

On July 19, 2002, Alaska Airlines issued Maintenance Information Letter

No. 05-00-02-21, ìMD-80 Rudder Trim Tab Hinge Support Bearings Lubrication,î76

72 According to Alaska Airlines records, 34 additional lubrication task cards, including lubrication tasks

for flaps, slats, spoilers, ailerons, and landing gear components, were revised in December 1997. These task

cards also reflected the change to Aeroshell 33.

73 The GMM was not modified to reflect the grease change to Aeroshell 33; it still specified the use of

Mobilgrease 28 to lubricate the elevators and horizontal stabilizer components.

74 According to the PMI, changes to ìaccepted [maintenance] manualsî used as part of an overall

FAA-approved maintenance program can be made and then submitted to the FAA as part of a routine change

notification process. The PMI stated, ìin accepted manuals, they go ahead and make the change, publish it,

and send you a copy and you read it. And if you have any objection to it, [you] notify them in writing that

you have objections. If you donít tell them, then itís accepted.î Referring to the Alaska Airlines lubrication

change and the task card change notification in 1997, the PMI stated, ìI donít know that anybody caught that

or noticed it or bought off on it or looked into it at all.î 

75 The copies provided to the Safety Board of these documents included a trade magazine article on 

Aeroshell 33, excerpts of Boeing 737 and McDonnell Douglas MD-80 maintenance manuals, Boeing

service letters, internal correspondence and messages between Alaska Airlines and Boeing, and existing

specifications on MIL-G-81322 and MIL-G-23827 greases. None of the documents provided any

information or performance data specifically applicable to the use of Aeroshell 33 on McDonnell Douglas

airplanes or these airplanesí horizontal stabilizer trim systems.

76 Maintenance Information Letter No. 05-00-02-21 is valid for 1 year, expiring on July 19, 2003, or

when rescinded.
Factual Information 37 Aircraft Accident Report

because of ìfindings [that] have revealed that the MD-80 rudder trim tab support bearings

may not always be receiving adequate lubrication.î Alaska Airlines maintenance task card

No. 24612000 specifies lubrication of each of the four bearings that support the MD-80

rudder trim tab at every C check. However, the letter stated that ìshop inspection of tabs

from two aircraftÖonly recently out of C-check indicated extreme bearing wear, with

little or no evidence of new greaseî and that ìone bearing fell apart upon removal.î The

letter stated that the solution was adherence to the lubrication instruction in task card

No. 24612000. In an October 3, 2002, letter to the Safety Board, Alaska Airlines indicated

that the ì[maintenance information letter] was incorrect when it described these aircraft as

ëonly recently out of C-check.íî Alaska Airlines stated that one of the airplanes had last

been in for a C check about 6 months before the discovery of the worn bearing and that the

other airplane had last been in for a C check about 15 months before the discovery of the

worn bearing. In an October 24, 2002, facsimile to the Safety Board, Alaska Airlines

stated further that as a result of additional analysis, the rudder trim tab lubrication interval

had been reduced ìfrom the current ëCí check (approximately 4,350 flight hours) to the

Boeing (McDonnell Douglas) MSG-2 On Aircraft Maintenance PlanningÖëLowí end

interval of 1,200 flight hours.î

1.6.3.3 Procedures for Monitoring Acme Nut Thread Wear

1.6.3.3.1 Defining and Calculating Wear Rate

 The only portion of the jackscrew assembly that wears significantly is the

aluminum-bronze acme nut. Therefore, the terms ìwearî and ìwear rateî refer to the wear

of the acme nut. 

As will be discussed in section 1.6.3.3.2, acme nut thread wear can be inferred by

measuring the amount of movement, or end play, between the acme nut and screw threads.

In other words, end play is an indirect measurement of acme nut wear. End play

measurements are typically performed either on-wing or after removal from the airplane

during a bench check. The Safety Board conducted a statistical analysis of end play data

that concluded that the reliability and validity of the on-wing end play check is very low

and is susceptible to measurement error.77 Because the bench end play check is conducted

in a more controlled environment, it is presumably less susceptible to measurement error;

however, the reliability and validity of the bench end play check was not statistically

evaluated. 

Unless otherwise specified, the term ìwear rateî used in this report refers to an

approximate wear rate determined by comparing end play measurements from the

beginning and the end of an elapsed time. The amount of the change in end play over that

period of time is used to calculate the approximate rate of wear. (Wear rate calculations are

generally converted so that they can be expressed as an average wear rate per 1,000 flight

hours.) The wear rate at any given point during the elapsed time is not known; it may be

higher or lower than the approximated wear rate.

77For more information about the Safety Boardís end play data study, see section 1.18.2.
Factual Information 38 Aircraft Accident Report

1.6.3.3.2 Development of End Play Check Procedures

The DC-9 jackscrew assembly was originally certified with an expected service

life of 30,000 hours. According to Douglas documents,78 after 30,000 hours, the gap

between the acme screw and nut threads was expected to be 0.0265 inch. Specifications

for newly manufactured jackscrew assemblies stipulate that this gap be between 0.003 to

0.010 inch. No regular inspections to monitor acme nut thread wear were recommended at

the time the DC-9 was certified. However, when the DC-9 entered service in 1965,

Douglas initiated a program, which closely monitored a sample of DC-9 airplanes. As part

of this program, an end play check procedure was used to monitor wear by measuring the

gap, or end play, between the acme screw and nut threads.79 This procedure was conducted

during bench checks to determine whether the wear rate of the acme nut threads was

comparable to the DC-8 wear rate of 0.001 inch per 1,000 hours.80

As a result of excessive wear rates discovered through this sampling program,

Douglas issued Douglas Design Memorandum C1-250-DES-DC9-683 in February 1967.

The memorandum stated that DC-9 operators had reported six jackscrew assemblies that

indicated ìa wear rate considerably in excess of that predictedî81 and provided the

following information about the flight hours and end play measurements from the six

jackscrew assemblies: (The Safety Board calculated the wear rate per 1,000 hours, taking

into account the end play measurement of the jackscrew assembly when new. If the end

play measurement when new was known, that value was used. Otherwise, a presumed end

play measurement of 0.007 inch was used.)

78 The documents were All Operators Letter (AOL) 9-48, dated November 4, 1966, and Douglas Design

Memorandum C1-250-DES-DC9-683, dated February 28, 1967.

79 The sampling program ended in 1970. As will be discussed, by this time, Douglas had determined that

wear rates for the DC-9 jackscrew assembly made with new materials and manufacturing procedures were

0.001 inch per 1,000 hours or lower. 

80 A 1967 Douglas test report on DC-8 jackscrew assemblies indicated an average wear rate of not

greater than 0.001 inch per 1,000 hours.

81 The design memorandum also noted, ìthere is evidence from screws in service, which have been

returned to [Douglas], that lubrication practices as defined in the Maintenance Manual have not been

followed. Operators should be advised of the importance of relubrication as specified.î

Airplane No. Flight Hours End Play (inches)

Wear Rate (inches per 

1,000 hours)

1 4,086 0.030 0.0056

2 3,300 0.026 0.0058

3 3,322 0.026 0.0057

4 3,390 0.029 0.0065

5 3,400 0.028 0.0062

6 2,200 0.023 0.0073
Factual Information 39 Aircraft Accident Report

As a result of these operator reports, Douglas conducted a series of full-scale tests,

which involved operating several jackscrew assemblies under varying conditions in a test

fixture. The Douglas engineering test report stated that the purposes of the tests were ìto

simulate a rate of wear in the laboratory that is comparable to that being experienced in

service on the DC-9 horizontal stabilizer screw and nut [and to] determine the right

material of the jackscrew end nut, and/or lubrication, which would generate a minimum of

wear to provide a service life of 30,000 flight hours.î As a result of these tests, Douglas

established the end play measurement limit of 0.040 inch after determining that this limit

would not create aerodynamic flutter or other structural problems.82 Douglas also changed

some of the materials specifications and manufacturing processes for the acme screw

(increased heat treating and nitriding) to reduce wear. Douglas conducted laboratory tests

of the DC-9 acme screws made with the newly specified materials and manufacturing

processes, which revealed that the screws had an average wear rate of about 0.001 inch per

1,000 hours. 

To allow operators to monitor acme nut wear without removing the jackscrew

assembly from the airplane, Douglas developed the on-wing end play check procedure.

The procedure calls for pulling down on the horizontal stabilizer by applying a specified

amount of torque to a tool known as a restraining fixture to change the load on the acme

screw from tension to compression.83 The load reversal allows movement, or end play,

between the acme screw and nut threads. This play is measured using a dial indicator,

which is mounted on the lower rotational stop (fixed to the screw) with the movable

plunger set against the lower surface of the acme nut, that measures the vertical movement

of the acme screw when the load is applied. The amount of end play is used as an

indication of the amount of acme nut thread wear (and acme screw thread wear, if there is

any). The procedure calls for the restraining fixture load to be applied and removed

several times until consistent measurements are achieved. Figure 8 shows how the

restraining fixture should be placed on the horizontal stabilizer during the end play check

procedure. Figure 9 shows a view of a typical dial indicator set up for end play checks.

82 In AOL 9-48A, Douglas stated that jackscrew assemblies could remain in service as long as the end

play measurement remained within tolerances (between 0.003 and 0.040 inch). End play checks that

indicated measurements within the specified limits were not required to be recorded before the issuance of

AD 2000-15-15 in July 2000. 

83 For more information about restraining fixtures, see section 1.6.3.3.3.
Factual Information 40 Aircraft Accident Report

Figure 8. Depiction of how restraining fixture should be placed on the horizontal stabilizer 

during an end play check.
Factual Information 41 Aircraft Accident Report

Figure 9. View of a typical dial indicator set up for end play checks.

On May 29, 1984, McDonnell Douglas issued AOL 9-1526, which reported that

ìtwo operators have reported three instances of premature removal/replacement of a

horizontal stabilizer actuator assemblyî and reiterated the OAMP documentís

recommendation that all DC-9 operators lubricate the jackscrew assembly at

600-flight-hour intervals. AOL 9-1526 added, ìthese assemblies, which had accumulated

less than 6,000 flight hours each since new, were replaced due to excessive end play

between the acme screw and acme nut.î AOL 9-1526 stated that the jackscrew assemblies

were returned to McDonnell Douglas for investigation and that ìthe acme nut installed in

each assembly exhibited severe wear of the thread surfaces. In addition, grease samples

taken from the lubrication passagesÖon two of these [jackscrew] assemblies were dry and

without evidence of recent renewal. Accordingly, Douglas is of the opinion that the most

probable cause of the observed acme nut thread wear and subsequent excessive end play

was inadequate lubrication of the actuator assemblies.î84 The AOL concluded, ìIn view of

the foregoing, Douglas wishes to emphasize the importance of maintaining a

conscientious lubrication program to minimize acme nut thread wear and extend the

service life of the actuator assembly.î

In 1990, another airline reported that a DC-9 acme nut had worn beyond the

0.040-inch end play measurement limit after accumulating 5,780 flight hours, a wear rate
Factual Information 42 Aircraft Accident Report

of 0.0057 inch per 1,000 flight hours. In AOL 9-2120, dated December 6, 1990,

McDonnell Douglas asked operators of DC-9 and MD-80 airplanes to report on the

lubrication frequency of the jackscrew assembly, the mean time between removal

(MTBR) and mean time between unscheduled removal (MTBUR), and the average wear

rate of the assembly as measured by an on-wing end play check. On the basis of responses

from 10 DC-9 operators and 11 MD-80 operators,85 on September 5, 1991, McDonnell

Douglas issued AOL 9-2120A, which provided the following results: 

In AOL 9-2120A, McDonnell Douglas noted that the prematurely worn jackscrew

assembly was not typical but, because the assembly was not examined by McDonnell

Douglas, the cause of the premature wear was unknown.86 However, McDonnell Douglas

again reiterated its 600-flight-hour lubrication interval recommendations.87

On April 13 and November 20, 2000, Boeing issued revisions to the end play

check procedure, including the following:

ï Clarification of which part number (P/N) restraining fixture could be used on

specific airplanes;

ï Clarification of how and where to install dial indicator mounting bracket and

probe;

84 According to a McDonnell Douglas field service representative report dated March 13, 1984, the end

play measurement of one of the jackscrew assemblies removed and returned to the company was 0.043 inch.

The report stated that the acme nut threads were ìworn and serrated.î The report added that ìit should be

noted that lubrication cycle of jackscrew has been sporadic on this and other DC-9 [aircraft] at this

location.î The maintenance facility returned the assembly to McDonnell Douglas with a letter attached to

the field representative report, which was addressed to product support and asked McDonnell Douglas to

compare the returned assembly to drawing specifications and check the ìtrueness of the screw shaft

assembly.î The letter also asked, ìWhat is the hardness spec of the nut?î The letter concluded, ìI would

appreciate an expeditious response in this matter for I have a later delivered aircraft with only 4,000 hours

and within .001 of the maximum backlash [end play] limit.î 

85 No information was available about reported wear rates for each of the operators that responded to the

survey or for any individual jackscrew assembly that was included in the survey results. Therefore, the

variability among operators or assemblies is unknown. 

Model Lube Interval MTBR MTBUR Wear Rate

DC-9 1,329 hours 34,054 hours 34,395 hours 0.0011 inch per 1,000 hours

MD-80 804 hours 24,397 hours 28,397 hours 0.0013 inch per 1,000 hours

86 Before issuing AOL 9-1526 in 1984, McDonnell Douglas examined six jackscrew assemblies that

had been reported to have excessive wear. However, McDonnell Douglas could not examine the jackscrew

assembly removed in 1990 because it was not delivered for inspection.

87 The Safety Board confirmed that Alaska Airlinesí maintenance department had received a copy of

AOL 9-2120A from McDonnell Douglas. 
Factual Information 43 Aircraft Accident Report

ï Addition of a note to specify that restraining fixture threads are to be cleaned

and lubricated before the procedure is performed;

ï Addition of a step to apply 100 inch-pounds to the restraining fixture in the

lengthening direction and then 300 inch-pounds (instead of a range from 250 to

300 inch-pounds as previously stated in step [8]) on the restraining fixture in

the shortening direction to get the end play; and

ï Specification of the allowable end play limits in step (11) as ìnot less than

0.003 and not more than 0.040 inchî instead of ìbetween 0.003 and 0.040.î

1.6.3.3.2.1 Alaska Airlinesí End Play Check Procedure

The Alaska Airlines maintenance task card for the acme screw and nut end play

check, dated October 7, 1996, which was in effect at the time of the accident airplaneís last

end play check, described the end play check procedure as follows:

(1) Using control wheel trim switches, move horizontal stabilizer to

approximately 1 degree airplane noseup position.

WARNING:TAG AND SAFETY OPEN CIRCUIT BREAKERS.

(2) Open the following circuit breakers.

(3) Remove panel and stabilizer fairing to gain access to work area.

(4) Install horizontal stabilizer restraining fixture.

(5) Clamp dial indicator mounting bracket to jackscrew torque tube restraining

nut and position dial indicator probe against acme nut. 

(6) Clamp dial indicator mounting bracket to upper stop on jackscrew and

position dial indicator probe against lower plate of support assembly. 

(7) Preload indicator probe to a least .100 inch and record dial indicator reading.

Circuit Breaker Panel

Location

Panel Area

Autopilot & Alternate upper epc Left Radio

Longitudinal Trim (3) AC Bus

Primary

Longitudinal

Trim (2) upper epc Left Radio

DC Bus
Factual Information 44 Aircraft Accident Report

(8) Apply 250 to 300 inch pounds of torque to horizontal stabilizer by shortening

restraining fixture and record dial indicator readings.

(9) Relieve torque on restraining fixture. Check that dial indicator has returned to

initial reading.

(10) Repeat steps (8) and (9) several times to insure consistent results (within

.001 inch).

(11) Check that end play limits are between .003 and .040 inch. Readings in

excess of above are cause for replacement of the acme jackscrew and nut.

Should replacement become necessary accomplish E.O. [engineering order]

8-55-10-01 if not previously accomplished.

(12) Check that free play between jackscrew and upper support does not exceed

.010 inch. If free play exceeds this dimension, upper bearing must be

replaced.

(13) Remove restraining fixture, dial indicator and attaching brackets and clamps.

Install stabilizer fairing and panel.

(14) Close circuit breakers identified in step (2).

(15) Return horizontal stabilizer to neutral.

1.6.3.3.3 Horizontal Stabilizer Restraining Fixture

1.6.3.3.3.1 General 

The McDonnell Douglas MD-80 generic task card for the acme screw and nut

operation (No. 0855, dated December 1991) describes the end play check procedure and

identifies a McDonnell Douglas-manufactured horizontal stabilizer restraining fixture,

P/N 4916750-1, for use in the procedure. The task card also noted that equivalent

substitutes could be used in place of these tools. The original Douglas engineering

drawing for the restraining fixture indicates that the -503 configuration was to be used on

MD-80 series airplanes, which would include the accident airplane. 

1.6.3.3.3.2 Alaska Airlines-Manufactured Restraining Fixtures

The Safety Boardís investigation revealed that up until the time of the accident,

Alaska Airlines had only one restraining fixture in its inventory (at its OAK maintenance

facility). The fixture was manufactured in-house; it was not manufactured by Boeing.88

Since June 30, 1984, when Alaska Airlines began operating MD-80s, the restraining

fixture was tracked through a general component tracking system as P/N 0-1301-0-0169

(manufacturerís P/N 4916750-1). There were no records of an initial inspection or any

recurrent inspections of the restraining fixture. According to Alaska Airlines records, after

the accident, it manufactured 11 additional restraining fixtures similar in design to its
Factual Information 45 Aircraft Accident Report

original fixture and purchased 7 fixtures manufactured by Boeing. The Alaska Airlines

manager of tool control, who participated in the fabrication of the fixtures ordered by

airline management after the accident, told Safety Board investigators in an interview that

ìwhat they [Alaska Airlines personnel] were making [the 11 restraining fixtures] wasnít

even closeî to Boeingís engineering drawing requirements.89 He added that ìwe were

directed to build the tools, and we did exactly what we were told.î

During the investigation, Safety Board investigators conducted several on-wing

end play checks using both restraining fixtures fabricated by Alaska Airlines90 and those

manufactured by Boeing. Examination of the restraining fixtures indicated that none of the

fixtures manufactured by Alaska Airlines met Boeingís engineering drawing requirements

for the fixtures. During several of these tests, the restraining fixtures fabricated by Alaska

Airlines yielded lower end play measurements than the measurements obtained using the

Boeing-manufactured fixture.91 Figure 10 shows two of the restraining fixtures fabricated

by Alaska Airlines and three of the fixtures manufactured by Boeing. 

After the accident, on April 13, 2000, Boeing sent Message M-7200-00-00975 to

all DC-9, MD-80, MD-90, and 717 operators to ensure that horizontal stabilizer inspection

tooling conformed to the toolís engineering drawing requirements. Regarding the

restraining fixture, the Boeing message stated the following:

recent reports received by Boeing indicate that tooling not manufactured by

Boeing is being utilized to perform the subject checks. In some cases, this tooling

may not conform to the toolís drawing requirements...any variation in the tooling

thread quality, pitch or amount of thread engagement can [a]ffect the wear check

results.

The Boeing message asked operators to ìensure the restraining fixtures being utilized

fully conform to the toolís drawing requirementsî to ìmaintain consistent results from

wear checks.î

88 In its submission, Alaska Airlines does not dispute that it had only one restraining fixture during this

time. However, it contends that this fixture was manufactured by Douglas. As support for this contention,

Alaska Airlines cites the July 11, 2002, deposition testimony (taken in connection with civil litigation

arising from this accident) of the mechanic who conducted the September 1997 end play check of the

accident jackscrew assembly that he thought the restraining fixture at the OAK maintenance facility in 1997

was ìa Douglas part.î

89 Alaska Airlines did not have the Boeing engineering drawing requirements. The manager of tool

control was given a copy of the engineering drawing for the in-house manufactured fixture for use in

fabricating the additional 11 fixtures.

90 The Safety Board did not use the actual restraining fixture that was used for the accident airplaneís

last end play check because it was not in serviceable condition.

91 Discrepancies up to a maximum of about 0.005 inch were recorded.
Factual Information 46 Aircraft Accident Report

Figure 10. Two Alaska Airlines-fabricated restraining fixtures and three Boeingmanufactured fixtures.

On June 26, 2000, Safety Board investigators submitted a written request to

Alaska Airlines for information about the restraining fixture it used during the

September 1997 end play check on the accident airplane. The Board also requested

information and documents regarding the physical properties of all of Alaska Airlinesí

in-house fabricated restraining fixtures. Investigators contacted Alaska Airlines on several

occasions to follow up on these requests; however, Alaska Airlines did not provide the

requested information until August 2, 2000. 

On August 2, 2000, Alaska Airlines notified the FAA that it was concerned that the

restraining fixtures it had manufactured in-house might not be ìan equivalent substituteî

for a Boeing fixture and, therefore, could produce erroneous measurements. Alaska

Airlines thereafter suspended use of its in-house manufactured fixtures and temporarily

grounded 18 of its 34 MD-80 series airplanes after determining that these airplanes might

have received end play checks with the in-house manufactured tools. Before placing the

airplanes back in service, Alaska Airlines performed end play checks on the grounded

airplanes using Boeing-manufactured restraining fixtures; all of the rechecked assemblies

had end play measurements within permissible limits. According to Alaska Airlines

records, on August 4, 2000, it acquired 15 Boeing-manufactured restraining fixtures, all of

which conformed to Boeingís engineering drawing requirements.

On December 4, 2000, the FAA issued Flight Standards Handbook Bulletin for

Airworthiness 00-20A, ìEquivalency of Special Equipment or Test Apparatus as Provided
Factual Information 47 Aircraft Accident Report

by Parts 43 and 145,î which provides guidance for PMIs regarding the acceptability for

equivalency of special equipment and/or test apparatus used in maintaining aircraft and

their components. According to the FAA, the handbook bulletin was issued in connection

with AD 2000-03-51, which referenced Boeing Alert Service Bulletin (ASB)

DC9-27A362. ASB DC9-27A362, which specified the use of a restraining fixture for the

required end play check, stated that the tool was currently unavailable from Boeing but

that Boeing would supply the drawing necessary for operators to produce the tool. The

ASB included eight items of guidance to PMIs for determining that repair stations and

operators complied with the equivalency requirements, but it also stated, ì[i]n instances

involving the use of equivalent equipment and/or apparatus, the determination of

equivalency for such equipment is the primary responsibility of the repair station or the air

carrier, not the FAA.î

1.6.3.3.4 End Play Check Intervals 

1.6.3.3.4.1 Manufacturer-Recommended End Play Check Intervals

In the initial OAMP document for the DC-9, Douglas recommended that end play

checks be performed at C-check intervals, or every 3,600 flight hours. Subsequently, in

both the MSG-2 and MSG-3 MRB reports and OAMP documents for the DC-9 and

MD-80 airplanes, the recommended end play check interval was increased to every other

C check. Therefore, the MSG-2 OAMP document recommended that end play checks be

performed every 7,000 flight hours, or 30 months, whichever comes first, and the MSG-3

OAMP document recommended that end play checks be performed every 7,200 flight

hours, or 30 months, whichever comes first.92

1.6.3.3.4.2 Alaska Airlinesí End Play Check Intervals

At the time of the flight 261 accident, Alaska Airlinesí end play check interval was

every 30 months, or about every 9,550 flight hours. The following is a chronology of acme

screw and nut end play check intervals at Alaska Airlines:

ï In March 1985, end play checks were scheduled for every other C check,

which were accomplished every 2,500 flight hours; therefore, end play checks

were to be accomplished every 5,000 flight hours.

ï By July 1988, C-check intervals had been extended to 13 months, with no

accompanying flight-hour limit; therefore, end play checks were to be

accomplished every 26 calendar months, or about every 6,400 flight hours.

ï In April 1996, C-check intervals were extended to 15 months, or about every

4,775 flight hours. Alaska Airlinesí director of reliability and maintenance

programs testified at the public hearing that this C-check interval extension

was approved after the airline prepared a data package ìthat looked at five

separate tail numbers [airplanes] that we selected as a stratified sample of the

92As stated previously, the MSG-2 recommended C-check interval was 3,500 flight hours, and the 

MSG-3 recommended C-check interval was 3,600 flight hours.
Factual Information 48 Aircraft Accident Report

fleet. At that time I believe there were a little over 40 aircraft. We reviewed in

detail all of the nonroutine discrepancies that had been written against the

aircraft during those C checks.î He indicated that individual maintenance tasks

tied to C-check intervals (such as the end play check) were not separately

considered in connection with the extension. However, he stated that based on

the history of maintenance discrepancies noted for the five sample airplanes,

two lubrication tasks were identified as inappropriate for extension along with

the C check, and these tasks were converted to time-controlled (stand-alone)

items to be performed at shorter intervals.93 He indicated that all of the

information was submitted to the FAA and that, approximately 1 month later,

the extension was approved and implemented. The end play check interval

remained at every other C check, which was then 30 calendar months, or about

9,550 flight hours. 

ï On April 6, 2000, after the accident, in accordance with ADs 2000-03-51 and

2000-15-15, the end play check became a time-controlled item required to be

performed every 2,000 flight hours. Table 3 shows a comparison of

manufacturer-recommended end play check intervals with Alaska Airlinesí

intervals.

Table 3. Comparison of manufacturer-recommended end play check intervals with 

Alaska Airlinesí intervals.

93 The two tasks identified as requiring shorter intervals based on service history were (1) lubrication of 

the bent-up trailing-edge wing doors and (2) lubrication of bearings and bushings in the elevator hinges.

 Task 

Description

Original 

Douglas-Recommended 

Interval

MSG-2 MRB 

and OAMP

MSG-3 

MRB

MSG-3 

OAMP 

C Check 3,600 flight hours 3,500 flight 

hours or 15 

months, 

whichever 

comes first

3,600 flight 

hours or 15 

months, 

whichever 

comes first

3,600 flight 

hours

End Play 

Check

C check 

(3,600 flight hours)

Every other C 

check

(7,000 flight 

hours or 30 

months, 

whichever 

comes first)

Every other 

C check

(7,200 flight 

hours or 30 

months, 

whichever 

comes first)

Every other 

C check

(7,200 flight 

hours)
Factual Information 49 Aircraft Accident Report

1.6.3.4 Accident Airplaneís Maintenance Information

Alaska Airlines maintenance records documented that the following maintenance

tasks and checks relevant to the jackscrew assembly were performed on the accident

airplane:

ï On May 27, 1993, a C check was accomplished at the OAK maintenance

facility. The airplane had accumulated 2,674 flight hours. This C check

included an end play check. Four nonroutine discrepancies were noted and

corrected. 

ï On April 27, 1994, a C check was accomplished at the OAK maintenance

facility. The airplane had accumulated 5,484 flight hours. No end play check

was required, and no nonroutine discrepancies related to the horizontal

stabilizer were noted. 

ï On May 17, 1995, a C check was accomplished at the OAK maintenance

facility, including an end play check. The airplane had accumulated

9,194 flight hours. Five nonroutine discrepancies related to the horizontal

stabilizer were noted. Maintenance action taken included cleaning the

horizontal stabilizer jackscrew compartment around the vertical stabilizer and

the areas around the right and left elevator hydraulic actuators.

ï On June 20, 1996, a C check was accomplished at the OAK maintenance

facility. The airplane had accumulated 12,906 flight hours. No end play check

was required. Seven nonroutine discrepancies were noted and corrected. The

records indicated that the elevators and horizontal stabilizer, including the

jackscrew assembly, were lubricated.

 Task 

Description

Alaska 

Airlines 1985

Alaska 

Airlines 1988

Alaska Airlines 

1996 to April 

2000

Alaska 

Airlines 

Currently

C Check 2,500 flight 

hours

13 months

(About 3,200 

flight hours)

15 months

(About 4,775 

flight hours)

15 months

End Play Check Every other C 

check

(5,000 flight 

hours)

Every other C 

check

26 months

(About 6,400 

flight hours)

Every other C 

check

30 months

(About 9,550 

flight hours)

2,000 flight 

hoursa

a. All carriers currently meet this requirement.
Factual Information 50 Aircraft Accident Report

ï On February 27, 1997, according to maintenance records, the elevators and

horizontal stabilizer, including the jackscrew assembly, were lubricated at the

SFO maintenance facility.

ï On September 26, 1997, the airplane entered a C check (which was completed

on October 1, 1997) at the OAK maintenance facility, which included an end

play check.94 The airplane had accumulated 17,699 flight hours. The records

indicated that the elevators and horizontal stabilizer, including the jackscrew

assembly, were lubricated. The task card (No. 28312000) specified

Mobilgrease 28 as the lubricant grease.

ï On June 26, 1998, according to maintenance records, the elevators and

horizontal stabilizer, including the jackscrew assembly, were lubricated at the

SFO maintenance facility. The task card specified Aeroshell 33 as the lubricant

grease. 

ï On January 13, 1999, a C check was accomplished at the OAK maintenance

facility. The airplane had accumulated 22,407 flight hours. No end play check

was required. No nonroutine discrepancies related to the horizontal stabilizer

were noted. The records indicated that the elevators and horizontal stabilizer,

including the jackscrew assembly, were lubricated. The task card specified

Aeroshell 33 as the lubricant grease.

ï On September 24, 1999, according to maintenance records, the elevators and

horizontal stabilizer, including the jackscrew assembly, were lubricated at the

SFO maintenance facility.95 The task card specified Aeroshell 33 as the

lubricant grease.

The Safety Boardís review also revealed that, between July 4, 1999, and January 11,

2000, nine A checks were performed on the accident airplane. No nonroutine discrepancies

related to the horizontal stabilizer were noted during any of these inspections. The accident

airplaneís last walk-around checks were performed at Anchorage International Airport,

Anchorage, Alaska, and SEA on January 30, 2000. The airplaneís last service check was

also performed at SEA on January 30, 2000.

1.6.3.4.1 Accident Airplaneís Last C Check Requiring an End Play Check

As stated previously, the accident airplane entered a C check, which included an

acme screw and nut end play check (and several rechecks), at Alaska Airlinesí OAK

maintenance facility96 on September 26, 1997. Completion of the C check and estimated

time of release was scheduled for September 30, 1997, at 2300. According to Alaska

94 For more details about the September 1997 C check, including the end play measurement, see

section 1.6.3.5.1.

95 Alaska Airlines maintenance records indicated that this was the last time that the accident airplaneís

jackscrew assembly was lubricated.

96 The Alaska Airlines OAK maintenance facility operates 24 hours a day, 7 days a week. The work day

is divided into the following three shifts: the graveyard shift, from about 2230 to 0630; the day shift, from

about 0600 to 1430; and the swing shift, from about 1430 to 2300.
Factual Information 51 Aircraft Accident Report

Airlines records, on September 30, 1997, the estimated time of release was changed to

October 1st at 2300. According to the October 2nd graveyard-shift turnover plan, the

accident airplane was released on October 2nd at 2300. The following items were noted

during the Safety Boardís review of Alaska Airlines maintenance records for this

C check:97

ï September 27th (Saturday)óa nonroutine work card (MIG-4) was generated

following an initial end play check on the accident airplane by a day-shift

mechanic and an inspector. The MIG-4 noted the following discrepancy:

ìHorizontal Stabóacme screw and nut has maximum allowable end play limit

(.040 in.).î The ìplanned actionî box, which was filled out by the day-shift

lead mechanic and inspector, stated, ìReplace nut and perform

E.O. 8-55-10-01.î The swing-shift supervisor also signed off on the planned

action. See figure 11.

Figure 11. The September 27, 1997, MIG-4.

97All of the shift turnover plans were available except for the September 27 and 28, 2000, day-shift 

turnover plans. When asked by Safety Board investigators why these shift turnover plans were not available,

Alaska Airlines responded that they were most likely not generated because of personnel problems at the

time.
Factual Information 52 Aircraft Accident Report

ï September 28th (Sunday)óaccording to the day-shift turnover plan, the

horizontal stabilizer was lubricated per task card No. 24312000 (which called

for Mobilgrease 28).98 The swing-shift lead mechanic made the following

entry in the swing-shift turnover plan: ìRef MIGÖhorz stab:Öget copy of

E.O. & see what we need to do & order.î The swing-shift turnover plan also

contained the following entry related to the tail area: ìN/Rís [nonroutines],

lube & treat C/W [complied with], still parts to order.î

ï September 29th (Monday)óthe graveyard-shift lead mechanic made several

entries in the graveyard-shift turnover plan, including ìcontinue parts ordering

(PANIC),î99 ìneed copy of E.O. 8-55-10-01 for horiz stab acme screw and nut

excessive end play,î and ìTuesday [September 30th] departure looks doubtful.î

The day-shift turnover plan contained the following entry related to the tail

area: ìE.O. 8-55-10-01 needed for acme screw nut [excessive] end play.î The

swing-shift lead mechanic made the following entry on the swing-shift

turnover plan: ìre-do acme screw [check] to confirm problemÖE.O. # is

invalid.[100]Öhave [dayshift]Öfollow up.î 

ï September 30th (Tuesday)óthe graveyard-shift lead mechanic marked through

the line on the MIG-4 that called for replacement of the jackscrew assembly

and wrote, ìre-evaluate test per WC [work card] 24627000.î A different

mechanic and inspector made an entry in the MIG-4ís ìcorrective actionî

section, which stated, ìRechecked acme screw and nut end play per WC

24627000. Found end play to be within limits .033 for step 11 and .001 for step

12. Rechecked five times with same result.î (See figure 11.) The MIG-4 was

then reviewed by the graveyard-shift lead mechanic, who made the following

entry in the graveyard-shift turnover plan: ìstabilizer jackscrew acme nut

C/W.î The day-shift lead mechanic noted in the day-shift turnover plan, ìtail

close ñ I/W.î101 The day-shift turnover plan also noted, ìpartial O.K. to close

up.î The swing-shift turnover plan noted, ìtail close (I/W).î 

98Alaska Airlines maintenance records did not include the task card for the lubrication of the horizontal 

stabilizer. Because lubrication of the jackscrew assembly was a ìrepeatedî maintenance item, the

maintenance records only contained a complete task card for the most recent lubrication on September 24,

1999, at SFO.

99Alaska Airlines officials stated that jackscrew assemblies were not stocked as inventory items at the 

time of this C check and were only available through third-party vendors. Alaska Airlinesí director of safety,

quality control, and training told Safety Board investigators that ìwe do not believe a jackscrew was

ordered. Our procedures call for the creation of a ëfield requisitioní whenever we order aircraft parts that are

not in stock, such as the subject jackscrew.î He added that field requisitions are noted in a field request log.

He stated that a review of requisitions and logs related to the C check found no ìjackscrew components

listed in these documents. Based on this information, we do not believe a jackscrew or jackscrew

components were ordered.î He added, ìwe believe that the reference in the planned action to ëreplace nutí

caused some confusion since the nut should only be replaced as part of the jackscrew assemblyÖWe also

believe that the incorrect reference in the MIG-4 to perform E.O. 8-55-10-01 caused further confusion and

interrupted the normal parts ordering process.î

100The E.O. reference number was incorrect because it applied to another airplane model.

101ìI/Wî generally means ìin accordance with.î
Factual Information 53 Aircraft Accident Report

ï October 1st (Wednesday)óthe graveyard-shift turnover plan noted, ì[tail] close

up needs work, 2 items closed without QA [quality assurance] OK.î102 The

day-shift turnover plan noted, ìtail closed up.î

ï October 2nd (Thursday)óthe graveyard-shift turnover plan noted that the

accident airplane ìdeparted 0300 local time, so far so good.î

1.6.3.4.2 Maintenance Personnel Statements Regarding the Accident 

Airplaneís Last End Play Check

During postaccident interviews, the Alaska Airlines mechanic who conducted the

end play check on September 27, 1997, stated that he did not recall what end play

measurements were obtained, adding, ìI donít remember the exact number, but it was

below 40 [thousandths of an inch] I believe.î He stated that he had previously

accomplished about six end play checks and that ìthe ones I have done were well below

the limits.î He explained that during the procedure, ìwhile Iím torqueing, heís [the

inspector] looking at the dial indicator gauge.î He also stated that he could not recall how

many measurements were taken during the procedure. He stated that he noted the

discrepancy on the MIG-4, adding that he did not consider the measurement to be beyond

allowable limits. Referring to the decision to generate the MIG-4, he stated, ìI donít really

remember why. But, you know, maybeÖmaybe to have somebody take a lookÖyou

know, like a second opinion, maybe.î 

The September 27th day-shift inspector who was working with the mechanic

(noting dial indicator readings) and who wrote the ìplanned actionî instructions to replace

the jackscrew assembly stated that he considered a 0.040-inch end play check result to be

ìat the end of the limit.î He added, ìItís not in excess. I would consider anything over 40

thousandths to be excess. So 40 thousandths would be right on the edge. How much

further do you have to go before youíre out of limits, especially if you got another two

years before youíre going to check it again?î The mechanic stated that he had never

encountered similar wear during other end play checks and added the following:

Iíve worked on every MD-80 that Alaska Airlines owns extensively. I have never

come across any jackscrew that was worn to that extent. And we have aircraft, at

that time we had aircraft that were 15 years old. We have what would be a

relatively young aircraft [the accident airplane] that, for whatever reason, is, if itís

at its limit or if itís at the endÖit has a significant amount more wear than Iíve

ever noticed in the past, and thatís compared against aircraft that are three times

its age. 

The September 30th graveyard-shift lead mechanic who crossed out the planned

action to replace the jackscrew assembly stated that he did so ìbecause the MIG should

not have been generated to begin with. There shouldnít have been a[n] MIG at all for the

problem because it was within the allowable limits on the work card.î He stated that he

102 The Safety Boardís investigation did not determine which two items were closed without quality

assurance approval because Alaska Airlines records did not provide this information.
Factual Information 54 Aircraft Accident Report

ordered a recheck of the end play ìto take it one step beyond and prove that, in fact, it was

as stated in the spec, 0.040 or less.î He added the following:

At that point in time, the planned action was inappropriate for the discrepancy.

The discrepancy should have never existed. It fell within the work card limits, so,

therefore, the discrepancy should have never been written. The planned action

was inappropriate andÖwe could have opted to go ahead and sign it off right then

and there. But I opted to re-evaluate to make sure that the limits stated were, in

fact, either at 0.040 or below.

The inspector who worked with the mechanic who performed the recheck on

September 30th stated that he had performed about three end play checks and never

encountered results that were not within limits. He stated that the recheck ìverified that

the jackscrew was within limits.î The mechanic who performed the recheck stated that all

five end play measurements of 0.033 inch, as noted on the MIG-4, were ìwithin a

thousandths of it.î He stated that the inspector and the lead mechanic were present during

the recheck procedure. 

1.7 Meteorological Information

According to the automatic terminal information service weather information

recorded by the CVR at 1559:50, the following LAX weather conditions were current

beginning about 1550:

Wind two three zero at eight. Visibility eight. Few clouds at two thousand eight

hundred. One two thousand scattered. Ceiling two zero thousand overcast

temperature one six dew point one one. Altimeter three zero one seven.

Simultaneous ILS [instrument landing system] approaches in progress runway

two four right and two five left or vector for visual approach will be provided.

Simultaneous visual approaches to all runways are in progress and parallel

localizer approaches are in progress between Los Angeles International and

Hawthorne airports. Simultaneous instrument departure in progress runway two

four and two five. 

1.8 Aids to Navigation

No problems with navigational aids were reported.

1.9 Communications

No external communication difficulties were reported.
Factual Information 55 Aircraft Accident Report

1.10 Airport Information

Not applicable.

1.11 Flight Recorders

1.11.1 Cockpit Voice Recorder

The accident airplane was equipped with a Fairchild model A-100A CVR,

S/N 62892. On February 2, 2000, the CVR was transported to the Safety Boardís

Washington, D.C., headquarters for readout and evaluation.103 The exterior case was

severely damaged by impact forces, but it had not been punctured. The tape and spool

were wet but were otherwise intact and in good condition. The recording comprised four

channels of good quality audio information:104 one channel contained audio information

recorded by the cockpit area microphone (CAM),105 one channel contained audio

information recorded by the public address system, and the other two channels contained

audio information recorded through the radio/intercom selector panels at the captain and

first officer positions. The CVR recording began about 1549:49 during cruise flight and

ended about 1620:57, when the airplane impacted the water. A transcript was prepared of

the 31-minute recording (see appendix B).

1.11.1.1 CVR Sound Spectrum Study 

The Safety Board further examined the audio information recorded by the four

CVR channels using a spectrum analyzer (which provides a visual representation of the

frequency of the sound signals) and a computer signal analyzer (which allows analysis of

the analog wave form and frequency content of the sounds and provides detailed timing

information of the events).

The Safety Board analyzed the sound of two distinct ìclicksî recorded on the CVR

(referred to as ìsnapsî in the sound spectrum study report). The first click was recorded at

1549:54.8 and lasted about 0.01 second.106 The first click was compared with a second

103 The CVR was transported to Safety Board headquarters immersed in water in a sealed, plastic

container. 

104 The Safety Board ranks the quality of CVR recordings in five categories: excellent, good, fair, poor

and unusable. In a recording of good quality, most of the crew conversations can be accurately and easily

understood. The transcript that was developed might indicate several words or phrases that were not

intelligible. Any loss in the transcript can be attributed to minor technical difficulties or momentary dropouts

in the recording system or to a large number of simultaneous cockpit/radio transmissions that obscure each

other. 

105 The CAM is mounted in the overhead panel between the captain and first officer. The CAM is

designed to capture sounds and conversations in the cockpit whenever the CVR is powered.

106 According to the FDR, the autopilot disengaged about 1549:56.
Factual Information 56 Aircraft Accident Report

click recorded at 1609:14.8 (just before the accident airplaneís initial dive),107 and the

Board determined that the two sounds had identical frequency and energy characteristics.

The Safety Board also analyzed several distinct, very faint ìthumpî sounds

recorded on the CAM channel. A set of thump sounds was recorded at 1609:16.9 (also just

before the accident airplaneís initial dive) and comprised two distinct thumps that were

about 0.4 second apart. A series of distinct thumps was recorded at 1619:21.1 and

comprised four thumps that were less than 0.1 second apart108 and were recorded about

15 seconds before the second and final dive occurred at 1619:36.6. Analysis determined

that all of the thumps had low-to-mid range frequency characteristics, which were not

consistent with impulse sounds associated with metal-to-metal tapping or metal

failure/tearing sounds. The frequency characteristics were determined to be similar to

sounds produced by airflow.109

The Safety Board conducted a test to determine whether the failure of the acme

screw torque tube would produce sounds consistent with those recorded on the CVR. In

the laboratory, a hydraulic ram was used to pull the torque tube to failure, and the sounds

associated with the torque tube failure were recorded.110 The sounds recorded during the

test were then compared with the sounds recorded on the accident airplaneís CVR.

Comparison of the data determined that the sound signatures recorded during the failure

test did not match any of the sound signatures recorded by the accident airplaneís CVR. 

1.11.2 Flight Data Recorder

The accident airplane was equipped with a Sundstrand Data Control Universal

FDR, S/N 9182. The FDR recorded information digitally on eight tracks using a

1/4-inch-wide magnetic tape that had a recording duration of 25 hours. On February 3,

2000, the FDR was transported to the Safety Boardís Washington, D.C., headquarters for

readout and evaluation. Although the FDRís outer protective case was severely damaged

by high-impact forces, data were retrieved and analyzed.111 The FDR began recording data

from the accident flight about 1332 (after airplane startup and shortly before takeoff).

During its examination of the data, the Safety Board determined that two

parameters, angle-of-attack and left brake-pressure, were not functioning properly and

were recording faulty engineering values. 

107According to the FDR, the autopilot disengaged about 1609:16.

108These four thump sounds were referred to as the ìsound of faint thumpî in the CVR transcript.

109 Although the Safety Board could not determine the direct source of the sounds, they were similar to

sounds recorded during previously conducted flight tests involving wake vortex encounters.

110 The data were collected using a calibrated sound pressure transducer and an accelerometer attached to

a mount used to hold the torque tube in place during the test.

111 For information on airplane performance studies based on an analysis of the FDR data, see sections

1.16.1 and 1.16.1.1.
Factual Information 57 Aircraft Accident Report

1.12 Wreckage and Impact Information

1.12.1 General

The on-site phase of the investigation, including wreckage recovery, examination,

and documentation, was accomplished between February 1 and March 15, 2000.112

Throughout most of the on-site investigation, remote-operated vehicles and side-scan

sonar equipment were used to search the underwater debris field.113 The majority of the

debris field was cleared using the remote-operated vehicles to either load baskets with, or

wrap cables around, the wreckage. In the late stages of wreckage recovery, a commercial

trawler was used to drag the ocean bottom and recover smaller wreckage pieces.114 All of

the recovered wreckage was unloaded at Port Hueneme, California, for examination and

documentation. 

About 85 percent of the fuselage was recovered. The largest sections were from

below the airplaneís floor line. The fuselageís upper crown structure was broken into

smaller pieces and had substantial compression damage. The recovered fuselage segments

increased in size from the forward section of the airplane (where they were the smallest) to

the aft section of the airplane (where they were the largest).

No evidence indicated any pre- or postimpact fire damage. Fractures found on

pieces of fuselage wreckage were consistent with failures generated by high-energy

impact. Examination of the fracture surfaces found no evidence of preexisting cracking or

foreign impact damage.

The majority of both wings were recovered. The right wing exhibited more

damage than the left wing. 

About 85 percent of the empennage, which comprises the horizontal and vertical

stabilizers and the engines, was recovered. Most of the horizontal stabilizer was

recovered, including the center box and the left and right outer sections. All of the

empennageís control surfaces were recovered except for a 4-foot section of the left

elevator and the geared tab. 

The Safety Board examined the horizontal stabilizer wreckage for evidence of

preimpact failures and structural damage that might have occurred during the accident

flight. Damage and/or witness marks on the horizontal stabilizer were visually examined

to determine what degree of horizontal stabilizer movement (travel) would have been

necessary to create the witness marks or damage. 

112 Floating wreckage debris was recovered throughout the on-site phase of the investigation.

113 Side-scan sonar searching preceded the underwater wreckage recovery to determine the boundaries of

the underwater debris field, which was spread over a 1/4-mile area of the ocean bottom at a depth of about

700 feet.

114 About 15 percent of the wreckage was recovered using this method. Ten percent of the wreckage was

not recovered.
Factual Information 58 Aircraft Accident Report

The horizontal stabilizer was found fragmented into several pieces. The right side

of the horizontal stabilizer was more fragmented than the left side. A 1.5-foot section of

the vertical stabilizer was found attached to the horizontal stabilizerís pivot fitting lugs.

The bottom side of both lugs were scraped, and the grease fittings located on the bottom

side of each lug were found sheared off. Examination determined that the leading edge of

the horizontal stabilizer would have to rotate up to a position about 16.2° leading edge up

for the grease fittings to sheer off. The adjacent vertical stabilizer pivot structure had

contact marks (in line with the grease fitting location) consistent with the leading edge of

the horizontal stabilizer having rotated to a position of about 80°. The vertical stabilizer

was found fragmented, consistent with high-energy impact damage. The vertical

stabilizerís tip fairing was recovered broken into two segments. The recovered segments

of the vertical stabilizerís tip fairing were placed together, and the two fracture surfaces

were matched. The forward 25 percent of the vertical tip fairing was not recovered.

Examination of the matched fairings portions showed upward deformation damage that

was also consistent with the leading edge of the horizontal stabilizer having rotated to a

position of about 80°.

The horizontal stabilizer front spar lower cap had a 1.5-inch semicircular

indentation about 3.5 inches left of the centerline, consistent with contact by the acme

screw. The jackscrew assembly side attachment plates were bent about 1.25 inches to the

left, and the side plate lugs were partially fractured, consistent with high-energy impact

damage. 

Both engines were recovered separated. The left (No. 1) engine was found

separated into three sections at the high-speed compressor.115 No evidence indicated an

engine fire or an uncontained engine failure. Examination of the recovered compressor

groups from the left engine revealed marks and signatures consistent with the engine

producing significant rotational energy at impact. The thrust reverser was found in the

stowed position.

The right (No. 2) engine was recovered in one piece except for the entire fan front

and rear cases, which were not recovered. None of the recovered fan blades exhibited

significant blade tip rubbing. No evidence indicated integral fan hub and disk distress or

rupture. No evidence indicated an engine fire or an uncontained engine failure. The

recovered compressor blades and vanes did not exhibit signs of distress from engine

rotation. Examination of the recovered compressor groups from the right engine revealed

signatures consistent with the engine producing little rotational energy at impact. The

thrust reverser was found in the stowed position.

1.12.2 Horizontal Stabilizer Jackscrew Assembly Components 

 The main supporting elements of the horizontal stabilizer jackscrew assembly

were found intact and attached to the horizontal stabilizerís front spar. The acme screw

was found cracked but attached to the support assembly. Metallic filaments were found

115 The high-speed compressor was not recovered.
Factual Information 59 Aircraft Accident Report

wrapped around the central part of the acme screw.116 (Figures 12a and 12b show the acme

screw immediately after it was brought on board the recovery ship and immediately after

it was brought to shore, respectively; figure 12c shows the acme screw during initial

inspection.) The acme screw was rinsed with fresh water immediately after its recovery by

directing the water from a hose similar to a garden hose over the gearbox and allowing the

water to flow down over the screw. To preserve the acme screw and the thread remnants

wrapped around the screw in their as-recovered condition, the stream of water used to

rinse the screw was not directly sprayed on it. The acme screw might have been rinsed

again with a garden-type hose after it was brought to shore. 

Figure 12a. The acme screw immediately after it was brought on board the recovery ship.

116These metallic filaments were later identified as acme nut thread remnants. After initial mechanical 

and metallurgical examinations, the acme screw, thread remnants, and acme nut were removed from the

surrounding structure and sent to the Safety Boardís Materials Laboratory for examination. For details about

the metallurgical examinations of these components, see section 1.16.2. 
Factual Information 60 Aircraft Accident Report

Figure 12b. The acme screw immediately after it was brought to shore.
Factual Information 61 Aircraft Accident Report

Figure 12c. The acme screw during initial inspection.

The upper mechanical stop was found attached to the splines of the acme screw,

and the clamp bolt for this stop was in place. The lower mechanical stop and its clamp bolt

were recovered as a separate unit from the acme screw. Several impact/scrape marks were

found on the upper surface of the lower mechanical stop, and a round impact mark was

found on the lower surface of the stop. Evidence of damage to the splines on the inside

diameter of the lower mechanical stop was observed.117

The portion of the torque tube that extended beyond the lower end of the acme

screw was fractured. The lower end of the torque tube, the torque tube nut washer, and the

torque tube nut were not recovered. 

The acme nut and gimbal assembly were found attached in the vertical stabilizer

structure. The gimbal assembly was found to be movable on both rotational axes, except

in areas distorted by surrounding structure.

The primary trim motor was found detached from the differential gearbox, but it

was mostly intact. The alternate gearbox was found attached to the primary motor along

with a portion of the upper housing of the differential gearbox. Both electrical connectors

to the primary motor were present; however, the connection wires were missing. 

117For more information about the damage to the splines on the inside diameter of the lower mechanical 

stop, see section 1.16.2.4.
Factual Information 62 Aircraft Accident Report

All recovered pieces of the accident airplaneís primary trim motor were examined.

No evidence of any thermal damage was noted. The primary trim motorís auxiliary

gearbox was removed from the motor, disassembled, and examined. No evidence

indicated any preimpact mechanical damage.118 All recovered pieces of the accident

airplaneís alternate trim motor were examined. Impact damage and gouging were

predominantly located toward the left rear portion of the motor. No evidence indicated any

preimpact mechanical damage. 

The primary up-trim relay, the down-trim relay, the up-trim brake relay, the

down-trim brake relay, and the trim brake override switch were examined. No evidence of

thermal damage or excessive wear was observed on either the down-trim or the up-trim

relay contacts. Corrosion was observed on all metal components of both trim relays. No

evidence of any preimpact mechanical malfunction was found on either relay.

Examination and testing of the up-trim and down-trim brake relays found no evidence of

any preimpact mechanical malfunction or abnormal wear. An electrical continuity check

of the trim brake override switch determined that it was in the neutral position. No

evidence indicated any preimpact mechanical malfunction.

1.13 Medical and Pathological Information

Tissue specimens from both pilots were transported to the FAAís Civil Aerospace

Medical Institute laboratory for toxicological analysis. The laboratory performed its

routine analysis for major drugs of abuse119 and prescription and over-the-counter

medications, and the results were negative. Although the analysis detected ethanol in the

tissue specimens of both pilots, the analysis report noted that the ethanol found was

consistent with postmortem ethanol production. 

1.14 Fire

There was no evidence of an in-flight fire.

1.15 Survival Aspects

The airplane was destroyed by high-impact forces, and no occupiable space

remained intact. The Ventura County, California, Coronerís report stated that all of the

accident airplaneís occupants died as a result of blunt force impact trauma.

118 According to the manufacturer of the primary trim motor (Sunstrand), it had overhauled the accident

airplaneís primary trim motor on February 22, 1994, and again on October 4, 1996.

119 The five drugs of abuse tested are marijuana, cocaine, opiates, phencyclidine, and amphetamines.
Factual Information 63 Aircraft Accident Report

1.16 Tests and Research 

1.16.1 Airplane Performance

The Safety Board used FDR, CVR, ATC transcript, and radar120 data to determine

the accident airplaneís motions and performance during the accident sequence. These data

(and associated calculations) indicated the following:

ï The last horizontal stabilizer movement before the initial dive was recorded at

1349:51 (about 2 hours and 20 minutes before the initial dive) during the

climbout from PVR when the airplane was climbing through approximately

23,400 feet at 331 KIAS. At that time, the horizontal stabilizer moved from

0.25° to 0.4° airplane nose down.

ï Following the horizontal stabilizer movement at 1359:51, the airplane

continued to climb at 330 KIAS. The airplane began to level off at

approximately 26,000 feet and slowed to 320 KIAS. At 1351:51, the airplane

began to climb again, and, by 1352:50, it had slowed to about 285 KIAS as it

climbed to approximately 28,600 feet and then leveled off.

ï At 1353:12, the airplane was at approximately 28,557 feet and 296 KIAS, and

the autopilot disengaged.

ï The airplane began to climb at a much slower rate over the next 7 minutes.

During this part of the ascent, the elevators were deflected between -1° and -3°,

and the airplane was flown manually using up to 50 pounds of pulling force.

ï By 1400:00, the airplane had reached an altitude of approximately 31,050 feet.

For about the next 24 minutes, the airplane was flown manually using

approximately 30 pounds of pulling force while maintaining level flight at

280 KIAS. Starting at 1424:30, the airspeed increased, eventually reaching

301 KIAS,121 at which time the airplane was flown using about 10 pounds of

pulling force. At 1546:59, the autopilot was re-engaged.

ï At 1609:16, the autopilot disengaged, and the CVR recorded the sound of a

clunk. At 1609:16.9, the CVR recorded the sound of two faint thumps in

succession. During the 3 to 4 seconds after the autopilot was disengaged, the

horizontal stabilizer moved from 0.4° airplane nose down to a recorded

position of 2.5° airplane nose down,122 and the airplane began to pitch down,

starting a dive that lasted about 80 seconds as the airplane went from

approximately 31,050 feet to between 23,000 and 24,000 feet. During the

120 Air Route Sur\veillance Radar data were obtained from the FAAís Los Angeles ARTCC, and Airport

Surveillance Radar data were collected from several airport facilities in the Southern California Terminal

Radar Control area and from the U.S. Air Force 84th Radar Evaluation Squadron.

121 The flight plan called for a cruise speed of 283 knots calibrated airspeed.

122 The horizontal stabilizer remained at the recorded position of 2.5° until the last minute of the flight.

For details about simulation studies conducted by the Safety Board to determine the actual position of the

horizontal stabilizer during this time, see section 1.16.1.1.
Factual Information 64 Aircraft Accident Report

movement of the horizontal stabilizer, the CVR recorded the sound of the

stabilizer-in-motion tone for 3 to 4 seconds. During the next several seconds,

the load factor oscillated between 0.4 and 1.0 G, and the elevators moved to the

trailing-edge-up direction from -0.4° to -7.0°,123 consistent with control inputs

being made to bring the nose up. 

ï By 1609:42, the airplane had descended to approximately 29,450 feet and had

accelerated to about 322 KIAS. At this time, the airplaneís pitch angle was

-7.9° airplane nose down, and the elevators had moved to about a -8° position.

The speed brakes deployed 1 second later and remained deployed for the next

1 minute 30 seconds.

ï At 1610:00, the airplane was accelerating through 343 KIAS (the airplaneís

maximum allowable airspeed). At 1610:01.9, the CVR recorded the sound of

the overspeed warning.

ï By 1610:16, the airplane had accelerated to 353 KIAS, the maximum airspeed

it reached during the initial dive, and this airspeed remained nearly constant

until 1610:23 as the airplane was descending through 24,200 feet. By this time,

the airplaneís pitch angle had increased from -8° to -5° airplane nose down,

and the elevators had moved to about a -11° position.

ï About 1611:13, the speed brakes were stowed. By this time, the airplaneís

airspeed had decreased to 262 KIAS, and the airplane was maintaining an

altitude of approximately 24,400 feet with a pitch angle of 4.4°. During the

next 10 seconds, the airplaneís airspeed decreased to 254 KIAS. The airplane

remained between 23,000 and 24,000 feet for the next 5 minutes, and the

airspeed gradually increased to 335 KIAS. During this period, elevator

movements ranged from between -7° to -9°. Airplane performance calculations

indicated that a control column force of between about 130 and 140 pounds

was required to recover from the dive.124

ï At 1614:44, the speed brakes deployed again, and the airplaneís airspeed began

to decrease. 

ï At 1617:50, the speed brakes were being stowed. By this time, the airplane had

leveled off at approximately 18,000 feet, and the airspeed had decreased to 253

KIAS. At this time, the elevators were operating between -10° and -13°, and

the airplaneís pitch angle was between -2° and 0°, with a constant indicated

horizontal stabilizer angle of 2.5° airplane nose down.

ï At 1617:54, the CVR recorded the captain order deployment of the slats.

About 1618:00, when the airplane was passing through 17,800 feet at

252 KIAS, the FDR recorded the extension of the leading edge slats.

About 1618:05, the CVR recorded the captain order deployment of the flaps.

About 3 seconds later, the flaps began extending to 11°, and the airspeed

123 The available range of the airplaneís elevators is from +15° to -25°.

124 As stated previously, this estimated force could be applied by either the captain, the first officer, or

both. 
Factual Information 65 Aircraft Accident Report

decreased about 4 knots. The flaps remained at this value for about 20 seconds,

during which time the airplaneís pitch angle was about 0°, with changes of less

than 1°. About 1618:26, the CVR recorded the captain order retraction of the

slats and flaps. During the 50 seconds after retraction of the slats and flaps, the

airplaneís indicated airspeed increased from 245 to 270 knots, and the airplane

climbed from approximately 17,400 to 17,900 feet with a pitch angle of about

4°. 

ï Beginning at 1619:09, the FDR recorded small oscillations in elevator angle,

pitch angle, and vertical acceleration values for about 15 seconds.

ï At 1619:21, the CVR recorded the sound of a faint thump. At 1619:21.5, the

FDR recorded a 0.09° change, from 2.51° to 2.60°, in the horizontal stabilizer

position (this was the first change in recorded horizontal stabilizer position

since the one that coincided with the airplaneís initial dive from 31,050 feet

about 10 minutes earlier) and elevator deflections increasing from -9° to -13°.

Pitch angle and elevator oscillations ceased 2 seconds after the stabilizer

position changed. 

ï At 1619:29, the CVR recorded the captain order redeployment of the slats and

flaps. At 1619:35, when the airplane was at an altitude of 17,900 feet, an

indicated airspeed of 270 knots, and a pitch angle of 2°, the flaps were

transitioning from 7° to 11°.

ï At 1619:36, the left elevator value was -12°, the right elevator value was -11°,

and the load factor was 1 G. At 1619:36.6, the CVR recorded an extremely

loud noise, and the FDR recorded a right elevator position of -25°.125 By

1619:36.6, the load factor had increased to 1.45 Gs. At 1619:36.9, the pitch

angle began to change to the airplane-nose-down direction, and the load factor

decreased to 0.67 G. At this time, radar data detected several small primary

returns. Within 3 seconds, the load factor quickly decreased to -3 Gs. The next

several seconds of FDR data indicated a maximum airplane-nose-down pitch

rate of nearly 25° per second. 

ï By 1619:40, the airplane was rolling left wing down, and the rudder was

deflected 3° to the right. 

ï By 1619:42, the airplane had reached its maximum valid recorded

airplane-nose-down pitch angle of -70°, and the load factor had increased to

-1.45 Gs. At this time, the roll angle was passing through -76° left wing down.

During the next 2 seconds, the load factor decreased to -2.1 Gs.

ï By 1619:45, the load factor had increased to -1.75 Gs, the pitch angle had

increased to -28°, and the airplane had rolled to a -180° inverted position.

Further, the airplane had descended to approximately 16,420 feet, and the

indicated airspeed had decreased to 208 knots.

125 All subsequent recorded elevator values exceeded the physical ranges of the airplaneís elevator

system and were considered invalid.
Factual Information 66 Aircraft Accident Report

ï At 1619:54, the left aileron moved to more than 16° (to command right wing

down), and, during the next 6 seconds, moved in the opposite direction to -13°

(to command left wing down).

ï At 1619:57, the rudder was returned to the near 0° position, the flaps were

retracted, and the airplane was rolling through -150° with an

airplane-nose-down pitch angle of -9°. 

ï After 1619:57, the airplane remained near inverted, and its pitch oscillated in

the nose-down position until it impacted the ocean. FDR data indicated large

rudder and aileron movements during the final minute before impact. Further,

engine pressure ratio126 parameters fluctuated erratically during the final dive,

which is consistent with airflow disturbances associated with extreme

angles-of-attack.

1.16.1.1 Airplane Performance Simulation Studies 

 The Safety Board conducted several performance studies to reproduce, in

simulation models, the flight data parameters on the accident airplaneís FDR using the

same control inputs recorded on the FDR (for example, aileron, elevator, horizontal

stabilizer angle, and engine torque). Additionally, kinematics studies127 using MD-80

aerodynamic data obtained from Boeing calculated the control deflections required to

match the airplane response on the FDR. These calculations provided an estimate of

control inputs for use in simulations when control inputs recorded on the FDR did not

produce the same response in the simulations as those recorded on the FDR. These

kinematics calculations also provided the tension load on the jackscrew assembly.

 The airplane performance studies focused on the airplaneís initial dive

(about 1609:16), which occurred after the autopilot was disengaged, and the second and

final dive (about 1619:37), which occurred after an extremely loud noise was recorded by

the CVR. Results of simulation and kinematics studies indicated that, before the autopilot

was disengaged, the values recorded for the horizontal stabilizer, elevator, aileron, and

rudder produced the same airplane responses as those recorded on the FDR during cruise

flight. After the autopilot was disengaged and the airplane subsequently pitched down, the

simulation results showed considerable discrepancies between FDR data and simulated

airplane response. With the use of the recorded horizontal stabilizer angle, simulations

were conducted using control inputs based on those recorded by the FDR. The simulations

showed that the airplane would have recovered quickly and climbed using the stabilizer

angle and elevator positions and movements recorded on the FDR, indicating that

recorded values of the longitudinal controls were not accurate representations of the actual

positions on the accident airplane after the initial dive.

126 Engine pressure is a measure of engine thrust, comparing total turbine discharge pressure to the total

pressure of the air entering the compressor.

127 Kinematics is a process that involves fitting curves through available FDR data (such as heading,

pitch, and roll) and calculating accelerations, forces, and moments from these rates. The flight control

surface positions required to match the FDR angular and position data are then calculated using

aerodynamic data for the airplane that have been provided by the manufacturer.
Factual Information 67 Aircraft Accident Report

Kinematics studies were conducted to account for the discrepancies between the

FDR data and the simulation results. One study assumed that the stabilizer value was

accurate and calculated the elevator movement required to match the FDR-recorded

values. A second study assumed that the elevator values were accurate and calculated the

stabilizer movement required to match the FDR-recorded values. Simulations indicated

favorable matching with FDR values using a constant, increased offset in the horizontal

stabilizer position compared with the recorded elevator position. On the basis of these

findings, further kinematics and simulator studies assumed that the elevator positions

recorded by the FDR after the initial dive were accurate and that the recorded horizontal

stabilizer positions recorded by the FDR were inaccurate.

Results of kinematics and simulation studies indicated that the accident airplaneís

performance was consistent with the horizontal stabilizer position moving beyond the 2.5°

airplane-nose-down value recorded on the FDR during the initial dive (causing the

airplane to pitch nose down as recorded, assuming the elevator positions were as recorded

on the FDR). Kinematics study results indicated a relatively steady and consistent

airplane-nose-down horizontal stabilizer angle in the 10 minutes between the initial dive

and the second dive, followed by a large increase in stabilizer angle at about the time valid

elevator data were lost on the FDR and the loud noise was recorded on the CVR.

Calculated tension loads on the acme screw between the first and second dives were

considerably larger than the loads encountered in the normal operational envelope.

The studies indicated that the performance of the accident airplane during the second dive

was consistent with the horizontal stabilizer moving quickly to a higher leading-edge-up

position (which corresponds to airplane nose down) of at least 14° just after the loud noise

was recorded by the CVR. 

1.16.2 Metallurgical Examinations

The Safety Boardís metallurgical investigation included on-scene and laboratory

examinations. Initially, recovered wreckage was examined at a dockside facility at Port

Hueneme, California, from February 9 to 11, 2000. Metallurgical examinations focused on

the horizontal stabilizer, the jackscrew assembly, and related components. After the

on-scene examinations concluded, laboratory examinations were conducted at the Safety

Boardís Materials Laboratory, Washington, D.C.; the Boeing Commercial Aircraft

Materials and Process Engineering Laboratory, Long Beach, California; and Integrated

Aerospace, Santa Ana, California. 

1.16.2.1 Horizontal Stabilizer Forward Spar

The jackscrew assembly (not including the lower mechanical stop, the lower end

of the torque tube, the torque tube retaining nut and washer, and the trim motors) was

recovered attached to the front of the horizontal stabilizer forward spar.128 Spar brackets

through which the acme screw attachment bolts pass were found severely bent. The lower

128 The jackscrew assembly is attached to spar brackets on the front of the horizontal stabilizer by six

bolts, three on each side of the spar.
Factual Information 68 Aircraft Accident Report

legs of both brackets were fractured at bend locations. A visual inspection determined that

the fractures were consistent with overstress separations.

1.16.2.2 Acme Screw 

Several contact marks were found on the acme screw surface. A downward-facing,

C-shaped contact mark on cadmium plating was found overlaying one end of a crack at

the upper end of the acme screw. An inspection of the adjacent horizontal stabilizer

structure found a mating dent in the horizontal stabilizer forward sparís lower flange. The

spar flange dent was smoothly curved with a radius that matched the acme screw at the

C-shaped contact mark location. A visual inspection also determined that the upper end of

the acme screw had contacted the bore of the pass-through hole in the lower gearbox

support plate. Marks indicated several major impacts at various aft positions and minor

impacts at the forward positions within the bore. 

Corrosion pitting and red rust areas were found along most of the acme screwís

length, with the heaviest areas of corrosion and rust along its lower half. Large areas of the

acme screw were also found covered by white deposits, which chemical analysis

determined were consistent with corrosion debris from the magnesium gearbox case

attached to the top of the acme screw assembly. Figure 13 shows the acme screw thread

remnants wrapped around the screw, the red rust areas, and the white deposits.

Figure 13. A photograph of the acme screw thread remnants wrapped around the screw, 

the red rust areas, and the white deposits.
Factual Information 69 Aircraft Accident Report

On-scene visual and tactile inspections by Safety Board metallurgists of the acme

screwís threaded areas found no evidence of grease in any condition, either semi-fluid

(that is, fresh) or solid/dry (that is, old or degraded), or other lubricants in the central

ìworking regionî of the screw threads.129 Laboratory examinations found small flakes of

dried and hardened grease attached to some of the thread remnants in this region. The

acme screwís lower threads (which are outside of the working region) were found partially

packed with a mixture consistent with sand and grease. (Figure 14 shows the sand/grease

mixture packed between the acme screwís lower threads.) Parts of the acme screwís upper

six to eight threads had an oily sheen, and small deposits of greaselike material were found

between the threads. 

Figure 14. A photograph of the sand/grease mixture packed between the acme screwís 

lower threads.

129The acme screw working region is that part of the screw that can come in contact with the acme nut 

during its operation between the upper and lower electrical stop limits.
Factual Information 70 Aircraft Accident Report

The acme screw was locally bent about 5° at the point of a crack in the area of the

C-shaped contact mark above the acme screw threads and below the upper stop splines.

The crack was gapped open about 1/16 inch and extended about 270° around the acme

screwís circumference. An initial visual inspection of the crack surfaces and subsequent

microscopic laboratory examination determined that the crack was on a 45° plane,

consistent with an overstress fracture. No evidence indicated any preexisting cracks.

The acme screw was cleaned with soap and water for more detailed laboratory

examination. Cleaning removed most of the dried and hardened grease and some of the

white magnesium deposits; however, most of the red-rusted, corrosion-pitted areas

remained. Laboratory examination revealed that there was less pitting at the upper and

lower parts of the acme screw where the screwís black-oxide coating was more intact.130

No wear was visible on the flanks of the acme screwís 10 lower and 7 upper threads. The

upper flanks of the threads in the working region showed evidence of polishing and

removal of the black-oxide coating that increased toward the center of the acme screw. 

Examination of the surfaces through the black-oxide coating found no machining

steps (indentations worn into the surface) or sharp features. Thread flank surfaces in areas

that did not have a black-oxide coating appeared circumferentially worn and lustrous with

no wear steps or sharp features found in these areas. Surface roughness was measured at

four locations on the flanks of the acme screw threads, and all locations recorded a surface

finish of 32 root mean squared (RMS) value131 or finer, as required by the manufacturerís

specifications. 

The lower end of the acme screw, including about 19 threads, was cleaned

electrolytically,132 which completely removed the corrosion debris and most of the white

magnesium deposits but left the black-oxide coating, paint, and cadmium plating (on the

splined areas) intact. Subsequent microscopic examination determined that corrosion had

deeply pitted the acme screw surface. Surface finish was measured on the thread flank in

the cleaned lower end and determined to be 32 RMS or finer. Chemical and metallurgical

tests established that the acme screw met material and process specifications.

1.16.2.3 Acme Screw Torque Tube

The acme screw torque tube was fractured approximately in line with the bottom

of the screw, through the first full thread just below the external splines at the lower end of

the tube. Neither the mating lower piece of the torque tube nor the retaining nut and

washer from this area of the assembly were recovered. The upper end of the torque tube

130 A black-oxide coating is a thin surface finish treatment that provides limited corrosion and wear

protection.

131 RMS is a numerical measurement of surface roughness. The acme screwís engineering drawing

specifications require a surface finish of 32 RMS or finer in the threaded area. In later specifications, RMS

has been replaced by RA (arithmetic average).

132 Electrolytic cleaning is accomplished by applying a small amount of voltage to the part while it is

immersed in a mildly caustic solution.
Factual Information 71 Aircraft Accident Report

was bent about 5° in a location corresponding to the location of the crack in the acme

screw. 

Macroscopic fracture traces indicated that the torque tube fracture initiated from a

broad front at the tubeís right forward quadrant (with the torque tube oriented as

recovered) and propagated through the tubeís cross-section. Macroscopic fracture features

emanated from a thin reflective fracture band adjacent to the thread root that was

determined to be a preexisting fatigue crack. Fatigue striations were identified throughout

the band, consistent with low-cycle fatigue cracking.133 The fatigue band followed the first

thread root around almost 330° of the torque tubeís circumference and extended about

0.03 inch onto the fracture surface in the initiation area. The remaining fracture area was

ductile overstress stemming from the fatigue. Chemical and metallurgical tests established

that the torque tube material met material and process specifications. 

The large diameter upper end of the torque tube displayed a 0.1-inch-wide and

0.028-inch-deep wear band. The band ran completely around the torque tube at the

location corresponding to the upper end of the acme screw. 

1.16.2.4 Lower Mechanical Stop and Corresponding Spline Area on the 

Acme Screw

The lower mechanical stop of the acme screw assembly was recovered with a

section of the vertical stabilizer that had been separated from the jackscrew assembly and

related structure. The lower mechanical stop was covered by dark, partially dried grease,

and the stop bolt and nut were in place.

Numerous dents and mechanical marks were found on the upper and lower

surfaces of the lower mechanical stop. Mechanical damage was also found on the lower

mechanical stopís internal splines and on the clamp bolt. The lower mechanical stopís

clamp bolt and nut were found in place.

Optical microscopy, scanning electron microscopy (SEM), and x-ray energy

dispersive spectroscopy (EDS) examinations identified multiple damage features on the

lower mechanical stopís upper surface, consistent with contact with the lower end of the

acme nut and the nutís lower stop lug that protrudes below the main body of the nut. The

features ranged from light marks in the primer paint to severe dents and substantial

deformation of the metal. The damage surface was consistent with both clockwise

(rotational) scoring and substantial deformation and rotational sliding contact (in both

directions) with the acme nut.134

Rotational stripping was observed in circumferential bands at the upper and lower

ends of the internal spline teeth in the lower mechanical stop.135 The middle area of the

133 For more information on torque tube testing, see section 1.16.5.

134 In an intact jackscrew assembly, clockwise rotation of the acme screw results in movement of the

horizontal stabilizer toward airplane-nose-down pitch, and counter-clockwise rotation of the acme screw

results in movement of the horizontal stabilizer toward airplane-nose-up pitch.
Factual Information 72 Aircraft Accident Report

splines, which was about 1/4 inch wide, was damaged but not stripped at any

circumferential location. Circumferential scoring marks associated with the stripped areas

of the spline teeth were on a plane slightly offset from directly circumferential. The

stripping pattern and offset circumferential marks were consistent with the lower

mechanical stop being at two or more skewed angles to the acme screwís splines during

stripping. Most spline tooth peaks were found smeared in a counter-clockwise direction,

consistent with a clockwise rotation of the acme screw (as viewed from above). However,

many splines were damaged consistent with acme screw rotation in both directions.

Imprint marks created by upward motion of the acme screw spline teeth through

the lower mechanical stop were observed across the peaks of the stopís spline teeth that

were not stripped (the middle portion of the splines). The orientation of the marks was

consistent with the lower mechanical stop being skewed (at an angle) to the splines of the

acme screw as the screw pulled upward through the stop.

Damage along the length of the portion of the shank of the lower mechanical stop

clamp bolt that was exposed to the interior of the stop was consistent with rotational

contact with acme screw spline teeth. The exposed area of the bolt shank also had imprint

marks oriented transverse to the length of the bolt, consistent with upward motion of the

acme screw relative to the lower mechanical stop. These transverse marks on the bolt

shank were superimposed upon the rotational damage, consistent with this damage being

created, at least in part, after the rotational damage of the lower mechanical stop spline

teeth.

1.16.2.5 Acme Nut 

The acme nut was recovered attached to vertical stabilizer wreckage in its normal

position but separate from the acme screw and the remainder of the jackscrew assembly.

On-scene visual inspection of the thread area on the inside diameter of the acme nut

showed a relatively smooth, flat surface, with only small ridges of the acme thread

remaining. (Figure 15 shows an overall view of the recovered acme nut assembly.

Figure 16 shows a closer view of the interior of the acme nut.) In addition, a pattern of dry,

black deposits also spiraled around the diameter, following the raised ridges. The inside

surface also contained random areas of white deposits. No evidence of grease was found

inside the acme nut during on-scene visual and tactile inspections or during

high-magnification examination in the laboratory. Various quantities of reddish-brown and

black grease were found on the exterior of the acme nut and gimbal ring, and areas of

135 Stripping was considered present in any region where individual spline teeth were indistinguishable

from each other.
Factual Information 73 Aircraft Accident Report

green-tinted, black and red grease deposits were found on the surrounding airplane

structure. 

Figure 15. A photograph of an overall view of the recovered acme nut assembly.

Figure 16. A photograph of a closer view of the interior of the acme nut.

The acme nut has a grease fitting on the forward, upper side of the nut through

which grease can be applied to the acme nut threads. A solid, black, claylike, dry residue

coated with white, powdery material was found in the internal counterbore of the acme

nutís grease passageway where it intersected the inside surface of the nut, as well as in the

smaller diameter grease passageway behind the counterbore. When the grease fitting was
Factual Information 74 Aircraft Accident Report

removed to allow collection of material samples from the grease passageway, translucent,

reddish grease consistent with the appearance of Mobilgrease 28 was observed on the

fittingís external threads. (Figure 17 shows a cross-section through the acme nutís grease

passageway and fitting.) Traces of translucent, reddish grease were also found inside the

grease passageway in the acme nut and in the bore of the grease fitting, a section distinct

from the grease passageway. (Figure 18 shows the grease passageway and counterbore

after the acme nut was sectioned.) Microscopic examination determined that the sample

from the counterbore consisted mostly of black powdery material, larger chunks of black

and white material, and particles of copper-colored metallic debris identified as acme nut

material.

Figure 17. A diagram of a cross-section through the acme nutís grease passageway and 

fitting. Red areas denote the locations where red grease was found, and gray areas 

denote the location where the dry residue was found.
Factual Information 75 Aircraft Accident Report

Figure 18. A photograph of the grease passageway and counterbore after the acme nut 

was sectioned.

Magnified visual inspections of the acme nutís interior after the nut was

longitudinally sectioned confirmed the presence of the spiraling pattern consistent with

the original location of the internal nut threads. For comparison with the accident acme nut

interior, an exemplar nut was similarly sectioned.136 The spiral patterns comprised a small

raised ridge followed by a lustrous depression with spiral machining or wear marks

transitioning into a dark band of surface deposits that rose to the next ridge. Under higher

magnifications, longitudinal smearing and shear fractures were identified on the raised

edges. The smearing and shearing directions were consistent with upward movement of

the acme screw threads in relation to the nut body. Material was observed folded upward

and smeared over adjacent surfaces along most of the ridges. Typical ridges were between

0.013 and 0.020 inch wide.

Most of the markings observed on the acme nutís interior surface spiraled around

the bore in the same manner as the original thread pattern. However, some circumferential

and longitudinal marks were observed in isolated locations overlaying the spiral marks.

Two circumferentially oriented bands of scratches were observed partially around the

136 The exemplar nut, which was removed during the overhaul of a jackscrew assembly, had a reported

0.0036-inch end play measurement at the time of overhaul.
Factual Information 76 Aircraft Accident Report

inner diameter near the top of the acme nut on the forward side. Several longitudinal scuff

marks were also observed on the upper aft side of the acme nutís interior above the grease

passageway. The scuff marks appeared to have been produced by the contacting object

moving vertically upward. 

1.16.2.6 Acme Nut Thread Remnant Examination

Twelve acme nut thread remnants137 were found wrapped around the central part of

the acme screw when it was recovered. Most of the thread remnants were found spiraled

around the acme screw oriented primarily along the path of the screw threads, although

some were found tangled together and bent in multiple directions. The thread remnants

were clustered into two closely spaced groups extending about 6 to 8 inches along the

acme screw. The thread remnants were copper colored with a greenish tint, consistent with

copper-alloy oxidation. After recovery, each thread remnant was numbered, and its width,

maximum thickness, estimated length, and number of 360° turns were measured. The

accumulated length of the thread remnants was equivalent to about 75 percent of the total

length of the original threads. 

High-magnification optical examinations revealed that all of the thread remnants

were roughly rectangular in shape with a mostly flat upper side and a multibeveled lower

side. The outer diameterís edges were fractured, corresponding to the approximate major

(outer) diameter of the acme nut. Examination also revealed that some of the thread

remnants had corrosion pitting.

A representative sample of a thread remnant (No. 4) was further examined by SEM

and EDS. EDS examination determined that thread remnant No. 4 was composed of an

aluminum-nickel-bronze alloy, consistent with acme nut material specified in the

manufacturerís engineering drawing (unified numbering system [UNS] alloy C95500).

138

High-magnification SEM examination revealed smearing deformation and shearing

dimples along thread remnant No. 4ís outer edge fracture surface. The orientation of the

shear dimples was consistent with thread remnant No. 4 moving upward relative to the

body of the acme nut. The following measurements were made at one of the fractured ends

of thread remnant No. 4: the shear fracture on the outer edge measured 0.0131 inch, the

nearby minimum section thickness measured 0.0076 inch, and the maximum section

thickness at the central ridge measured 0.0184 inch. 

Additionally, two metallographic cross-sections were cut radially through another

representative remnant (No. 6). The metallographic sections showed that thread remnant

No. 6ís flat face exhibited a slight, concave curvature, which was also identified on thread

remnant No. 4. The following measurements were made on one of the metallographic

sections of thread remnant No. 6: the shear fracture on the outer edge measured 0.0174

137 One of these acme screw thread remnants was removed by Safety Board investigators on scene.

138 UNS alloy C95500 is a copper alloy containing aluminum, nickel, iron, and manganese. The

engineering drawing specified an aluminum-nickel-bronze alloy per QQ-B-671, type 2, class 4 Cond HT.

This designation has been superseded with UNS alloy C95500.
Factual Information 77 Aircraft Accident Report

inch, the nearby minimum section thickness measured 0.0099 inch, and the maximum

section thickness at the central ridge measured 0.0180 inch. 

1.16.3 Shear Load Capability of the Acme Nut Threads

The ultimate shear load capability of the acme nut threads was approximated by

assuming that the thread root cross-section was uniformly at the materialís ultimate shear

stress. This calculation was performed for three thread thicknesses: 0.15-inch-thick

threads, as measured on newly manufactured threads; 0.0131-inch-thick threads, as

measured on thread remnant No. 4; and 0.0174-inch-thick threads, as measured on the

metallographic section of thread remnant No. 6. The calculations were based on the

specified minimum ultimate strength (Ftu) for the nut material and assumed a shear

strength equal to 77 percent Ftu. Table 4 shows the results of the shear load calculations.139

Table 4. Shear load calculations for three acme nut thread thicknesses.

1.16.4 Studies of Thread Stress and Deformation

In connection with this investigation, the Safety Board asked Boeing to conduct a

study to estimate the maximum wear that the acme nut threads could sustain and still

maintain their ultimate load capability. Additionally, the Board, in collaboration with the

State University of New York, Stony Brook, New York, initiated a study of the stresses

and deformations in the acme nut. Both studies utilized the ABAQUS140 finite element

analysis (FEA) code and axisymmetric models of the acme screw and nut to address

thread-by-thread distribution of the load transfer from the screw to the nut. The Boeing

study also used an axisymmetric model to evaluate the stresses in a loaded thread section

of the same geometry as one of the remnants recovered from the accident airplane. The

Safety Board/Stony Brook study used a three-dimensional FEA to examine the change in

deformation and stress in a single thread revolution as a function of acme nut thread wear. 

139 For more information about these calculations, see the Materials Laboratory Group Chairmanís

Factual Report in the public docket for this accident.

Shear Load Calculations of the Acme Nut Threads

Shear load for 

0.15-inch-thick thread 

(newly manufactured)

Shear load for 

.0131-inch-thick thread 

(as measured on 

thread remnant No. 4) 

Shear load for 

.0174-inch-thick thread 

(as measured on the 

metallographic section of 

thread remnant No. 6)

Load for 32 

threads

2,200,999 pounds 191,263 pounds 249,782 pounds

Load for 1 thread 68,781 pounds 5,977 pounds 6,806 pounds

140 Hibbitt, Karlsson & Sorensen, Inc. ABAQUS/Standard Userís Manual. Version 6.2. Pawtucket, Rhode

Island. 2001.
Factual Information 78 Aircraft Accident Report

Both studies indicated that the load transfer among the 32 thread revolutions was

not uniform along the length of the acme nut, with thread revolutions near the top and

bottom of the nut receiving greater loads than those near the center of the nut. Further, the

studies showed that the load distribution varied depending on specific temperature and

load conditions. The Boeing study concluded that the maximum load carried by any

individual thread revolution was approximately 30 percent of the applied load, whereas

the Safety Board/Stony Brook study concluded that the maximum load carried by any

individual thread revolution was approximately 23 percent of the applied load. On the

basis of the results of the thread remnant stress analysis, Boeing concluded that the only

way to develop the large plastic bending observed in the thread remnant was for the thread

loading to be concentrated near the tip of the thread rather than near the root.

The Safety Board/Stony Brook study examined the acme screw-to-nut contact

pressure distribution on a single nut thread revolution and determined that the pressure

distribution was nonuniform across the thread surface (radial to the nut), with a small

region near the thread root (major diameter) having a much higher pressure than the

remainder of the contact surface. When material removal was modeled in the region of

highest stress, the stress pattern stayed the same with the high-contact pressure region

moving radially inward across the thread from the material removal region to the adjacent

material on the remaining thread surface. The Safety Board/Stony Brook study concluded

that the wear mechanism consisted of material removal proceeding on a layer-by-layer

removal process along the thread surface and that, within each layer, wear progressed

from the root to the tip (major diameter to minor [inner] diameter).

The Safety Board/Stony Brook study ran a series of FEAs to evaluate the effect of

the sequential wear process on the stress and deformation in acme nut threads. The study

found that the stresses in the acme nut remained low compared to the yield stress141 of the

aluminum-bronze acme nut material, even at wear levels exceeding the maximum amount

allowed for an acme screw to remain in service. At an axial wear level of approximately

0.090 inch, the maximum stress was beginning to approach the yield stress under a load of

2,000 pounds per thread revolution. When the axial wear level reached 0.09265 inch and

approximately 50 percent of the radial thread surface had worn, the bending deflection of

the thread tip began to increase rapidly with load increase. When approximately

75 percent of the radial thread surface was removed, the bending reduced the available

contact surface to a value too small for the FEA to provide valid data. The study

concluded that, at this point, the acme screw thread would begin to slide axially across the

thread surface rather than slide during rotation.

1.16.5 Torque Tube Testing

Under the Safety Boardís supervision, Boeing conducted static load tests on torque

tubes. Torque tube assemblies from MD-80 airplanes other than the accident airplane were

141 Yield stress is the largest value of stress for which all deformation within the material is recoverable

when the material is unloaded.
Factual Information 79 Aircraft Accident Report

tested to failure in tension to generate the sound signature142 of the predicted torque tube

fracture and to validate and generate data for a subsequent FEA. 

To conduct the static load tests, the threads on two acme nuts were machined away

to their roots (the major diameter) so that the acme screw could move freely through the

nut vertically (without rotating). The tests were conducted in a computer-controlled

hydraulic load frame by pulling the acme screw upward through the altered nuts until the

torque tube fractured. 

In the first static load test, the lower mechanical stop on the acme screw contacted

the acme nut in a manner that did not create any bending loads in the torque tube. This

produced pure tension loading in the torque tube similar to loading that occurs during

normal flight conditions. The first static load test produced a tension failure of the torque

tube at a load of 74,400 pounds.143

In the second static load test, the lower mechanical stop on the acme screw

contacted the acme nut in a manner that caused the entire load to be applied to one side of

the stop in an offset manner. This type of load was similar to that indicated by lower

mechanical stop damage on the accident airplane; however, the load was applied on top of

the stop lug, creating a moment arm slightly greater than that indicated on the accident

lower mechanical stop. This loading condition produced a combination of tension and

bending in the torque tube. The second static load test produced a tension failure of the

torque tube at a load of 25,576 pounds.144

For the FEA, Boeing generated a numerical model for the geometry and material

parameters of the accident acme screw, torque tube, lower mechanical stop and clamp

bolt, washer, and lower assembly nut. This model was used to analytically calculate the

static offset load capability of the accident torque tube using the loads derived from the

two static load tests. The Boeing model predicted that the accident torque tube fractured at

a load of 25,500 pounds.

Neither of the two static load test conditions accounted for fatigue damage to the

torque tube (similar to damage found on the accident torque tube). Low-cycle fatigue

testing of small diameter torque tube material specimens was performed to estimate the

stress conditions that would generate a similar low-cycle fatigue fracture surface. To test

the torque tube material, tensile specimens were machined from torque tubes; threads

were added to the center portions of the specimens (simulating the threads at the lower end

of the torque tube); and constant-amplitude cyclic loading at calculated stress levels145 of

140,000, 166,000, and 185,000 pounds per square inch (psi)146 were applied to the

142 For more information on the Safety Boardís sound spectrum study, see section 1.11.1.1.

143 When the airplane is subjected to the ultimate design loads specified for certification, the torque tube

sustains a 57,000-pound load.

144 According to the Boeing torque tube test report summary, ìthe fracture load of just over

25,000 [pounds]Öis based on a loading configuration that provides for the largest offset moment arm with

the nut stop resting directly upon the lower rotational stop.î Boeingís summary noted that the accident

airplaneís ìlower rotational stop exhibits evidence that the orientation of the two stops provides a smaller

moment armî and thus had a higher predicted fracture load.
Factual Information 80 Aircraft Accident Report

individual test specimens until a fracture was produced. Low-cycle fatigue fractures were

created under all three of the test conditions. The fracture surfaces on the accident torque

tube were more similar to those produced when 140,000- and 166,000-psi cyclic loads

were applied than to those produced when a 185,000-psi cyclic load was applied. The

140,000-psi specimen fractured after application of 1,549 load cycles, the 166,000-psi

specimen fractured after application of 389 load cycles, and the 185,000-psi specimen

fractured after application of 100 load cycles.

1.16.6 Additional Safety Board Jackscrew Assembly 

Examinations

1.16.6.1 February 2000 Post-AD 2000-03-51 Jackscrew Assembly 

Examinations

The Safety Board examined eight jackscrew assemblies removed from airplanes

shortly after the issuance of FAA AD 2000-03-51 in February 2000,147 including two

assemblies from Alaska Airlinesí airplanes.148 The jackscrew assemblies were removed in

accordance with AD 2000-03-51 for various reasons, including wear and the presence of

metal shavings. In addition, the Board examined five Alaska Airlines acme nuts that had

been removed from jackscrew assemblies that were being overhauled.

The Safety Board performed a loaded acme nut end play test on all eight of the

removed jackscrew assemblies by mounting each jackscrew assembly (in as-received

condition) in a frame and attaching the gimbal ring to a moment arm. During the tests, the

acme nut was loaded by applying 180 pounds in both directions (up and down).

Movement of the acme nut on the screw was measured using a dial indicator and recorded

as end play. End play measurements were made with the acme nut positioned in three

locations: near the center of the acme screw, near the lower mechanical stop, and near the

upper mechanical stop. The free play was also measured at the middle position during the

loaded end play test. 

After lubricating the middle section of the acme screw with Aeroshell 33 by hand

and injecting it through the acme nutís grease fitting (the original grease was not

removed), end play measurements were repeated on each jackscrew assembly with the nut

in the middle position. The procedures were repeated again after removing by hand all of

the grease from the middle section of the acme screw. The test data show that the

as-received end play measurements changed slightly in both directions for both the

regreased and new grease conditions. 

145 The stress level was calculated by dividing the applied load by the measured cross-sectional area in

the threaded region of the test specimens.

146 These values were used to ensure that testing occurred in the high-stress region of the torque tube and

produced a fracture in a short timeframe. The values were first estimates of the possible stress range that the

torque tube might have experienced and were not intended to reproduce actual conditions. 

147 For more information about postaccident ADs, see section 1.18.1.

148 These jackscrew assemblies were DCA 3000 from N982AS and DCA 3008 from N981AS.
Factual Information 81 Aircraft Accident Report

All eight of the removed jackscrew assemblies were disassembled to separate the

gimbal ring from the acme nut, the acme nut from the acme screw, the acme screw from

the torque tube, and the torque tube from the spherical bearing. Individual items of each

jackscrew assembly were cleaned and visually examined. Visual inspections of the

disassembled acme nuts from the eight removed jackscrew assemblies and the other five

Alaska Airlines acme nuts identified sharp edges, or burrs, at the minor diameter of all of

the threads on all of the nuts. The burrs on all of the acme nuts extended inward from the

lower flanks of the threads. Some burrs were partially disconnected, forming long, thin

metal slivers. The lower flanks and major diameters of the threads were bright and shiny

with a finely scratched and worn appearance. The scratches followed the spiral threads

and had obliterated the original surface finish.

One of the acme nuts from an Alaska Airlines jackscrew assembly that was being

overhauled was cut along its longitudinal axis to inspect its thread surfaces. Burrs and

slivers were identified along the entire length of the cut. In many areas, slivers were

separating from the threads in thin, wide flakes. The lower flanks and the major diameter

of the threads were worn along the entire acme nut. Almost no wear marks were identified

on the minor diameters or on the upper flanks. 

Most of the acme screws from the disassembled assemblies were very similar in

wear characteristics, showing only polishing of the lower thread flanks and major

diameter. However, the acme screw from one of the removed jackscrew assemblies

(S/N DL-96) showed much heavier wear with steps worn into both the upper and lower

flanks of the threads. During the disassembly of DL-96, gritty, gray-colored material was

noted on the upper surfaces of the gearbox support plates.

The torque tubes from all of the jackscrew assemblies were undamaged, except for

DCA 3000 from N982AS. The upper end of this torque tube displayed a .0065-inch-deep

wear band all around the large diameter section.

1.16.6.2 Examinations of Hawaiian Airlinesí Jackscrew Assemblies 

On February 17, 2000, a jackscrew assembly, S/N DCA 110, was removed from a

Hawaiian Airlines DC-9 (N601AP) after an on-wing end play measurement of 0.043 inch

was recorded.149 The assembly had accumulated 2,582 flight hours (6,377 flight cycles)

since a 1997 overhaul.150 A bench end play check conducted by Integrated Aerospace151

after the jackscrew assembly was removed and cleaned yielded an end play measurement

of 0.048 inch. The acme nut and screw were forwarded to the Safety Boardís Materials

Laboratory for examination. 

149Before the flight 261 accident, Hawaiian Airlinesí lubrication interval for the jackscrew assembly 

was every 1,000 flight hours, and its end play check interval was every 3,000 flight hours.

150After the 1997 overhaul, the DCA 110 jackscrew assembly had a recorded end play measurement of 

0.010 inch. The jackscrew assembly was sent to Hawaiian Airlines, where it was stored until its installation

on N601AP on June 22, 1998.

151Integrated Aerospace, formerly Trig Aerospace, Inc., is the only original manufacturer and supplier 

of new jackscrew assemblies. 
Factual Information 82 Aircraft Accident Report

Metallurgical examination of the acme screw identified visible wear steps in both

the upper and lower flanks of the threads. No other mechanical or corrosion damage was

noted on the acme screw. Examination of the acme nut showed that both flanks of the nut

threads were lustrous with circumferential marks consistent with wear. However, no burr

was noted at the minor diameter as seen on other acme nuts previously examined. The

major diameter of the threads (the root area) was also worn and lustrous with a fine pattern

of parallel scratches on both the upper and lower flank surfaces. The parallel scratch

pattern ran circumferentially around the thread flanks. The major diameter surfaces also

showed heavy localized wear that enlarged the diameter in some areas of the acme nut.

The diametrical wear was determined to be nonuniform in both the circumferential and

vertical directions, consistent with an angular misalignment of the acme screw axis to the

acme nut axis with the upper end of the screw displaced to the right and the lower end to

the left. The calculated wear rate of this jackscrew assembly (using the 0.048-inch n end

play measurement obtained during the overhaul bench check and accounting for the

0.010-inch end play measurement obtained when the jackscrew assembly was first

installed) was 0.0147 inch per 1,000 flight hours.

On June 18, 2001, another jackscrew assembly, also partially stamped with

S/N DCA 110,152 was removed from the same airplane after an on-wing end play

measurement of 0.036 inch. The acme screw and nut had accumulated 2,514 flight hours

(6,071 flight cycles). Metallurgical examination at the Safety Boardís Materials

Laboratory identified polishing on the acme screwís upper thread flanks and major

diameter in the central part of the screw. No steps in the thread flanks were identified. The

lower flanks of the acme nut threads were worn with a circular pattern, and the shape of

the acme screw thread major diameter chamfer was worn into the nut threads.

Examination of the acme nutís profile and interior surfaces after sectioning identified

small burrs on all threads extending from the lower thread flank into the minor diameter

area. The major diameter surfaces displayed some localized contact damage, but the

diameter was not enlarged at any location. The calculated wear rate of this jackscrew

assembly (assuming a starting end play measurement of 0.006 inch after overhaul and a

final bench-check end play measurement of 0.027 inch) was 0.0084 inch per 1,000 flight

hours.

The Safety Boardís examination also revealed evidence of pink-colored grit

material inside the counterbore of the acme nut grease passageway in the jackscrew

assembly removed in February 2000. A Hawaiian Airlines maintenance work card dated

December 4, 1999, stated, ìGrit blast accumulated in vertical stab as seen thru access

panel 6401 and panel 3703 (left lower banana fairing). Area to be inspected after

cleaning.î The corrective action noted on the card was the following: ìCleaned area.

Inspected area. No discrepancies noted.î According to Hawaiian Airlines personnel, the

152Although S/N DCA 110 was stamped on the gearbox support assembly, the S/N stamped on the acme 

screw and nut was D 3351, indicating that the acme screw and nut from the first DCA 110 assembly were

replaced by a new acme screw and nut. Hawaiian Airlines personnel stated that they kept the DCA 110

gearbox support assembly, torque tube, and stop nut with N601AP (after sending the acme screw and nut for

overhaul) when they installed the newly acquired D 3351 acme nut and screw in February 2000. Records

indicated that the D 3351 assembly was manufactured in 1998 and delivered to Boeing with an end play

measurement of 0.006 inch. 
Factual Information 83 Aircraft Accident Report

area noted on the work card was located inside the vertical stabilizer where the jackscrew

assembly is installed. 

Several grease samples taken from the original acme nut and screw of jackscrew

assembly DCA 110 and from the area of the assembly near the inside of the vertical

stabilizer contained translucent, pink particles, consistent with the fine grains of a sandlike

material. EDS chemical analysis indicated that the grains were composed of iron, silicon,

aluminum, calcium, oxygen, and magnesium. Examination of grease samples taken from

the D 3351 assembly also revealed pink particles, consistent with those found in the grease

sample taken from DCA 110. 

An on-site inspection of Hawaiian Airlinesí maintenance facilities in August 2001

revealed that it had been using either Australian garnet or glass beads to remove corrosion

from airplane structure. Examinations of a sample of the garnet material found that it

visually and chemically matched the pink, sandlike material found in the grease of both

jackscrew assemblies removed from Hawaiian Airlines. 

1.16.6.3 Examinations of Alaska Airlines Reports of Acme Screw ìWobbleî

On November 26, 2000, Alaska Airlines maintenance personnel observed the

acme screw ìwobbleî while operating the horizontal stabilizer trim system on N965AS.

According to the personnel, the wobble appeared as a slight rocking motion of the acme

nut as the acme screw was rotated within it. Alaska Airlines removed the jackscrew

assembly from N965AS and notified the Safety Board about the wobble. 

On February 21, 2001, Safety Board investigators observed the jackscrew

assembly as it was operated on an Integrated Aerospace fixture and a wobble was noted.

The end play of the wobbling acme screw was checked several times (before disassembly)

while installed on the Integrated Aerospace fixture. The acme screw was rotated before

each end play check to identify any variance in end play caused by wobble-induced acme

nut and screw position changes. No differences were noted, and the end play measured

0.018 inch consistently. 

After the jackscrew assembly was disassembled, the technician who removed the

lower torque tube nut indicated that the torque nut was looser than what he considered to

be typical.153 A wear band was noted on the large diameter section near the top portion of

the torque tube. The depth of the wear band was not uniform around the periphery of the

groove and measured 0.035 inch at its deepest point. According to Boeing, a wear band

frequently occurs in this area because the clearances are small and, at times, the torque

tube contacts the inside wall of the acme screw. The Boeing DC-9 Overhaul Maintenance

Manual requires a steel-sleeve repair of the wear band during overhaul if the wear band is

deeper than 0.025 inch.

153 The technician used a wrench to remove the torque nut. No measuring device was used to measure the

torque required to loosen the nut. The technicianís comments were based on his personal experience of feel.
Factual Information 84 Aircraft Accident Report

In a March 27, 2001, letter, Alaska Airlines asked the Safety Board to investigate

the wear band. Alaska Airlines noted that the torque tubes from N982AS and the accident

airplane had similar wear bands.154 Alaska Airlines also expressed concern that the

wobble could produce erroneous end play measurements and/or excessive acme nut wear.

The Board collected data from Integrated Aerospace and Boeing to determine if a

relationship existed between acme nut wear rate and torque tube wear band depth. The

collected data included information about 161 jackscrew assemblies examined by

Integrated Aerospace in 2000. Of these assemblies, only 77 had torque tubes. Of the

77 jackscrew assemblies with torque tubes, only 49 had a recorded bench-check end play

measurement, a torque tube wear band measurement, and a valid flight-hour total. A

Board examination found no statistically significant difference regarding acme nut wear

between the torque tubes with no (or a slight) wear band and the torque tubes with deeper

wear bands.

1.16.7 Chemical and Microscopic Analyses of Grease Residues 

from the Accident Jackscrew Assembly

The Safety Board contracted the U.S. Navyís Air System Commandís Aerospace

Materials Laboratory, Patuxent River, Maryland,155 to conduct chemical and microscopic

analyses of eight grease residue samples taken from the accident jackscrew assembly. An

organic analysis method, Fourier Transform Infrared Spectroscopy, was used to compare

the grease residue samples taken from the accident jackscrew assembly to control samples

of Aeroshell 33 and Mobilgrease 28. The spectra resulting from the residue samples were

weak, which was attributed to the degraded quality of the grease. None of the grease

residue samples could be matched perfectly to either Aeroshell 33 or Mobilgrease 28.

However, the resulting analysis indicated that most of the residue samples displayed

infrared spectra consistent with the presence of both Aeroshell 33 and Mobilgrease 28. On

the basis of a comparison of the residue samples to the control samples, the Naval

laboratory concluded that both greases were likely present on the accident jackscrew

assembly. An independent review of this work conducted by a private consultant

commissioned by the Board also concluded that the residue samples had infrared

signatures consistent with the presence of both Aeroshell 33 and Mobilgrease 28. 

According to the Naval laboratory report, the microscopic analysis of the residue

samples found that the ìsample grease was heavily loaded with particles[156] originating

fromÖaluminum bronzeî and that ìthese particles were consistent with wear debris.î The

154 Metallurgical examination determined that the wear band on the torque tube from jackscrew assembly

DCA 3000 was 0.0065 inch deep and that the wear band on the accident torque tube was 0.028 inch deep.

155 The U.S. Navyís Aerospace Materials Laboratory is the Department of Defenseís (DoD) steward of

the MIL PRF specifications for both Aeroshell 33 and Mobilgrease 28 and qualifies greases to these

specifications. 

156The Naval laboratory report stated that ìa couple of larger metal particles were extracted from the 

grease with tweezers and examined individually. One was a large flake and the other was a long shaving.

Both were aluminum bronze, equivalent in composition to the smaller particles.î 
Factual Information 85 Aircraft Accident Report

Naval report stated that ìno other particle contamination was found other than that

attributed to the environment of the crash site and the recovery operation.î 

1.16.8 Additional Grease Testing, Experiments, and Analysis

1.16.8.1 Standardized Grease Testing 

The Safety Board also contracted the U.S. Navyís Aerospace Materials Laboratory

to conduct a series of standardized grease tests. Pure quantities of Aeroshell 33 and

Mobilgrease 28, as well as several mixture ratios of each grease, were tested using several

American Society for Testing and Materials standard test methods for lubricating greases.

These tests examined the greasesí firmness (when new and after being mixed), storage

stability, and oil bleedout caused by elevated temperature and elapsed time. The results of

these tests indicated that, in general, the physical properties of Aeroshell 33 and

Mobilgrease 28 did not change significantly when mixed; however, the change in the

physical properties of the two greases noted in 90/10 and 10/90 mixture ratios exceeded

that of the standard for compatibility of mixed greases. The Naval laboratory report

concluded that the change in properties was insignificant and would not significantly

affect the greasesí performance. 

The Naval laboratory also conducted seawater immersion tests on control samples

of Aeroshell 33 and Mobilgrease 28 and on a 50/50 mixture of the two grease types. The

Naval report concluded that, after being immersed in seawater for 2 weeks, the three test

samples ìexhibited only slight color changes, with no other adverse effects noted.î The

report concluded that ìdeterioration of the aircraft grease sample [from the accident

airplane] was not due to exposure to seawater for two weeks following the incident.î157

1.16.8.2 Experiments and Analysis on the Surface Chemistry of Acme Nut 

Material When Exposed to Various Greases or Grease Mixtures

The Safety Board contracted the U.S. Navyís Aerospace Materials Laboratory and

Science Applications International Corporation, San Diego, California, to conduct a series

of experiments to examine the surface chemistry of acme nut material when exposed to

Mobilgrease 28, Aeroshell 33, and mixtures of the two greases under various conditions

that can occur during flight, including exposure to ambient and elevated temperatures and

fluid contamination (water, deicing fluid, and anti-icing fluid). 

During the experiments, it was found that under certain circumstances

Aeroshell 33 would produce a visible brownish discoloration to small, localized regions of

the acme nut material. The surface chemistry of the discolored regions was analyzed using

x-ray photoelectric spectroscopy. The analysis indicated that the discolored region was

157In another example, the Safety Board observed that an acme screw recovered from a China Northern 

MD-80 airplane that crashed into a salt water bay near Dalian, China, on May 7, 2002, still had a coating of

grease clearly visible on it after it had been immersed in seawater for 5 days. According to the Civil Aviation

Administration of China, the last scheduled lubrication of the jackscrew assembly was during an A check

that took place March 27 through April 2, 2002, and the specified grease was Aeroshell 5 (MIL-G-3545C).
Factual Information 86 Aircraft Accident Report

caused by additives in the Aeroshell 33 grease chemically reacting with each other and

depositing on the surface of the acme nut material, which is an intended chemical reaction

between antiwear additives in the grease. Science Applications International Corporation

concluded that this was not a corrosion process and, therefore, would not corrode the acme

nut material. 

1.16.8.3 Wear Testing Under Various Grease Conditions

The Safety Board contracted the Tribological Materials Laboratory at Rensselaer

Polytechnic Institute, Troy, New York, and the tribology division of Battelle Memorial

Institute, Columbus, Ohio, to conduct a series of wear tests on grease-lubricated,

aluminum-bronze and steel couples in sliding wear. The test program concentrated on the

American Society for Testing and Materials D3704 Block-on-Ring wear test method,

which involves a steel ring sliding in reciprocating motion upon an aluminum bronze

block. Contact pressure ranges and sliding speeds used in these tests reflected those

experienced in an operational jackscrew under flight and static loads. Additionally, test

specimens were fabricated from actual jackscrew assembly steel and aluminum-bronze

components. 

Because Mobilgrease 28 has been demonstrated to effectively lubricate the

jackscrew assembly, the wear test program examined the lubricating effectiveness of

Aeroshell 33 relative to Mobilgrease 28. The wear tests were run under the following

conditions: (1) with a mixture of Aeroshell 33 and Mobilgrease 28; (2) with greases

contaminated with water, deicing fluid, and salt water; (3) with aged and wear-debris

laden grease; (4) with greases exposed to subzero temperatures; and (5) with a complete

lack of lubrication. Over 50 wear tests were performed. During the wear tests, the wear

rates were continuously monitored so that the steady-state wear behavior, which governs

the long-term jackscrew assembly wear, could be identified separately from the transient

wear behavior experienced during the break-in period that began each test. 

 The results of the wear tests showed that, in all ranges of contact pressure, lower

wear rates were generated when Aeroshell 33 was used than when Mobilgrease 28 was

used. Additionally, the effects of contamination, aging, and subzero temperature exposure

were determined to have little effect on the lubricating effectiveness of Aeroshell 33. Only

the tests run without any lubrication generated wear rates significantly higher than those

generated when Mobilgrease 28 was used. In these cases, wear rates were seen to

transition up to 10 times higher than those with equivalent conditions lubricated with

Mobilgrease 28. Figure 19 shows a summary of the results of the high-load wear tests,

including the contamination and low-temperature tests.
Factual Information 87 Aircraft Accident Report

Figure 19. A summary of the results of the high-load wear tests, including the 

contamination and low-temperature tests.

1.17 Organizational and Management Information

1.17.1 Alaska Airlines, Inc.

Alaska Airlines, Inc., was organized in 1932 and incorporated in 1937 and, at the

time of the accident, was one of two subsidiaries of Alaska Air Group, Inc., which was

incorporated in 1985. At the time of the accident, Alaska Airlines employed about

10,000 people and served more than 40 domestic and international destinations (including

5 cities in Mexico) from hubs in Anchorage, Alaska; Seattle, Washington (the location of

the companyís headquarters); and Portland, Oregon. According to FAA documents, at the

time of the accident, Alaska Airlines operated a fleet of 88 airplanes, including 49 737s

and 39 MD-80 series airplanes.158

158 According to a June 20, 2000, FAA special inspection report on Alaska Airlines, the airline was

operating a fleet of 89 airplanes, of which, 55 were 737s and 34 were MD-80 series airplanes. The FAA

special inspection report added that Alaska Airlines planned to add 9 737-700 and 10 737-900 series

airplanes to its fleet over the next 2 years. For more information about the FAA special inspection report, see

section 1.17.3.4. Steady-State Wear Factor (mm3/N.m) x106

Neat Grease Tests Fluid Contaminated Tests Low

Temperature

Tests (-29C)

Used Grease Test

Avg. 3 tests

80

70

60

50

40

30

20

10

0

A33

M28

50/50 Mix

Avg. 4 TestsAvg. 5 TestsAvg. 3 Tests

A33

M28

A33A33

M28

Dry

Grease+5% Salt WaterGrease+5% De-Icing Fluid

50/50 Mix

Used A33

A33

Grease

+5% DI Water

DRY
Factual Information 88 Aircraft Accident Report

1.17.2 Alaska Airlines Flight Crew Training

According to FAA and Alaska Airlines documents, Alaska Airlines conducted

training for its MD-80 fleet under the Advanced Qualification Program, which was

effective March 1, 1985. A Safety Board review of the airlineís MD-80 training

curriculum indicated that the primary and alternate horizontal trim system components

were covered during the airplane-specific ground school. During the second segment of

maneuver briefing (before the full-flight simulator session), pilots were asked to describe

situations that might require use of the runaway stabilizer checklist and to perform the

checklist.159 Application and performance of the runaway stabilizer checklist was also

required during the third and fourth segments of the curriculum.

Classroom materials for the MD-80 continuing qualification program, which were

effective in 1999, included normal and abnormal procedures for flight control systems.

Supplemental instructional material given to pilots contained a quiz that included

questions addressing electrical failures and associated stabilizer inoperative indications. 

1.17.2.1 Stabilizer Trim Check Procedures

The stabilizer trim check procedures listed in the Alaska Airlines MD-80 Flight

Handbook included checking both the primary and alternate trim systems. Specifically, the

ìBefore Start Expanded Proceduresî included the following steps:

Move PRIMARY MOTOR BRAKE switch to ëSTOP.í Operate both Captainís

control wheel trim switches in NOSE UP and NOSE DN [down] directions.

Observe stabilizer does not move and aural warning does not sound. Unless

stabilizer is running away, NEVER PLACE THE PRIMARY MOTOR

BRAKE SWITCH TO ëSTOPí WITH THE STABILIZER IN MOTION.

Damage will result. Move the PRIMARY MOTOR BRAKE switch to NORM

and verify guard is down. The PRIMARY MOTOR BRAKE switch does not

affect operation of the alternate stabilizer trim system. While observing the

stabilizer indicator, hold Captainís control wheel left trim switch to NOSE UP and

then to NOSE DN momentarily. 

CAUTION ñ Do not hold single switch position longer than 3 seconds.

The stabilizer should not move and aural warning should not sound. Repeat

this check using Captainís control wheel right trim switch. Observe stabilizer does

not move and aural warning does not sound.

Operate both Captainís control wheel trim switches in NOSE UP and then NOSE

DN directions and note corresponding direction of stabilizer indicator and aural

warning. Operate First Officerís control wheel trim switches and check stabilizer

indicator. Moving Captainís and First Officerís control wheel trim switches in

opposition directions will stop stabilizer movement. This is not a required

test. If test is used, do not exceed 3-second limit.

159 For more information about the runaway stabilizer checklist, see section 1.17.2.2.
Factual Information 89 Aircraft Accident Report

Operate both LONG TRIM HANDLES in the same direction and check for

corresponding trim indicator movement, release handles to neutral. While

observing horizontal trim indicator, momentarily operate each ALT LONG TRIM

switch individually to NOSE UP and NOSE DN. Observe stabilizer does not

move and aural warning does not sound. Operate both ALT LONG TRIM

switches and check for direction of travel at slower rate. To obtain maximum

service life, do not maintain switch positions longer than necessary to check

response. If there is no response, release switches and determine cause.

Should either trim handle or any control switch bind, hesitate or stick when

released to neutral, it should be corrected before flight. Any time LONG

TRIM handles, control wheel trim switches, or ALT LONG TRIM

(secondary) controls are used, they should be moved to their fully deflected

switch/lever position to ensure both motor and brake circuits are actuated.

The ìTaxi Expanded Proceduresî in the Flight Handbook outlined the following

steps: 

Rotate the FLAP thumbwheel, in the LONG TRIM Takeoff Position Display, until

the computed TO (takeoff) FLAP value appears in the FLAP window. 

Rotate the CG thumbwheel, in the LONG TRIM Takeoff Position Display, until

the computed CG value appears in the CG window. 

ï when the FLAP and CG values are set for takeoff, the Stabilizer TAKEOFF

CONDTN LONG TRIM Readout window will display the proper trim setting. 

ï using the Primary LONG TRIM Control Wheel Switches, or LONG TRIM

Handles, set the LONG TRIM (White) Indicator opposite the Long Trim

Takeoff Position (Green) Indicator.

ï if the indicators are not within specified tolerance, after takeoff flaps have

been selected, the aural warning will sound when the throttles are advanced

for takeoff. 

1.17.2.2 Runaway Stabilizer Checklist Procedures 

The Alaska Airlines MD-80 Quick Reference Handbook (QRH) included the

following checklist procedures to troubleshoot a runaway stabilizer:
Factual Information 90 Aircraft Accident Report
Factual Information 91 Aircraft Accident Report

1.17.2.3 Stabilizer Inoperative Checklist Procedures

The Alaska Airlines MD-80 QRH included the following checklist procedures to

troubleshoot an inoperative stabilizer:
Factual Information 92 Aircraft Accident Report

1.17.2.3.1 Postaccident Boeing Flight Operations Bulletin on Stabilizer Trim 

Inoperative/Malfunction Procedures

On February 10, 2000, Boeing issued a flight operations bulletin for DC-9,

MD-80, MD-90, and 717 airplanes that addressed stabilizer trim inoperative/malfunction

procedures. On the basis of information gathered in the flight 261 accident investigation to

that date, the bulletin recommended the following:

If a horizontal stabilizer trim system malfunction is encountered, complete the

flight crew operating manual (FCOM) checklist(s). Do not attempt additional

actions beyond that contained in the checklist(s). If completing the checklist

procedures does not result in an operable trim system, consider landing at the

nearest suitable airport. If an operable trim system is restored, the captain should

consider proceeding to an airport where suitable maintenance is available, or to

the original destination based on such factors as distance, weather, etc.

The Boeing bulletin added the following:

The primary and alternate trim motors are each equipped with a thermal cut-off

device which interrupts electrical current to the motor if that trim motor overheats.

Repeated or continuous use of the trim motor may cause a thermal cutoff. After

the motor cools, it will automatically restore trim function when the thermal cutoff

resets. Because an overheat cutoff in one trim motor does not affect the functions

of the other, if the alternate trim motor overheats, the primary trim system may be

used to retrim the stabilizer; the reverse is also true. However, if the flight crew

uses the primary trim system repeatedly to resist a runaway in the alternate trim

system, the primary motor could overheat, and the crew may be left with a

runaway alternate trim if the primary trim thermal cutoff occurs. This action could

also cause both trim motors to overheat, and result in a temporarily inoperative

stabilizer.

Also, if a runaway trim motor overheats and stops, it could again runaway once it

has cooled and the thermal cutoff resets. If the crew determines that the stabilizer

is inoperative, and suspects that they may have a thermal cutoff, and if flight

conditions permit, the captain may delay the diversion to an alternate airfield long

enough to allow for a cooling period/thermal reset. After a reset, the crew should

refer to the recommendations paragraph above.

The Boeing bulletin also advised flight crews that ìexcessive or prolonged testing

of the trim system on the ground before departure may generate enough heat to produce a

thermal cutoff during routine trimming shortly after takeoff.î160

160 After the flight 261 accident, there were reports of flight crews overtesting the trim systems, causing

them to overheat.
Factual Information 93 Aircraft Accident Report

1.17.3 FAA Oversight of Alaska Airlines

1.17.3.1 General

The Alaska Airlines operating certificate was issued in September 1946 and was

managed by the FAAís Certificate Management Office (CMO) in Seattle, Washington. The

CMO reported to the Seattle FSDO. The Seattle CMO staffing at the time of the flight 261

accident included a supervisor, five operations inspectors, and three principal inspectors:

the PMI, the principal operations inspector, and the principal avionics inspector. The

Seattle CMO staffing also included three geographic inspectors to assist with surveillance

of Alaska Airlines: one from the Alaska Region, one from the Northwest Mountain

Region, and one from the Western Pacific Region. 

1.17.3.2 Preaccident FAA National Aviation Safety Inspection Program 

Inspection

From August 13 to August 23, 1995, a nine-member FAA National Aviation

Safety Inspection Program (NASIP) inspection team conducted operations and

airworthiness inspections at Alaska Airlines. During the inspection, the FAA team found

ìpotential problems [with the airlineís] systems for assuring compliance with FAR

requirementsî in the areas of ìoperations training, flight control, maintenance training

programs, and contractual arrangements.î 

The final NASIP inspection report listed 16 noncompliance findings: 8 of the

findings were related to operations, and the other 8 findings were related to airworthiness

issues. Five of the operations findings were deficiencies in the flight attendant training

manual, emergency drill training, and record-keeping procedures. Two operations findings

were related to flight dispatcher and preflight briefings and guidance to flight crews

related to weather issues. One of the airworthiness findings related to a maintenance

action that was contrary to the certificate holderís manual and the FARs. The report stated

that the mechanic tested the wrong system (the system tested was not the one reported to

be inoperative) and that the mechanic was ìnot trained and qualified to perform

maintenance onÖ[low visibility landing] maintenance program systems.î

In addition, the report noted several deficiencies related to maintenance

record-keeping and replacement part storage and found that the airlineís GMM, which

stated that repairs and overhauls could be conducted by another approved carrier, did ìnot

contain a procedure, or a requirement, that will assure that the other carrier must meet the

manufacturerís approved repair procedures.î 

1.17.3.3 FAA Air Transportation Oversight System

Alaska Airlines transitioned from the FAAís existing oversight system, the

Program Tracking and Reporting System (PTRS), to the Air Transportation Oversight

System (ATOS) in October 1998.161 According to the FAA, ATOS was designed to focus

161 The 10 largest U.S. air carriers were selected for the implementation of ATOS.
Factual Information 94 Aircraft Accident Report

on ìsystem failuresî rather than ìrandom eventsî and to improve the reporting and

evaluation of system failures and trends for more aggressive intervention. Further, ATOS

was designed to conduct inspections in 7 air carrier systems (aircraft configuration

control, manuals, flight operations, personnel training and qualifications, route structure,

flight rest and duty time, and technical administration), 14 air carrier subsystems, and

88 elements. 

The ATOS program office manager testified that the transition to ATOS at some

airlines, including Alaska Airlines, was difficult because of staff levels and training issues.

He stated, ìI think it would have been real prudent for us maybe to have overlaid some

type of transition plan when we did implement ATOS, so that maybe we had a few of

those comfortable things that worked well under our traditional surveillance still available

when we implemented ATOSÖWe didnít really have that transition plan. We shut off the

old one and started the new one.î He also stated that, although some ATOS modules were

ìstill at a rudimentary level of developmentÖweíre still far, far above where we were with

just a compliance-based, event-based PTRS surveillance system. We have a lot better and

a lot richer and a lot more robust data in that ATOS data repository right now than weíve

ever had under our traditional surveillance for these 10 carriers.î

The former PMI for Alaska Airlines, who retired from the FAA in November 1999

after 8 1/2 years in that position, stated that the implementation of ATOS resulted ìin a

terrible transition.î He also stated the following: 

The first thing that happened was the geographic support that we were

accustomed to getting totally disappeared. We ended up with three other offices

supporting us geographically; airworthiness, avionics and an operations inspector

from each region that Alaska operated into. And between them coming up here

and getting training and ATOS training and learning how to understandÖall this

stuff, they actuallyÖ[werenít] doing any surveillance. None of us wereÖwe were

too caught up doing ATOS things to actually go out and do any surveillance, do

the system evaluations. They wouldnít let any geographic inspectors that werenít

assigned to the carrierÖlook at the carrier. We used to get PTRS reports regularly

from all over the field out there. We had about nine officers that were giving us

geographic support. And after ATOS we ended up with about three officers, one in

each region. And they were too busy doing all these complexÖstuff. Nobody was

out there looking at the carrier.

The Seattle CMO supervisor at the time of the ATOS implementation at Alaska

Airlines stated that ìthe amount of surveillance that we have done since the introduction

of ATOS has probably generally decreased. The [ATOS] concept is great. Translating that

into some real life implementation is tougher than it sounds.î

1.17.3.3.1 Seattle CMO Memorandum on Staff Shortages for Surveillance of 

Alaska Airlines

A November 12, 1999, memorandum, from the supervisor of the FAAís CMO in

Seattle to the FAAís Director, Flight Standards Service, stated that staffing at the Seattle

CMO ìhas reached a critical pointî and added that ìto accomplish our assigned activities,
Factual Information 95 Aircraft Accident Report

we require at least four additional airworthiness inspectors and one cabin safety

inspector.î The memorandum noted the following: 

[Alaska Airlines] one of the countryís top ten air carriers, is undergoing a state of

aggressive growth and expansionÖWe are not able to properly meet the workload

demands. Alaska Airlines has expressed continued concern over our inability to

serve it in a timely manner. Some program approvals have been delayed or

accomplished in a rushed manner at the ëeleventh hourí and we anticipate this

problem will intensify with time. Also, many enforcement investigations,

particularly in the area of cabin safety, have been delayed as a result of resource

shortages.

The memorandum stated that the Seattle CMO had implemented an ATOS

comprehensive surveillance plan (CSP) and that the

current CSP for airworthiness consists of 50 safety attribute inspectionsÖand 176

element performance inspections...The CSP for operations consists of 50 [safety

attribute inspections] and 103 [element performance inspections]. Effective

execution of the ATOS inspections requires a significant expenditure of inspector

resources. In order for the Seattle FSDO [CMO] to accommodate the significant

volume of Alaska Airlineís ëdemandí work and effectively meet the objectives of

the ATOS program, additional inspector staffing must be made available.

The memorandum concluded that, if the Seattle CMO ìcontinues to operate with

the existing limited number of airworthiness inspectors and without a cabin safety

inspector, diminished surveillance is imminent and the risk of incidents or accidents at

Alaska Airlines is heightened.î 

A May 3, 2002, memorandum from the FAAís Director, Flight Standards Service,

in response to the Safety Boardís request for information regarding staffing changes at the

Seattle CMO since the flight 261 accident indicated that the authorized staffing level had

increased from 15 to 37 and that the on-board staffing level had increased from 12 to 30

and would increase to 32 by May 19, 2002.162 The memorandum indicated that the staffing

included an operations research analyst and a cabin safety inspector.

1.17.3.4 FAA Postaccident Special Inspection of Alaska Airlines

As a result of the flight 261 accident, the FAA conducted a special inspection of

Alaska Airlines from April 3 to April 19, 2000, to determine its compliance with the

FARs. The inspection team was headed by a member of the FAAís System Process Audit

staff from Washington, D.C., and team members included several FAA inspectors from

the Seattle FSDO and CMO. According to the special inspection report, dated June 20,

162 The Safety Board confirmed that the Seattle CMO on-board staffing level had increased to 32 by

May 19, 2002, and determined that, as of September 2002, its on-board staffing level had further increased

to 35.
Factual Information 96 Aircraft Accident Report

2000,163 the 15-member inspection team focused on Alaska Airlines maintenance and

operations at its SEA and OAK facilities.

The special inspection report included the following findings: 

Lack of Management Personnel:

ï Director of Maintenance position has been vacant for nearly two years (since

6/12/98). Currently two individuals are sharing the position.

ï Director of Operations position is currently vacant.

ï The Director of Safety is also the Director of Quality Control and Training.

This position also does not report directly to the highest level of management.

Maintenance Training:

ï Alaska Airlines training] manual does not specify maintenance training

curriculums or on-the-job training (OJT) procedures or objectives.

ï The OJT program is informal, and is administered at the discretion of the

appointed instructors. The program is not structured, there is no identification

of subjects to be covered, and there are no criteria for successful completion

provided.

Maintenance Program:

ï General Maintenance Manual (GMM) does not reflect the procedures that the

company is actually using to perform maintenance on its aircraft at the

companyís maintenance facilities. 

ï The GMM does not include ëhow toí procedures regarding heavy check

planning and/or production control.

ï The GMM does not contain complete procedures for the issuance of an

airworthiness release to an aircraft coming out of a heavy check.

ï Two aircraft were released to service from a ëCí check without the completion

of all necessary paperwork. This causes the inspection team to question the

completion of all work on the aircraft before [it is] issu[ed] an airworthiness

release andÖused in revenue service.

ï Spot checks of [Alaska Airlinesí] shelf-life program for consumables revealed

numerous discrepancies with the expiration dates that were exceeded or

mislabeled.

163 Details of the FAA special inspection report were officially released in a June 2, 2000, FAA press

release.
Factual Information 97 Aircraft Accident Report

ï Shift turnover forms (MIG-48) are not being used consistently within the

company as required by the GMM. Forms that are being used are incomplete

and/or missing required signatures. There were also various other methods of

shift turnover forms in use within the company that are not listed in the GMM.

ï Numerous nonroutine work cards (MIG-4) were signed off using a company

issued stamp, rather than an actual signature, as required by the GMM.

ï Numerous MIG-4 forms had the box checked for ëpartial work completedí but

there were no entries on the back of the card for any partial work initiated or

completed.

ï Work cards being used for heavy checks are being modified or deleted without

approval of engineering or quality control. Mechanics on the floor are making

these changes on their own.

ï There are no procedures in the GMM on how routine and nonroutine work

cards will be distributed and controlled during a heavy check.

Continuous Analysis and Surveillance Program:

ï [Alaska Airlines] manuals do not contain facsimiles of audit checklists to be

used to administer the program.

ï Data gathering is not continuous, but periodic. Audits are performed at 12 or

24-month intervals and an allowance for a six-month extension is available.

ï Audit methods and techniques do not address compliance with regulatory

safety standards. Rather, audit checklists are modeled after the CASE audit

program, which is a generic program that is not specifically designed or

tailored for [Alaska Airlines].164

The report also stated, ìthe area that showed the highest potential of systems

breakdowns is in the Maintenance Program (15 findings). This is probably because Alaska

Airlines maintenance personnel are not following the procedures that the company has in

its manuals thereby increasing the probability for errors.î The report added that ìby not

having a functional [continuing analysis and surveillance program], numerous other areas

suffer from the lack of oversight and reform. This was evident within [Alaska Airlines]. It

is controlled by the Quality Assurance department, which appears to be understaffed.

Audits are not being completed in a timely manner and are incomplete, so problem areas

are not being identified.î

The special inspection report concluded the following:

ï The procedures that are in place at [Alaska Airlines] are not being followed.

164 The FAAís former director of Flight Standards Service stated at the Safety Boardís public hearing that

he was ìstaggered at the results of that inspection that showed that there were broad and systemic problems

in the companyís continuous analysis and surveillance program.î 
Factual Information 98 Aircraft Accident Report

ï The controls that are in place are clearly not effective, as measured by the

number of findings that the team had during the inspection. 

ï The authority and responsibilities are not very well defined. This situation is

aggravated [by] the fact that three positions [the director of maintenance, the

director of operations, and the director of safety] are not filled. One of the

positions, [director of maintenance], is being filled by two people, but the

division of duties and responsibilities has not been made in the GMM;

consequently, there is confusion as to who is responsible for what tasks.

ï Control of the deferral system is missing. Items are being deferred without

using the approved MEL [minimum equipment list]/CDL [configurations

deviations list], resulting in items not being repaired for long periods of time.

ï Quality Control and Quality Assurance Programs are ineffective. This is

evident through things such as ëCí check packages that are missing signatures,

open work cards, partial work completed, forms incomplete, etc.

The special inspection reportís final assessment of Alaska Airlinesí condition was

that ìmost of the findings can be attributed to (a) the processes (both those in place and

those that are missing) at the carrier and (b) ineffective quality control and quality

assurance departments. Correcting these areas would eliminate the majority of the issues

identified in this report.î 

The special inspection team also evaluated Alaska Airlines based on FAA criteria

ìfor monitoring operators during periods of growth or major change.î The inspection

report stated that the airline planned to add additional aircraft, ìwhich would result in a 20

percent increase over 3 years,î165 and that the ìcompany has a very high utilization rate,

particularly with the MD-80 fleet, to accommodate its very aggressive flight schedule.î

The report noted that there was an ìapparent shortage of [flight] instructorsî at the airline,

which ìwill become an even more prominent issue when aircraft are added to the fleet and

more flight crews are needed, along with the increase in the summer schedule.î The report

further noted that because of a shortage of maintenance personnel at OAK, ìcontractors

will be used more for accomplishing their ëCí checksî and that ìQuality AssuranceÖand

Quality ControlÖare not adequately staffed at contract maintenance facilities.î The report

suggested that the key elements of Alaska Airlinesí system, including the maintenance

deferrals, quality assurance and control, internal evaluation, maintenance control, and

GMM ìshould be monitored to determine if the carrierís system is able to support

additional aircraft.î

1.17.3.4.1 FAA Proposed Suspension of Alaska Airlinesí Heavy 

Maintenance Authority

In its June 2, 2000, press release regarding the special inspection findings, the FAA

proposed the suspension of Alaska Airlinesí heavy maintenance authority. The FAA press

165 These criteria are found in handbook bulletins HBAW 98-21 and HBAT 98-36. The inspection teamís

report noted that, according to these handbook bulletins, ìa growth rate of 10 percent or more is high.î
Factual Information 99 Aircraft Accident Report

release stated, ìapproximately six to seven aircraft, of the airlineís fleet of 89 aircraft, are

in heavy maintenance in any given month.î The FAA stated that Alaska Airlines was

working with the FAA ìto correct the deficiencies outlined in the inspection.î On June 29,

2000, the FAA accepted an Airworthiness and Operations Action Plan submitted by

Alaska Airlines, and its authority to conduct heavy maintenance was not suspended.

1.17.3.4.2 Alaska Airlines Airworthiness and Operations Action Plan

Alaska Airlinesí June 29, 2000, Airworthiness and Operations Action Plan stated

that the airline had ìinstituted both interim and long-term measures that will ensure that its

maintenance operations meet or exceed all Federal Aviation Regulations and that all

aircraft released from heavy maintenance are safe and airworthy with all maintenance

properly documented.î The action plan stated that the airline had ìinitiated a

top-to-bottom review of all operationsî and was developing a continuing analysis and

surveillance program ìthat will not only detect and correct any deficiencies in Alaskaís

maintenance program, but will constantly improve the program through a process of data

collection, analysis and corresponding changes.î166

The action plan stated that the Alaska Airlines maintenance training program had

been reorganized to develop a ìtraining profile for all maintenance personnel by areaî and

that OJT would be ìstandardized and formalized.î The action plan stated that Alaska

Airlines would take the following actions regarding its maintenance program:

audit 100 percent of the maintenance documents produced during a heavy

maintenance check, for every airplane, at every location, whether accomplished in

house or by an approved vendor. We also conduct random audits on every airplane

during each heavy check. We analyze the findings to validate the new heavy

maintenance procedures. If the analysis indicates a problem with a new procedure,

it is corrected immediatelyÖheavy maintenance procedures contained in our

GMM have been completely revised utilizing a meticulous process implemented

by Alaska Airlines personnel in conjunction with highly-qualified outside

consultants under the supervision of the FAA.

Further, the action plan stated that Alaska Airlines had ìcreated an executive level

safety position, vice president of safety, that reports directly to our CEO.î In addition, the

plan stated that Alaska Airlines had created more than 130 new positions in its

maintenance and engineering division, 28 new positions in its flight operations division,

and 11 new positions in its safety division.

The action plan also stated that new procedures and policies had been

implemented to improve monitoring of contract maintenance vendors and ìto ensure that

all heavy maintenance performed at a vendor meets or exceeds requirements of applicable

166 A Department of Transportation (DOT) Office of the Inspector General report on FAA oversight of

continuing analysis and surveillance programs, dated December 12, 2001, concluded that, if the continuing

analysis and surveillance program ìhad been operating effectively, Alaska Airlinesí own internal monitoring

process should have identified the deficiencies in its maintenance program.î The FAA acknowledged the

findings in the DOT report. For more information about the DOT report, see section 1.17.3.4.4. 
Factual Information 100 Aircraft Accident Report

FARs and the Alaska maintenance program.î The plan added that a policies and

procedures manual was developed for heavy maintenance vendors to ìensure

consistencyÖand consideration of industry best practices. This manual provides the basis

for vendor representative accountability, consistency of policies and training of new

vendor representatives.î 

Finally, the action plan stated that Alaska Airlines had developed an internal audit

program ìto provide oversight of all operating departments within Alaska AirlinesÖ[and]

to develop an effective process to validate major changes to process or procedures at

Alaska Airlines.î The plan stated that this internal audit would be ìmandatory for flight

operations, in-flight services, maintenance and engineering and airport operations.î

1.17.3.4.3 FAA Followup Evaluation 

 During the week of September 18, 2000, the FAA conducted a focused followup

evaluation of Alaska Airlinesí SEA and LAX maintenance facilities and a contract

maintenance facility in Everett, Washington.

According to a September 22, 2000, FAA memorandum, the purpose of the

evaluation was to ìassist the [FAAís] Alaska Airlines Certificate Management Office

(CMO) in completing an assessment of the carrierís revised heavy maintenance process,

nonroutine work cards (MIG-4), vendor maintenance process, quality assurance program,

and continuing analysis and surveillance.î The memorandum added that ìduring the

second day of this evaluation it was determined that the new programs were not

completely implemented and the scope of the evaluation would be limited to a ëprogress

check.íî 

The September 22, 2000, memorandum listed procedural discrepancies and several

recurring findings, including the following:

Alaska Airlines does record and transmit information and instructions to their

counterparts for shift turnovers as required by the GMM. Two Alaska Airlines

forms titled, Priority Items and Engineering Report, are being utilized during shift

turnovers. Alaska Airlines has neither identified the aforementioned forms [n]or

implemented procedures in their [GMM] for the use of the forms.

Not all of the quality assurance (QA) auditors have been trained on the new

[continuing analysis and surveillance] procedures, and the company personnel

that were explaining the new [continuing analysis and surveillance] procedures to

us [the evaluation team] did not appear to understand their own process of

qualifying and authorizing QA auditors.

There is no formal course or syllabus for [continuing analysis and surveillance]

auditor training. The training program consists of only informal instruction

accomplished during OJT. There are no checklists or guides being used to

standardize the OJT. There are no controls or testing requirements in place to

determine how much the students were comprehending and retaining. 
Factual Information 101 Aircraft Accident Report

The September 22, 2000, memorandum also noted that Alaska Airlines appeared

not to have ìimplemented procedures for documenting the qualifications and

authorizations of QA auditorsî and that ìthere was a lack of standardization between

facilities, both internal and external, on following the instructions for reference material

attached to the MIG-4.î The memorandum concluded that ìthese [finding] areas should be

re-visited after the manuals have been re-written, the programs have been fully

implemented, and all personnel have been trained. This is projected to be in

January 2001.î 

A September 25, 2000, FAA memorandum summarized an inspection team

evaluation of Alaska Airlinesí maintenance reliability program document, which was the

basis of the airlineís reliability analysis program.167 The memorandum listed irregularities

concerning the document, which included that the document did not

ï define all of the aircraft components controlled by the program,

ï identify the individual responsible for analyzing trend-related information,

ï address the individuals responsible for conducting further analysis for

corrective action of program deficiencies or negative trends as identified by the

programís analytical process,

ï contain a specific method to follow so that data are routed to the proper

organizational element for review,

ï contain a method for ensuring that changes in operating flight hours or cycles

as appropriate to the trend level of reliability experienced occurs,

ï contain a procedure to assign the appropriate personnel to find the cause of all

identified areas that exceed performance standards,

ï contain a method for establishing extension limits, and

ï contain a method for determining the analysis of nonroutine maintenance work

forms as a significant analytical factor inÖmaintenance program

effectiveness.

The September 25, 2000, memorandum concluded the following:

Of concern is that the maintenance reliability program [document] adequately

depicts an accurate portrait of the maintenance programís total effectiveness in a

single source document. And further, that changes to the maintenance as identified

through the reliability program analysis, are reviewed, controlled, and FAA

CMO-approved as warranted. The validity of a maintenance reliability program,

as reflected in Operations Specifications, does not imply carte blanche to an

operator for maintenance program changes without sufficient analytical data

presented as justification. The analytical data should accompany any notice or

request for a maintenance program change. Sufficient FAA guidance in

publications exists in support of this assertion.

167 For a description of Alaska Airlinesí reliability analysis program, see section 1.6.3.1.3.
Factual Information 102 Aircraft Accident Report

In July 2001, an FAA 12-member panel again reviewed Alaska Airlinesí progress

in implementing its action plan.168 According to an FAA briefing document on the review,

the review consisted of presentations by Alaska Airlines representatives, a visit by some

of the panel members to Alaska Airlinesí SEA maintenance facility, and discussions about

Alaska Airlinesí progress. According to the briefing document, Alaska Airlines

ìconvincingly demonstrated [its] completion of all initiatives and commitments set forth

in their action plan.î The briefing document indicated that the panel members who visited

the SEA maintenance facility found that, ìwith one exception, Alaska Airlines technicians

[had] an acceptable working knowledge of the automated General Procedures Manual.î

Further, the briefing document stated that the panel had ìreached a consensus that Alaska

Airlines met or exceeded all commitments set forth in their action plan...[and that the

airline had demonstrated] strong and visible leadership from the top executivesÖfor

safety, compliance and a strong and healthy safety culture.î

The FAA briefing document stated that Alaska Airlines had added

341 maintenance positions, enhanced its internal evaluation program, created a Board of

Directorsí Safety Committee ìthat interfaces with the director of safety,î improved

General Procedures Manual written procedures, and revised ìflight operations procedures

and corresponding training that address critical operational control issues and the interface

between maintenance and flight operations with emphasis on abnormal and emergency

procedures.î The briefing document concluded that Alaska Airlines had

ìdemonstratedÖthat previously identified systemic deficiencies have been corrected and

verified as correctedî and that the FAA had verified ìsignificant changes and

improvements at Alaska.î The briefing document was signed by the manager of the FAAís

Northwest Mountain Region Flight Standards Division.

1.17.3.4.4 DOT Office of the Inspector General Report on FAA Oversight of 

Continuing Analysis and Surveillance Programs

In its December 12, 2001, report on FAA oversight of continuing analysis and

surveillance programs, the DOT Office of the Inspector General stated that the findings of

the FAAís postaccident inspection of Alaska Airlines ìraised questions as to why the

FAAís routine surveillance had not identified the deficiencies in Alaska Airlinesí

[continuing analysis and surveillance program] and ensured that they were corrected.î The

DOT report stated that the FAA ìneeds to place greater emphasis on [continuing analysis

and surveillance program] oversightî and must ìensure [that continuing analysis and

surveillance program] deficiencies identified through its oversight inspections are

corrected.î

168 Four of the panel members were participants in the April 2000 special inspection.
Factual Information 103 Aircraft Accident Report

The DOT report added the following:

The lack of effective oversight of air carriersí [continuing analysis and

surveillance programs] perpetuates a system where [the] FAA relies on its own

inspections to ensure carriers maintain their aircraft in an airworthy condition.

This system is ineffective because the FAA does not have sufficient resources to

physically inspect every aircraft. While it is clearly the responsibility of air

carriers to ensure day-to-day safe operation and maintenance of their aircraft,

[the] FAA must be more proactive in identifying deficiencies in air carriersí

[continuing analysis and surveillance programs].

The DOT report also noted that to properly evaluate a maintenance program, an

airline should have the following:

procedures in its [continuing analysis and surveillance program] to assess aircraft

mechanical performance. For example, the carrier should review data such as

engine removal rates and pilot reports of mechanical disruptions to identify

negative trends or premature failures. Mechanical monitoring programs can help

carriers maintain reliable aircraft operating rates by identifying [the] cause of

maintenance-related delays and cancellations. More importantly, a properly

designed and utilized [continuing analysis and surveillance program] establishes a

culture of safety with an airlineís operations.

The DOT report concluded that FAA inspectors ìneed to better document their

[continuing analysis and surveillance program] inspectionsî and ìbetter train its

inspectors to evaluate carriersí [continuing analysis and surveillance programs] for

systemic weaknesses.î

1.17.4 Other Safety Evaluations of Alaska Airlines 

1.17.4.1 DoD Capability Survey of Alaska Airlines

In September 1998, the DoDís Air Carrier Survey and Analysis Office conducted a

biennial capability survey of Alaska Airlinesí operations.169 The DoD survey report stated

the following:

The commendable practices we observed during the survey were Alaska Airlinesí

overall training program to include the excellent crew coordination exhibited

during DoD cockpit observations. Operations management conducts ongoing

internal audits that include effective cross-divisional audits. Alaskaís safety

program also exceeds standards with a highly motivated staff and exceptional

projects like their flight operational quality assuranceÖprogram.

Regarding operations management, the DoD survey report stated that Alaska

Airlines had a ìwell-defined organizational structure, with staffing levels commensurate

169 DoD capability surveys of air carriers are part of a routine approval process for participation in the

DoDís air transportation program. 
Factual Information 104 Aircraft Accident Report

for their scope of operationsî and ìvery experienced personnel in key positions with

minimal management turnover.î Regarding maintenance management, the survey report

stated that Alaska Airlines management provided ìstrong oversight of maintenanceî and

that ìcompany structure and lines of authority [were] satisfactorily defined.î The report

added that ìexcellent communications exist[ed] with both operations and mechanics.î

The DoD survey report stated that Alaska Airlines had a ìstrong safety program

that is responsive to employee inputsî and that ìinteraction between safety, functional

managers, and unions occurs frequently through formal meetings, written

communications, and informal discussion.î The survey report further stated that Alaska

Airlines ì[continuing analysis and surveillance] program provides strong oversight of all

maintenance activitiesî and that a formal internal audit program provided ìexcellent

supervision of the companyís internal maintenance functions.î The report noted that

ìduring summer 1998, [the] carrier experienced difficulty accomplishing some heavy

checks on time due to high summer operations tempo and fluctuating heavy check flow.î

The report concluded that the Alaska Airlinesí operations and maintenance management

and operations safety program and training ìexceed [DoD] standards.î

1.17.4.2 Criminal Investigation of Alaska Airlines

In October 1998, an Alaska Airlines senior lead mechanic alleged to the FAA that

maintenance records had been falsified at Alaska Airlinesí OAK maintenance facility. His

statements triggered a 3-year criminal investigation into the allegations by a Federal grand

jury. In December 2001, Federal prosecutors stated that no charges would be filed against

Alaska Airlines as a result of the investigation.

1.17.4.3 Postaccident Independent Safety Assessment of Alaska Airlines

After the accident, Alaska Airlines hired a consulting firm to conduct an

independent safety assessment of the airline. The firm assembled a 13-member team of

subject matter experts in flight operations, hazardous materials handling, security, and

maintenance and engineering. These areas included organizational structure and training,

flight dispatch, crew scheduling, station and ramp operations, cabin safety, maintenance

quality assurance programs, maintenance records systems, mechanical reliability

reporting, and maintenance contracting. The assessment teamís inspections of Alaska

Airlines were conducted from April 10 to May 19, 2000.

The final safety assessment report, dated June 19, 2000, stated that the purpose of

the assessment was to examine ìthe prevailing operational situation from a safety

perspective (i.e., the ësafety cultureí) and to identify areas where Alaska Airlinesí overall

operational safety and efficiencies could be enhanced or improved through strengthening

or modifying existing policies, procedures or practices, or through new initiatives.î The

report added that the assessment focused on ìexamination of safety issues from the

standpoint of organizational functionality and management oversight and the evaluation of

Alaska Airlinesí internal policies, procedures and practices.î
Factual Information 105 Aircraft Accident Report

The safety assessment team made more than 170 recommendations that it stated

were intended to ìguide both strategic planning and tactical action by Alaska Airlines as

they move to strengthen the foundation of an effective safety culture that anticipates the

potential for problems and acts swiftly and effectively to mitigate such threats.î The

recommendations suggested, in part, that Alaska Airlines develop clearer lines of

managerial authority and responsibilities; increase staffing; enhance standardization of

operating and training manuals; revise its GMM to clarify and update policies,

responsibilities, and procedures; increase maintenance training; and create a new

corporate safety department that reports directly to the highest levels of the airline. The

report also recommended that Alaska Airlines appoint a vice president of corporate safety

to manage the department.

1.18 Additional Information

1.18.1 Postaccident Airworthiness Directives

On February 11, 2000, as a result of information gathered immediately after the

flight 261 accident, the FAA issued AD 2000-03-51 to all operators of DC-9, MD-90-30,

MD-88, and 717-200 series airplanes. AD 2000-03-51 stated the following:

[after the Alaska Airlines crash, the] FAAÖreceived a report from an operator

that indicated two instances of metallic shavings in the vicinity of the jackscrew

assembly and gimbal [acme] nut of the horizontal stabilizer. Metallic shavings in

the vicinity of the horizontal stabilizer indicate excessive wear of the jackscrew

assembly. Such excessive wear, if not corrected, could result in possible loss of

pitch trim capability, which could result in loss of vertical control of the airplane.

AD 2000-03-51 required operators to ìperform a general visual inspection of the

lubricating grease on the jackscrew assembly and the area directly below the jackscrew

and surrounding areas for the presence of metal shavings and flakesî and to replace the

assembly before further flight if shavings or flakes were found.170 The AD also required

inspection and lubrication of the jackscrew assembly, ìprior to the accumulation of

650 hours total time-in-serviceÖor within 72 hoursî after receipt of the AD and were to

be repeated ìat intervals not to exceed 650 flight hours.î 

In addition, AD 2000-03-51 required operators to ìperform a general visual

inspection of the jackscrew assembly to detect the presence of corrosion, pitting or

distressî and to replace the assembly if these conditions were found. Further, the AD

required an inspection of the lubrication of the jackscrew assembly, an inspection of the

acme screw and nut upper and lower mechanical stops, testing of the horizontal stabilizer

shutoff control, and the performance of end play checks ìwithin 2,000 flight hours since

170 On February 11, 2000, Boeing issued ASBs for DC-9, MD-90-30, MD-88, and 717-200 series

airplanes that described the ìprocedures for inspecting the general condition of the jackscrew assembly and

the area around the jackscrew assembly to detect the presence of metal shavings and flakes.î
Factual Information 106 Aircraft Accident Report

the last acme screw and nut wear checkî and, thereafter, to be repeated at intervals not to

exceed 2,000 flight hours.

On July 28, 2000, the FAA issued AD 2000-15-15, which superseded

AD 2000-03-51. AD 2000-15-15, which had an effective date of August 23, 2000,

included the same requirements as AD 2000-03-15, but AD 2000-15-15 also required

operators to look for the presence of metallic particles in the jackscrew assembly

lubrication. The AD stated the following:

Since the issuance of AD 2000-03-51, the FAA has received numerous reports of

incidents in which metallic particles (including slivers and dust, as well as

shavings and flakes) were found [e]mbedded within the grease on the threaded

portion of the jackscrew assembly of the horizontal stabilizer actuator and on the

area directly below the jackscrew assembly. Findings by the manufacturer indicate

that such metallic particles can be identified as a non-magnetic metallic substance

which is golden in color...in consideration of new findings by the manufacturer

regarding the types of material found in the jackscrew assembly of the horizontal

stabilizer since issuance of AD 2000-03-51, the FAA has determined that the

required inspections should be expanded to include metallic particles such as

slivers and dust, as well as the metal shavings and flakes identified in

AD 2000-03-51. 

AD 2000-15-15 concluded that ìthe FAA has determined that it is necessary for

operators to report the results of the end play checksî performed at the 2,000-flight-hour

intervals prescribed in AD 2000-03-51 ìto provide information regarding the wear rates of

the jackscrew assembly.î The AD stated that the FAA would use this reported end play

data ìto confirm that the repetitive intervals of 650 flight hours [for the general visual

inspection of the jackscrew assembly and lubrication]Öand the repetitive intervals of

2,000 flight hours [for the end play checks]Öare appropriate compliance times for

accomplishment of the end play check and are adequate for ensuring the safety of the

fleet.î

1.18.1.1 Safety Board and FAA Correspondence Regarding the 2,000-Flight 

-Hour End Play Check Interval Specified in ADs 2000-03-51 and 2000-15-15

In a May 14, 2001, letter to the FAA, the Safety Board indicated that the results of

a February 17, 2000, end play check on a Hawaiian Airlines DC-9,171 which yielded an

end play measurement of 0.043 inch after having accumulated only about 2,500 flight

hours, called into question the adequacy of the 2,000-flight-hour end play check interval

required by AD 2000-15-15. The Board stated that, because of the wear observed on the

Hawaiian Airlines jackscrew assembly and the potential that other assemblies with

higher-than-normal wear rates are in service, it was concerned that the 2,000-flight-hour

end play check interval might not be adequate to ensure the safety of the fleet affected by

AD 2000-15-15. The Board requested any additional information regarding the

background data and analysis that the FAA used to establish the 2,000-flight-hour end

171 For more information about the Hawaiian Airlines DC-9 jackscrew assembly, see section 1.16.6.1.
Factual Information 107 Aircraft Accident Report

play check interval identified in ADs 2000-03-51 and 2000-15-15 and any final or

terminating actions it had identified in this case.

In a June 26, 2001, letter, the FAA responded that Boeing initially recommended

the 2,000-flight-hour end play check interval in ASB DC-9-27A362. The FAA stated that,

while evaluating this recommendation, it had numerous discussions with Boeing

regarding the acceptability of the 2,000-flight-hour interval. The FAA added the

following:

These discussions focused on the robust design of the acme nut, structural test

data, structural analysis, existing wear rate data, and the service history of the

fleet. The original Boeing wear rate data showed a wear rate of 0.001 inch per

1,000 flight hours when the jackscrew was lubricated at 400-hour intervals. The

Boeing data was supplemented with recent wear rate information provided to

Boeing by one of the largest MD-80 operators. These new data showed a wear rate

which closely correlated with the existing Boeing wear rate but was obtained from

aircraft with jackscrews that were being lubricated at approximately

940-flight-hour intervalsÖanalysis showed that a jackscrew worn beyond

0.080-inch end play could still safely carry limit load. Based on FAA review of

the analytical data and on the discussions with Boeing, we concluded that

Boeingís recommendation for endplay checks at 2,000-flight-hour intervals

supported by lubrication and inspections at 650-flight-hour intervals was

sufficient to produce predictable wear rates and detect worn jackscrew assemblies

well before their structural capability would be compromised.

In response to the Safety Boardís concern that the 2,000-flight-hour end play check

interval would be inadequate to identify a jackscrew assembly that is wearing at a

higher-than-normal rate, the FAA stated, in part, the following: 

The FAA is also concerned about jackscrews found with excessive wear rates

because these are seen as indicators not only of possible improper lubrication or

contaminated grease but also of a potential design problem. With one

exceptionÖthe current endplay check intervals have shown to be adequate to

detect jackscrews with accelerated wear ratesÖ.If [the Hawaiian Airlines]

jackscrew were to have remained in serviceÖand if it continued to wear at the

same rate, the lubrication and inspections requirements of AD 2000-15-15 would

have provided at least two opportunities for maintenance personnel to detect

excessive wear before the next required endplay check. This is because

jackscrews wearing at an excessive rate will produce an excessive amount of

debris. The AD requires an inspection for debris at each lubrication interval and, if

found, a requirement to perform an endplay check. Were the excessive wear not

detected before the next endplay check, the jackscrew assembly with the amount

of endplay identified in your letter...would retain sufficient structural capability to

safely support limit load. 

In response to the Safety Boardís query about whether any terminating actions had

been identified on this issue, the FAA stated, in part, the following:
Factual Information 108 Aircraft Accident Report

The FAA has had discussions with Boeing regarding the effectiveness of the

existing inspections and the appropriateness of design changes to the jackscrew

assembly. To date, these discussions have yielded no firm conclusions that a

design change is either appropriate or warranted.

1.18.2 Safety Board Statistical End Play Data Study

The Safety Board statistically examined the end play data collected as a result of

ADs 2000-03-51 and 2000-15-15. The Board received a total of 3,174 end play

measurements from 1,493 airplanes at 44 operators. 

Because end play was measured at repeated intervals for individual jackscrew

assemblies, it was possible to estimate whether the end play measurement was reliable.172

According to Boeing estimates, very little change in end play was expected to occur over

the 2,000-flight-hour interval between checks because end play was expected to increase

at a rate of about 0.001 inch per 1,000 flight hours. Because this expected change was

small and unidirectional, the Safety Board used the test-retest method to estimate

measurement reliability.173

End play measurements reported on three consecutive occasions174 for the same

airplanes were compared using a correlation analysis that measures the strength of the

relationship between two sets of numbers and produces a value known as the correlation

coefficient (represented in notation as ìrî). The correlation coefficient ranges between

±1.0.175 The correlation coefficient for the first and second measurements was +0.553 and

+0.416 for the second and third measurements. The coefficients of determination,

(represented in notation as ìR2î), representing shared, or common, variance, were

0.306 for the first and second measurements and 0.173 for the second and third

measurements, indicating low measurement reliability.176

In addition to analyzing on-wing end play measurements collected as a result of

ADs 2000-03-51 and 2000-15-15, a second portion of the end play data study focused on

172 Reliability refers to the ability of a measurement tool to yield reproducible results. 

173 Measurement reliability is often assessed using a test-retest method in which two consecutive

measurements are recorded and, if the entity being measured has not changed (or has changed in a consistent

manner) during the measurement interval, subsequent correlation of the two measurements should reveal a

strong relationship. The absence of such a correlation suggests that other variables, such as measurement

error, may have caused variability in the observed measurement.

174 After screening the data for errors (missing data, duplicate cases, multiple measurement conventions,

incorrect flight hours, and incorrect fuselage numbers), the resulting set contained 1,388 cases at the first

measurement; 852 cases at the second measurement; and 482 cases at the third measurement. 

175 A coefficient of +1.0, known as a perfect positive correlation, means that in each case changes in one

measurement resulted in an identical change in the other measurement. A coefficient of -1.0, known as a

perfect negative correlation, means that changes in one measurement resulted in an identical change in the

other measurement but that the change was in the opposite direction. A coefficient of 0 means that no

relationship existed between the two measurements and that changes in one measurement had no effect on

the second measurement.

176 Two alternative test-retest analyses that accounted for varying time intervals between checks for

individual aircraft were performed and yielded similar results.
Factual Information 109 Aircraft Accident Report

an examination of a small subset of cases in which both on-wing and bench-check end

play measurement data were available. Boeing collected and reported data on jackscrew

assemblies that were returned to Integrated Aerospace during 2000. Integrated Aerospace

reported that 157 jackscrew assemblies were returned for overhaul during the 2000

calendar year, a marked increase over those received in previous years,177 and likely

affected by the AD requiring end play checks for the entire fleet of DC-9, MD-90-30,

MD-88, and 717-200 series airplanes. Of the 157 jackscrew assemblies, 49 were removed

because of end play measurements that exceeded operatorsí criteria for removal. Other

reasons for removal included the presence of metal flakes, excessive free play, or damage

to the assembly. 

Jackscrew assemblies returned to Integrated Aerospace are cleaned and then bench

end play checks are conducted. Because some of the conditions that can create variation in

the end play measurement are controlled during bench checks, they are expected to yield

more accurate end play measurements than on-wing checks conducted by maintenance

personnel. Of the 157 jackscrew assemblies removed in 2000, bench end play checks for

142 of the assemblies resulted in end play measurements that fell within in-service end

play tolerances (between 0.003 and 0.040 inch). Twelve assemblies had bench-check end

play measurements greater than 0.040 inch. No bench-check end play measurements were

reported for three assemblies. Boeing contacted operators and obtained a matched sample

(both on-wing and bench-check end play measurements) for 64 jackscrew assemblies. 

After correcting documentation errors, the Safety Board calculated the difference

in end play measurements between the on-wing and the bench-check settings. In 45 of the

64 cases (70.3 percent), the on-wing end play measurements were higher than the

bench-check measurements; in 6 of the 64 cases (9.4 percent), the on-wing and

bench-check end play measurements were equal; and in 13 of the 64 cases (20.3 percent),

the on-wing end play measurements were lower than the bench-check measurements. 

 After removing one outlier,178 the validity of the on-wing end play measurement

was approximated using a correlation analysis between the on-wing and bench-check end

play measurements.179 The analysis determined that the resulting correlation coefficient

was +0.442 (R2

=0.195), suggesting low measurement validity. The Safety Board end play

data study report, dated March 18, 2002, concluded, ìin the absence of additional

information such as the rate of acme nut wear and the thread thickness at which failure

may occur, the observed level of measurement error raises doubts about the utility of the

existing end play measurement procedure.î 

177 According to Integrated Aerospace records, 59 jackscrew assemblies were returned in 1999, 54 in

1998, 59 in 1997, and 42 in 1996.

178 An outlier is an extreme measurement that is not representative of the rest of the data set.

179 This method assumed that the bench-check end play measurement was a criterion, or a standard, that

represented the true end play in the jackscrew assembly. If the on-wing end play measurement is valid

(representing the actual state of end play), then the correlation coefficient representing the relationship

between the on-wing and bench-check end play measurements should be close to +1.0. 
Factual Information 110 Aircraft Accident Report

1.18.3 Previous Safety Recommendations Resulting from the 

Alaska Airlines Flight 261 Investigation 

On October 1, 2001, the Safety Board issued Safety Recommendations A-01-41

through -48. In the safety recommendation letter, the Board expressed concern that

lubrication procedures for the acme screw and nut were not sufficient to ensure thorough

lubrications and that end play check procedures were not adequate to ensure accurate

measurements of acme screw and nut wear. In addition, the Board expressed concern that

use of an inappropriate grease type or a mixture of incompatible grease types could cause

adverse effects and that technical information and practical experience about aviation

lubrication was not being shared by the aviation industry. 

Safety Recommendation A-01-41 asked the FAA to do the following:

Require the Boeing Commercial Airplane Group to revise the lubrication

procedure for the horizontal stabilizer trim system of Douglas DC-9, McDonnell

Douglas MD-80/90, and Boeing 717 series airplanes to minimize the probability

of inadequate lubrication.

In a December 12, 2001, response letter, the FAA stated that it agreed with the

intent of Safety Recommendation A-01-41 and that it was ìworking with the Boeing

Commercial Airplane Group to rewrite the lubrication procedures to the optimal

standard.î The FAA also stated that it was ìevaluating the necessity of an additional task

to perform a detailed inspection of the lubricant at a ëCí check interval to check for

indications of metal shavings.î

On June 14, 2002, the Safety Board classified Safety Recommendation A-01-41

ìOpenóAcceptable Response,î pending the revisions to the lubrication procedure for the

horizontal stabilizer trim system of DC-9, MD-80/90, and 717 series airplanes. On

July 25, 2002, Boeing representatives provided the Board with a briefing on new

lubrication procedures currently being developed and tested.

Safety Recommendation A-01-42 asked the FAA to do the following:

Require the Boeing Commercial Airplane Group to revise the end play check

procedure for the horizontal stabilizer trim system of Douglas DC-9, McDonnell

Douglas MD-80/90, and Boeing 717 series airplanes to minimize the probability

of measurement error and conduct a study to empirically validate the revised

procedure against an appropriate physical standard of actual acme screw and acme

nut wear. This study should also establish that the procedure produces a

measurement that is reliable when conducted on-wing.

In its December 2001 response letter, the FAA stated that it agreed with the intent

of Safety Recommendation A-01-42 and that it had asked Boeing to ìrevise the end play

check procedureÖto minimize the probability of measurement error.î The FAA further

stated that it had asked Boeing ìto conduct a study to validate the revised procedure

empirically against an appropriate physical standard of actual acme screw and acme nut
Factual Information 111 Aircraft Accident Report

wear. The study will also establish procedures that produce a measurement that is reliable

when conducted on-wing.î

On June 14, 2002, the Safety Board classified Safety Recommendation A-01-42

ìOpenóAcceptable Response,î pending the revisions to the end play check procedure

and the validation of the revised procedure. On July 25, 2002, Boeing representatives

provided the Board with a briefing on revised end play check procedures currently being

developed and tested.

Safety Recommendation A-01-43 asked the FAA to do the following:

Require maintenance personnel who lubricate the horizontal stabilizer trim system

of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series

airplanes to undergo specialized training for this task.

Safety Recommendation A-01-44 asked the FAA to do the following:

Require maintenance personnel who inspect the horizontal stabilizer trim system

of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series

airplanes to undergo specialized training for this task. This training should include

familiarization with the selection, inspection, and proper use of the tooling to

perform the end play check.

In its December 2001 response letter, the FAA stated that it believed that ìcurrent

regulatory requirements of 14 CFR 121.375 adequately address maintenance training

programs.î The FAA added that the agency was ìrevising [AC] 120-16C, Continuous

Airworthiness Maintenance Programs, to expand the intent of the requirement for

maintenance training programs.î The FAA stated that the AC revisions would address

basic training, initial training, recurrent training, specialized training, vendor training, and

competence-based training. In addition, the FAA asserted that ìcurrent regulatory

requirements of 14 CFR 65.81 and 14 CFR 65.103 adequately address the inspection

requirements for maintenance personnel and repairmen.î The FAA concluded that

ìcurrent regulatory requirements in place to require maintenance training programs and

inspection requirements for maintenance personnel and repairmen address the full intent

of these safety recommendations.î

In its June 14, 2002, response letter to the FAA, the Safety Board stated that it

ìcontinues to believe that current FAA regulations do not result in maintenance personnel

being properly trained to perform lubrication and end play inspection of the acme screw

and nut assemblyî and urged the FAA to reconsider its position. The Board classified

Safety Recommendations A-01-43 and -44 ìOpenóUnacceptable Response,î pending

accomplishment of the recommended actions. 
Factual Information 112 Aircraft Accident Report

Safety Recommendation A-01-45 asked the FAA to do the following:

Before the implementation of any proposed changes in allowable lubrication

applications for critical aircraft systems, require operators to supply to the FAA

technical data (including performance information and test results) demonstrating

that the proposed changes will not present any potential hazards and obtain

approval of the proposed changes from the principal maintenance inspector and

concurrence from the FAA applicable [Aircraft Certification Office].

Safety Recommendation A-01-46 asked the FAA to do the following: 

Issue guidance to principal maintenance inspectors to notify all operators about

the potential hazards of using inappropriate grease types and mixing incompatible

grease types. 

In its December 2001 response letter, the FAA stated that it was ìissuing a flight

standards information bulletin to address these safety recommendations.î The FAA added

that the bulletin would do the following:

provide inspectors with guidance on processing an operatorís proposed

substitution of a lubricant or a lubrication application method change. The bulletin

will direct aviation safety inspectors to request that the operator supply technical

data on the substitute lubricant or [lubrication application method change] that

affects critical aircraft systems and include performance information, along with

test results, demonstrating that the proposed change will not present any potential

hazard. The bulletin will also direct aviation safety inspectors to have the analysis

test data reviewed by the appropriate FAA Aircraft Certification Office to

determine if the substitute lubricant meets the required specifications.

In addition, the FAA stated that it was ìincorporating a revision in Order 8300.10,

Airworthiness Inspectorís Handbook, to state that if there is any doubt as to the soundness

of the request, the aviation safety inspector should coordinate the request with the

appropriate Aircraft Certification Office.î In an April 2, 2002, letter, the FAA further

responded to Safety Recommendations A-01-45 and -46, stating that, on February 15,

2002, it had issued Flight Standards Information Bulletin for Airworthiness

(FSAW) 02-02, ìThe Potential Adverse Effects of Grease Substitutionî and revised Order

8300.10 per its previous letter. 

On July 29, 2002, the Safety Board classified Safety Recommendations A-01-45

and -46 ìClosedóAcceptable Actionî based on the issuance of FSAW 02-02 and the

revisions to Order 8300.10.

Safety Recommendation A-01-47 asked the FAA to do the following:

Survey all operators to identify any lubrication practices that deviate from those

specified in the manufacturerís airplane maintenance manual, determine whether

any of those deviations involve the current use of inappropriate grease types or

incompatible grease mixtures on critical aircraft systems and, if so, eliminate the

use of any such inappropriate grease types or incompatible mixtures.
Factual Information 113 Aircraft Accident Report

Safety Recommendation A-01-48 asked the FAA to do the following:

Within the next 120 days, convene an industrywide forum to disseminate

information about and discuss issues pertaining to the lubrication of aircraft

components, including the qualification, selection, application methods,

performance, inspection, testing, and incompatibility of grease types used on

aircraft components. 

In its December 2001 response letter, the FAA stated that it agreed with the intent

of Safety Recommendations A-01-47 and -48 and had scheduled an FAA/industry forum

that would include ìrepresentatives from domestic and international airlines [and] will

address the issues raised in these safety recommendations.î The FAA/industry forum was

held in January 2002.

In its June 14, 2002, response letter to the FAA, the Safety Board requested that the

FAA provide copies of any proceedings or recommendations that resulted from the

FAA/industry forum.180 The Board also requested that the FAA indicate whether it was

able to survey all operatorsí lubrication practices, as called for in Safety Recommendation

A-01-47, and to describe any long-term actions planned as a result of the forum. Pending

receipt of the requested information, the Board classified Safety Recommendations

A-01-47 and -48 ìOpenóAcceptable Response.î 

1.18.4 Alaska Airlines Fleetwide MD-80 Jackscrew Assembly 

Data and Tracking History

Alaska Airlines and Boeing provided data to the Safety Board so that it could

produce a complete inventory of all jackscrew assemblies used in the Alaska Airlines

fleet. The data included the tail numbers of the airplanes in which the jackscrew

assemblies were installed, information about whether the airplanes were original airline

airplanes, the date that the airplanes were delivered to Alaska Airlines, the total flight

hours on the jackscrew assemblies, the bench-check end play measurements for the

jackscrew assemblies,181 the date of removal and replacement of the jackscrew assemblies,

and the computed wear rate of the jackscrew assemblies. 

The data indicated that three jackscrew assemblies, including the accident

assembly and the assemblies from N981AS and N982AS, had excessive wear

rates. According to maintenance records and information provided by Alaska Airlines, the

180 FSAW 02-02C, ìThe Potential Adverse Effects of Grease Substitution,î (amended on May 22, 2002)

stated that presentations at the FAA/industry forum indicated that ìthere have been no equipment failures

attributed to changing greases when lubrication practices (lubrication type, frequency, and technique) are per

the original equipment manufacturerís recommendations; and each lubrication point is properly purgedî and

that ìwhen properly purged, the resulting intermixing of greases does not pose an airworthiness concern.

However, the lack of lubrication can result in damage to mechanisms that require lubrication for normal

operation.î

181 Most of the end play measurements were taken during bench checks after the jackscrew assemblies

were removed. If no bench-check end play measurements could be obtained, the measurement from the most

recent on-wing end play check was used. 
Factual Information 114 Aircraft Accident Report

jackscrew assemblies from N981AS and N982AS received their most recent lubrications

at Alaska Airlinesí maintenance facility at OAK (during C checks) and the lubrications

before that at its maintenance facility at SFO.182

N981AS

On February 9, 2000, metal shavings were found in another Alaska Airlines

MD-83, N981AS, during a fleet inspection in Portland, Oregon. An on-wing end play

check was performed on N981AS, and the end play measurement was 0.055 inch. At the

time of the end play check, the airplane had accumulated 10,201 flight hours.

The jackscrew assembly was removed and sent to the Safety Boardís Materials

Laboratory for examination. Safety Board investigators found metal shavings, some

measuring 1/8 to 1/4 inch long, in the area directly beneath the acme screw and in the

grease on the acme screw. Several semicircular shavings about 2 1/5 inches long were also

found. After the assembly was cleaned, an end play check was performed; the end play

measurement was 0.053 inch in a loaded condition.

N982AS

Also on February 9, 2000, metal shavings were found in an Alaska Airlines

MD-83, N982AS, during a fleet inspection at SEA. Several on-wing end play checks were

performed on N982AS, which had accumulated 9,980 flight hours. At a torque of

275 inch-pounds, the end play measurements were 0.0510, 0.0525, and 0.0525 inch on

three successive attempts. At a torque of 250 inch-pounds, the end play measurement was

0.082 inch on three successive attempts.

The jackscrew assembly was removed and sent to the Safety Boardís Materials

Laboratory for examination. Safety Board investigators found metal shavings, some

measuring 1/8 inch long, in the area directly beneath the acme screw and in the grease on

the acme screw. A semicircular shaving about 2 1/5 inches long was also found. After the

assembly was cleaned, an end play check was performed; the end play measurement was

0.048 inch in a loaded condition. 

The data also indicated that, in 1999, Alaska Airlines had removed and replaced

the jackscrew assemblies from the following MD-80s in its fleet: N947AS, N975AS, and

N951AS.

N947AS

Jackscrew assembly S/N DCA 1935 was removed from N947AS in June 1999 at

Alaska Airlinesí OAK maintenance facility during a 30,000-flight-hour check because of

excessive wear. N947AS was delivered new to Alaska Airlines in December 1990. At the

182 As previously mentioned, the accident jackscrew assembly received its last lubrication at Alaska

Airlinesí SFO maintenance facility. According to Alaska Airlines, nearly all of its MD-80s had received at

least one jackscrew assembly lubrication at its SFO or OAK maintenance facility during the 3 years before

the flight 261 accident.
Factual Information 115 Aircraft Accident Report

time of the jackscrew assembly removal, N947AS had accumulated 27,860 flight hours

(15,906 flight cycles). No end play measurements were recorded. However, a bench end

play check conducted at Trig Aerospace (currently Integrated Aerospace) on February 27,

2000, indicated an end play measurement of 0.029 inch. 

N975AS

Jackscrew assembly S/N DCA 2272 was removed from N975AS on November 17,

1999, at Alaska Airlinesí OAK maintenance facility during a C check because the

assembly had a 0.042-inch end play measurement. N975AS was delivered new to Alaska

Airlines in May 1994. At the time of the jackscrew assembly removal, N975AS had

accumulated 19,960 flight hours (10,786 flight cycles).

Mitchell Aircraft Spares had jackscrew assembly DCA 2272 overhauled by

Aerotron AirPower on January 17, 2000. Assembly DCA 2272 was repurchased by

Alaska Airlines and sent to Trig Aerospace with an order stating that the unit ìneeds

overhaul to bring to serviceable condition.î The assembly was overhauled by Trig

Aerospace on February 27, 2000, according to specifications outlined in the

Boeing/Douglas Overhaul Maintenance Manual 27-41-1, revision 24, dated April 15,

1998. 

N951AS

Jackscrew assembly S/N ìAC951î was removed from N951AS on November 26,

1999, by Aviation Management Systems, Phoenix, Arizona, during a 30,000-flight-hour

check because the assembly had a 0.045-inch end play measurement. Alaska Airlines

purchased N951AS from JetAmerica in October 1987. At the time of the jackscrew

assembly removal, N951AS had accumulated 52,894 flight hours (30,284 flight cycles).

No maintenance history or information about the jackscrew assembly before or after the

acquisition was found.

According to records from a computer-based, statistical component reliability (or

alert) program that Alaska Airlines used to track the purchasing of appliances, parts, and

components, the airline had not replaced a jackscrew assembly on any airplanes in its

MD-80 fleet until 1999.183 The Alaska Airlines director of reliability and maintenance

programs testified at the Safety Boardís public hearing that the program, which began in

1995, tracks components ìat a rate of 1,000 unit hours in service.î 

Alaska Airlinesí reliability analysis program database indicated that the airline had

purchased three replacement jackscrew assemblies in 1999. After replacement of the first

jackscrew assembly in June 1999, Alaska Airlines began tracking jackscrew assemblies as

rotable components.184 No earlier maintenance histories (for example, overhaul status, last

maintenance, and vendor tear down reports) were available for Alaska Airlinesí jackscrew

assemblies. 

183 Alaska Airlines started operating MD-80s in 1985.
Factual Information 116 Aircraft Accident Report

1.18.5 MD-11 Jackscrew Assembly Acme Nut Wear History

In 1995, five operators of McDonnell Douglas MD-11 (MD-11) airplanes reported

excessive wear of acme nuts on these airplanes. Subsequent investigations by McDonnell

Douglas and operators determined that the excessive wear was a result of improper acme

screw thread surface finish.185 On the basis of this evidence, McDonnell Douglas issued

Service Bulletin (SB) MD11-27-067 on July 31, 1997, which provided instructions for

measurement, inspection for excessive wear on the actuator assemblies, and repair and

replacement, if necessary. 

In 1998, another MD-11 operator reported that the acme nut threads on one

jackscrew assembly had completely worn away. Boeing determined that this excessive

wear was also a result of improper acme screw thread surface finish. The trim system was

rendered inoperative by the subsequent failure of the shear pins installed in the drive

mechanism that connects the two acme nuts. As a result of this incident, on November 19,

1999, SB MD11-27-067 was escalated to an ìAlertî status (ASB MD11-27A-067,

revision 5). The summary section of the ASB stated, ìIf not corrected, this condition, in

conjunction with a failure of the opposite side jackscrew assembly, could result in a free

horizontal stabilizer that would cause loss of airplane pitch control.î On July 30, 1998, the

FAA issued an AD that reiterated this concern and required MD-11 operators to comply

with ASB MD11-27A-067.

1.18.6 Safety Board Observations of Jackscrew Assembly 

Lubrications 

 Safety Board investigators observed lubrications of jackscrew assemblies

performed by maintenance personnel from two MD-80 operators and discussed the

lubrication procedure with these and other maintenance personnel from these operators.

Investigators observed that different methods were used by maintenance personnel to

accomplish certain steps in the lubrication procedure, including the manner in which

grease was applied to the acme nut fitting and the acme screw186 and the number of times

the trim system was cycled to distribute the grease immediately after its application. In

addition, Safety Board investigators who attempted to perform the lubrication procedure

noted that, because of the access panelís size, it could be difficult to insert a hand into the

access panel and that after a hand was inserted, it blocked their view of the jackscrew

assembly, thereby requiring them to accomplish the task primarily by ìfeel.î

184 Before its first jackscrew assembly replacement in June 1999, Alaska Airlines tracked the assemblies

as nonstocked items. After the first jackscrew assembly replacement, the assemblies were added to the

airlineís reliability program rotable component statistical alerting system. Rotables are parts and appliances

that are economically repairable and are periodically overhauled to a fully serviceable condition. These parts

and appliances have specific requirements to track time and location at a serialized level. 

185 No evidence of improper surface finish was found on the accident acme screw threads. For details

about the accident acme screw thread surface finish, see section 1.16.2.2.

186The various grease application procedures included the use of a brush, a grease gun nozzle, a glove, a 

bare hand, or a rag. 
Factual Information 117 Aircraft Accident Report

A lubrication test performed under Safety Board supervision determined that,

when only the grease fitting was lubricated, grease went into the top six or seven acme nut

threads before extruding out of the top of the nut. Grease began to extrude out of the top of

the acme nut after seven pumps of the grease gun. Lubricating the acme nut through the

grease fitting did not provide lubrication to all of the acme screw and nut pressure flanks.

Demonstrations187 designed to compare the effectiveness of various methods of

lubricating the acme screw and nut assembly found that a thorough application of grease

onto the entire length of the acme screw, followed by the cycling of the trim several times,

maximized the distribution of lubrication on the acme nut threads. Several of the methods

observed by or reported to Safety Board investigators did not involve either the

application of grease to the entire length of the acme screw or cycling of the trim several

times.

1.18.7 End Play Check Anomalies

1.18.7.1 Safety Board End Play Check Observations

Safety Board investigators observed end play checks performed by maintenance

personnel from several MD-80 operators, including Alaska Airlines (at its OAK

maintenance facility),188 and observed bench end play checks performed by Integrated

Aerospace. Results of hundreds of end play checks submitted to the FAA in accordance

with ADs 2000-03-51 and 2000-15-15 were also reviewed.189

In its October 2001 safety recommendation letter, the Safety Board noted that the

accuracy of the end play results could be affected by deviations in one or more of the

following areas: (1) calibration and interpretation of the dial indicator; (2) installation of

the dial indicator; (3) application and direction of the specified torque to the restraining

fixture;190 (4) calibration of the torque wrench; (5) fabrication, lubrication, and

maintenance of the restraining fixture; (6) rotation of the acme screw within its gearbox

during the procedure;191 and (7) the individual mechanicís knowledge of the procedures. 

187 The demonstrations involved the use of acme nuts manufactured from a transparent plastic, which

allowed a view of the acme nut threads during lubrication and operation of an acme screw being rotated

through the nut under a static load. 

188 In March 2000, Safety Board investigators observed jackscrew assembly end play checks on

N932AS, which was undergoing a 17-day C check. The end play check was performed by two sets of

mechanics and inspectors.

189 As stated previously, AD 2000 15-15 requires all U.S. operators of DC-9, MD-80/90, and 717 series

airplanes to perform end play checks every 2,000 flight hours and report the end play measurements to the

manufacturer after every check. Before the flight 261 accident, operators performed end play checks at

various intervals, all but one of which exceeded 2,000 hours. These intervals were based on each airlineís

maintenance program and were approved by the airlineís PMI. For more information about AD 2000-15-15

and the Safety Boardís statistical end play data study, see sections 1.18.1 and 1.18.2, respectively.

190 Torquing the restraining fixture in the wrong direction results in end play measurements of 0.

191 Rotation of the acme screw can effect the movement of the dial indicator plunger and cause higher

end play readings.
Factual Information 118 Aircraft Accident Report

During some of the end play check observations, Safety Board investigators

evaluated different methods of accomplishing the end play check procedure. Investigators

observed that, when the dial indicator was mounted above the acme nut instead of below it

(as is called for in the current end play check procedure), the dial indicator was easier to

install, its face could be seen more easily (without the need for an inspection mirror, as

was the case when it was installed below the acme nut), and the end play measurement

could be more easily discerned from the movement of the indicator needle.192

Investigators also observed that, when the dial indicator was mounted such that the

plunger contacted the canted surface of the acme nut stop lug at a skewed angle to the

plunger axis, the end play measurement was lower than it was when it contacted a level

surface on the acme nut at a right angle to the plunger axis. On April 13 and November 20,

2000, Boeing issued revisions to the end play check procedure.193

1.18.7.2 Postaccident Alaska Airlines-Reported Near-Zero End Play 

Measurements

In March and April 2002, Alaska Airlines removed two jackscrew assemblies from

MD-80 airplanes in its fleet following near-zero end play measurements. One jackscrew

assembly was removed after a scheduled end play check at Alaska Airlinesí maintenance

facility at OAK, and the other assembly was removed during a scheduled C check at a

contract facility near SEA. Both jackscrew assemblies were removed because of reported

end play measurements of 0.001 inch during all iterations of the end play check.194 Safety

Board investigators examined these jackscrew assemblies and determined that the end

play measurements were actually higher (within normal and expected limits) and that the

0.001-inch end play measurements were caused by procedural errors made by

maintenance personnel while performing the end play check procedure.195 Specifically,

the investigators determined that the maintenance personnel must have torqued the

restraining fixture in the wrong direction before measuring the end play. 

After these two anomalous end play measurements,196 the FAA conducted

followup inspections at the two facilities. According to the FAA inspectors, all of the

restraining fixtures, torque wrenches, and dial indicators used to perform the end play

192 When the dial indicator was mounted below the acme nut, as called for in the current procedure, the

dial indicator plunger moved upward as the acme nut moved, causing the needle to move counter-clockwise,

or opposite the intended direction. This movement required maintenance personnel to read the dial indicator

backward and interpret the total needle movement to obtain the end play measurement. 

193 For more information about Boeingís revisions to the end play check procedure, see section

1.6.3.3.2.1. 

194 The end play check was reiterated about eight times on the jackscrew assembly removed at OAK and

about six times on the jackscrew assembly removed at the contract facility near SEA.

195 A Safety Board examination of the bore of the acme nut from one of the jackscrew assemblies

revealed the presence of green grease near the grease fitting entrance and red grease along the bore in all

other locations. Grease samples taken from the other jackscrew assembly, from various locations along the

acme screw threads immediately above and below the acme nut, exhibited the appearance of a mixture of

green and red grease.

196 Because the manufacturing specifications require a minimum end play of 0.003 inch, the 0.001-inch

end play measurements were considered anomalous.
Factual Information 119 Aircraft Accident Report

checks were the correct tools and properly calibrated. Further, FAA inspectors indicated

that, before these anomalous end play measurements were reported, Alaska Airlines had

provided 4 hours of specialized training to all maintenance personnel who perform end

play checks at these two facilities. The specialized training focused on how to properly

conduct the end play check procedure. 

 According to Boeing, during the 6-month period from December 2001 to

May 2002, six end play measurements of less than 0.004 inch were reported, not including

the two 0.001-inch end play measurements reported by Alaska Airlines.

1.18.8 Jackscrew Assembly Overhaul Information

1.18.8.1 Jackscrew Overhaul Specifications and Authority

Airlines and maintenance facilities that have been issued class 1 accessory ratings

under 14 CFR 145.31(f) can overhaul jackscrew assemblies without additional FAA

authorization. A class 1 accessory rating authorizes the holder to repair and alter

ìmechanical accessories that depend on friction, hydraulics, mechanical linkage, or

pneumatic pressure for operation, including aircraft wheel brakes, mechanically driven

pumps, carburetors, aircraft wheel assemblies, shock absorber struts and hydraulic servo

units.î 

According to Boeing, Integrated Aerospace is the only contract facility it uses to

perform jackscrew assembly overhauls.197 When Integrated Aerospace receives a

jackscrew assembly with an end play measurement greater than 0.015 inch,198 it will

discard the acme nut and gimbal ring assembly, check that the dimensions of the acme

screw are within tolerances, mate a new acme nut and gimbal ring to the acme screw, and

assign a new S/N to the nut assembly.199 Following the buildup of the jackscrew assembly,

Boeing requires that the overhauled assembly be functionally tested in a weighted fixture

after the assembly is temporarily outfitted to additionally support hardware and electric

motors. Boeing also requires new jackscrew assemblies to undergo an operational test in

the same test fixture. 

197 The instructions and specifications Boeing requires of its maintenance facilities are contained in a

service rework drawing. Before the flight 261 accident, information collected from numerous production

drawings and proprietary documents formed the basis for the rework instructions. Following the accident,

the FAA required Boeing to more formally organize the rework procedures, which resulted in Boeingís

publication of the service rework drawing. Integrated Aerospace was the only maintenance facility to

possess the drawing. According to Boeing officials, the drawing is proprietary to ensure that the company

maintains confidence and control over the precise machining tolerances required.

198 Jackscrew assemblies may also be overhauled because of excessive free play or damage to the lower

mechanical stop because of an electric shutoff failure or misrigging. Not all jackscrew assemblies submitted

for overhaul have a high degree of acme nut thread wear. 

199 According to Boeing, the installation of the gimbal ring supports involves precision machining at

tolerances as small as five ten-thousandths of an inch to ensure that the alignment of the fastener bores

within the gimbal ring supports, the gimbal rings, and the acme nut are adequate to maintain the integrity of

the dual load path design.
Factual Information 120 Aircraft Accident Report

1.18.8.2 Review of the Boeing DC-9 Overhaul Maintenance Manual

The Safety Board reviewed the section of the Boeing DC-9 Overhaul Maintenance

Manual (in 27-41-1, revision 24, dated April 15, 1998, which was not required to be

approved by the FAA) titled, ìHorizontal Stabilizer Actuator Control Installation.î The

following observations were noted:

ï The manual contained no requirements to functionally test overhauled

jackscrew assemblies in a weighted fixture after the assembly is temporarily

outfitted to additionally support hardware and electric motors nor to undergo

an operational test in the same test fixture, as was required by maintenance

facilities per Boeingís service rework drawing.

ï The manual contained no requirement for the use of work cards documenting

the tasks for overhaul.

ï The manual provided no procedures nor equipment listings for bench-check

end play measurements.

ï The manual contained no requirement to document the bench-check end play

measurement after overhaul nor to ensure that the receiver of the overhauled

unit was provided with the measurement.

ï The manual contained a requirement that, if the axial end play measurement

exceeded 0.040 inch, the ìnut assemblyî (defined in the manual as ìthe acme

screw, acme nut, and gimbal ring support assemblyî) must be replaced. The

manual stipulated that the replacement nut assembly could be new (purchased

from Boeing with an end play measurement of between 0.003 and 0.010 inch)

or used (with an end play measurement of 0.040 inch or lower). The manual

also stated that the overhaul shop could send nut assemblies to Boeing for

overhaul.

ï The manual contained instructions to check the jackscrew assembly on-wing

every 1,000 flight hours if the measured bench-check end play measurement

was ìbetween 0.0340 and 0.0390 inch.î No such requirement exists in the

airplane maintenance manual, which is used by operators. 

ï The manual contained no procedures or equipment listings pertaining to the

surface finish of the acme screw threads. No mention was made of the

importance of proper acme screw thread surface finish.

ï The manual provided no detail on how to apply grease to the jackscrew

assembly.

ï The manual did not specify which type of grease should be used to lubricate

the acme screw and nut or other assembly components, including the torque

tube and gearbox.

ï The manual did not specify how to pack the jackscrew assembly for immediate

shipping after overhaul; protective packaging instructions were given only for

units going into ìstorage.î
Factual Information 121 Aircraft Accident Report

1.18.8.3 Safety Board Maintenance Facility Observations

Safety Board investigators visited Integrated Aerospace and three other FAA

Part 145 maintenance facilities that overhaul jackscrew assemblies. The visits revealed

that two of the three maintenance facilities had self-imposed end play measurement limits

at which the acme nut had to be replaced. One maintenance facilityís end play

measurement limit was 0.025 inch; the other maintenance facilityís limit was 0.030 inch.

The third maintenance facility had no self-imposed limit at which the acme nut had to be

replaced; however, if the end play measurement was less than 0.040 inch, the maintenance

facility would return the overhauled assembly to the customer. 

The Safety Boardís visits also revealed that the maintenance facilities (except for

Integrated Aerospace) performed the end play check using different methods. One

maintenance facility used a unique, fabricated, self-contained fixture200 that exerted a

force on the acme nut and eliminated acme screw rotation. The end play measurement was

taken by a direct dial indicator reading. Another maintenance facility placed the jackscrew

assembly on a wooden cradle and moved the acme nut by hand onto the screw while

observing a dial indicator. The last maintenance facility used a magnet to hold the dial

indicator in place on the acme screw while the nut was held on a table by hand as the end

play was checked manually. Only Integrated Aerospace recorded the received and final

end play measurements and used a detailed set of work cards to document each step in the

overhaul process. The other maintenance facilities documented only that an assembly had

been overhauled and used the DC-9 Overhaul Maintenance Manual. Further, two of the

four maintenance facilities (not Integrated Aerospace) used subcontractors to apply the

black-oxide coating on the acme screw and to ensure that the acme screw thread surface

finish met specifications. 

The FAA PMI assigned to one of the maintenance facilities stated that he was not

aware that the maintenance facility performed overhauls of DC-9 jackscrew assemblies

until the facility informed him that it had received a suspect assembly from a foreign

airline. Representatives of the maintenance facility stated that they overhauled about six

jackscrew assemblies per year and that their customers included aircraft parts brokers and

foreign airlines. Representatives of another maintenance facility stated that the jackscrew

assemblies were not typically lubricated after overhaul.201 Representatives of the last

maintenance facility indicated that they did not pack the assembly for storage as specified

by the DC-9 Overhaul Maintenance Manual because the overhauled assembly was

shipped out immediately and not put into storage.202 This maintenance facilityís operators

stated that they did not track assemblies after shipment to determine if overhauled

assemblies would be stored for long periods of time by parts brokers or airlines.

200 The fixture was not referenced in the DC-9 Overhaul Maintenance Manual. 

201 The DC-9 Overhaul Maintenance Manual instructions call for lubrication of overhauled units.

202 The storage procedures in the DC-9 Overhaul Maintenance Manual stipulated that jackscrew

assemblies be wrapped in barrier material and heat sealed and that 80 percent of the air be evacuated from

the wrapping. The manual contained no instructions for packing jackscrew assemblies for shipping.
Factual Information 122 Aircraft Accident Report

Safety Board investigators also visited an aircraft parts broker in southern

California on February 24, 2001. The broker was listed on an ìILSî subscription listing of

organizations that sell jackscrew assemblies and other aircraft components.203

Investigators examined a recently overhauled acme screw in stock. The jackscrew

assembly was stored in an open cardboard box and was wrapped loosely in plastic.

Documents that accompanied the assembly indicated that it had been overhauled by a

maintenance facility located in the Los Angeles area in November 2000 because of a

broken upper mechanical stop. The documents did not contain end play measurements or

information about the overhaul. The assembly had red grease on it. 

1.18.8.4 Overhauled Jackscrew Assembly Data From 1996 to 1999

Boeing provided the Safety Board with bench-check end play measurement data of

all jackscrew assemblies that had been shipped to Integrated Aerospace for overhaul from

1996 to 1999. A review of the data indicated that, out of a total of 214 jackscrew

assemblies, 13 had a recorded bench-check end play measurement of 0.045 inch or

greater, and 5 had a recorded bench-check end play measurement of 0.055 inch or greater.

One jackscrew assembly had a recorded bench-check end play measurement of

0.075 inch, which was the highest measurement reported to investigators during the

investigation.

According to Delta Air Lines representatives, the jackscrew assembly

(S/N DCA 2239) with the 0.075-inch end play measurement was originally installed on an

MD-88 airplane that was delivered new to Delta Air Lines from McDonnell Douglas on

March 17, 1993. An on-wing end play check conducted on July 18, 1999, yielded the

0.075-inch end play measurement, and Delta Air Lines removed the jackscrew assembly

for overhaul. At the time of the 0.075-inch end play measurement, the airplane had

accumulated 18,417 flight hours and 14,520 cycles with the same jackscrew assembly.

Integrated Aerospace received the jackscrew assembly on October 29, 1999, and the

company also measured a bench-check end play of 0.075 inch. Integrated Aerospace

overhauled the jackscrew assembly (with a new acme nut) and returned it to Delta Air

Lines on December 17, 1999. According to Delta Air Lines, no further information related

to the maintenance history of this jackscrew assembly was found. 

1.18.9 Industrywide Jackscrew Assembly Maintenance 

Procedures

The Safety Board surveyed all 13 domestic airlines that operated at least 10 DC-9,

MD-80/90, and/or 717 airplanes in their fleets. The airlines were asked to report their

jackscrew assembly lubrication and end play intervals before the flight 261 accident. In

addition, the airlines were asked to report the type of grease and the internally imposed

maximum allowable end play limit they used before the accident and the number of

jackscrew assemblies they removed in 1998 and 1999. The airlines were also asked to

203 The ILS subscription listing is used by overhaul shops and brokers to advertise overhauled parts for

purchase by airlines. 
Factual Information 123 Aircraft Accident Report

provide the maintenance work cards for both the lubrication and end play check of the

jackscrew assembly. A review of the survey results revealed the following pertinent

information:

ï Alaska Airlines was the only airline that used Aeroshell 33 to lubricate the

jackscrew assembly. All of the other operators used an MIL-G-23827-qualified

grease, such as Mobilgrease 28, Aeroshell 7, or Aeroshell 22.

ï Alaska Airlines had the highest MD-80 jackscrew assembly lubrication

interval (approximately 2,550 flight hours).

ï Alaska Airlines had the second highest MD-80 end play check interval

(approximately 9,550 flight hours). The highest end play check interval at all

of the airlines was approximately 10,500 flight hours.

ï The lowest MD-80 jackscrew assembly lubrication interval was 600 flight

hours.

ï The operatorsí MD-80 end play check intervals varied from 7,000 to

10,500 flight hours.

ï The operatorsí DC-9 jackscrew assembly lubrication intervals varied from 550

to 2,530 flight hours.

ï The operatorsí DC-9 end play check intervals varied from 2,000 to 7,720 flight

hours. One operator did not use the end play check and instead had a required a

hard-time removal at 15,500 hours.

ï All of the MD-90 and 717 operatorsí jackscrew assembly lubrication intervals

were set at 3,600 flight hours.

ï All of the MD-90 and 717 operatorsí end play check intervals were set at 7,200

flight hours.

1.18.10 FAA Commercial Airplane Certification Process Study

In February 2002, the FAA issued a report titled, ìCommercial Airplane

Certification Process Study ñ An Evaluation of Selected Aircraft Certification, Operation

and Maintenance Processes.î The report was based on a study that was led by the FAA

and conducted by a team that included representatives from U.S. and non-U.S. airplane

manufacturers, the U.S. aviation industry, the National Aeronautics and Space

Administration, the Air Line Pilots Association, the DoD, Sandia National Laboratories,

the Fed Ex Pilots Association, and an international airworthiness consultant. 

The airplane certification process study report stated that the study was conducted

because ìrecent accidents such as the Alaska Airlines MD-80 in January 2000, or the

Trans World Airways B-747 in July 1996, have brought into question the adequacy of

certain processes related to the certification of the airplane.î The report focused on

numerous process categories, including human factors in airplane design, operations and

maintenance, flight critical systems and structure, safety data management,
Factual Information 124 Aircraft Accident Report

maintenance/operations coordination, major repairs and modifications, and safety

oversight. The report added, ìspecial emphasis was also placed upon analyzing how the

various major certification processes in the airplaneís life cycle relate to each other, and

evaluating the relationship between these certification processes and the maintenance and

operating processes being applied in service.î

The airplane certification process study reportís findings and observations

included the following:

ï Design techniques, safety assessments, and regulations do not adequately

address the subject of human error in design or in operations and maintenance.

ï There is no reliable process to ensure that the assumptions made in the safety

assessments are valid with respect to operations and maintenance activities and

that operators are aware of these assumptions when developing their

operations and maintenance procedures. In addition, certification standards

may not reflect the actual operating environment.

ï Several catastrophic events may have been avoided by use of a more robust

design and in-depth safety analysis that challenged the assumptions of the

design process. This is particularly important when the catastrophic event is

the end-point of a short fault tree (two or three failures) or where very specific

human actions are required to prevent an accident.

ï Processes for identification of safety critical features of the airplane do not

ensure that future alterations, maintenance, repairs, or changes to operational

procedures can be made with cognizance of those safety features.

ï Multiple FAA-sponsored data collection and analysis programs exist without

adequate interdepartmental coordination or executive oversight.

ï There is no widely accepted process for analyzing service data or events to

identify potential accident precursors.

ï Adequate processes do not exist within the FAA or in most segments of the

commercial aviation industry to ensure that the lessons learned from specific

experiences in airplane design, manufacturing, maintenance, and flight

operations are captured permanently and made readily available to the aviation

industry. The failure to capture and disseminate lessons learned has allowed

airplane accidents to occur for causes similar to those in past accidents.

ï There are currently no industry processes or guidance materials available

which ensure that (1) safety-related maintenance or operational

recommendations developed by the manufacturer are evaluated by the operator

for incorporation into their maintenance or operational programs and (2)

safety-related maintenance or operational procedures developed or modified

by the operator are coordinated with the manufacturer to ensure that they do

not compromise the type design safety standard of the airplane and its systems. 
Factual Information 125 Aircraft Accident Report

ï The lack of adequate formal business processes between FAA Aircraft

Certification Service and Flight Standards Service limits effective

communication and coordination between the two that often results in

inadequate communications with the commercial aviation industry.

ï Processes to detect and correct errors made by individuals in the design,

certification, installation, repair, alteration, and operation of transport-category

airplanes are inconsistent, allowing unacceptable errors in critical

airworthiness areas.

Referring to the lubrication-related safety recommendations the Safety Board

issued on October 1, 2001, the report stated that ìwhile the relevance of mixing grease

types in this accident has not been determined, both types of grease were discovered in the

jackscrew assembly of the accident aircraft. It appears that the communication between

operator, [airplane manufacturer,] and the grease manufacturers was not adequate to

ensure that the grease mixing recommendations [by Boeing and the grease manufacturers]

were communicated and incorporated.î204

1.18.11 Fail-Safe Jackscrew Assembly Designs

Representatives of Nook Industries, which manufactures jackscrew assemblies for

nonaviation applications, indicated to Safety Board investigators that the jackscrew

assemblies it designs for safety-critical functions are equipped with a secondary acme nut.

The secondary acme nut, which is also referred to as a ìfollower nut,î is threaded on the

acme screw and follows the primary load-bearing acme nut as it rotates longitudinally

along the acme screw. Engineers at the National Aeronautics Space Administrationís

(NASA) John F. Kennedy Space Center have developed a similar mechanism for use in

the jackscrew assembly used for the gaseous oxygen vent arm on its space shuttle launch

pads. The secondary acme nut in both the Nook and NASA designs does not carry any

load unless the primary nut threads shear off. If this occurs, the secondary acme nut

assumes the load and allows the system to continue to operate. The addition of the

secondary acme nut also provides an alternate method for monitoring end play between

the acme screw and nut threads that does not require relieving the load on the primary nut.

As the threads of the primary acme nut wear, the distance between the primary and

secondary nut decreases; therefore, wear of the primary nut threads, or end play, can be

gauged by monitoring changes in this distance.

204Boeing service letters issued on August 12, 1993, and June 30, 1997, noted that these grease types 

were incompatible ìand should not be mixed.î
Analysis 126 Aircraft Accident Report

2. Analysis

2.1 General

The flight crewmembers on Alaska Airlines flight 261 were properly certificated

and qualified and had received the training and off-duty time prescribed by Federal

regulations. No evidence indicated any preexisting medical or other condition that might

have adversely affected the flight crewís performance during the accident flight.

 The airplane was dispatched in accordance with Federal Aviation Administration

(FAA) regulations and approved Alaska Airlines procedures. The weight and balance of

the airplane were within limits for dispatch, takeoff, climb, and cruise. 

Daylight visual meteorological conditions prevailed, and weather was not a factor

in the accident.

Examination of recovered wreckage and cockpit voice recorder (CVR), flight data

recorder (FDR), and air traffic control (ATC) data revealed no evidence of a fire or of

impact with birds or any other foreign object. 

No evidence indicated that the airplane experienced any preimpact structural or

system failures, other than those associated with the longitudinal trim control system, the

horizontal stabilizer, and its surrounding structure.205

FDR and CVR information indicated that both engines were operating normally

before the final dive. Examination of the recovered thrust reversers and FDR data

indicated that the thrust reversers were stowed at impact. Examination of the wreckage

indicated that the left engine was producing significant rotational energy at impact and

that the right engine was producing very little rotational energy at impact. Erratic engine

pressure ratio data recorded on the FDR during the final dive were consistent with airflow

disturbances associated with extreme angles-of-attack. Such extreme angles-of-attack can

cause fuel unporting and can interrupt the airflow into the engine inlet, either of which

could lead to an engine flameout. Therefore, the lack of rotation on the right engine was

likely the result of an engine flameout during the final dive. 

ATC personnel involved with the accident flight were properly certificated and

qualified for their assigned duty stations. ATC radar and communications equipment were

functioning properly during the flight. 

Analysis of the structural damage and FDR data indicated that the airplane was

nearly inverted206 and was in a steep nose-down, right-wing-low attitude when it impacted

the water, which is consistent with uncontrolled flight. 

205 For an analysis of this failure sequence, see sections 2.2.2 through 2.2.4.
Analysis 127 Aircraft Accident Report

This analysis examines the accident scenario, including the catastrophic failure of

the longitudinal trim control system, flight crew decision-making, possible reasons for the

excessive acme nut thread wear, the importance of monitoring acme nut thread wear,

jackscrew assembly overhaul procedures and facilities, adequacy of Alaska Airlinesí

maintenance program, FAA oversight, and horizontal stabilizer design issues. 

2.2 Accident Sequence

2.2.1 Takeoff and Climbout

FDR data indicated that the accident airplaneís longitudinal trim control system

was functioning normally during the airplaneís descent into Lic Gustavo Diaz Ordaz

International Airport (PVR), Puerto Vallarta, Mexico, on the flight just before the accident

flight. FDR data indicated that the airplane landed at PVR with a horizontal stabilizer

angle of 6o airplane nose up.

FDR data indicated that the accident flight crew had trimmed the airplane to a

7o airplane-nose-up position before takeoff from PVR. After the airplane took off about

1337, FDR data indicated that the horizontal stabilizer moved at the normal primary trim

motor rate of 1/3o per second from 7o to 2o airplane nose up. Thereafter, as the airplane

continued to climb from 6,200 feet to about 23,400 feet, the FDR recorded horizontal

stabilizer movement at the normal alternate trim motor rate of 1/10o per second from

2o airplane nose up to 0.4o airplane nose down. Operation of the alternate trim motor

during this period is consistent with pitch control being commanded by the autopilot,

which was activated at 1340:12 while the airplane was climbing through 6,200 feet. On

the basis of these data, the Safety Board concludes that the longitudinal trim control

system on the accident airplane was functioning normally during the initial phase of the

accident flight. 

2.2.2 Jamming of the Horizontal Stabilizer

FDR data indicated that the horizontal stabilizerís last movement during the

climbout was to 0.4o airplane nose down at 1349:51 as the airplane was climbing through

23,400 feet at 331 knots indicated airspeed (KIAS). After this, no horizontal stabilizer

movement was recorded on the FDR until the airplaneís initial dive 2 hours and

20 minutes later. This cessation of horizontal stabilizer movement is not consistent with a

typical MD-80 climb profile, which normally would require additional stabilizer

movements to maintain trim. Thus, as the accident airplane continued to climb above

23,400 feet without any horizontal stabilizer movement, the autopilot would have

attempted to achieve trim by continuing to add elevator input to compensate for the lack of

movement of the horizontal stabilizer.

206 The largest intact sections of the recovered fuselage were from below the airplaneís floor line,

indicating that the airplane was rolling toward inverted at impact.
Analysis 128 Aircraft Accident Report

After the last horizontal stabilizer movement, the airplane continued to climb at

330 KIAS. The airplane began to level off at approximately 26,000 feet and slowed to 320

KIAS. At 1351:51, the airplane began to climb again. By 1352:50, the airplane had slowed

to about 285 knots as it climbed to approximately 28,600 feet and then leveled off. 

At 1353:12, when the airplane was at approximately 28,557 feet and 296 KIAS,

the autopilot was disconnected, most likely by the flight crew in response to the trim

annunciator warning light, which would have illuminated to indicate the airplaneís

out-of-trim condition.207 In a typical climb profile, a reduction in speed during the ascent

would result in an airplane-nose-up stabilizer movement to trim the airplane. At the time

of the autopilot disconnect, the airplane was still trimmed for an airspeed of 331 knots (the

airspeed when the last horizontal stabilizer movement occurred), 35 knots faster than the

airspeed at that time. After the autopilot was disconnected, the elevators were deflected in

the airplane-nose-up direction from 0o to approximately -1.5o. 

For almost 7 minutes after the autopilot disconnect, the airplane continued to climb

at a much slower rate than before the disconnect, reaching about 31,050 feet by 1400:00.

During this part of the ascent, the elevators were deflected in the airplane-nose-up

direction between -1o and -3o, which, according to airplane performance calculations,

would have required up to 50 pounds of pulling force on the control column(s).208 The

calculations further indicated that, after leveling off at 31,050 feet, only about 30 pounds

of pulling force would have been required to maintain level flight for the next 24 minutes

while the airplane was flying at 280 KIAS. This was followed by an increase in airspeed209

beginning at 1424:30 that, along with the decreased weight of the airplane because of fuel

usage, would have eased the amount of pulling force needed to maintain level flight to

roughly 10 pounds until the autopilot was switched back on about 1547.

After the autopilot disconnect at 1353:12, the flight crew would have noticed that

the airplane was mistrimmed because significant pilot control forces would have initially

been needed to maintain the desired climb and cruise configuration, and the flight crew

would have likely attempted to correct the problem by manually activating the primary or

alternate trim systems to move the horizontal stabilizer. However, the horizontal stabilizer

remained at 0.4o airplane nose down, indicating that the flight crew was unable to

manually command movement of the horizontal stabilizer. During discussions with

Alaska Airlines maintenance personnel at Los Angeles International Airport, (LAX), Los

Angeles, California, later in the flight, the captain indicated that the flight crew had tried

numerous troubleshooting strategies to move the horizontal stabilizer, including checking

the circuit breakers and using the control wheel trim switches and longitudinal trim

handles on the center pedestal separately and together.210 The captain told the maintenance

207 The airplaneís speed decreased (from 331 to 296 KIAS) during the minutes preceding the autopilot

disconnect, indicating that the airplane was mistrimmed. 

208 As stated previously, all estimates of pulling force in this report refer to the combined column forces

from both the captainís and the first officerís control columns. It is not known whether these forces were

applied by the captain, the first officer, or both.

209 Although the flight plan called for a cruise speed of 283 knots calibrated airspeed, FDR data indicated

that the airplaneís cruise speed increased, starting at 1424:30, eventually reaching 301 KIAS.
Analysis 129 Aircraft Accident Report

personnel, ìit appears to be jammedÖthe whole thing, it spikes out when we use the

primary, we get AC load that tells me the motorís tryin to run but the brake wonít move it.

when we use the alternate, nothing happens.î211

In sum, the Safety Board concludes that the horizontal stabilizer stopped

responding to autopilot and pilot commands after the airplane passed through 23,400 feet.

The pilots recognized that the longitudinal trim control system was jammed, but neither

they nor the Alaska Airlines maintenance personnel could determine the cause of the jam.

2.2.2.1 Cause of the Jam

As previously mentioned, the horizontal stabilizerís jackscrew assembly includes

an acme screw and nut, both of which have two threads. After the accident, severely worn

and sheared remnants of the acme nut threads were recovered wrapped around the acme

screw. The condition of the recovered acme nut thread remnants indicated that

approximately 90 percent of the thread thickness had worn away before the remainder of

the threads sheared off.212 In comparison, a jackscrew assembly with the maximum

amount of wear permitted in service, as indicated by an end play measurement of

0.040 inch (that is, 0.030 to 0.037 inch of wear), would only have about 22 percent of the

thread thickness worn away.213

The interior of the recovered acme nut contained no intact threads, although there

were small ridges where the worn nut thread remnants had previously been attached. The

inner diameter of those ridges was almost identical to the outer diameter of the acme

screw threads, which is consistent with the worn acme nut threads having been sheared off

by the acme screw threads during airplane-nose-down trimming movements. 

Each of the acme nut threads makes 16 360° spiral revolutions, for a total of 32

360° revolutions. Calculations indicated that loads of more than 190,000 pounds would be

necessary to simultaneously shear all 32 360o spiral revolutions of the acme nut threads if

they were worn to the same degree as the accident acme nut thread remnants. However,

airplane performance data determined that the flight loads on the jackscrew assembly

were only 4,000 to 5,000 pounds during the climbout from PVR. Therefore, it is very

unlikely that the worn acme nut thread spiral revolutions were simultaneously sheared

along their entire length.

210 When Alaska Airlines maintenance personnel asked the captain whether the pilots had tried ìthe

suitcase handles [colloquial term for longitudinal trim handles] and the pickle switches [colloquial term for

control wheel trim switches]î in their attempts to troubleshoot the horizontal stabilizer trim problem, the

captain replied that they had ìtried everything together.î When the maintenance technician subsequently

asked again whether the flight crew had tried the control wheel trim switches and longitudinal trim handles,

the captain replied, ìyea we tried just about every iteration.î

211 The alternate trim motor uses less power than the primary trim motor; therefore, activation of that

system does not normally produce enough electrical current to cause any appreciable movement of the

needle on the AC load meter. 

212 For more information about the condition of the recovered acme nut threads, see section 1.16.3.6.

213 This calculation was based on a comparison with the manufacturing specifications for the thickness of

the threads at the major diameter (0.15 inch).
Analysis 130 Aircraft Accident Report

The calculations further indicated that approximately 6,000 to 7,000 pounds would

be necessary to shear one 360o spiral revolution of an acme nut thread if it were worn to

the same degree as the accident acme nut thread remnants. However, the Safety Boardís

finite element analysis (FEA) study showed that, at high levels of thread wear, such as

those of the accident acme nut threads, bending stresses develop that are significantly

larger than the stresses caused by direct contact pressure. The study showed that the

bending deformations resulting from a 2,000-pound load on a single highly worn thread

spiral caused a drastic change in the threadís behavior, indicating that a structural failure,

rather than shearing failure, is possible.

Further, the results of the Safety Boardís and Boeingís FEA of the stress and

deformation in the acme nut threads under loading indicated that the wear process is an

incremental one in which the loads and resulting wear shift and move among all of the

32 spiral revolutions in the acme nut.214 Thus, an incremental bending-induced shear

fracture of the threads most likely propagated along the thread spiral, eventually leading to

complete failure. Figure 20 depicts the stages of acme nut thread wear to the point of

fracture. 

The Safety Board concludes that the worn threads inside the horizontal stabilizer

acme nut were incrementally sheared off by the acme screw and were completely sheared

off during the accident flight. The Safety Board further concludes that, as the airplane

passed through 23,400 feet, the acme screw and nut jammed, preventing further

movement of the horizontal stabilizer until the initial dive. Safety Board investigators

considered numerous factors that might have played a role in the jamming of the acme

screw to the nut, including bending of the worn threads before or during the shearing,

distortion of the remnants after they were sheared,215 and loads resulting from the acme

screw threads pulling upward across the remaining ridges after the shearing;216 however,

the Board could not determine the exact cause of the jam.

214 For more information about these studies, see section 1.16.4.

215 Because the thinnest part of the sheared thread remnants was too large to fit in the space between the

crowns of the acme screw threads and the stripped surface of the nut, the remnants themselves probably

played only a minor role, if any, in the creation of the jam. 

216 It was theorized that, as the last of the acme nut thread remnants shear fractured and the screw threads

tried to override the resulting ridges in the nut as the screw was pulled upward by aerodynamic loads and

rotated circumferentially by the primary trim motor, the combined pinching and friction loads became

greater than the combined aerodynamic and primary trim motor forces.
Analysis 131 Aircraft Accident Report

Figure 20. A graphical depiction of the stages of acme nut thread wear to the point of 

fracture.
Analysis 132 Aircraft Accident Report

2.2.3 Release of the Jam and the Initial Dive

At 1608:59, following discussions with Alaska Airlines maintenance personnel

about whether the fight crew had tried the longitudinal trim handles and the control wheel

trim switches, the captain told the first officer, ìIím going to click it off you got it [the

airplane]Ölets do that.î At this time, the airplane was cruising at 31,050 feet.

At 1609:14.8, the captain stated, ìthisíll click it off,î and, about 1609:16, the autopilot

parameter on the FDR switched from ìengagedî to ìoff.î FDR and airplane performance

data indicated that during the 3 to 4 seconds following the autopilot disconnect, the

horizontal stabilizer traveled from its jammed position of 0.4o airplane nose down to

beyond the maximum airplane-nose-down position that could be recorded by the FDR and

remained beyond this position for the remainder of the flight.217 Analysis of the FDR data

showed that the trim movement rate was not uniform, which was inconsistent with normal

trim operation commanded by either the primary or alternate trim motor.

As the jam was overcome, the acme screw was being pulled upward through the

acme nut by aerodynamic loads, causing upward movement of the horizontal stabilizer,

resulting in greater airplane-nose-down motion. This upward pulling motion would have

continued until the lower mechanical stop on the acme screw contacted the lower surface

of the acme nut, preventing further upward motion of the horizontal stabilizer.

Boeing engineering data, computer modeling, and ground demonstrations on an

actual airplane indicated that the maximum possible horizontal stabilizer position when

the acme screw lower mechanical stop is pulling up against the acme nut is about

3.1o airplane nose down. This is consistent with airplane performance studies of the

airplaneís movements at this point in the accident sequence that showed the airplane

beginning to pitch nose down to nearly -8o. FDR data showed that during this 3- to

4-second period, the elevators were deflected from -0.4o to -4.0o, indicating that the flight

crew made control inputs to bring the nose up and counteract the movement of the

horizontal stabilizer. The speed brakes were deployed at 1609:43, 27 seconds after the

autopilot disconnect, most likely in an attempt by the flight crew to retard the airplaneís

rapidly increasing airspeed. The speedbrakes remained deployed for the next 1 minute

30 seconds. The airplane began to recover from this initial dive and leveled off at

about 1611 at approximately 24,000 feet. Airplane-nose-up elevator inputs continued for

about the next 8 minutes (until the final dive about 1619).

Safety Board investigators attempted to determine what caused the horizontal

stabilizer to move immediately after the autopilot disconnect. Normally, the autopilot is

disconnected by the pilot flying using the autopilot disconnect switch located on the

outboard side of the control wheel. However, the autopilot will also disconnect when the

primary trim control system is activated by either the control wheel trim switches or the

longitudinal trim handles on the center pedestal. After the initial dive, the captain told the

217 The maximum airplane-nose-down position of the horizontal stabilizer during normal operation

is 2.1o. The FDR recorded airplane-nose-down positions up to about 2.5o. Airplane performance data

indicated that the stabilizer position had to have been beyond 2.5o airplane nose down to result in the

airplaneís pitch movements during and after the initial dive.
Analysis 133 Aircraft Accident Report

Alaska Airlines LAX maintenance technician, ìwe did both the pickle switch [control

wheel trim switches] and the suitcase handles [longitudinal trim handles] and it ran away

full nose trim down [sic].î Based on this comment and those made by the Alaska Airlines

LAX maintenance personnel immediately preceding the autopilot disconnect, it appears

that the captain disconnected the autopilot when he activated the primary trim control

system by using either the control wheel trim switches, the longitudinal trim handles, or

both. 

Consequently, although operation of the primary trim motor as part of

troubleshooting attempts earlier in the flight did not release the jam, the torque created by

the primary trim motor when the captain activated the primary trim system at 1609:16

apparently provided enough force to overcome the jam between the acme nut and screw.

Over the next 3 to 4 seconds, the increasing angle-of-attack of the horizontal stabilizer and

the increased elevator deflections would have in turn increased the tension loads on the

acme screw, contributing to the motion of the acme screw upward through the acme nut. 

Therefore, the Safety Board concludes that the accident airplaneís initial dive from

31,050 feet began when the jam between the acme screw and nut was overcome as a result

of the operation of the primary trim motor. Release of the jam allowed the acme screw to

pull up through the acme nut, causing the horizontal stabilizer leading edge to move

upward, thus causing the airplane to pitch rapidly downward. 

Markings on the recovered jackscrew assembly lower mechanical stop and the

lower surface of the acme nut indicated that the nut repeatedly contacted the stop in

various positions and that the acme screw and attached lower mechanical stop had rotated

in both directions while the stop was in contact with the nut. Further, the rotationally

sheared splines on the lower mechanical stop indicated that the acme screw rotated within

the lower mechanical stop after the stop contacted the lower surface of the acme nut.

Metallurgical analysis determined that the marks on the lower mechanical stop could have

been created by rotation of the acme screw only after complete loss of the acme nut

threads. Circumferential marks were found in the acme nut bore, also indicating acme

screw rotation after the threads had sheared off. Such rotation of the acme screw could

only be caused by operation of a trim motor. Therefore, the Safety Board concludes that

the acme screw did not completely separate from the acme nut during the initial dive

because the screwís lower mechanical stop was restrained by the lower surface of the

acme nut until just before the second and final dive about 10 minutes later. 

2.2.4 The Second and Final Dive

After the flight crew recovered from the initial dive and leveled off at

approximately 18,000 feet, the captain instructed the first officer to deploy the slats and

flaps (at 1617:54 and 1618:05, respectively), most likely to evaluate flight control

pressures. FDR data indicated that the airspeed decreased about 8 knots and that the

airplane remained stable and controllable following the slat and flap deployment.

At 1618:26, the captain asked the first officer to retract the slats and flaps for unknown
Analysis 134 Aircraft Accident Report

reasons. During the 50 seconds after the slats and flaps were retracted, the airplane

accelerated from about 245 to 270 KIAS. 

Beginning at 1619:09, the FDR recorded small oscillations in elevator angle, pitch

angle, and vertical acceleration values for about 15 seconds. At 1619:21, the CVR

recorded a series of faint thumps.218 Although the horizontal stabilizer position had

already exceeded the maximum position capable of being sensed and recorded by the

FDR, the FDR nonetheless indicated an increase in the horizontal stabilizer position at the

time of the faint thump.219 This increase was the first horizontal stabilizer movement since

the one that precipitated the initial dive about 10 minutes earlier. The FDR contained no

direct data about the actual position to which the horizontal stabilizer moved at this time.

However, Boeing engineering design data and computer modeling of the horizontal and

vertical stabilizers indicated that if the acme screw was free to travel through the acme nut,

the horizontal stabilizer would move from the 3.1o airplane-nose-down position it was

being held at by the lower mechanical stop and would contact the vertical stabilizer tip

fairing brackets at a 3.6o airplane-nose-down angle. Structural examinations determined

that this additional movement of the horizontal stabilizer could only result from a fracture

of the acme screw torque tube. 

Metallurgical analysis of the recovered section of the fractured torque tube showed

that the final separation initiated from an area of low-cycle fatigue cracking. On the basis

of an evaluation of the static loading conditions and performance data, the Safety Board

determined that the torque tube was capable of sustaining a single application of the

maximum load applied to the torque tube between the initial and final dives.220 However,

low-cycle fatigue tests and analysis, performance data, and FEA established that the

magnitude and frequency of the loads on the torque tube after complete shearing of the

acme nut threads, in combination with an offset loading condition,221 would be sufficient

to initiate and propagate low-cycle fatigue cracking in the torque tube. The low-cycle

fatigue cracking severely reduced the load capability of the torque tube during the

10-minute period between the initial and second and final dive, causing it to fracture at

loads considerably less than it was capable of sustaining if it did not have fatigue damage.

At 1619:29, the captain ordered redeployment of the slats and flaps. At 1619:35,

the flaps were transitioning from 7o to 11o. At 1619:36.6, the CVR recorded the sound of

an ìextremely loud noise.î Immediately thereafter, the airplane began its final dive. At the

time of this pitchover, radar detected several small primary returns, consistent with parts

of the vertical stabilizerís tip fairing being torn from the airplane as the fairing brackets

218 According to the Safety Boardís sound spectrum study, the ìfaint thumpî noted on the CVR transcript

at 1619:21was actually a series of thumps.

219 After the sound of the faint thump, the elevator deflections increased from -9o to -13o, indicating the

pilots were making additional airplane-nose-up elevator inputs to maintain level flight with the increased

airplane-nose-down horizontal stabilizer angle. 

220 For more information about these loads, see section 1.16.5

221 The torque tube was subjected to a combination of bending and tension as a result of an offset loading

condition created in the torque tube because the stop lugs on the lower mechanical stop and the acme screw

prevented application of equal load around the axis of the tube. 
Analysis 135 Aircraft Accident Report

broke,222 which would have allowed the horizontal stabilizer to move well beyond the 3.6o

airplane-nose-down position it was being held at by the brackets. Therefore, the extremely

loud noise recorded on the CVR at 1619:36.6 was likely made as the fairing brackets

failed and caused the loss of the tip fairing and structural deformation of the tail under

flight loads, resulting in local aerodynamic disturbances.223

Valid airplane performance data do not exist for horizontal stabilizer positions

beyond 14o airplane nose down. Although the exact horizontal stabilizer angle required to

produce the final pitchover could not be determined, the Safety Boardís airplane

performance studies determined that the airplaneís pitch rate during the final dive could

only have resulted from a rapid increase to a stabilizer angle significantly beyond

14o airplane nose down.224

In sum, the Safety Board concludes that the cause of the final dive was the

low-cycle fatigue fracture of the torque tube, followed by the failure of the vertical

stabilizer tip fairing brackets, which allowed the horizontal stabilizer leading edge to

move upward significantly beyond what is permitted by a normally operating jackscrew

assembly. The resulting upward movement of the horizontal stabilizer leading edge

created an excessive upward aerodynamic tail load, which caused an uncontrollable

downward pitching of the airplane from which recovery was not possible.

2.2.5 Flight Crew Decision-Making

2.2.5.1 Decision to Continue Flying Rather than Return to PVR 

Safety Board investigators considered several reasons that might explain the

captainís decision not to return immediately to PVR after he experienced problems with

the horizontal stabilizer trim system during the climbout from PVR.

Neither the Alaska Airlines MD-80 Quick Reference Handbook (QRH) Stabilizer

Inoperative checklist nor the companyís QRH Runaway Stabilizer emergency checklist

required landing at the nearest suitable airport if corrective actions were not successful.

These checklist procedures were the only stabilizer-related checklist procedures contained

in the QRH, and the flight crew most likely followed these checklist procedures in their

initial attempts to correct the airplaneís jammed stabilizer. 

222 The Safety Board notes that the fairing brackets were not intended to carry the load of the horizontal

stabilizer.

223 As discussed in section 1.11.1.1, sound tests indicated that neither this sound nor any of the other

sounds recorded by the accident airplaneís CVR were similar to sounds associated with the failure of the

torque tube. The investigation could not determine exactly what caused the sound. 

224 Structural analysis indicated that the horizontal stabilizer must travel to a position of greater than

15o airplane nose down to allow the acme screw to completely exit the top of the acme nut, as it did during

the final dive. Symmetrical structural damage was found on the tail, indicating that the horizontal stabilizerís

hinge fittings contacted the vertical stabilizerís rear spar, which would occur at a horizontal stabilizer

position of 16.2o airplane nose down.
Analysis 136 Aircraft Accident Report

The airplaneís takeoff weight of 136,513 pounds was well below the takeoff and

climb limits for the departure runway, but it exceeded the airplaneís maximum landing

weight of 130,000 pounds. Because the airplane did not have an in-flight fuel dumping

system, the airplane would have had to remain in flight for about 45 minutes after takeoff

until enough fuel had burned to reduce the airplaneís weight by the 6,500 pounds needed

to reach the airplaneís maximum landing weight. A return to PVR to execute an

overweight landing would have required higher-than-normal approach speeds for landing

and would have created additional workload and risk. An overweight landing at PVR

would have been appropriate if the flight crew had realized the potentially catastrophic

nature of the trim anomaly. However, in light of the airplaneís handling characteristics

from the time of the initial detection of a problem to the initial dive, the flight crew would

not have been aware that they were experiencing a progressive, and ultimately

catastrophic, failure of the horizontal stabilizer trim system.

The flight crew would have been aware that Alaska Airlinesí dispatch and

maintenance control in Seattle-Tacoma International Airport (SEA), Seattle, Washington,

and LAX could be contacted by radio (via ground-based repeater stations) when the

airplane neared the United States. However, even though the last horizontal stabilizer

trimming movement was recorded by the FDR about 1349:51, the flight crew did not

contact Alaska Airlinesí maintenance until shortly before the beginning of the CVR

transcript about 1549,225 which suggests that control problems caused by the jammed

horizontal stabilizer remained manageable for some time.226 Further, as previously

mentioned, the positive aerodynamic effects of the higher cruise airspeed and fuel burn

would have reduced the necessary flight control pressures to roughly 10 pounds and made

the airplane easier to control. Therefore, the Safety Board concludes that, in light of the

absence of a checklist requirement to land as soon as possible and the circumstances

confronting the flight crew, the flight crewís decision not to return to PVR immediately

after recognizing the horizontal stabilizer trim system malfunction was understandable. 

Although they elected not to return to PVR, later in the flight the flight crew

decided to divert to LAX, rather than continue to San Francisco International Airport

(SFO), San Francisco, California, where the flight was originally scheduled to make an

intermediate stop before continuing to SEA. Comments recorded by the CVR indicated

that the flight crew may have felt pressure from Alaska Airlines dispatch personnel to land

in SFO.227 However, after discussing the malfunctioning trim system and current and

expected weather conditions at SFO and LAX with Alaska Airlines dispatch and

maintenance personnel, the captain decided to land at LAX rather than continue to SFO.

225 The FDR indicates that a continuing series of radio transmissions began over the very-high frequency

2 channel (used for all non-ATC radio transmissions) at 1521. These transmissions continued until the end of

the FDR recording, which suggests that the flight crew began contacting Alaska Airlines maintenance

personnel at this time.

226 The Safety Board notes that the 30-minute CVR recording did not capture the flight crewís earlier

troubleshooting efforts or the beginning of the flight crewís discussions with maintenance personnel. A

longer CVR recording that captured these events would have aided in this investigation. In Safety

Recommendation A-99-16, the Board recommended that CVRs on all airplanes required to carry both a

CVR and an FDR be capable of recording the last 2 hours of audio. This safety recommendation is currently

classified ìOpenóUnacceptable Response.î
Analysis 137 Aircraft Accident Report

The decision to divert to LAX was apparently based on several factors, including more

favorable wind conditions at LAX (compared to a direct crosswind at SFO) that would

reduce the airplaneís ground speed on approach and landing228 and the captainís concern,

expressed to Alaska Airlines dispatch personnel, about ìoverflying suitable airports.î The

Safety Board concludes that the flight crewís decision to divert the flight to LAX rather

than continue to SFO as originally planned was prudent and appropriate. Further, the

Safety Board concludes that Alaska Airlines dispatch personnel appear to have attempted

to influence the flight crew to continue to SFO instead of diverting to LAX. 

2.2.5.2 Use of the Autopilot

After the flight crew had manually flown the airplane for almost 2 hours, the

autopilot was engaged about 1547, disengaged at 1549:56, and re-engaged 19 seconds

later. The autopilot remained engaged until 1609:16, just before the initial dive. No

discussion on the CVR (which began at 1549) indicated why the autopilot was engaged in

either instance. As discussed previously, the increased airspeed and reduced weight would

have brought the airplane closer to a trimmed condition, thus allowing autopilot

engagement to maintain altitude and heading while further troubleshooting was attempted.

However, Alaska Airlinesí MD-80 QRH Stabilizer Inoperative checklist states, ìdo not

use autopilotî if both trim systems are inoperative. In light of the autopilotís inability to

maintain trim (using the alternate trim motor) during the climbout from PVR and the flight

crewís subsequent unsuccessful attempts to manually activate the primary trim control

system (using the primary trim motor), the flight crew should have known that both the

alternate and the primary trim control systems were inoperative. Thus, the flight crewís

use of the autopilot was contrary to company procedures. 

Because the AC load meter registered electrical spikes when the crew attempted to

activate the primary trim system, the flight crew should have realized that the primary trim

motor was operational and that the system was jammed beyond the trim motorís

capability. Further, engagement of the autopilot, which would have been making

automatic elevator corrections to the airplaneís mistrimmed condition, masked the

airplaneís true condition from the flight crew. If the autopilot had subsequently been

disconnected without one of the flight crewmembers holding the control wheel and

making immediate corrective inputs, the airplaneís out-of-trim condition would have

resulted in a severe pitch maneuver immediately after the autopilot disconnect. Therefore,

227 At 1552:02, after the captain had stated his intention to divert to LAX, Alaska Airlines dispatch

personnel cautioned that if the flight landed at LAX rather than SFO, ìweíll be looking at probably an hour

to an hour and a half [before the airplane could depart again] we have a major flow program going right

now.î At 1552:41, the captain responded, ìI really didnít want to hear about the flow being the reason youíre

calling us cause Iím concerned about overflying suitable airports.î At 1555:00, the captain commented to a

flight attendant, ìit just blows me away they think weíre gonna land, theyíre gonna fix it, now theyíre

worried about the flow, Iím sorry this airplaneís [not] gonna go anywhere for a whileÖso you know.î After

a flight attendant replied, ìso theyíre trying to put the pressure on you,î the captain stated, ìwell no, yea.î

228 According to Alaska Airlinesí MD-80 QRH Stabilizer Inoperative checklist, 15o of flaps would have

been the appropriate flap setting for the accident airplaneís approach and landing with a jammed stabilizer;

this reduced flap setting would have increased approach speeds and required a corresponding increase in the

amount of runway needed during landing. 
Analysis 138 Aircraft Accident Report

the Safety Board concludes that the flight crewís use of the autopilot while the horizontal

stabilizer was jammed was not appropriate.229

2.2.5.3 Configuration Changes

At 1615:56, after recovery from the initial dive, the captain told the air traffic

controller that he wanted to ìchange my configuration, make sure I can control the jet and

Iíd like to do that out here over the bay if I may.î As previously mentioned, the captain

then ordered extension of the slats at 1617:54 and the flaps at 1618:05. The captain did not

brief the first officer about what to expect or what to do if these configuration changes

resulted in excessive flight control pressures or loss of control of the airplane. Further, the

captain did not specify that the flaps should be extended at a slower-than-normal rate,

which would have been a prudent precaution to minimize the possibility of the

configuration change causing abrupt airplane movements that could be difficult to control.

Nevertheless, at 1618:17, after the slats and flaps were extended, the captain noted that the

airplane was ìpretty stable right here.î The captain added that the airspeed needed to

decrease to 180 KIAS (the airplane was then at 250 KIAS). Nine seconds later, at 1618:26,

the captain ordered retraction of the slats and flaps, and the airspeed began to subsequently

increase. It was not clear from the CVR recording why the captain ordered retraction of

the slats and flaps and allowed the airspeed to increase nor did the CVR recording indicate

any discussion about the possible effects of the slat and flap extension.

The Safety Board notes that an airplane with flight control problems should be

handled in a slow and methodical manner and that any configuration that would aid a

landing should be maintained if possible. On the basis of the captainís comment, the

airplane was stable after the slat and flap extension at 1618:05. This configuration would

have aided the approach and landing process. The Safety Board concludes that flight

crews dealing with an in-flight control problem should maintain any configuration change

that would aid in accomplishing a safe approach and landing, unless that configuration

change adversely affects the airplaneís controllability. 

2.2.5.4 Activation of the Primary Trim Motor

At 1618:49, after the slats and flaps were retracted, the captain stated that he

wanted to ìget the nose upÖand then let the nose fall through and see if we can stab it

when itís unloaded.î The first officer responded, ìyou mean use this again? I donít think

we shouldÖif it can fly.î These statements suggest that the captain may have been

indicating his intention to retry the primary trim system after reducing aerodynamic forces

on the horizontal stabilizer. However, after the first officerís statement at 1619:14, ìI think

if itís controllable, we oughta just try to land it,î the captain abandoned his plan and

responded, ìok letís head for LA.î

229 The Safety Board also notes that the captainís disconnection of the autopilot when he was the pilot not

flying was also contrary to standard industry procedures. Normally, the pilot flying would use the autopilot

disconnect switch on the control column and then assess the airplaneís controllability. 
Analysis 139 Aircraft Accident Report

The Safety Board notes that the earlier repeated attempts to activate the primary

trim system went well beyond what was called for in Alaska Airlinesí checklist

procedures and ultimately precipitated the release of the jam of the acme screw and nut,

resulting in the lower mechanical stop impacting the bottom of the nut. An additional

attempt to use the trim switches at this point would not have been prudent. The severity of

the initial dive changed the situation aboard the airplane to an emergency, which required

a more deliberate and cautious approach. 

The Safety Board recognizes that, from an operational perspective, the flight crew

could not have known the extent of airplane damage. Although flight crews are trained in

jammed stabilizer and runaway stabilizer scenarios, the loss of acme nut and screw

engagement exceeded any events anticipated in emergency training scenarios, and the

flight crew was not trained to devise or execute appropriate configurations and procedures

to minimize further damage to the airplane or to prevent the accident. However, the flight

crewís earlier attempts to activate the trim motor and configuration changes may have

worsened the situation. As previously discussed, the captainís activation of the primary

trim motor at 1609:16 precipitated the release of the jam and the initiation of the initial

dive. However, it was not clear how many times previous to that the flight crew activated

the primary trim motor nor was it clear whether or to what extent the prior activations

hastened the release of the jam. Therefore, the Board could not determine the extent to

which the activation of the primary trim motor played a role in causing or contributing to

the accident. 

2.2.5.5 Adequacy of Current Guidance

 The Safety Board notes that after the flight 261 accident, Boeing issued a flight

operations bulletin outlining procedures to be followed in the event of an inoperative or

malfunctioning horizontal stabilizer trim system. The bulletin advised flight crews to 

complete the flight crew operating manual (FCOM) checklist(s). Do not attempt

additional actions beyond that contained in the checklist(s). If completing the

checklist procedures does not result in operable trim system, consider landing at

the nearest suitable airport.

 The Safety Board agrees that this advice is generally appropriate. However, the

Board does not agree that the flight crew should merely ìconsiderî landing at the nearest

suitable airport if accomplishing the checklist items does not result in an operational trim

system. In such a case, the flight crew should always land at the nearest suitable airport as

expeditiously and safely as possible. Further, the bulletin provides additional information

regarding the possibility that repeated or continuous use of the trim motors may result in

thermal cutoff and states that the motor may reset after a cooling period.230 The Board is

concerned that this additional information addressing repeated or continuous use of the

trim motors may weaken or confuse the initial guidance to refrain from attempting

troubleshooting measures beyond those specified in the checklist procedures. 

230 For additional information about the content of the Boeing flight operations bulletin, see

section 1.17.2.3.1.
Analysis 140 Aircraft Accident Report

The Safety Board concludes that, without clearer guidance to flight crews

regarding which actions are appropriate and which are inappropriate in the event of an

inoperative or malfunctioning flight control system, pilots may experiment with

improvised troubleshooting measures that could inadvertently worsen the condition of a

controllable airplane. Accordingly, the Safety Board believes that the FAA should issue a

flight standards information bulletin directing air carriers to instruct pilots that in the event

of an inoperative or malfunctioning flight control system, if the airplane is controllable

they should complete only the applicable checklist procedures and should not attempt any

corrective actions beyond those specified. In particular, in the event of an inoperative or

malfunctioning horizontal stabilizer trim system, after a final determination has been

made in accordance with the applicable checklist that both the primary and alternate trim

systems are inoperative, neither the primary nor the alternate trim motor should be

activated, either by engaging the autopilot or using any other trim control switch or

handle. Pilots should further be instructed that if checklist procedures are not effective,

they should land at the nearest suitable airport.The Safety Board also believes that the

FAA should direct all Certificate Management Offices (CMO) to instruct inspectors to

conduct surveillance of airline dispatch and maintenance control personnel to ensure that

their training and operations directives provide appropriate dispatch support to pilots who

are experiencing a malfunction threatening safety of flight and instruct them to refrain

from suggesting continued flight in the interest of airline flight scheduling.

2.3 Evaluation of Potential Reasons for Excessive 

Acme Nut Thread Wear

As previously mentioned, the condition of the recovered acme nut thread remnants

indicated that approximately 90 percent of the threads had worn away before the

remainder of the threads sheared off.231 Further, calculations using the 0.033-inch end play

measurement from the accident jackscrew assemblyís last end play check in September

1997 and an assumed total wear of 0.133 inch at the time the worn threads were sheared

off indicated that the average wear rate of the accident acme nut threads between

September 1997 and the time of the accident was 0.012 inch per 1,000 flight hours. As

previously discussed, the expected wear rate is about 0.001 inch per 1,000 flight hours.232

Therefore, the Safety Board concludes that the acme nut threads on the accident airplaneís

horizontal stabilizer jackscrew assembly wore at an excessive rate. There is, however, no

reason to believe that this excessive rate was constant throughout this time period.

In evaluating what could have caused the excessive wear of the accident acme nut

threads, the Safety Board considered the potential effects of the following factors: (1) the

use of Aeroshell 33 for lubrication of the jackscrew assembly, (2) acme screw thread

231 For more information about the condition of the recovered acme nut threads, see section 1.16.2.6.

232 This was the wear rate documented in 1967 test results for DC-8 and DC-9 jackscrew assemblies and

was also the rate reported by McDonnell Douglas All Operators Letter (AOL) 9-2120A. For more

information on these expected wear rates, see section 1.6.3.3.2.
Analysis 141 Aircraft Accident Report

surface finish, (3) foreign debris, (4) abnormal loading of the acme nut threads, and (5)

insufficient lubrication of the jackscrew assembly.

2.3.1 Use of Aeroshell 33 for Lubrication of the Jackscrew 

Assembly

Task cards applicable to the lubrication of the accident jackscrew assembly for its

last three scheduled lubrications (September 1999, January 1999, and June 1998)

specified the use of Aeroshell 33. Findings from laboratory examinations of degraded

grease found on the accident acme screw were consistent with the presence of

Aeroshell 33, as well as the previously specified grease, Mobilgrease 28. Aeroshell 33 has

properties that are different from those of Mobilgrease 28.233 Therefore, Safety Board

investigators considered whether Alaska Airlinesí December 1997 switch to Aeroshell 33

might have caused the acme nut threads to wear faster. However, laboratory tests showed

no significant adverse effects when Aeroshell 33 was used. Specifically, those tests

indicated that the wear rates of test specimens lubricated with Aeroshell 33, or mixtures

containing Aeroshell 33, were lower than those of test specimens lubricated with

Mobilgrease 28.234 This finding is consistent with the fact that only three of Alaska

Airlinesí acme nuts wore at an excessive rate after Aeroshell 33 was introduced in

December 1997.235 None of the rest of the MD-80s in Alaska Airlinesí fleet showed any

indication of excessive acme nut wear or any other anomalies that could be associated

with the use of Aeroshell 33.

Standardized grease testing conducted by the U.S. Navyís Aerospace Materials

Laboratory on mixtures of the two greases indicated that the grease mixture did not meet

the industry standard for compatibility at the 90/10 and 10/90 ratios. However, test results

showed that the incompatibility had an insignificant effect on the lubrication performance

of the grease. These results were again confirmed by the limited number of acme nuts that

wore at an excessive rate after Aeroshell 33 was introduced in September 1997. These

grease mixtures would have been present on all Alaska Airlines MD-80s for some time

during the switch from Mobilgrease 28 to Aeroshell 33; if Aeroshell 33 or mixtures

containing Aeroshell 33 had a negative effect on acme nut wear, these effects would have

been apparent throughout the fleet. 

Because the recovered accident acme screw and sheared nut thread remnants had

some evidence of corrosion, the surface chemistry of aluminum-bronze acme nut material

was evaluated. Tests conducted by the U.S. Navyís Aerospace Materials Laboratory and

Science Applications International Corporation determined that Aeroshell 33 was not

233 Boeingís MD-80 Maintenance Manual specified the use of lubricants that meet the specifications of

MIL-G-81322. Mobilgrease 28 meets these specifications, whereas Aeroshell 33 meets the specifications of

MIL-G-23827. 

234 For more information about these tests, see section 1.16.8.2.

235 Only three Alaska Airlines jackscrew assemblies are known to have worn at a higher-than-expected

rate: the accident assembly and the assemblies from airplanes N981AS and N982AS. For more information

about the acme nuts from airplanes N981AS and N982AS, see section 1.18.4.
Analysis 142 Aircraft Accident Report

corrosive to aluminum bronze.236 It is noteworthy that none of the Alaska Airlines acme

screws or nuts examined by the Safety Board that had been lubricated with Aeroshell 33,

including those from two other Alaska Airlines airplanes that wore at an excessive rate,

revealed evidence of corrosion. However, immersion of unlubricated metal surfaces in salt

water is known to cause corrosion. Therefore, the corrosion on the acme screw and nut

thread remnants was very likely caused by exposure to seawater during the 7 1/2 days

before they were recovered, and the use of Aeroshell 33 to lubricate the jackscrew

assembly was not a factor in the excessive wear of the accident acme nut.

Nonetheless, because of the potential for improper lubrication practices and

inappropriate grease selection to have adverse safety effects, the Safety Board continues to

be concerned that Alaska Airlines began using Aeroshell 33, and the FAA did not object to

its use, without sufficient research, testing, or followup tracking and evaluation of its

in-service performance to demonstrate its acceptability.237 This lack of research, testing,

and followup in connection with the December 1997 grease change is all the more

disconcerting considering that, in April 1996, Alaska Airlines had significantly extended

the lubrication interval for the horizontal stabilizer and associated components from 1,600

flight hours to 8 months (the equivalent of about 2,250 flight hours). That interval

extension was presumably based on the conclusion that the previous long-term

performance of the grease then in use (Mobilgrease 28) demonstrated that it would remain

effective in those areas for the extended interval. However, Alaska Airlines had no

previous experience with Aeroshell 33 in those areas and, therefore, could not have known

whether this extended interval would also be appropriate for the performance of

Aeroshell 33.

2.3.2 Acme Screw Thread Surface Finish

Because of previous instances of excessive acme nut thread wear on MD-11

jackscrew assemblies resulting from improper acme screw thread surface finish,238 Safety

Board investigators considered this as a possible factor in the excessive wear of the

accident nut threads. However, a detailed metallurgical examination of the accident acme

screw found no evidence that the thread surface finish was out of production

specifications. 

2.3.3 Foreign Debris 

Because the excessive wear of two Hawaiian Airlines DC-9 acme nuts was

apparently associated with the presence of abrasive grit-blasting material in the jackscrew

assembly,239 Safety Board investigators considered whether the abrasive effects of foreign

236 For more information about these tests, see section 1.16.8.2.

237 For more information about the circumstances regarding and the procedures followed for this grease

change, see section 1.6.3.2.2.

238 For more information about these instances of premature wear on MD-11 jackscrew assemblies, see

section 1.18.5.

239 For more information about these two acme nuts, see section 1.16.6.1.
Analysis 143 Aircraft Accident Report

debris might have been a factor in the excessive wear of the accident acme nut threads.

However, examinations and tests of grease samples taken from the accident acme screw

found no evidence of foreign debris that would have abraded the acme nut threads during

operation. Metallic particles found in grease residue from the acme nutís grease fitting

counterbore were from the nut itself and were consistent with sliding wear, not abrasive

wear. Further, although the Hawaiian Airlines jackscrew assemblies showed evidence of

abrasive wear on the acme screw threads and the underside of the acme nut threads, which

normally do not wear, there was no such evidence on the accident acme screw and nut

threads.

Although the abrasive effects of foreign debris was not a factor in the excessive

wear of the accident jackscrew assembly, the Safety Board remains concerned that this

could play a role in the excessive wear of other jackscrew assemblies. This issue, and the

related issue of the adequacy of the current end play check interval, are further discussed

in section 2.4.2.

2.3.4 Abnormal Loading of the Acme Nut Threads

Safety Board investigators also considered the possibility that abnormal loading on

the jackscrew assembly may have caused the acme nut threads to wear excessively. This

scenario was evaluated following reports from Alaska Airlines of a ìwobblingî acme

screw.240 The wobble appeared as a slight rocking motion of the acme nut as the screw was

rotated within it. The investigators determined that the wobble in that and other jackscrew

assemblies, including the accident jackscrew assembly, was associated with a torque tube

wear band. It was hypothesized that this wobble might alter the engagement of the acme

nut and screw threads, thus accelerating thread wear. However, analysis of the jackscrew

assembly revealed that the gimbal rings, which hold the acme nut in place, accommodate

any such wobble and prevent wobbling from resulting in any abnormal loading. This was

confirmed when comparing wear rates of acme nuts from assemblies that had torque tubes

with little or no wear band and those with deeper wear bands. The comparison showed no

statistically significant difference in corresponding average wear rates. 

2.3.5 Summary of Possibilities Ruled Out as Reasons for the 

Excessive Acme Nut Thread Wear

As a result of the findings discussed in sections 2.3.1 through 2.3.4, the Safety

Board concludes that Alaska Airlinesí use of Aeroshell 33 for lubrication of the jackscrew

assembly, acme screw thread surface finish, foreign debris, and abnormal loading of the

acme nut threads were not factors in the excessive wear of the accident acme nut threads. 

240 For more information about the wobbling acme screw, see section 1.16.6.2.
Analysis 144 Aircraft Accident Report

2.3.6 Insufficient Lubrication of the Jackscrew Assembly 

It is generally understood that a lack of lubrication of moving parts significantly

increases wear. Laboratory tests confirmed that the unlubricated wear rate of

aluminum-bronze test specimens, under pressure conditions associated with the normal

flight loads on the jackscrew assembly, was roughly 10 times higher than the wear rate of

test specimens lubricated with any tested grease type or combination of greases.241 As

discussed in section 1.6.3.3.2, previous instances of excessive and accelerated wear of

DC-9 and MD-80 acme nuts has been attributed to, or associated with concerns about,

insufficient lubrication. Thus, the accident acme nut threadsí excessive and accelerated

wear is consistent with a lack of sufficient lubrication. Furthermore, as discussed below,

there were additional indications that the accident jackscrew assembly had not been

adequately lubricated before the accident.

First, no evidence of any type of grease in any condition, either semi-fluid (that is,

fresh) or dry/solid (that is, old/degraded), was found inside the acme nut or on the working

region of the recovered acme screw.242 Further, in photographs taken immediately after

recovery and during initial inspection of the jackscrew assembly, no grease is visible on

the working areas of the acme screw. (See figures 12a and 12b.) Detailed metallurgical

examination revealed only small flakes of dried and hardened grease attached to some of

the acme nut thread remnants wrapped around the acme screw, but no evidence of grease

capable of providing significant lubrication.

These findings contradict the assertions of some Alaska Airlines representatives

who stated that they observed or felt a greaselike substance on the surface of the acme

screw immediately after it was recovered. However, the Safety Board notes that seawater,

fuel, or hydraulic fluid from the accident wreckage could have caused the acme screw to

appear shiny and that this sheen might have been mistaken for grease. To preserve the

condition of the acme screw and thread remnants, they were rinsed with fresh water

immediately after recovery, but that rinsing was accomplished by directing the water from

a hose similar to a garden hose over the gearbox and allowing the water to flow down over

the acme screw; the stream of water used to rinse the screw was not directly sprayed on it.

The acme screw may have been rinsed again with a garden-type hose after it was brought

on shore, but it was never subjected to any type of high-pressure wash. It should be noted

that photographs of the acme screw taken after these rinses show sand still adhered to the

surfaces. (See figure 14.)

Although the jackscrew assembly had been immersed in seawater for 7 1/2 days

before it was recovered, laboratory tests at the U.S. Navyís Aerospace Materials

Laboratory showed that immersion in seawater does not significantly alter or remove

grease.243 This was demonstrated by the fact that semi-fluid grease was found adhered to

241 For more information about these tests, see section 1.16.8.3.

242 As previously stated, the acme screw working region is that part of the screw that can come in contact

with the acme nut during its operation between the upper and lower mechanical stop limits.

243 For more information about these tests, see section 1.16.8.1.
Analysis 145 Aircraft Accident Report

airplane components on the accident airplane even after they had been immersed in

seawater for 7 1/2 days. Specifically, some evidence of semi-fluid grease was found

outside the working region on the extreme lower end of the acme screw, and significant

amounts of intact semi-fluid grease were visible on other airplane components. The Safety

Board notes that a coating of grease was still clearly visible on an acme screw recovered

from a China Northern MD-80 airplane that crashed on May 7, 2002, into a bay near

Dalian, China.244 The acme screw from that airplane had been immersed in seawater for 5

days. 

Second, the recovered acme nutís grease fitting passageway and counterbore

contained a dried, black, claylike substance consistent with degraded grease. If the acme

nutís grease fitting had recently been used to lubricate the jackscrew assembly, as it should

have been during the jackscrew assemblyís last lubrication on September 14, 1999 (as

well as during all previous lubrications), the counterbore should have contained fresh

grease. The contents of the counterbore area should not have been affected by impact or

salvage damage because the area was protected externally by the grease fitting on one side

and was isolated internally from direct contact on the bore side. 

Third, postaccident interviews with the SFO mechanic who was responsible for

performing the last lubrication of the accident airplaneís horizontal stabilizer components,

including the jackscrew assembly, revealed his lack of knowledge about how to properly

perform the procedure. For example, he indicated that the lubrication task took about 1

hour, whereas Boeing documents and testimony indicated that, when properly done, the

task should take more than 4 person hours. The SFO mechanic also stated that he did not

recall checking to see if grease was coming out of the top of the acme nut, as specified in

the general lubrication guidance. Further, in the recovered wreckage, about 40 percent of

the other grease fittings on the elevators and horizontal stabilizer included on the same

task card as the acme screw and nut contained either dry or semi-dry grease. In

comparison, grease fittings on the rudder that had been lubricated in January 1999 by a

different mechanic contained mostly fresh or semi-fresh grease. This suggests that the

SFO mechanic who was responsible for lubricating the jackscrew assembly in September

1999 did not adequately perform the task. It is noteworthy that the other two Alaska

Airlines jackscrew assemblies with higher-than-expected wear rates had received their last

horizontal stabilizer lubrication servicing at OAK during C checks, but both had received

the lubrication before that at SFO. The lubrication of one of those two assemblies at SFO

was accomplished by the same mechanic who was responsible for the last lubrication of

the accident airplaneís jackscrew assembly.

On the basis of all of the factors discussed in this section, the Safety Board

concludes that there was no effective lubrication on the acme screw and nut interface at

the time of the Alaska Airlines flight 261 accident. Further, the Board notes that the

inadequate lubrication of the accident jackscrew assembly was not an isolated occurrence.

The recent discovery by Alaska Airlines of inadequately lubricated MD-80 rudder trim tab

244 The last scheduled lubrication of the jackscrew assembly on the China Northern airplane was during

an A check that took place sometime between March 27 and April 2, 2002, and the specified grease was

Aeroshell 5 (MIL-G-3545C). 
Analysis 146 Aircraft Accident Report

hinge support bearings245 demonstrates that deficiencies continue to exist in Alaska

Airlinesí lubrication practices.

2.3.6.1 Analysis of How Many Recently Scheduled Lubrications Might Have 

Been Missed or Inadequately Performed Before the Accident

The service history of Alaska Airlinesí MD-80 fleet indicates that jackscrew

assembly lubrications remained effective for the 8-month lubrication interval (equivalent

to approximately 2,550 flight hours) in effect before the accident. Otherwise, Alaska

Airlinesí entire MD-80 fleet would have experienced excessive acme nut wear, which was

not the case. At the time of the accident, only 4 months had elapsed since the accident

airplaneís last scheduled jackscrew assembly lubrication in September 1999. The

condition of the recovered jackscrew assembly, specifically, the absence of any grease

inside the acme nut or on the working areas of the acme screw, and the degraded grease

inside the acme nutís grease fitting passageway and counterbore, indicates that at least that

lubrication was missed or inadequate.

Then number of lubrication opportunities beyond the one in September 1999 that

might have been performed inadequately or missed entirely is less certain.246 However, it

is apparent that the abnormally high wear of the accident acme nut threads likely occurred

over a period of time that extended beyond the 4 months between the September 1999

lubrication and the accident date. Laboratory tests indicate that the wear rate for

unlubricated parts247 cannot account for the amount of wear exhibited by the accident

acme nut thread remnants if it had occurred only since the last scheduled lubrication. This

indicates that unlubricated wear rates were likely experienced over a longer period of time

and, therefore, more than just the last scheduled lubrication was missed or inadequately

performed. The Safety Board could not determine exactly how many more scheduled

lubrications were missed or inadequately performed. However, the Safety Board

concludes that the excessive and accelerated wear of the accident jackscrew assembly

acme nut threads was the result of insufficient lubrication, which was directly causal to the

Alaska Airlines flight 261 accident. 

2.3.6.2 Alaska Airlinesí Lubrication Interval Extension

DC-9 test results and DC-8 service history indicated that frequent lubrication of

the jackscrew assembly would allow the acme screw to meet its original design life of

30,000 flight hours. DC-9 certification documents, including Douglas Process

Standard 3.17-49 (issued August 1, 1964), specified a lubrication interval for the

245 For more information, see section 1.6.3.2.3. 

246 Because of the absence of any end play measurements since the October 1997 measurement, there is

little upon which to base a wear rate estimate for periods of time beginning after the

October 1997 measurement. Further uncertainty is introduced by the amount of wear exhibited by the thread

remnants (approximately 90 percent of their original thickness had worn). The Safety Boardís FEA

indicated that the load distribution, and thus the associated acme nut thread wear rates, changes dramatically

after a wear of approximately 0.095 inch (approximately 74 percent worn), thereby invalidating the

traditionally assumed relationship between end play and wear rate.

247 For more information about these tests, see section 1.16.8.3.
Analysis 147 Aircraft Accident Report

jackscrew assembly of 300 to 350 flight hours. However, the 300- to 350-flight-hour

recommended lubrication interval for the DC-9 was not contained in the manufacturerís

initial on-aircraft maintenance planning (OAMP) documents for the DC-9 or the MD-80.

Instead, those documents specified a lubrication interval of 600 to 900 flight hours.

As discussed in section 1.6.3.2.1.2, in 1987, Alaska Airlinesí lubrication interval

for horizontal stabilizer components, including the jackscrew assembly, was every

500 flight hours, consistent with the manufacturerís recommendation in the Maintenance

Review Board (MRB) report and OAMP documents derived from Maintenance Steering

Group (MSG)-2 guidance, which recommended a lubrication interval of 600 to 900 flight

hours. In 1988, Alaska Airlinesí lubrication interval increased to every 1,000 flight hours

(a 100 percent increase); in 1991, to every 1,200 flight hours (an additional 20 percent

increase); and, in 1994, to every 1,600 flight hours (an additional 33 percent increase).

In 1996, the interval was changed to 8 months with no specified flight-hour limit. Based

on fleet utilization at the time, 8 calendar months equated to about 2,550 flight hours, an

additional increase of greater than 59 percent. Thus, at the time of the accident, Alaska

Airlinesí lubrication interval for the jackscrew assembly was more than 400 percent

greater than it was in 1987.

The investigation did not determine what type of information, if any, was

presented as justification for the lubrication interval extensions in 1988, 1991, and 1994.

However, according to the FAA principal maintenance inspector (PMI) for Alaska

Airlines, who reviewed and accepted the 1996 interval extension, Alaska Airlines

presented documentation of the manufacturerís recently extended recommended interval

as justification for its increase. (The extended recommended lubrication interval in the

MRB report and OAMP document derived from the MSG-3 guidance was every C check,

or every 3,600 flight hours.)

Testimony at the Safety Boardís public hearing and Boeing documents indicated

that the original design engineersí recommended lubrication interval was not considered

during the MRB-3 decision-making process regarding the extension of the manufacturerís

recommended interval. Further, Boeing design engineers were not consulted about nor

aware of the extended 3,600-flight-hour MRB-3 recommended lubrication interval.

Although Alaska Airlinesí extended lubrication and end play check intervals have now

been superseded by the 650-flight-hour lubrication and 2,000-flight-hour end play check

intervals specified in Airworthiness Directive (AD) 2000-15-15, the Safety Board is

concerned that there is no mechanism in place to prevent similar unsafe interval

extensions for other maintenance tasks. This issue and associated safety recommendations

are discussed further in section 2.4.1. 

2.3.6.2.1 Safety Implications of Lubrication Interval Extension

As grease is used in a system, it loses its effectiveness over time and requires

replacement. Longer lubrication intervals increase the likelihood that a missed or

inadequate lubrication will result in excessive wear. Conversely, shorter lubrication

intervals increase the likelihood that, even if a lubrication is missed or inadequately

performed, the existing grease will remain effective until the next scheduled lubrication. 
Analysis 148 Aircraft Accident Report

The Safety Board notes that at the time of Alaska Airlinesí increase to an 8-month

lubrication interval, it was the only U.S. airline that had a calendar-time lubrication

interval with no accompanying flight-hour limit and no specification, ìwhichever comes

first.î A calendar-time lubrication interval can degrade the margin of safety because wear

is directly related to aircraft usage, or flight hours, and not to calendar time. Also, a purely

calendar-based interval does not account for increases in flight hours that result from

increased airplane utilization. Conversely, intervals based on flight hours, or on calendar

time with an accompanying flight-hour time limit and the proviso, ìwhichever comes

first,î ensure that the flight-hour limit will not be exceeded as a result of increased

airplane utilization. Thus, unless a maximum utilization is also specified, calendar-time

intervals are inappropriate for certain tasks, such as lubrication or inspections for fatigue,

when component deterioration is related to usage, not time.248

In sum, at the time of the accident, Boeingís recommended lubrication interval for

the MD-80 jackscrew assembly was every 3,600 flight hours, about 4 to 6 times longer

than Douglasí original recommendation in the MSG-2 OAMP document of every 600 to

900 flight hours. Alaska Airlinesí lubrication interval for the MD-80 jackscrew

assemblyóalthough it was still less than the manufacturerís recommended interval in the

MSG-3 OAMP document of 3,600 flight hoursówas about 3 to 4 times longer than

Douglasí originally recommended lubrication interval, resulting in a significant decrease

in the MD-80 fleetís ability to tolerate missed or inadequate lubrications. The Board notes

that the negative safety implications of the ongoing lubrication interval extensions were

magnified by the simultaneous ongoing extensions of the end play check interval in that

there would be fewer and fewer opportunities to discover and address any excessive wear

resulting from lubrication deficiencies.

Therefore, the Safety Board concludes that Alaska Airlinesí extensions of its

lubrication interval for its MD-80 horizontal stabilizer components and the FAAís

approval of these extensions, the last of which was based on Boeingís extension of the

recommended lubrication interval, increased the likelihood that a missed or inadequate

lubrication would result in excessive wear of jackscrew assembly acme nut threads and,

therefore, was a direct cause of the excessive wear and contributed to the Alaska Airlines

flight 261 accident.

2.3.6.3 Adequacy of Lubrication Procedures

The horizontal stabilizer lubrication procedure specifies that, after the access doors

are opened, grease is to be applied to the acme nut grease fitting under pressure. The

procedure further specifies the brush application of a light coat of grease to the acme

screw threads and operation of the trim system through its full range of travel to distribute

the grease over the length of the acme screw.

248 In contrast, some maintenance tasks are intended to prevent or control conditions (such as corrosion

or deterioration based on aging) that are based solely on the passage of time. Such tasks might appropriately

be tied to calendar-time intervals without any flight-hour limits.
Analysis 149 Aircraft Accident Report

Safety Board investigators observed maintenance personnel from two MD-80

operators perform jackscrew assembly lubrications and discussed the lubrication

procedure with those and other maintenance personnel from those operators. Investigators

noted many differences in the methods used by the personnel to accomplish certain steps

in the lubrication procedure, including the manner in which grease was applied to the

acme nut fitting and screw and the number of times the trim system was cycled to

distribute the grease. Several of the methods observed by or reported to investigators did

not involve application of grease to the entire length of the acme screw and cycling the

trim system several times.

Laboratory demonstrations designed to compare the effectiveness of various

methods of lubricating the jackscrew assembly found that application of a complete

coating of grease over all exposed threads, filling the thread valleys, followed by the

cycling of the trim system several times, maximized the distribution of the grease over the

length of the acme screw. In contrast, the observations and demonstrations established that

applying grease only through the acme nut grease fitting and then cycling the trim system

several times did not distribute an adequate amount of grease over the remainder of the

acme screw. Although both methods of grease application are specified in the lubrication

procedure, if a mechanic mistakenly believed that lubricating only through the acme nut

grease fitting was adequate, the acme screw and nut would receive insufficient lubrication. 

The extent to which these deficiencies in the lubrication procedure may have

played a role in the inadequate lubrication of the accident jackscrew assembly could not

be determined. However, in an October 1, 2001, safety recommendation letter, the Safety

Board expressed its concern that the current lubrication procedure was not adequate to

ensure consistent and thorough lubrication of the jackscrew assembly by all operators and

issued Safety Recommendation A-01-41, which asked the FAA to require Boeing to

ìrevise the lubrication procedure for the horizontal stabilizer trim system of Douglas

DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to minimize the

probability of inadequate lubrication.î In a December 12, 2001, letter, the FAA responded

that it agreed with the intent of Safety Recommendation A-01-41 and that it was ìworking

with the Boeing Commercial Airplane Group to rewrite the lubrication procedures to the

optimal standard.î On June 14, 2002, the Safety Board classified the recommendation

ìOpenóAcceptable Response.î

After the issuance of Safety Recommendation A-01-41, Safety Board investigators

continued to evaluate the adequacy of the current lubrication procedure and identified

three additional areas that should be addressed.

 First, the Safety Board is concerned about the wear debris produced as a result of

the wear process and other foreign debris that may accumulate in the grease over time.

Because material that infiltrates the working grease reduces its lubricating effectiveness, it

is desirable to flush out these materials before fresh grease is added (that is, to completely

replace used, less effective grease with fresh, more effective grease).249 The Board notes

that the current jackscrew lubrication procedure does not stipulate the removal of used

grease from the acme screw before the application of fresh grease.250 The Board is aware

that Boeing is developing a revised lubrication procedure for the jackscrew assembly that
Analysis 150 Aircraft Accident Report

includes removal of the used grease before application of the fresh grease. Although the

revised procedure, if properly performed, should improve the effectiveness of the

lubrication, it is not yet known whether the FAA will require the use of this improved

procedure.

The Safety Board concludes that when lubricating the jackscrew assembly,

removal of degraded grease from the acme screw before application of fresh grease will

increase the effectiveness of the lubrication. Therefore, the Safety Board believes that as

part of the response to Safety Recommendation A-01-41, the FAA should require

operators of DC-9, MD-80/90, and 717 series airplanes to remove used grease from the

jackscrew assembly acme screw and flush degraded grease and particulates from the acme

nut before applying fresh grease. 

Second, Safety Board investigators noted that when they attempted to perform the

lubrication procedure, it was difficult to insert a hand through the access panel openings

because of their size.251 They further noted that after a hand was inserted, it blocked the

view of the jackscrew assembly, requiring the task to be accomplished primarily by ìfeel.î

The Board is aware that Boeing is developing a modification for an expanded access

panel. The Safety Board concludes that a larger access panel would facilitate the proper

accomplishment of the jackscrew assembly lubrication task. Therefore, the Safety Board

believes that, as part of the response to Safety Recommendation A-01-41, the FAA should

require operators of DC-9, MD-80/90, and 717 series airplanes, in coordination with

Boeing, to increase the size of the access panels that are used to accomplish the jackscrew

assembly lubrication procedure.

Third, this investigation has highlighted the need for improved methods for

ensuring that jackscrew assembly lubrications are accomplished properly at scheduled

lubrication intervals. Currently, the lubrication procedure is generally performed and

signed off by a single maintenance technician, and that technicianís work is not required to

be inspected. 

Although the Safety Board cannot be certain that such a requirement would have

prevented the Alaska Airlines flight 261 accident, the Safety Board concludes that if the

jackscrew assembly lubrication procedure were a required inspection item for which an

inspectorís signoff is needed, the potential for unperformed or improperly performed

lubrications would be reduced. Therefore, because of the critical importance of adequately

lubricating the jackscrew assembly and the potentially catastrophic effects of excessive

acme nut wear resulting from insufficient lubrication, the Safety Board believes that the

249 This finding was further supported by interviews with representatives of the largest manufacturer of

nonaviation jackscrew assemblies, Nook Industries. Nook representatives told Safety Board investigators

that purging old grease out of the jackscrew assembly, and then refreshing the acme screw with new grease,

increases the life of the acme nut. 

250 The current jackscrew lubrication procedure does call for the application of new grease into the nut,

via the grease fitting, until grease is observed extruding from the nut; however, this does not remove the old

grease from the acme screw surface or a large portion of the nut.

251 Two access panel openings (one 4 inches by 6 inches and the other 8 inches by 7 inches) are generally

used to accomplish the lubrication procedure. 
Analysis 151 Aircraft Accident Report

FAA should establish the jackscrew assembly lubrication procedure as a required

inspection item that must have an inspectorís signoff before the task can be considered

complete.

2.4 Monitoring Acme Nut Thread Wear 

Because the MD-80 jackscrew assemblyís structural function is critical to the

safety of flight, and that structural function cannot be maintained without proper acme nut

and screw thread engagement, it is essential that acme nut thread wear be regularly

monitored. The failure to adequately monitor acme nut thread wear may result in

continued operation of an airplane with excessive nut thread wear. As demonstrated by the

Alaska Airlines flight 261 accident, because no other structure performs the function of

the jackscrew assembly, the loss of acme nut and screw engagement as a result of

excessive wear will most likely have catastrophic results.

As previously discussed, in-service acme nut thread wear is monitored by

performing an on-wing end play check procedure at specified intervals. Before 1967, there

were no required periodic inspections. The jackscrew assembly had an expected service

life of 30,000 hours, at which time the end play measurement was expected to be 0.0265

inch. However, as a result of higher-than-expected wear rates reported in 1966, in 1967,

Douglas developed the end play check procedure and increased the maximum permissible

end play measurement at which a jackscrew assembly could remain in service to 0.040

inch. (The minimum end play measurement remained at 0.003 inch.)252 In AOL 9-48A,

Douglas stated that jackscrew assemblies could remain in service as long as the end play

measurement remained within these tolerances. 

2.4.1 Alaska Airlinesí Preaccident End Play Check Intervals

Alaska Airlines has consistently required end play checks at every other C check.

However, the length of its C-check interval has changed over time. As a result of those

C-check interval changes, between 1985 and 1996 Alaska Airlines increased its end play

check interval by almost 200 percent.

In 1985, when Alaska Airlinesí MD-80 maintenance program was initially

approved by the FAA, C checks were conducted every 2,500 flight hours; therefore, end

play checks were performed every 5,000 flight hours. By July 1988, C-check intervals had

been extended to 13 months, with no corresponding flight-time limit; therefore, end play

checks were performed every 26 months, which was approximately 6,400 flight hours,

based on the airplane utilization rate at that time. Although FAA approval would have

been required for this C-check interval extension, no information was available regarding

what documentation, if any, Alaska Airlines presented to the FAA to justify the extension. 

252 As discussed in section 1.6.3.3.2, as a result of these higher-than-expected wear rates, Douglas also

changed some of the materials specifications and manufacturing processes for the acme screw (increased

heat treating and nitriding) to reduce wear. 
Analysis 152 Aircraft Accident Report

In 1996, Alaska Airlinesí C-check interval was extended to 15 months; therefore,

end play checks were only required every 30 months. Because Alaska Airlinesí airplane

utilization had also increased, this 30-month interval was equivalent to approximately

9,550 flight hours. Alaska Airlines sought and obtained advance FAA approval for this

C-check interval extension. Alaska Airlinesí director of reliability and maintenance

programs testified at the public hearing that Alaska Airlines had presented the FAA with a

data analysis package based on the maintenance histories of five sample airplanes to

justify the C-check interval extension. He indicated that individual maintenance tasks tied

to C-check intervals (such as the end play check) were not separately considered in

connection with the extension.253 Thus, the FAAís approval of the 1996 C-check extension

also effectively constituted approval to extend the end play check from 26 to 30 months, a

15 percent increase, and, more importantly, a 55 percent increase in flight-hour intervals,

from approximately 6,400 to 9,550 flight hours. 

Although Alaska Airlinesí extended 30-month end play check interval was

consistent with the manufacturerís recommended calendar-time limit, the resulting

9,550-flight-hour interval far exceeded the manufacturerís recommended flight-hour

interval, which at that time was 7,000 or 7,200 flight hours (depending on whether MSG-2

or MSG-3 guidance was used). At that time, Alaska Airlines had the second highest end

play check interval of all operators of DC-9, MD-80/90, and 717 series airplanes. Further,

Alaska Airlines was the only U.S. carrier at that time that did not have an accompanying

flight-hour limit with the specification, ìwhichever comes first,î for its end play check

interval. As discussed previously in connection with Alaska Airlinesí lubrication interval,

use of only a calendar-time interval does not account for increased airplane utilization

rates and could result in a lower safety margin than intended. 

The Safety Board notes that, because of Alaska Airlinesí extended end play check

interval of 30 months, or about 9,550 flight hours, after the accident airplaneís last end

play check in September 1997, the jackscrew assembly would not have been required to

undergo another end play check until March 2000. Between the time that the 0.040- and

0.033-inch end play measurements were recorded in September 1997 and the time of the

accident, the airplane had been flown for 28 months, nearly 9,000 flight hours. During this

time, the acme nut thread wear progressed to failure. 

In light of what has been learned in this investigation, it is now apparent that the

manufacturerís previously recommended end play check intervals of 7,000 or 7,200 flight

hours were not adequate.254 Nonetheless, the Safety Board notes that if Alaska Airlines

had not extended its end play check interval to beyond the recommended interval, the

airplane would have been required to undergo an end play check at least 1,800 to 2,000

flight hours before the accident, and the excessive end play could have been identified at

253 However, based on the history of maintenance discrepancies noted for the five sample airplanes, two

maintenance tasks were identified as inappropriate for extension, and these tasks were converted to

stand-alone items to be performed at shorter intervals. The two maintenance tasks identified as requiring

shorter intervals based on service history were (1) lubrication of the bent-up, trailing-edge wing doors and

(2) lubrication of the bearings and bushings in the elevator hinges.

254 This issue is discussed further in the next section.
Analysis 153 Aircraft Accident Report

that time. Thus, the Safety Board concludes that Alaska Airlinesí extension of the end

play check interval and the FAAís approval of that extension allowed the accident acme

nut threads to wear to failure without the opportunity for detection and, therefore, was a

direct cause of the excessive wear and contributed to the Alaska Airlines flight 261

accident.

2.4.1.1 Adequacy of Existing Process for Establishing Maintenance Task 

Intervals

The Safety Board is concerned that the absence of any significant maintenance

history pertaining to the jackscrew assembly was apparently considered by Alaska

Airlines and the FAA as sufficient justification to extend the end play check interval as

part of the C-check interval extension. In general, the absence of maintenance history

should not be considered adequate justification to extend the interval for the performance

of a critical maintenance task. Any significant maintenance change associated with a

critical flight control system should be independently analyzed and supported by technical

data demonstrating that the proposed change will not present a potential hazard.

Therefore, any maintenance task change related to the jackscrew assembly, which is an

essential element of a critical flight control system, should be handled in this manner.

The Safety Board is further concerned that the MSG and MRB-based process by

which manufacturers develop initial and revised recommended maintenance task

intervals255 resulted in significant extensions of both the lubrication and end play check

intervals without any such analysis or support. As discussed in section 1.6.3.2.1,

testimony at the Safety Boardís public hearing and Boeing documents indicated that

Douglasí original recommended lubrication interval of 600 to 900 flight hours was not

considered during the MSG-3 decision-making process to extend the recommended

interval to 3,600 hours. Further, Boeing design engineers were not consulted about nor

aware of the escalated lubrication interval specified in the MSG-3 documents. The FAAís

MD-80 MRB chairman testified at the public hearing that the escalation of C-check

intervals in the MSG-3 MD-80 MRB did not involve a task-by-task analysis of each task

(such as the jackscrew lubrication task and end play check) that would be affected by the

changed interval. 

The Safety Board concludes that Alaska Airlinesí end play check interval

extension should have been, but was not, supported by adequate technical data to

demonstrate that the extension would not present a potential hazard. The Safety Board

further concludes that the existing process by which manufacturers revise recommended

maintenance task intervals and by which airlines establish and revise these intervals does

not include task-by-task engineering analysis and justification and, therefore, allows for

the possibility of inappropriate interval extensions for potentially critical maintenance

tasks. In addition, the Board notes that the FAA plays a limited role in this process

compared to the role it plays in the initial certification process.

255 For more information about this process, see section 1.6.3.1.1.
Analysis 154 Aircraft Accident Report

Therefore, the Safety Board believes that the FAA should review all existing

maintenance intervals for tasks that could affect critical aircraft components and identify

those that have been extended without adequate engineering justification in the form of

technical data and analysis demonstrating that the extended interval will not present any

increased risk and require modification of those intervals to ensure that they (1) take into

account assumptions made by the original designers, (2) are supported by adequate

technical data and analysis, and (3) include an appropriate safety margin that takes into

account the possibility of missed or inadequate accomplishment of the maintenance task.

In conducting this review, the FAA should also consider original intervals recommended

or established for new aircraft models that are derivatives of earlier models and, if the

aircraft component and the task are substantially the same and the recommended interval

for the new model is greater than that recommended for the earlier model, treat such

original intervals for the derivative model as ìextendedî intervals. The Safety Board

further believes that the FAA should conduct a systemic industrywide evaluation and issue

a report on the process by which manufacturers recommend and airlines establish and

revise maintenance task intervals and make changes to the process to ensure that, in the

future, intervals for each task (1) take into account assumptions made by the original

designers, (2) are supported by adequate technical data and analysis, and (3) include an

appropriate safety margin that takes into account the possibility of missed or inadequate

accomplishment of the maintenance task.

The Safety Board also believes that the FAA should require operators to supply the

FAA, before the implementation of any changes in maintenance task intervals that could

affect critical aircraft components, technical data and analysis for each task demonstrating

that none of the proposed changes will present any potential hazards, and obtain written

approval of the proposed changes from the PMI and written concurrence from the

appropriate FAA Aircraft Certification Office. 

2.4.2 Adequacy of Current End Play Check Intervals

The Safety Board recognizes that the FAA acted promptly following the Alaska

Airlines flight 261 accident by issuing ADs that shortened the end play check interval to

2,000 flight hours. However, evidence collected during this investigation suggests that

acme nut thread wear at a higher-than-expected rate could allow a potentially dangerous

level of wear to occur in less than 2,000 hours. 

As previously discussed, two jackscrew assemblies installed sequentially on the

same Hawaiian Airlines DC-9 airplane that were removed because of high end play

measurements were found to have worn at an unprecedented rate. The first acme nut had

an approximate wear rate of 0.015 inch per 1,000 flight hours, and the second had an

approximate wear rate of 0.008 inch per 1,000 flight hours.256 These wear rates are about

15 and 8 times greater, respectively, than the expected wear rate of about 0.001 inch per

1,000 flight hours. This accelerated wear was attributed to the presence of grit-blasting

256 For more information about these jackscrew assemblies, see section 1.16.6.1.
Analysis 155 Aircraft Accident Report

material that had been introduced inadvertently into the jackscrew assembly and become

embedded in the grease on the acme screw.

If the first excessively worn jackscrew assembly had measured just within, rather

than beyond, the 0.040-inch limit at the last end play check, and assuming the acme nut

threads had continued to wear at an approximate rate of 0.015 inch per 1,000 flight hours,

the end play measurement would have been about 0.069 inch by the time of its next

scheduled end play check 2,000 flight hours later. Further, assuming that this end play

check was either missed or improperly accomplished and the jackscrew assembly

remained in service despite an end play of 0.069 inch and continued to wear at the same

rate, the end play measurement would have been about 0.099 inch by the time of the next

scheduled end play check. Moreover, there is no basis for assuming that the wear rate of

this Hawaiian Airlines jackscrew assembly represents the maximum possible acme nut

thread wear rate. Just as the wear rate and wear mechanism on the Hawaiian Airlines

jackscrew assembly was unprecedented and unanticipated, there may be other

unprecedented and unanticipated wear rates and mechanisms that could also result in

excessive or accelerated wear. Therefore, it is possible that acme nut threads could wear at

an even faster rate than the Hawaiian Airlines acme nut threads, possibly even to

catastrophic limits257 in 2,000 flight hours. 

To establish an appropriately conservative end play check interval, the

uncertainties regarding possible wear mechanisms and the maximum possible wear rate

must be considered in addition to the significant possibility of inaccurate end play

measurements. The Safety Board notes that, when failure mechanisms are known and

clearly defined, standard industry practice to ensure a damage tolerant design is to have a

safety margin that allows for two complete inspection cycles before the predicted failure

time.258 Thus, even if one inspection is missed or inadequately performed, there will be at

least one other opportunity to detect and correct the condition. However, when

uncertainties exist in the failure mechanism, as in the case of acme nut thread wear,

standard industry practice is to increase the safety margin to account for the reduced level

of confidence in the predicted failure time.

In a June 26, 2001, letter to the Safety Board, the FAA stated that it believed the

2,000-flight-hour interval provided an acceptable level of safety, citing the ìrobust designî

of the acme nut and the fact that it could safely carry normal flight loads even when worn

beyond 0.080 inch. The FAA also stated that the 650-flight-hour inspection and

257 According to the Safety Boardís study of thread stress and deformation, the acme nut threads will

begin to deflect and begin the process of sliding over the acme screw threads at a wear level of about

0.093 inch.

258 According to the FAAís Damage Tolerance Assessment Handbook, Volume I, issued in

February 1999, ìdamage tolerance refers to the ability of the design to prevent structural cracks from

precipitating catastrophic fracture when the airframe is subjected to flight or ground loads. Transport

category airframe structure is generally made damage tolerant by means of redundant (ëfail safeí) designs

for which the inspection intervals are set to provide at least two inspection opportunities per number of

flights or flight hours it would take for a visually detectable crack to grow large enough to cause a failure in

flight.î Although this refers to cracks, the Safety Board notes that the damage tolerance principles can be

applied to acme nut wear.
Analysis 156 Aircraft Accident Report

lubrication interval in AD 2000-15-15 provided frequent opportunities (in addition to the

end play check every 2,000 flight hours) to detect wear debris and, therefore, possible

excessive wear. Nonetheless, the Safety Board is concerned that significantly

higher-than-expected wearówear even greater than that of the Hawaiian Airlines

jackscrew assembliesócould result from foreign-object contamination, such as grit blast,

or from other factors that have not yet been identified.

The Safety Board concludes that, because of the possibility that

higher-than-expected wear could cause excessive wear in less than 2,000 flight hours and

the additional possibility that an end play check could be not performed or improperly

performed, the current 2,000-flight-hour end play check interval specified in AD

2000-15-15 may be inadequate to ensure the safety of the DC-9, MD-80/90, and 717 fleet.

Therefore, the Safety Board believes that, pending the incorporation of a fail-safe

mechanism in the design of the DC-9, MD-80/90, and 717 horizontal stabilizer jackscrew

assembly, as recommended in Safety Recommendation A-02-49 in this report, the FAA

should establish an end play check interval that (1) accounts for the possibility of

higher-than-expected wear rates and measurement error in estimating acme nut thread

wear and (2) provides for at least two opportunities to detect excessive wear before a

potentially catastrophic wear condition becomes possible. 

To establish an appropriate end play check interval, it is necessary to monitor end

play measurements over time to identify any excessive or unanticipated wear rates and to

continue evaluating the reliability and validity of end play measurements. Therefore, the

FAA, Boeing, maintenance facilities that overhaul jackscrew assemblies, and operators

should closely evaluate the measurement data currently being reported pursuant to

AD 2000-15-15. The Safety Board concludes that the continued collection and analysis of

end play data are critical to monitoring acme nut thread wear and identifying excessive or

unexpected wear rates, trends, or anomalies. Therefore, the Safety Board believes that the

FAA should require operators to permanently (1) track end play measurements according

to airplane registration number and jackscrew assembly serial number, (2) calculate and

record average wear rates for each airplane based on end play measurements and flight

times, and (3) develop and implement a program to analyze these data to identify and

determine the cause of excessive or unexpected wear rates, trends, or anomalies. The

Safety Board further believes that the FAA should require operators to report this

information to the FAA for use in determining and evaluating an appropriate end play

check interval.

2.4.3 Adequacy of End Play Check Procedure

As discussed in the Safety Boardís October 1, 2001, safety recommendation letter

and its end play data study report, dated March 18, 2002, the current end play check

procedure259 is subject to considerable measurement error. Safety Board investigators

observed end play checks performed by maintenance personnel from several MD-80

operators and identified numerous potential sources for inaccurate end play

measurements. Specifically, it was noted that the accuracy of the results could be affected

by deviations in one or more of the following areas: (1) calibration and interpretation of
Analysis 157 Aircraft Accident Report

the dial indicator; (2) installation of the dial indicator; (3) application and direction of the

specified torque to the restraining fixture; (4) calibration of the torque wrench;

(5) fabrication, lubrication, and maintenance of the restraining fixture; (6) rotation of the

acme screw within its gearbox during the procedure; and (7) the individual mechanicís

knowledge of the procedures. According to the Boardís end play data study report, end

play measurement inaccuracies can be as high as 0.030 inch, and errors can occur in either

direction.260

Further, an additional source of end play measurement error was possible at Alaska

Airlines before August 2000 because its fabricated end play restraining fixture was not

based on Boeing design specifications.261 Although there was only one such fixture at

Alaska Airlinesí maintenance facilities at the time of the accident,262 11 additional

restraining fixtures were made in-house after the accident despite an April 2000 Boeing

message to all DC-9, MD-80, MD-90, and 717 operators asking them to ensure that their

horizontal stabilizer inspection tooling conformed ìto the toolís drawing requirements.î

The Alaska Airlines manager of tool control, who participated in the fabrication of the

restraining fixtures ordered by airline management after the accident, told Safety Board

investigators that ìwhat they were making wasnít even closeî to the Boeing drawing. In

August 2000, following inquiries from Safety Board staff about which restraining fixture

was used during the September 1997 end play check, Alaska Airlines notified the FAA

that it was concerned that the restraining fixtures it had manufactured in-house might not

be ìan equivalent substituteî for the Boeing fixture. Alaska Airlines suspended use of the

in-house manufactured restraining fixtures, quarantined all non-Boeing manufactured

fixtures, and temporarily removed from service 18 airplanes that had been checked with

these fixtures.

The results of Safety Board testing indicated that Alaska Airlinesí fabricated

restraining fixtures could yield end play measurements that were lower than

measurements obtained with a Boeing-manufactured fixture.263 Therefore, it is possible

that the 0.040- and 0.033-inch end play measurements obtained during the accident

airplaneís September 1997 end play check were less than the actual end play and that the

accident jackscrew assembly in fact exceeded the 0.040-inch end play measurement limit

259 As discussed in section 1.6.3.3.2, the end play procedure calls for pulling down on the horizontal

stabilizer by applying a specified amount of torque to a tool known as a restraining fixture to change the load

on the acme screw from tension to compression. The load reversal allows movement, or play, between the

acme screw and nut threads that is measured using a dial indicator, which is mounted on the lower rotational

stop (fixed to the screw) with the movable plunger set against the lower surface of the acme nut that

measures the relative vertical movement between the acme screw and the nut when the loads are applied.

The amount of movement, or end play, is used as an indication of the amount of acme nut thread wear (and

acme screw thread wear, if there is any). The procedure calls for the restraining fixture load to be applied and

removed several times until consistent measurements are achieved.

260 For more information, see table 5 in the Safety Boardís end play data study report in the public docket

for this accident.

261 For more information about Alaska Airlinesí fabricated restraining fixtures, see section 1.6.3.3.3.2.

262 The Safety Board recognizes that Alaska Airlines contends in its submission that the restraining

fixture used during the September 1997 end play check was a Douglas fixture. However, evidence gathered

in the Boardís investigation does not support this contention.
Analysis 158 Aircraft Accident Report

at the time of the 1997 check. It is also possible that a Boeing-manufactured restraining

fixture would have yielded a more accurate reading, which could have resulted in the

jackscrew assembly being removed from service. However, as discussed previously, even

when a restraining fixture manufactured to Boeing specifications is used, there is still a

potential for inaccurate measurements. Therefore, the Safety Board concludes that, until

August 2000, Alaska Airlines used a fabricated restraining fixture that did not meet

Boeing specifications; however, the Board could not determine whether the use of this

noncompliant fixture generated an inaccurate end play measurement during the last end

play check or whether the use of this fixture contributed to the accident.

In Safety Recommendations A-01-42 and -44, the Safety Board asked the FAA to

do the following:

Require the Boeing Commercial Airplane Group to revise the end play

check procedure for the horizontal stabilizer trim system of Douglas DC-9,

McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to

minimize the probability of measurement error and conduct a study to

empirically validate the revised procedure against an appropriate physical

standard of actual acme screw and acme nut wear. This study should also

establish that the procedure produces a measurement that is reliable when

conducted on-wing. (A-01-42)

Require maintenance personnel who inspect the horizontal stabilizer trim

system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717

series airplanes to undergo specialized training for this task. This training

should include familiarization with the selection, inspection, and proper

use of the tooling to perform the end play check. (A-01-44)

In its December 12, 2001, response letter, the FAA indicated that it had asked

Boeing to revise the end play check procedure, and the Safety Board is aware that Boeing

is currently developing improved end play check procedures. Regarding Safety

Recommendation A-01-44, the FAA stated that it believed that ìcurrent regulatory

requirements of 14 CFR [Code of Federal Regulations] 121.375 adequately address

maintenance training programsî but that the agency was ìrevising Advisory Circular [AC]

120-16C, Continuous Airworthiness Maintenance Programs, to expand the intent of the

requirement for maintenance training programs.î In addition, the FAA stated that it

believed that ìcurrent regulatory requirements of 14 CFR 65.81 and 14 CFR 65.103

adequately address the inspection requirements for maintenance personnel and

repairmen.î The FAA concluded that ìcurrent regulatory requirements in place to require

maintenance training programs and inspection requirements for maintenance personnel

and repairmen address the full intent of these safety recommendations.î

263 Alaska Airlinesí fabricated restraining fixture did not result in a lower measurement than the Boeing

fixture on every occasion during the testing. However, Safety Board investigators determined that the

discrepancies, when they existed, could be as high as 0.005 inch and were likely caused by the fabricated

restraining fixtureís increased length of thread engagement, which created friction that required more torque

than the Boeing-manufactured fixture.
Analysis 159 Aircraft Accident Report

On June 14, 2002, the Safety Board classified Safety Recommendation A-01-42

ìOpenóAcceptable Response,î pending revisions to the end play check procedure. In its

letter, the Board classified Safety Recommendation A-01-44 ìOpenóUnacceptable

Response,î stating that it ìcontinues to believe that current FAA regulations do not result

in maintenance personnel being properly trained to perform lubrication and end play

inspection of the acme screw and nut assemblyî and urged the FAA to reconsider its

decision. 

The Safety Board is aware of technologies that could potentially provide more

reliable and accurate indications of acme nut thread wear than the end play check

procedure. For example, as a result of this accident, Sandia National Laboratories, with

FAA funding, began exploring two new methods for monitoring thread wear that involved

the use of nondestructive technology. One of these methods used a portable x-ray fixture

that could be installed next to the acme screw and nut to provide a graphic depiction of all

of the threads. The other method used a small ultrasonic sensor that could be passed over

the acme nut to obtain a graphic depiction of the thread crests on a handheld screen. 

The Safety Board concludes that the on-wing end play check procedure, as

currently practiced, has not been validated and has low reliability. In this regard, the Board

will continue to monitor the FAAís and Boeingís actions in response to Safety

Recommendations A-01-42 and -44. 

2.5 Deficiencies of Jackscrew Assembly Overhaul 

Procedures and Practices

The accident jackscrew assembly was never overhauled nor was it required to be.

However, to determine the adequacy of maintenance and inspection procedures applicable

to all DC-9, MD-80/90, and 717 jackscrew assemblies, Safety Board investigators

evaluated jackscrew assembly overhaul procedures and practices. Specifically,

investigators reviewed the DC-9 Overhaul Maintenance Manual and visited several

maintenance facilities that overhaul jackscrew assemblies, including Integrated

Aerospace, the only contract facility currently used by Boeing to overhaul acme nut and

screw pairs to manufacturing end play specifications.264

Safety Board investigators identified several deficiencies in the DC-9 Overhaul

Maintenance Manual procedures and in the practices of several of the maintenance

facilities that were visited. Specifically, the overhaul manual did not require the use of

work cards documenting each step in the overhaul process, and (with the exception of

Integrated Aerospace) the facilities did not use such work cards. Also, the manual called

for replacement of the acme nut only if the end play measurement was more than

0.040 inch. Although some overhaul facilities had lower self-imposed end play

measurement limits for replacement of the acme nut, at least one facility indicated that it

264 For more information about the DC-9 Overhaul Maintenance Manual and Safety Board investigator

visits to maintenance facilities, see section 1.18.8.
Analysis 160 Aircraft Accident Report

would return an overhauled jackscrew assembly to a customer as long as the end play

measurement did not exceed 0.040 inch. Further, the manual contained no requirement to

record or inform the customer of the end play measurement of an overhauled jackscrew

assembly. This means that a jackscrew assembly with an end play measurement of up to

0.039 inch could be represented as ìoverhauledî and returned to service by an operator

that might not be aware of the high end play measurement.265 A jackscrew assembly could

require overhaul for reasons other than excessive end play; however, it would be

reasonable for a customer to expect that an assembly would be returned after an overhaul

with an end play close to manufacturing specifications.

Further, the required steps for properly conducting the end play check procedure

were not well described and the required equipment was not specified in the DC-9

Overhaul Maintenance Manual. During visits to maintenance facilities, Safety Board

investigators learned that the facilities used different methods and various tools for

measuring end play. (Although investigators saw no evidence that the differences affected

the accuracy of the results, the lack of standardization nonetheless increases the potential

for error to occur.) The overhaul manual also did not contain detailed instructions on how

to apply grease to the jackscrew assembly at the completion of the overhaul nor did it

clearly specify which type of grease to use. Further, although the manual does require that

an overhauled jackscrew assembly be lubricated before it can be returned to the customer,

investigators learned that at least one maintenance facility returns overhauled assemblies

without lubricating them first. 

In addition, the DC-9 Overhaul Maintenance Manual did not contain detailed

instructions nor specify appropriate equipment for checking that the proper acme screw

thread surface finish had been applied. Many of the maintenance facilities visited by

Safety Board investigators indicated that they relied on subvendors to ensure that the

proper acme screw thread surface finish was applied and had no standard method for

verifying that this action had occurred.

Finally, the DC-9 Overhaul Maintenance Manual did not clearly specify

appropriate packaging procedures for transporting jackscrew assemblies after overhaul.

Although the overhaul manual contained detailed protective packaging instructions for

jackscrew assemblies going into ìstorage,î it did not specify any such protective

packaging for jackscrew assemblies being transported. However, it would be prudent to

use protective packaging for all overhauled jackscrew assemblies being returned to a

customer because a maintenance facility cannot be expected to know what the customer

intends to do with an assembly after it is returned.

Integrated Aerospace, the only maintenance facility authorized by Boeing to

overhaul the acme nut and screw to an ìas newî end play condition that meets

265 Although the DC-9 Overhaul Maintenance Manual calls for more frequent end play checks (every

1,000 flight hours) to be conducted on an overhauled unit that was reinstalled with an end play measurement

between 0.034 to 0.039 inch, this provision is of limited value because there is no requirement for operators

to be informed of this elevated end play measurement. Further, operators receiving an overhauled jackscrew

assembly would not be expected to consult the overhaul manual for special maintenance instructions.
Analysis 161 Aircraft Accident Report

manufacturing specifications, uses more rigorous and reliable overhaul procedures and

significant quality control measures that are not used or required by the other facilities that

overhaul jackscrew assemblies. For example, Integrated Aerospace uses detailed work

cards to document each step of the overhaul process. Whenever Integrated Aerospace

receives a jackscrew assembly with an end play measurement greater than 0.015 inch, it

will install a new acme nut, thereby restoring the assembly to the manufacturing

specifications for a new jackscrew assembly (0.003 to 0.010 inch). In doing so, Integrated

Aerospace must comply with detailed specifications provided by Boeing in a service

rework drawing, which are not contained in the DC-9 Overhaul Maintenance Manual. 

In addition, the investigation revealed that no special authorization beyond a

class 1 accessory rating is required for a maintenance facility to overhaul jackscrew

assemblies in accordance with the DC-9 Overhaul Maintenance Manual. A class 1

accessory rating allows a facility to perform maintenance and alteration of a number of

mechanical accessories. The maintenance facility is not required to demonstrate that it has

the necessary capability and equipment to perform jackscrew assembly overhauls. Safety

Board investigators found that the PMI of an FAA-certified maintenance facility may not

even be aware that the facility is performing overhauls of jackscrew assemblies. 

The Safety Board concludes that deficiencies in the overhaul process increase the

likelihood that jackscrew assemblies may be improperly overhauled. The Safety Board

further concludes that the absence of a requirement to record or inform customers of the

end play measurement of an overhauled jackscrew assembly could result in an operator

unknowingly returning a jackscrew assembly to service with a higher-than-expected end

play measurement. Therefore, the Safety Board believes that the FAA should require that

maintenance facilities that overhaul jackscrew assemblies record and inform customers of

an overhauled jackscrew assemblyís end play measurement. 

In addition to recording the end play measurement information provided by a

maintenance facility when it returns an overhauled jackscrew assembly, it would also be

prudent for operators to record end play measurements for the same assembly after it is

installed on an airplane. The Safety Board notes that end play measurements recorded by

maintenance facilities are likely to be obtained during bench checks. However, after the

overhauled assembly is re-installed on an airplane, end play measurements will likely be

obtained through use of the on-wing end play check procedure, which may yield a slightly

different measurement because of differences in the procedure. A wear rate that is

calculated using a bench-check measurement at one point in time compared with an

on-wing measurement at a later point in time will not be as informative or useful as a wear

rate that is calculated using measurements obtained through use of the same end play

check method. Therefore, the Safety Board concludes that operators will maximize the

usefulness of end play measurements and wear rate calculations by recording on-wing end

play measurements whenever a jackscrew assembly is replaced on an airplane.

Accordingly, the Safety Board believes that the FAA should require operators to measure

and record the on-wing end play measurement whenever a jackscrew assembly is

replaced. 
Analysis 162 Aircraft Accident Report

Finally, the Safety Board concludes that, because the jackscrew assembly is an

integral and essential part of the horizontal stabilizer trim system, a critical flight system,

it is important to ensure that maintenance facilities authorized to overhaul these

assemblies possess the proper qualifications, equipment, and documentation. Therefore,

the Safety Board believes that the FAA should require that maintenance facilities that

overhaul DC-9, MD-80/90, and 717 series airplanesí jackscrew assemblies obtain specific

authorization to perform such overhauls, predicated on demonstrating that they possess

the necessary capability, documentation, and equipment for the task and that they have

procedures in place to (1) perform and document the detailed steps that must be followed

to properly accomplish the end play check procedure and lubrication of the jackscrew

assembly, including specification of appropriate tools and grease types; (2) perform and

document the appropriate steps for verifying that the proper acme screw thread surface

finish has been applied; and (3) ensure that appropriate packaging procedures are followed

for all overhauled jackscrew assemblies, regardless of whether the assembly has been

designated for storage or shipping. 

2.6 Horizontal Stabilizer Trim System Design and 

Certification Issues

2.6.1 Acme Nut Thread Loss as a Catastrophic Single-Point 

Failure Mode

The DC-9 horizontal stabilizer trim system (which was also incorporated in the

MD-80/90 and 717 series airplanes) is a critical flight system because certain failures of

the system can be catastrophic. One such failure is the loss of acme screw and nut thread

engagement. However, as previously mentioned, the designers of the system assumed that

at least one set of the jackscrew assemblyís acme screw and nut threads would always be

intact and engaged to act as a load path. Therefore, the repercussions of stripped acme nut

threads and the corresponding effect on the airplane (including the possibility of the acme

screw disengaging from the acme nut) were not considered in the design of the horizontal

stabilizer trim system. 

At the Safety Boardís public hearing, Boeing engineers stated that they considered

loss of the acme nut threads to be a ìmultiple failure eventî and that such loss caused by

excessive wear was not considered a ìreasonably probable single failureî for certification

purposes.266 The Boeing engineers indicated that the jackscrew assembly was designed to

accommodate thread wear but acknowledged that monitoring and managing thread wear

was essential to maintaining the integrity of the design. Similarly, an FAA certification

266 The certification basis for the DC-9, MD-80/90, and 717 horizontal stabilizer trim systems was Civil

Aeronautics Regulations (CAR) 4b. CAR 4b.320, ìControl Systems,î stated that ìan adjustable stabilizer

shall incorporate means to permit, after the occurrence of any reasonably probable single failure of the

actuating system, such adjustment as would be necessary for continued safety of the flight.î (Current

certification regulations specify, in 14 CFR 25.671, that the airplane must be shown to be capable of

continued safe flight and landing after ìany single failureî of the actuating system.)
Analysis 163 Aircraft Accident Report

engineer testified that thread wear ìwas not considered as a mode of failure for either a

systems safety analysis or for structural considerationsî and that the ìdesign of the acme

nut and screw provided enough over-strength so that regulatory requirements could be met

with a significant amount of wear.î However, as the Alaska Airlines flight 261 accident

demonstrates, complete loss of the acme nut threads because of excessive wear is possible.

Further, the accident jackscrew assembly is not the only jackscrew assembly in which

excessive acme nut thread wear has occurred. As discussed previously, excessive wear of

acme nut threads has occurred on other occasions as a result of inadequate lubrication,

improper acme screw thread surface finish, and contamination. In addition, other potential

rapid wear mechanisms may not have yet been identified.

The Safety Board notes that the dual-thread design of the acme screw and nut does

not adequately protect against excessive acme nut thread wear. Although Boeing contends

that the two thread spirals along the length of both the acme screw and nut provide

structural redundancy, the Board notes that each set of thread spirals is always carrying

loads in flight and that both sets of thread spirals are subject to the same wear

mechanisms. Thus, although the dual-thread design may prevent a crack in one thread set

from propagating through the other thread set, both sets of threads remain vulnerable to

simultaneous wear failure. Therefore, the Safety Board concludes that the dual-thread

design of the acme screw and nut does not provide redundancy with regard to wear.

The FAAís certification scheme is intended to protect against catastrophic

single-point failure conditions. Specifically, 14 CFR 25.1309 requires that airplane

systems and associated components be designed so that ìthe occurrence of any failure

condition which would prevent the continued safe flight and landing of the airplane is

extremely improbable.î Further, AC 25.1309-1A, ìSystem Design and Analysis,î defines

ìextremely improbableî failure conditions as ìthose so unlikely that they are not

anticipated to occur during the entire operational life of all airplanes of one typeî and

ìhaving a probability on the order of 1 x 10-9 or less each flight hour, based on a flight of

mean duration for the airplane type.î AC 25.1309-1A specifies that in demonstrating

compliance with 14 CFR 25.1309, ìthe failure of any single element, component, or

connection during any one flightÖshould be assumed, regardless of its probability,î and

ìsuch single failures should not prevent continued safe flight and landing, or significantly

reduce the capability of the airplane or the ability of the crew to cope with the resulting

failure condition.î

An FAA senior certification engineer testified at the public hearing that 14 CFR

25.1309267 did not apply to the jackscrew assembly acme nut because the FAA did not

consider it part of a system. Rather, he stated that the jackscrew assembly was a

ìcombination structural element and systems elementî and that each category was

governed by its respective regulatory requirements. He indicated that those portions of the

jackscrew assembly below the gearbox and trim motors carried primary flight loads and,

therefore, were covered by regulations pertaining to structure, not systems. He testified

that the acme nut met the applicable regulatory requirements pertaining to structure,

267 The predecessor to the current 14 CFR 25.1309 was CAR 4b.606, which required that all systems ìbe

designed to safeguard against hazards to the airplane in the event of their malfunctioning or failure.î 
Analysis 164 Aircraft Accident Report

specifically, CAR 4b.201(a), ìStrength and Deformation,î which states that structure

ìshall be capable of supporting limit loads without suffering detrimental permanent

deformation.î268 He stated that acme nut threads that are within manufacturing

specifications far exceed the requirements for ultimate strength and deflection limit load

and added that the FAA does not consider deflection of worn acme nut threads to be

deformation in the context of this regulation. 

In addition, a Boeing structures engineering manager testified that the acme nut

complied with certification regulations pertaining to fatigue evaluation of structure.

Specifically, CAR 4b.270, ìFatigue Evaluation of Flight Structure,î required, for ìthose

portions of the airplaneís flight structure in which fatigue may be critical,î an evaluation

of either fatigue strength (also referred to as ìsafe lifeî) or fail safe strength (also referred

to as ìdamage toleranceî).269 However, the Boeing manager testified that neither

evaluation was performed because the acme nut was not considered ìfatigue criticalî

because of its robust design.

It is unclear whether the design and certification of the DC-9 (and MD-80/90 and

717) horizontal stabilizer trim system would have been any different if the certification

requirements for aircraft systems, in addition to those applicable to structure, had been

applied to the jackscrew assembly acme nut during the design phase. Boeing engineers

testified at the public hearing that the horizontal stabilizer trim system design complied

with the requirements of 14 CFR 25.1309. However, the FAA certification engineer

indicated that, even if section 25.1309 had been applicable to the entire jackscrew

assembly, acme nut thread wear would not have been considered in the required systems

safety analysis. He explained, ìif you refer to a [section 25.1309] type safety analysis to

try and determine a failure rate for a wear item, there are not, and thereís really no such

thing as a wear-critical item, never has been. The question of wear being a quantifiable

element so that one could do a safety-type analysis for structureóitís not feasible. The

data to do such an evaluation is not available. It doesnít exist.î270

In sum, the Safety Board is concerned that Boeing and the FAA did not account for

the catastrophic effects of total acme nut thread loss in the design and certification of the

horizontal stabilizer trim control system. The Board is also concerned that the certification

requirements for aircraft systems were not considered applicable to the entire jackscrew

assembly, particularly the acme nut. Because the loss of acme nut threads in flight most

likely would result in the catastrophic loss of the airplane, the Board considers the acme

nut to be a critical element of the horizontal stabilizer trim control system; therefore, it

should have been covered by the certification philosophy and regulations applicable to all

other flight control systems. The Safety Board concludes that the design of the DC-9,

MD-80/90, and 717 horizontal stabilizer jackscrew assembly did not account for the loss

of the acme nut threads as a catastrophic single-point failure mode. The Safety Board

268 A substantially similar requirement is currently contained in 14 CFR 25.305(a).

269 Substantially similar requirements are currently contained in 14 CFR 25.571.

270 The Boeing manager confirmed that ìthe condition of wear or wear-out was not included in the

original DC-9 fault analysis.î 
Analysis 165 Aircraft Accident Report

further concludes that the absence of a fail-safe mechanism to prevent the catastrophic

effects of total acme nut thread loss contributed to the Alaska Airlines flight 261 accident. 

2.6.2 Prevention of Acme Nut Thread Loss Through Maintenance 

and Inspection

Currently, prevention of acme nut thread loss is dependent on regular application

of lubrication and on recurrent inspections of the jackscrew assembly to monitor acme nut

thread wear. However, this maintenance-based approach to maintaining the horizontal

stabilizer trim systemís structural integrity has weaknesses. 

First, current lubrication and end play check intervals may not be adequate, and

their length can change. Research and testing relating to lubrication effectiveness and the

prior service history of the MD-80 fleet suggests that the current 650-flight-hour

lubrication interval is probably adequate to ensure proper lubrication of the jackscrew

assembly. However, because of the potential for additional undiscovered rapid wear

mechanisms and uncertainty regarding the maximum possible wear rate even for known

wear mechanisms, there is no such basis for assuming that the current 2,000-flight-hour

end play check interval is sufficient to ensure fleetwide safety.271 Further, there is no

guarantee that these intervals will not eventually be extended, which would further reduce

the level of safety.

Second, and more importantly, all maintenance and inspection tasks are subject to

human error. As previously discussed, this investigation has identified several weaknesses

in the lubrication and inspection procedures that could affect their intended results and

compromise safety.272 Further, several Safety Board accident investigations, including the

Alaska Airlines flight 261 investigation, have demonstrated that even simple maintenance

tasks are sometimes missed or inadequately performed and can have catastrophic

results.273 Therefore, the current horizontal stabilizer trim system design remains

271 For a more detailed discussion of the Safety Boardís concerns in this area, see section 2.4.2. 

272 For more information about the weaknesses in the lubrication and inspection procedures, see sections

2.3.6.3 and 2.4.3, respectively. 

273 For example, following the September 11, 1991, crash of a Continental Express Embraer 120 in Eagle

Lake, Texas, the Safety Board concluded that the airlineís maintenance inspection and quality assurance

programs failed to detect that the upper row of screws on the leading edge of the left horizontal stabilizer had

been removed during maintenance and had not been replaced. The partially secured left horizontal stabilizer

leading edge separated in flight, causing a severe nose-down pitchover. The airplane broke up in flight, and

all 14 people on board were killed. For more information, see National Transportation Safety Board. Britt

Airways, Inc., d/b/a Continental Express Flight 2574, In-flight Structural Breakup, EMB-120RT, N33701,

Eagle Lake, Texas, September 11, 1991. NTSB/AAR-92/04. In addition, the Boardís investigation of the

July 6, 1996, uncontained engine failure on a Delta Air Lines MD-88 in Pensacola, Florida, determined that

a fluorescent inspection process used to detect fatigue cracks during maintenance was susceptible to error

because it involved multiple cleaning, processing, and inspection procedures dependent on several

individuals and because of a low expectation of finding a crack. Shrapnel from the uncontained engine

failure pierced the fuselage and entered the rear cabin. Two passengers were killed, and two others were

seriously injured. For more information, see National Transportation Safety Board. Uncontained Engine

Failure, Delta Airlines Flight 1288, McDonnell Douglas MD-88, N927DA, Pensacola, Florida, July 6,

1996. NTSB/AAR-98-01.
Analysis 166 Aircraft Accident Report

vulnerable to catastrophic failure if maintenance and inspection tasks are not performed

properly.

2.6.3 Elimination of Catastrophic Effects of Acme Nut Thread 

Loss Through Design

The Safety Board concludes that, when a single failure could have catastrophic

results and there is a practicable design alternative that could eliminate the catastrophic

effects of the failure mode, it is not appropriate to rely solely on maintenance and

inspection intervention to prevent the failure from occurring; if a practicable design

alternative does not exist, a comprehensive systemic maintenance and inspection process

is necessary. In the case of the horizontal stabilizer trim system, such a design would

incorporate a reliable, independent means for eliminating, overcoming, or counteracting

the catastrophic effects of acme nut thread loss. The Safety Board notes that such a design

change would not necessarily need to incorporate dual actuators, or any other form of

system redundancy; the design would only need to provide a mechanism for preventing

stripped acme nut threads from resulting in unrecoverable movement of the horizontal

stabilizer. The Board notes that among the several design concepts listed in AC

25.1309-1A that can be used to avoid catastrophic failure conditions are the following: (1)

ìdesigned failure effect limits, including the capability to sustain damage, to limit the

safety impact or effects of a failureî; and (2) ìdesigned failure path to control and direct

the effects of a failure in a way that limits its safety impact.î

The Safety Board concludes that transport-category airplanes should be modified,

if practicable, to ensure that horizontal stabilizer trim system failures do not preclude

continued safe flight and landing. Therefore, the Safety Board believes that the FAA

should conduct a systematic engineering review to (1) identify means to eliminate the

catastrophic effects of total acme nut thread failure in the horizontal stabilizer trim system

jackscrew assembly in DC-9, MD-80/90, and 717 series airplanes and require, if

practicable, that such fail-safe mechanisms be incorporated in the design of all existing

and future DC-9, MD-80/90, and 717 series airplanes and their derivatives; (2) evaluate

the horizontal stabilizer trim systems of all other transport-category airplanes to identify

any designs that have a catastrophic single-point failure mode and, for any such system;

(3) identify means to eliminate the catastrophic effects of that single-point failure mode

and, if practicable, require that such fail-safe mechanisms be incorporated in the design of

all existing and future airplanes that are equipped with such horizontal stabilizer trim

systems.

Further, the Safety Board is concerned that the FAA certified a horizontal stabilizer

trim system that had a single-point catastrophic failure mode. The Safety Board concludes

that catastrophic single-point failure modes should be prohibited in the design of all future

airplanes with horizontal stabilizer trim systems, regardless of whether any element of that

system is considered structure rather than system or is otherwise considered exempt from

certification standards for systems. Therefore, the Safety Board believes that the FAA

should modify the certification regulations, policies, or procedures to ensure that new

horizontal stabilizer trim control system designs are not certified if they have a
Analysis 167 Aircraft Accident Report

single-point catastrophic failure mode, regardless of whether any element of that system is

considered structure rather than system or is otherwise considered exempt from

certification standards for systems.

2.6.4 Consideration of Wear-Related Failures During Design and 

Certification 

The Alaska Airlines flight 261 investigation revealed that the FAA certification

processes and procedures did not adequately consider and address the consequences of

excessive wear in the context of certifying the DC-9, MD-80/90, and 717 horizontal

stabilizer trim system. In light of this finding, the Safety Board is concerned that the

consequences of excessive wear might not be considered in the contexts of other

certifications as well. One way to ensure that such consequences are considered would be

to include wear-related failures in the failure modes and effects analyses (FMEA) and

fault tree analyses that are required under 14 CFR 25.1309. As stated previously, Boeing

and the FAA have accepted the premise that wear cannot be considered a mode of failure

in systems safety analyses such as FMEAs and fault trees; however, the Safety Board

notes that standards developed by the Society of Automotive Engineers (SAE) specify that

wear should be considered in FMEAs.274 Design guidelines should require that a

wear-related failure be assumed and that the results of such a failure be evaluated. 

The Safety Board concludes that the certification requirements applicable to

transport-category airplanes should fully consider and address the consequences of

failures resulting from wear. Therefore, the Safety Board believes that the FAA should

review and revise aircraft certification regulations and associated guidance applicable to

the certification of transport-category airplanes to ensure that wear-related failures are

fully considered and addressed so that, to the maximum extent possible, they will not be

catastrophic. 

2.7 Deficiencies in Alaska Airlinesí Maintenance 

Program

2.7.1 April 2000 FAA Special Inspection Findings

As a result of the Alaska Airlines flight 261 accident, the FAA conducted a special

inspection of Alaska Airlines in April 2000, which was headed by a member of its System

Process Audit staff from Washington, D.C. The FAAís final report on this special

inspection, dated June 20, 2000, stated the following:

ï procedures in place at the company are not being followed;

274 SAEís Aerospace Recommended Practices 4761, ìGuidelines and Methods for Conducting the Safety

Assessment Process on Civil Airborne Systems and Equipment,î in paragraph G.3.2.2.1 of appendix G, lists

wear among the failure modes to consider in performing an FMEA.
Analysis 168 Aircraft Accident Report

ï controls in place are clearly not effective;

ï authority and responsibilities are not well defined;

ï control of the deferral systems is missing; and

ï quality control and quality assurance programs are ineffective.

The report also noted that the continuing analysis and surveillance program was

responsible for ìoverseeing the operations within the company to ensure that the programs

establishedÖare effectiveî but that because Alaska Airlines did not have a ìfunctional

[continuing analysis and surveillance program], numerous other areas suffer from the lack

of oversight and reform.î The report concluded that most of the findings could be

attributed to Alaska Airlinesí processes, or the lack thereof, and ìineffective quality

control and quality assurance departments.î275

Specifically, the special inspection report revealed that the Alaska Airlines General

Maintenance Manual (GMM) did ìnot reflect the procedures that the company is actually

using to perform maintenance on its aircraft at the companyís maintenance facilities.î The

report documented inadequacies in the GMM procedures for issuance of airworthiness

releases of airplanes coming out of heavy checks, incomplete C-check paperwork,

discrepancies of shelf-life expiration dates of consumables, a lack of engineering approval

and quality control of work card modifications in heavy checks, and inadequate tool

calibrations.

The special inspection report also stated that the GMM did not ìspecify

maintenance training curriculums or on-the-job training (OJT) procedures or objectivesî

and cited 14 CFR 121.375, which requires that Part 121 operators have a training program

to ensure competency. The report noted that the OJT program was ìinformal [and]

administered at the discretion of the appointed instructors. The program is not structured,

there is no identification of subjects to be covered, and there are no criteria for successful

completion provided.î 

As a result of the special inspection teamís findings, the FAA initially proposed the

suspension of Alaska Airlinesí authority to conduct heavy maintenance. Instead, the FAA

accepted an Airworthiness and Operations Action Plan submitted by Alaska Airlines and

did not suspend Alaska Airlinesí heavy maintenance authority. In September 2000, the

FAA conducted a followup inspection, which revealed that Alaska Airlines had not

satisfactorily resolved many of the deficiencies identified in the special inspection report.

After an additional review in July 2001, consisting of presentations by Alaska Airlines

representatives and a brief visit by some of the panel members to Alaska Airlinesí SEA

maintenance facility followed by discussions about Alaska Airlinesí progress, the FAA

found that the airline had ìmet or exceeded all commitments set forth in their action planî

and ìdemonstratedÖthat previously identified systemic deficiencies have been

corrected.î The FAA further found that Alaska Airlines had made ìsignificant changes

and improvements.î 

275 For more detailed information about the FAA special inspection teamís findings, see section 1.17.3.4. 
Analysis 169 Aircraft Accident Report

2.7.2 Maintenance-Related Deficiencies Identified During This 

Accident Investigation 

In the course of its investigation of the Alaska Airlines flight 261 accident, the

Safety Board learned about numerous maintenance-related actionsóboth general and

specificóby Alaska Airlines management and maintenance personnel. These actions

supported the overall findings in the FAAís special inspection report and were indicative

of procedural and quality control deficiencies, which concerned the Board.

2.7.2.1 General Policy Decisions 

Extension of End Play Check Interval

As previously discussed, in 1996, Alaska Airlines extended its C-check interval

from 13 to 15 months, which resulted in the end play check interval being increased from

26 to 30 months. Although 30 months was the manufacturer-recommended calendar-time

interval, because of Alaska Airlinesí increased airplane utilization rate, 30 months was

equivalent to 9,550 flight hours, which was well beyond the 7,000- or 7,200-flight-hour

interval that was recommended by Boeing at the time.276 As previously noted, the flight

261 accident occurred almost 9,000 flight hours after the accident jackscrew assemblyís

last end play check. Therefore, if Alaska Airlines had adopted an interval that complied

with either of the flight-hour intervals specified by Boeing, the accident jackscrew

assembly would have undergone an end play check 1,800 to 2,000 flight hours before the

accident occurred and might have been removed at that time for having an excessive end

play measurement. The decision to exceed the manufacturerís recommended end play

check interval was a poor maintenance decision and demonstrated a lack of appreciation

for the need to account for flight hours when setting maintenance intervals for those tasks

related to airplane usage rather than calendar time. 

Grease Change from Mobilgrease 28 to Aeroshell 33

The process by which Alaska Airlines replaced Mobilgrease 28 with Aeroshell 33

grease on its work maintenance task cards for the MD-80 was both procedurally and

substantively deficient in several respects.

First, the grease change was accomplished without following established

procedures. The form used to request and obtain internal approval for the grease change

was incomplete and inadequate in the following ways: (1) it was missing several required

signatures;277 (2) it contained no indication that the required internal approval from Alaska

Airlinesí Reliability Analysis Program Control Board had been obtained; and (3) it

contained no signature on the line labeled ìchange request accomplished,î although the

276 As previously mentioned, MSG-2 guidance (which was the basis for Alaska Airlinesí maintenance

program) specified an end play check interval of 7,000 flight hours, and the more recent MSG-3 guidance

specified an interval of 7,200 flight hours.

277 There was no signature from the manager of reliability, the director of line maintenance, or the

manager of quality control.
Analysis 170 Aircraft Accident Report

grease change was implemented in December 1997. At the Safety Boardís public hearing,

neither Alaska Airlinesí former director of engineering (who at the time the change was

processed was Alaska Airlinesí manager of maintenance programs and technical

publications) nor the current director of engineering could explain how the change was

implemented without the required signatures or Reliability Analysis Program Control

Board approval. Further, the GMM was not modified to reflect the grease change to

Aeroshell 33; it still specified the use of Mobilgrease 28. This lapse indicates either a lack

of procedures or a failure to follow procedures.

Second, Alaska Airlines did not conduct any formal followup to evaluate the

performance of the new grease (Aeroshell 33). McDonnell Douglas responded to Alaska

Airlinesí request to change greases in a letter stating that it had ìno technical objectionî to

the use of Aeroshell 33; however, it also stated that McDonnell Douglas could not ìyet

verify the performance of this greaseî and that it would ìbe the responsibility of Alaska

Airlines to monitor the areas where Aeroshell 33 grease is used for any adverse reactions.î

However, Alaska Airlines did not actively monitor the in-service performance of the

grease. Although Alaska Airlines asserts that it monitored the performance of Aeroshell

33 through its overall reliability program, the Safety Board notes that there is no evidence

that any specific data were collected and evaluated regarding the lubricating effectiveness

of Aeroshell 33. For example, it would have been prudent for Alaska Airlines to measure

and record end play and calculate the wear rate on a sample of airplanes after the

introduction of Aeroshell 33 and perhaps even to shorten the lubrication intervals in order

to verify that its performance was at least equivalent to that of Mobilgrease 28. Alaska

Airlinesí failure to actively monitor the performance of the new grease is especially

significant in light of the extension in lubrication intervals for the horizontal stabilizer

components (from 1,600 to 2,550 flight hours) that it had implemented the year before.

Third, Alaska Airlinesí technical justification for the grease change was

inadequate. When Alaska Airlines notified the FAA in December 1997 of the grease

change to Aeroshell 33, it did not provide any substantiating justification at that time nor

was such action required. However, when the FAA requested substantiating justification

after concerns were raised in connection with the flight 261 investigation, the only

documents Alaska Airlines submitted to the FAA were a trade magazine article on

Aeroshell 33, excerpts from Boeing 737 and MD-80 maintenance manuals, Boeing

service letters, internal correspondence and messages between Alaska Airlines and

Boeing, and existing MIL specifications for Aeroshell 33 and Mobilgrease 28. None of

these documents contained any information or performance data specifically applicable to

the use of Aeroshell 33 on MD-80 airplanes or on the horizontal stabilizer systems of such

airplanes. These documents were apparently the only basis Alaska Airlines had for

justifying the grease change internally.278

278Alaska Airlines officials did not offer any of the additional information that they claim was used to 

show internal justification for the grease change. Alaska Airlinesí director of engineering testified at the

hearing that he thought that the documents submitted to the FAA provided sufficient justification for the

change. 
Analysis 171 Aircraft Accident Report

The FAA determined that the substantiating documents submitted by Alaska

Airlines did not support the change and it disapproved the use of Aeroshell Grease 33 as a

substitute for Mobilgrease 28. The Safety Board agrees that the documents submitted to

the FAA did not adequately justify the grease change. Even though the use of Aeroshell 33

for lubrication of the horizontal stabilizer system was not determined to be a factor in the

Alaska Airlines flight 261 accident, the Board notes that Alaska Airlines disregarded the

potential safety impact of a change in grease.

Failure to Fill Required Senior Positions

As noted in the FAA special inspection report, three required positionsóthe

director of maintenance, the director of operations, and the director of safetyówere not

filled at the time of the special inspection.279 Although the duties of the director of

maintenance were being performed by two people, the division of duties and

responsibilities was not clearly defined. The report noted, ìconsequently there is

confusion as to who is responsible for what tasks.î The report also noted that the director

of safety did not report ìdirectly to the highest level of management.î The lack of a

full-time director of maintenance and director of safety indicates a lack of sufficient

emphasis on maintenance and safety.

Use of Nonconforming Restraining Fixtures 

At the time of the accident, Alaska Airlines had only one restraining fixture in its

inventory (at its OAK maintenance facility). The fixture was manufactured in-house and it

did not meet the manufacturerís required specifications for this fixture. About 1 month

after the accident, Alaska Airlines manufactured 11 additional restraining fixtures similar

in design to its original in-house manufactured fixture. Alaska Airlinesí manager of tool

control, who participated in the fabrication of the fixtures ordered by airline management

after the accident, told Safety Board investigators in an interview that ìwhat they were

making [the restraining fixtures] wasnít even closeî to the Boeing drawing. He added that

ìwe were directed to build the tools, and we did exactly what we were told.î

On August 2, 2000, in response to Safety Board queries regarding the physical

properties of the in-house manufactured restraining fixtures, Alaska Airlines notified the

FAA that the restraining fixtures might not be ìan equivalent substituteî for the Boeing

fixture and, therefore, could produce erroneous measurements. Alaska Airlines thereafter

suspended use of its in-house manufactured restraining fixtures and temporarily grounded

18 of its 34 MD-80 series airplanes after determining that these airplanes may have

received end play checks with the in-house manufactured fixtures.280

Although use of Alaska Airlinesí in-house manufactured restraining fixture to

perform the accident airplaneís last end play check might not have contributed to the

279 Title 14 CFR 119.65 requires airlines to have full-time, qualified personnel in each of these positions. 

280 Alaska Airlines performed end play checks on the grounded airplanes using Boeing-manufactured

restraining fixtures and found that they had acceptable end play measurements.
Analysis 172 Aircraft Accident Report

Alaska Airlines flight 261 accident, the use of this nonconforming fixture is further

evidence of a flawed maintenance program. 

2.7.2.2 Specific Maintenance Actions

In addition to the general policy-related deficiencies outlined above, the Alaska

Airlines flight 261 accident investigation also revealed several individual maintenance

actions indicative of poor adherence to procedures and/or poor quality control.

Inadequate Lubrication of Jackscrew Assemblies 

The investigation revealed that the jackscrew assemblies from three of Alaska

Airlines 34 MD-80 airplanes (the accident assembly and those from N981AS and

N982AS discovered in February 2000) had excessively high wear rates. The high wear

rates on these jackscrew assemblies could not be explained by contamination, improper

acme thread surface finish, or anything other than a lack of lubrication. As previously

discussed, at least one lubrication had to have been missed or inadequately performed on

the accident assembly.281 However, a scenario in which more than one lubrication was

missed or inadequate results in a wear rate more consistent with that which Safety Board

testing showed to be the wear rate of unlubricated test specimens. Using the same

reasoning, at least one lubrication, and possibly more, must also have been missed entirely

or inadequately performed on the jackscrew assemblies on N981AS and N982AS. This

indicates either poor training in this task or poor supervision of maintenance personnel

performing this task. 

Inadequate Lubrication of Rudder Trim Tab Hinge Support Bearings

On July 19, 2002, Alaska Airlines issued a Maintenance Information Letter stating

that ì[r]ecent shop findings have revealed that the MD-80 rudder trim tab support bearings

may not always be receiving adequate lubrication.î The letter indicated that two airplanes

that had recently come out of a C check, during which the bearings should have been

lubricated, showed ìextreme bearing wear, with little or no evidence of greaseî and that

one of the bearings ìfell apart upon removal.î Alaska Airlines subsequently indicated that,

contrary to what was stated in the letter, the airplanes had not recently undergone a C

check. However, regardless of how long it had been since the last C check or scheduled

lubrication, the condition of these bearings strongly suggests that they were not adequately

lubricated at the last scheduled lubrication opportunity. This raises significant concerns

about Alaska Airlinesí lubrication practices, especially in light of the findings from this

investigation relating to inadequate lubrication of jackscrew assemblies.

Failure to Properly Process Order for New Jackscrew Assembly 

Following the 0.040-inch end play measurement on the accident jackscrew

assembly on September 27, 1997, a nonroutine work card (MIG-4) was generated to begin

281 For more information on the analysis of how many lubrications might have been missed or

inadequately performed, see section 2.3.6.1.
Analysis 173 Aircraft Accident Report

the process of ordering a replacement jackscrew assembly. The ìplanned actionî noted on

the nonroutine work card was ìReplace nut and perform E.O. [engineering order]

8-55-10-01.î However, a new jackscrew assembly was never ordered. According to

Alaska Airlinesí director of safety, Alaska Airlinesí procedures called for the creation of a

field requisition to order a nonstocked part, such as a jackscrew assembly, but no such

requisition was found. Further, the E.O. referenced in the nonroutine work card (which

was also referenced in the Alaska Airlines maintenance task card) was incorrect because it

applied to another airplane model. 

The Safety Board notes that 3 days after the 0.040-inch end play measurement was

recorded, a 0.033-inch measurement was recorded, and the accident airplane was

thereafter returned to service. The Board recognizes that there was no requirement for

Alaska Airlines to order a new jackscrew assembly because it was permissible to return

the assembly to service with a recorded end play measurement of either 0.040 or

0.033 inch. Nonetheless, the failure of Alaska Airlines maintenance personnel to process

the order for a new part is yet another example of failure to comply with internal

procedures. Further, the incorrect E.O. reference is an additional example of poor

maintenance quality control.

Continued Errors in Performing End Play Checks 

Finally, the Safety Board notes that even after the attention that has been focused

on the end play check procedure as a result of the flight 261 accident, Alaska Airlines

maintenance personnel continue to have difficulty properly performing the procedure. In

March and April 2002, Alaska Airlines removed two jackscrew assemblies, one at OAK

and one at a contract facility near SEA, after reported end play measurements of only

0.001 inch. The manufacturing specifications require a minimum end play of 0.003 inch;

therefore, a lower end play measurement is suspect. Safety Board investigators examined

these jackscrew assemblies and determined that their end play measurements were within

normal and expected limits and that the 0.001-inch end play measurements were caused

by maintenance errors in performing the end play check procedure. Specifically, it was

determined that such inaccurate measurements would result from applying torque to the

restraining fixture in the wrong direction before measuring the end play.

According to Boeing, during the 6-month period from December 2001 to

May 2002, only six other end play measurements of less than 0.004 inch were reported.

Therefore, it appears that Alaska Airlines has experienced a relatively high percentage of

such erroneous readings. Further, Alaska Airlines indicated that all maintenance personnel

who would be performing end play checks had undergone 4 hours of specialized training

on how to properly conduct the end play check procedure. It is noteworthy that, given the

heightened awareness resulting from the Alaska Airlines flight 261 accident, and even

with this additional training, end play check procedural errors continued to occur at

Alaska Airlines.
Analysis 174 Aircraft Accident Report

2.7.3 Summary

On the basis of the information in sections 2.7.2.1 and 2.7.2.2, the Safety Board

concludes that at the time of the flight 261 accident, Alaska Airlinesí maintenance

program had widespread systemic deficiencies. As previously discussed, two specific

maintenance-related deficienciesóinsufficient lubrication of the accident jackscrew

assembly and extension of the end play check intervalówere causal to the accident. The

Board could not establish whether any of the other maintenance-related deficiencies

discovered in connection with this accident investigation were directly related to, or

contributed to, the accident. However, these deficiencies are significant departures from

expected and established safety standards and, therefore, could well have compromised

safety in other ways.

Alaska Airlines has made numerous changes in response to the FAAís postaccident

special inspection findings, and an FAA panel has indicated that it is satisfied that the

ìpreviously identified systemic deficiencies have been corrected.î Nonetheless, the Safety

Board is concerned that these widespread deficiencies existed at Alaska Airlines before

the accident and were not identified or corrected until after attention was focused on them

as a result of this accident investigation. The Board is also concerned, in light of the recent

maintenance errors during end play check procedures and evidence of continued poor

lubrication practices, that the deficiencies at Alaska Airlines may not have been fully

corrected.

2.8 FAA Oversight 

The FAAís April 2000 special inspection uncovered widespread significant

deficiencies that the FAA should have identified earlier. The fact that these deficiencies

were not discovered until special attention was focused on Alaska Airlines by a

headquarters-led team as result of the flight 261 accident indicates that the FAAís

surveillance was ineffective before the accident. 

Several FAA officials acknowledged that surveillance of Alaska Airlines had been

inadequate for at least a year before the accident. The former PMI for Alaska Airlines

testified that the replacement of the Program Tracking and Reporting System (PTRS) of

oversight at Alaska Airlines with the Air Transportation Oversight System (ATOS) in

October 1998 resulted in ìa terrible transitionî that drastically reduced the amount of time

inspectors had for actual surveillance activities. He indicated that as a result of additional

workload and other administrative changes associated with the implementation of ATOS,

ìnobody was out there looking at the carrier.î282

The Seattle Flight Standards District Office (FSDO) CMO supervisor confirmed

that ìthe amount of surveillance that we have done since the introduction of ATOS has

probably generally decreased.î His concern about this decrease in oversight is reflected in

282 For more information about the impact of the implementation of ATOS on the FAAís oversight of

Alaska Airlines, see section 1.17.3.3.
Analysis 175 Aircraft Accident Report

a November 12, 1999, memorandum that he sent to the FAAís Flight Standards Division

stating that additional staffing would be needed for the Seattle FSDO CMO to carry out its

assigned duties. The memorandum stated the following: 

[staffing at the Seattle FSDO CMO] has reached a critical pointÖwe are not able

to properly meet the workload demands. Alaska Airlines has expressed continued

concern over our inability to serve it in a timely manner. Some program approvals

have been delayed or accomplished in a rushed manner at the ëeleventh hourí and

we anticipate this problem will intensify with time. Also, many enforcement

investigations, particularly in the area of cabin safety, have been delayed as a

result of resource shortagesÖin order for the Seattle FSDO [CMO] to

accommodate the significant volume of Alaska Airlineís ëdemandí work and

effectively meet the objectives of the ATOS program, additional inspector staffing

must be made available.

The memorandum also indicated that if the Seattle FSDO CMO ìcontinues to operate with

the existing limited number of airworthiness inspectorsÖdiminished surveillance is

imminent and the risk of incidents or accidents at Alaska Airlines is heightened.î 

Therefore, the Safety Board concludes that the FAA did not fulfill its responsibility

to properly oversee the maintenance operations at Alaska Airlines, and that at the time of

the Alaska Airlines flight 261 accident, FAA surveillance of Alaska Airlines had been

deficient for at least several years. The Safety Board notes that after the accident,

PTRS-based oversight of Alaska Airlines was re-instituted, and staffing at the CMO was

increased significantly. Therefore, it appears that the FAA has taken steps to increase the

level of its surveillance of Alaska Airlines. In July 2001, an FAA panel determined that

Alaska Airlines had corrected the previously identified deficiencies. However, in light of

(1) the systemic nature and widespread impact of those deficiencies, (2) Alaska Airlinesí

failure to demonstrate during the FAAís first followup inspection (in September 2000) that

it had fully corrected the deficiencies, (3) the absence of any in-depth followup inspection

by an objective team of FAA inspectors not personally responsible for the oversight of

Alaska Airlines to verify that the deficiencies have in fact been corrected,283 and (4) recent

maintenance errors by Alaska Airlines maintenance personnel during end play check

procedures and evidence of continued poor lubrication practices,284 the Board questions

the depth and effectiveness of Alaska Airlinesí corrective actions and remains concerned

about the overall adequacy of Alaska Airlinesí maintenance program. 

283 The FAA panelís conclusion that the deficiencies had been corrected was based primarily on briefings

from Alaska Airlines officials.

284 The Safety Board notes that these maintenance errors seem to underscore the need for specialized

training for personnel who lubricate and perform end play checks, as the Board recommended in Safety

Recommendations A-01-43 and -44. Those recommendations are currently classified ìOpenóUnacceptable

Response.î
Conclusions 176 Aircraft Accident Report

3. Conclusions

3.1 Findings

1. The flight crewmembers on Alaska Airlines flight 261 were properly certificated and

qualified and had received the training and off-duty time prescribed by Federal

regulations. No evidence indicated any preexisting medical or other condition that

might have adversely affected the flight crewís performance during the accident

flight.

2. The airplane was dispatched in accordance with Federal Aviation Administration

regulations and approved Alaska Airlines procedures. The weight and balance of the

airplane were within limits for dispatch, takeoff, climb, and cruise. 

3. Weather was not a factor in the accident.

4. There was no evidence of a fire or of impact with birds or any other foreign object. 

5. No evidence indicated that the airplane experienced any preimpact structural or

system failures, other than those associated with the longitudinal trim control system,

the horizontal stabilizer, and its surrounding structure. 

6. Both engines were operating normally before the final dive.

7. Air traffic control personnel involved with the accident flight were properly

certificated and qualified for their assigned duty stations. 

8. The longitudinal trim control system on the accident airplane was functioning

normally during the initial phase of the accident flight. 

9. The horizontal stabilizer stopped responding to autopilot and pilot commands after

the airplane passed through 23,400 feet. The pilots recognized that the longitudinal

trim control system was jammed, but neither they nor the Alaska Airlines

maintenance personnel could determine the cause of the jam.

10. The worn threads inside the horizontal stabilizer acme nut were incrementally sheared

off by the acme screw and were completely sheared off during the accident flight. As

the airplane passed through 23,400 feet, the acme screw and nut jammed, preventing

further movement of the horizontal stabilizer until the initial dive.
Conclusions 177 Aircraft Accident Report

11. The accident airplaneís initial dive from 31,050 feet began when the jam between the

acme screw and nut was overcome as a result of operation of the primary trim motor.

Release of the jam allowed the acme screw to pull up through the acme nut, causing

the horizontal stabilizer leading edge to move upward, thus causing the airplane to

pitch rapidly downward. 

12. The acme screw did not completely separate from the acme nut during the initial dive

because the screwís lower mechanical stop was restrained by the lower surface of the

acme nut until just before the second and final dive about 10 minutes later. 

13. The cause of the final dive was the low-cycle fatigue fracture of the torque tube,

followed by the failure of the vertical stabilizer tip fairing brackets, which allowed the

horizontal stabilizer leading edge to move upward significantly beyond what is

permitted by a normally operating jackscrew assembly. The resulting upward

movement of the horizontal stabilizer leading edge created an excessive upward

aerodynamic tail load, which caused an uncontrollable downward pitching of the

airplane from which recovery was not possible.

14. In light of the absence of a checklist requirement to land as soon as possible and the

circumstances confronting the flight crew, the flight crewís decision not to return to

Lic Gustavo Diaz Ordaz International Airport, Puerto Vallarta, Mexico, immediately

after recognizing the horizontal stabilizer trim system malfunction was

understandable.

15. The flight crewís decision to divert the flight to Los Angeles International Airport,

Los Angeles, California, rather than continue to San Francisco International Airport,

San Francisco, California, as originally planned was prudent and appropriate.

16. Alaska Airlines dispatch personnel appear to have attempted to influence the flight

crew to continue to San Francisco International Airport, San Francisco, California,

instead of diverting to Los Angeles International Airport, Los Angeles, California. 

17. The flight crewís use of the autopilot while the horizontal stabilizer was jammed was

not appropriate.

18. Flight crews dealing with an in-flight control problem should maintain any

configuration change that would aid in accomplishing a safe approach and landing,

unless that configuration change adversely affects the airplaneís controllability. 

19. Without clearer guidance to flight crews regarding which actions are appropriate and

which are inappropriate in the event of an inoperative or malfunctioning flight control

system, pilots may experiment with improvised troubleshooting measures that could

inadvertently worsen the condition of a controllable airplane.

20. The acme nut threads on the accident airplaneís horizontal stabilizer jackscrew

assembly wore at an excessive rate.
Conclusions 178 Aircraft Accident Report

21. Alaska Airlinesí use of Aeroshell 33 for lubrication of the jackscrew assembly, acme

screw thread surface finish, foreign debris, and abnormal loading of the acme nut

threads were not factors in the excessive wear of the accident acme nut threads. 

22. There was no effective lubrication on the acme screw and nut interface at the time of

the Alaska Airlines flight 261 accident. 

23. The excessive and accelerated wear of the accident jackscrew assembly acme nut

threads was the result of insufficient lubrication, which was directly causal to the

Alaska Airlines flight 261 accident. 

24. Alaska Airlinesí extensions of its lubrication interval for its McDonnell Douglas

MD-80 horizontal stabilizer components and the Federal Aviation Administrationís

approval of these extensions, the last of which was based on Boeingís extension of the

recommended lubrication interval, increased the likelihood that a missed or

inadequate lubrication would result in excessive wear of jackscrew assembly acme

nut threads and, therefore, was a direct cause of the excessive wear and contributed to

the Alaska Airlines flight 261 accident.

25. When lubricating the jackscrew assembly, removal of used grease from the acme

screw before application of fresh grease will increase the effectiveness of the

lubrication. 

26. A larger access panel would facilitate the proper accomplishment of the jackscrew

assembly lubrication task. 

27. If the jackscrew assembly lubrication procedure were a required inspection item for

which an inspectorís signoff is needed, the potential for unperformed or improperly

performed lubrications would be reduced. 

28. Alaska Airlinesí extension of the end play check interval and the Federal Aviation

Administrationís approval of that extension allowed the accident acme nut threads to

wear to failure without the opportunity for detection and, therefore, was a direct cause

of the excessive wear and contributed to the Alaska Airlines flight 261 accident.

29. Alaska Airlinesí end play check interval extension should have been, but was not,

supported by adequate technical data to demonstrate that the extension would not

present a potential hazard.

30. The existing process by which manufacturers revise recommended maintenance task

intervals and by which airlines establish and revise these intervals does not include

task-by-task engineering analysis and justification and, therefore, allows for the

possibility of inappropriate interval extensions for potentially critical maintenance

tasks.
Conclusions 179 Aircraft Accident Report

31. Because of the possibility that higher-than-expected wear could cause excessive wear

in less than 2,000 flight hours and the additional possibility that an end play check

could be not performed or improperly performed, the current 2,000-flight-hour end

play check interval specified in Airworthiness Directive 2000-15-15 may be

inadequate to ensure the safety of the Douglas DC-9, McDonnell Douglas MD-80/90,

and Boeing 717 fleet. 

32. The continued collection and analysis of end play data are critical to monitoring acme

nut thread wear and identifying excessive or unexpected wear rates, trends, or

anomalies. 

33. Until August 2000, Alaska Airlines used a fabricated restraining fixture that did not

meet Boeing specifications; however, the Safety Board could not determine whether

the use of this noncompliant fixture generated an inaccurate end play measurement

during the last end play check or whether the use of this fixture contributed to the

accident.

34. The on-wing end play check procedure, as currently practiced, has not been validated

and has low reliability.

35. Deficiencies in the overhaul process increase the likelihood that jackscrew assemblies

may be improperly overhauled. 

36. The absence of a requirement to record or inform customers of the end play

measurement of an overhauled jackscrew assembly could result in an operator

unknowingly returning a jackscrew assembly to service with a higher-than-expected

end play measurement.

37. Operators will maximize the usefulness of end play measurements and wear rate

calculations by recording on-wing end play measurements whenever a jackscrew

assembly is replaced on an airplane. 

38. Because the jackscrew assembly is an integral and essential part of the horizontal

stabilizer trim system, a critical flight system, it is important to ensure that

maintenance facilities authorized to overhaul these assemblies possess the proper

qualifications, equipment, and documentation. 

39. The dual-thread design of the acme screw and nut does not provide redundancy with

regard to wear.

40. The design of the Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717

horizontal stabilizer jackscrew assembly did not account for the loss of the acme nut

threads as a catastrophic single-point failure mode. The absence of a fail-safe

mechanism to prevent the catastrophic effects of total acme nut thread loss

contributed to the Alaska Airlines flight 261 accident.
Conclusions 180 Aircraft Accident Report

41. When a single failure could have catastrophic results and there is a practicable design

alternative that could eliminate the catastrophic effects of the failure mode, it is not

appropriate to rely solely on maintenance and inspection intervention to prevent the

failure from occurring; if a practicable design alternative does not exist, a

comprehensive systemic maintenance and inspection process is necessary.

42. Transport-category airplanes should be modified, if practicable, to ensure that

horizontal stabilizer trim system failures do not preclude continued safe flight and

landing. 

43. Catastrophic single-point failure modes should be prohibited in the design of all

future airplanes with horizontal stabilizer trim systems, regardless of whether any

element of that system is considered structure rather than system or is otherwise

considered exempt from certification standards for systems. 

44. The certification requirements applicable to transport-category airplanes should fully

consider and address the consequences of failures resulting from wear. 

45. At the time of the flight 261 accident, Alaska Airlinesí maintenance program had

widespread systemic deficiencies. 

46. The Federal Aviation Administration (FAA) did not fulfill its responsibility to

properly oversee the maintenance operations at Alaska Airlines, and at the time of the

Alaska Airlines flight 261 accident, FAA surveillance of Alaska Airlines had been

deficient for at least several years. 

3.2 Probable Cause

The National Transportation Safety Board determines that the probable cause of

this accident was a loss of airplane pitch control resulting from the in-flight failure of the

horizontal stabilizer trim system jackscrew assemblyís acme nut threads. The thread

failure was caused by excessive wear resulting from Alaska Airlinesí insufficient

lubrication of the jackscrew assembly.

Contributing to the accident were Alaska Airlinesí extended lubrication interval

and the Federal Aviation Administrationís (FAA) approval of that extension, which

increased the likelihood that a missed or inadequate lubrication would result in excessive

wear of the acme nut threads, and Alaska Airlinesí extended end play check interval and

the FAAís approval of that extension, which allowed the excessive wear of the acme nut

threads to progress to failure without the opportunity for detection. Also contributing to

the accident was the absence on the McDonnell Douglas MD-80 of a fail-safe mechanism

to prevent the catastrophic effects of total acme nut thread loss. 
Recommendations 181 Aircraft Accident Report

4. Recommendations

4.1 New Recommendations

As a result of the investigation of the Alaska Airlines flight 261 accident, the

National Transportation Safety Board makes the following recommendations to the

Federal Aviation Administration:

Issue a flight standards information bulletin directing air carriers to instruct

pilots that in the event of an inoperative or malfunctioning flight control

system, if the airplane is controllable they should complete only the

applicable checklist procedures and should not attempt any corrective

actions beyond those specified. In particular, in the event of an inoperative

or malfunctioning horizontal stabilizer trim control system, after a final

determination has been made in accordance with the applicable checklist

that both the primary and alternate trim systems are inoperative, neither the

primary nor the alternate trim motor should be activated, either by

engaging the autopilot or using any other trim control switch or handle.

Pilots should further be instructed that if checklist procedures are not

effective, they should land at the nearest suitable airport. (A-02-36) 

Direct all certificate management offices to instruct inspectors to conduct

surveillance of airline dispatch and maintenance control personnel to

ensure that their training and operations directives provide appropriate

dispatch support to pilots who are experiencing a malfunction threatening

safety of flight and instruct them to refrain from suggesting continued

flight in the interest of airline flight scheduling. (A-02-37)

As part of the response to Safety Recommendation A-01-41, require

operators of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing

717 series airplanes to remove degraded grease from the jackscrew

assembly acme screw and flush degraded grease and particulates from the

acme nut before applying fresh grease. (A-02-38) 

As part of the response to Safety Recommendation A-01-41, require

operators of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing

717 series airplanes, in coordination with Boeing, to increase the size of the

access panels that are used to accomplish the jackscrew assembly

lubrication procedure. (A-02-39) 

Establish the jackscrew assembly lubrication procedure as a required

inspection item that must have an inspectorís signoff before the task can be

considered complete. (A-02-40) 
Recommendations 182 Aircraft Accident Report

Review all existing maintenance intervals for tasks that could affect critical

aircraft components and identify those that have been extended without

adequate engineering justification in the form of technical data and

analysis demonstrating that the extended interval will not present any

increased risk and require modifications of those intervals to ensure that

they (1) take into account assumptions made by the original designers, (2)

are supported by adequate technical data and analysis, and (3) include an

appropriate safety margin that takes into account the possibility of missed

or inadequate accomplishment of the maintenance task. In conducting this

review, the Federal Aviation Administration should also consider original

intervals recommended or established for new aircraft models that are

derivatives of earlier models and, if the aircraft component and the task are

substantially the same and the recommended interval for the new model is

greater than that recommended for the earlier model, treat such original

intervals for the derivative model as ìextendedî intervals. (A-02-41) 

Conduct a systematic industrywide evaluation and issue a report on the

process by which manufacturers recommend and airlines establish and

revise maintenance task intervals and make changes to the process to

ensure that, in the future, intervals for each task (1) take into account

assumptions made by the original designers, (2) are supported by adequate

technical data and analysis, and (3) include an appropriate safety margin

that takes into account the possibility of missed or inadequate

accomplishment of the maintenance task. (A-02-42) 

Require operators to supply the Federal Aviation Administration (FAA),

before the implementation of any changes in maintenance tasks intervals

that could affect critical aircraft components, technical data and analysis

for each task demonstrating that none of the proposed changes will present

any potential hazards, and obtain written approval of the proposed changes

from the principal maintenance inspector and written concurrence from the

appropriate FAA aircraft certification office. (A-02-43) 

Pending the incorporation of a fail-safe mechanism in the design of the

Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717 horizontal

stabilizer jackscrew assembly, as recommended in Safety

Recommendation A-02-49, establish an end play check interval

that (1) accounts for the possibility of higher-than-expected wear rates and

measurement error in estimating acme nut thread wear and (2) provides for

at least two opportunities to detect excessive wear before a potentially

catastrophic wear condition becomes possible. (A-02-44) 
Recommendations 183 Aircraft Accident Report

Require operators to permanently (1) track end play measures according to

airplane registration number and jackscrew assembly serial number, (2)

calculate and record average wear rates for each airplane based on end play

measurements and flight times, and (3) develop and implement a program

to analyze these data to identify and determine the cause of excessive or

unexpected wear rates, trends, or anomalies. The Federal Aviation

Administration (FAA) should also require operators to report this

information to the FAA for use in determining and evaluating an

appropriate end play check interval. (A-02-45) 

Require that maintenance facilities that overhaul jackscrew assemblies

record and inform customers of an overhauled jackscrew assemblyís end

play measurement. (A-02-46) 

Require operators to measure and record the on-wing end play

measurement whenever a jackscrew assembly is replaced. (A-02-47) 

Require that maintenance facilities that overhaul Douglas DC-9,

McDonnell Douglas MD-80/90, and Boeing 717 series airplanesí

jackscrew assemblies obtain specific authorization to perform such

overhauls, predicated on demonstrating that they possess the necessary

capability, documentation, and equipment for the task and that they have

procedures in place to (1) perform and document the detailed steps that

must be followed to properly accomplish the end play check procedure and

lubrication of the jackscrew assembly, including specification of

appropriate tools and grease types; (2) perform and document the

appropriate steps for verifying that the proper acme screw thread surface

finish has been applied; and (3) ensure that appropriate packing procedures

are followed for all returned overhauled jackscrew assemblies, regardless

of whether the assembly has been designated for storage or shipping.

(A-02-48) 

Conduct a systematic engineering review to (1) identify means to eliminate

the catastrophic effects of total acme nut thread failure in the horizontal

stabilizer trim system jackscrew assembly in Douglas DC-9 (DC-9),

McDonnel Douglas MD-80/90 (MD-80/90), and Boeing 717 (717) series

airplanes and require, if practicable, that such fail-safe mechanisms be

incorporated in the design of all existing and future DC-9, MD-80/90, and

717 series airplanes and their derivatives; (2) evaluate the horizontal

stabilizer trim systems of all other transport-category airplanes to identify

any designs that have a catastrophic single-point failure mode and, for any

such system; (3) identify means to eliminate the catastrophic effects of that

single-point failure mode and, if practicable, require that such fail-safe

mechanisms be incorporated in the design of all existing and future

airplanes that are equipped with such horizontal stabilizer trim systems

(A-02-49) 
Recommendations 184 Aircraft Accident Report

Modify the certification regulations, policies, or procedures to ensure that

new horizontal stabilizer trim control system designs are not certified if

they have a single-point catastrophic failure mode, regardless of whether

any element of that system is considered structure rather than system or is

otherwise considered exempt from certification standards for systems.

(A-02-50) 

Review and revise aircraft certification regulations and associated guidance

applicable to the certification of transport-category airplanes to ensure that

wear-related failures are fully considered and addressed so that, to the

maximum extent possible, they will not be catastrophic. (A-02-51) 

4.2 Previously Issued Recommendations Resulting 

From This Accident Investigation

As a result of the Alaska Airlines flight 261 accident investigation, the Safety

Board issued the following safety recommendations to the FAA on October 1, 2001:

Require the Boeing Commercial Airplane Group to revise the lubrication

procedure for the horizontal stabilizer trim system of Douglas DC-9,

McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to

minimize the probability of inadequate lubrication. (A-01-41) 

On June 14, 2002, the Safety Board classified Safety Recommendation

A-01-41 ìOpenóAcceptable Response.î 

Require the Boeing Commercial Airplane Group to revise the end play

check procedure for the horizontal stabilizer trim system of Douglas DC-9,

McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to

minimize the probability of measurement error and conduct a study to

empirically validate the revised procedure against an appropriate physical

standard of actual acme screw and acme nut wear. This study should also

establish that the procedure produces a measurement that is reliable when

conducted on-wing. (A-01-42)

On June 14, 2002, the Safety Board classified Safety Recommendation

A-01-42 ìOpenóAcceptable Response.î

Require maintenance personnel who lubricate the horizontal stabilizer trim

system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717

series airplanes to undergo specialized training for this task. (A-01-43)

On June 14, 2002, the Safety Board classified Safety Recommendation

A-01-43 ìOpenóUnacceptable Response.î
Recommendations 185 Aircraft Accident Report

Require maintenance personnel who inspect the horizontal stabilizer trim

system of Douglas DC-9, McDonnell Douglas MD-80/90, and Boeing 717

series airplanes to undergo specialized training for this task. This training

should include familiarization with the selection, inspection, and proper

use of the tooling to perform the end play check. (A-01-44)

On June 14, 2002, the Safety Board classified Safety Recommendation

A-01-44 ìOpenóUnacceptable Response.î

Before the implementation of any proposed changes in allowable

lubrication applications for critical aircraft systems, require operators to

supply to the FAA technical data (including performance information and

test results) demonstrating that the proposed changes will not present any

potential hazards and obtain approval of the proposed changes from the

principal maintenance inspector and concurrence from the FAA applicable

aircraft certification office. (A-01-45)

On July 29, 2002, the Safety Board classified Safety Recommendation

A-01-45 ìClosedóAcceptable Action.î

Issue guidance to principal maintenance inspectors to notify all operators

about the potential hazards of using inappropriate grease types and mixing

incompatible grease types. (A-01-46)

On July 29, 2002, the Safety Board classified Safety Recommendation

A-01-46 ìClosedóAcceptable Action.î

Survey all operators to identify any lubrication practices that deviate from

those specified in the manufacturerís airplane maintenance manual,

determine whether any of those deviations involve the current use of

inappropriate grease types or incompatible grease mixtures on critical

aircraft systems and, if so, eliminate the use of any such inappropriate

grease types or incompatible mixtures. (A-01-47)

On June 14, 2002, the Safety Board classified Safety Recommendation

A-01-47 ìOpenóAcceptable Response.î 

Within the next 120 days, convene an industrywide forum to disseminate

information about and discuss issues pertaining to the lubrication of

aircraft components, including the qualification, selection, application

methods, performance, inspection, testing, and incompatibility of grease

types used on aircraft components. (A-01-48)

On June 14, 2002, the Safety Board classified Safety Recommendation

A-01-48 ìOpenóAcceptable Response.î
Recommendations 186 Aircraft Accident Report

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

CAROL J. CARMODY

Acting Chairman

JOHN J. GOGLIA

Member

JOHN A. HAMMERSCHMIDT

Member

GEORGE W. BLACK, JR.

Member

Adopted: December 30, 2002
Board Member Statements 187 Aircraft Accident Report

Board Member Statements

Acting Chairman Carol J. Carmodyís Statement

I believe that NTSB staff presented a strong case in the draft report on Alaska

Airlines Flight 261, and during its presentation to the Board at our December 10, 2002

meeting, for recommending that the FAA should convene a headquartersí-led team to

conduct another in-depth, on-site follow-up inspection of Alaska Airlines to evaluate

whether adequate corrective measures have been fully implemented to cure the

deficiencies identified in the FAAís April 2000 special inspection report. I did not support

the motion to eliminate this recommendation from the report.

The accident report thoroughly discusses the maintenance failings, and the support

for convening a headquartersí-led team can be summarized as follows:

1. The systemic nature and widespread impact of previously identified

deficiencies;

2. Alaska Airlinesí failure to demonstrate during the FAAís first follow-up

inspection (in September 2000) that it had fully corrected the deficiencies;

3. The absence of any in-depth follow-up inspection by an objective team of FAA

inspectors not personally responsible for the oversight of Alaska Airlines to

verify that the deficiencies have in fact been corrected; and

4. Recent maintenance errors by Alaska Airlines maintenance personnel during

end play checks and evidence of continued poor lubrication practices.

The argument made during the meeting that yet another inspection of Alaska

Airlines would divert already stretched FAA resources from other important tasks is

actually an argument for the FAA to make. The Boardís responsibility is to make

recommendations to improve safety. In making these recommendations, we try to avoid

specific direction on the means to accomplish any given safety objective. I would expect

that a recommendation to the FAA to undertake an inspection would be considered as

would any other Board recommendation. The FAA would review the need and the

resources available and decide whether or not to pursue the recommendation. I would not

expect the FAA to pursue an inspection if that pursuit would divert FAA resources from

more important safety accomplishments. Moreover, I would expect that if that were a

possibility, the FAA would so advise the Board.
Board Member Statements 188 Aircraft Accident Report

There needs to be an assurance that Alaska Airlines has permanently remedied the

recurring maintenance problems we have documented in our exhaustive investigation.

The public expects and deserves no less. FAA should not have to be coaxed by the NTSB

into being more proactive, and I hope that we have not served to perpetuate any problems

at Alaska Airlines that the recommended inspection was intended to identify and correct.

Member Black concurred with this statement.

Member John J. Gogliaís Statement

This is a maintenance accident. Alaska Airlinesí maintenance and inspection of its

horizontal stabilizer activation system was poorly conceived and woefully executed. The

failure was compounded by poor oversight. Lubrication periods were extended, while

inspection intervals were simultaneously lengthened, neither with sound technical basis.

And, if logic and standard practice dictate that as risk increases so should monitoring,

Alaskaís program was otherwise. And in the midst of this, the accident aircraft was

dispatched from a C-check with a jackscrew of questionable serviceability that was, in all

probability, not greased. And the evidence is that it was never adequately greased again.

Had any of the managers, mechanics, inspectors, supervisors, or FAA overseers whose job

it was to protect this mechanism done their job conscientiously, this accident cannot

happen. The jackscrew is robust. Even at its wear limit condition, it is many times strong

enough to carry out its function. And it does not wear quickly when cared for. It is a

time-tested mainstay in the fleet, and one major carrier with a diligent approach to its

maintenance has never seen one seriously wear, much less wear out. A whole herd of

miscues was needed to allow it to fail. Virtually any system on an aircraft treated with the

indifference shown to this mechanism will break, many with equally catastrophic effect.

Aircraft simply must be maintained, and maintained with care and at all cost. 

I say these things because I will also admit that it may be possible to design a

better jackscrew. If there is an approach that makes this mechanism fail safe, and if the

designers feel confident that the dangers of unintended consequences in changing a

proven design are zero or less, then yes, letís by all means add a fail-safe device. But we

can easily overdue and over simplify our concern with design issues. We want now to

design a safer actuating system and we want to design a better wear measurement system.

Fine. But the existing design is safe, it just includes grease as a part of the load transferring

system and there wasnít any grease. And the measuring device we now have, no matter

how rudimentary it is thought to be, seems to work well for many airlines, and it identified

the fact that this jackscrew was wearing and needed to be replaced. The carrier simply

failed to do its job.

All the systems in an aircraft that involve large motions under high loads require

maintenance. Careful maintenance. And that is a part of the design equation, and once

designed, it has to be followed. Certainly we should not rely solely on maintenance where

mechanical design can compensate for maintenance failure. Obviously, two hands are

better than one. Yet it is seductive error to think that we can engineer out failure

permanently. We canít. Fail-safe components subject to neglect and abuse are themselves

susceptible to failure. So we need to be vigilant in our emphasis on both sides of this
Board Member Statements 189 Aircraft Accident Report

equation. This was not a design accident. This was a maintenance accident, perhaps more

purely so than any others we have seen. It would be sad and ironic indeed if some new and

supposedly fail-safe design were to come to lessen the concentration we put to keeping

these mechanisms well-maintained. 

Let me reiterate, aircraft must be maintained with care and at all cost. Alaska

Airlines expanded rapidly in the years before this accident. With the goal of becoming

more profitable, as they became bigger and busier the pressures to keep their planes on

schedule put increasing stress on their maintenance facilities. And this took its toll. The

Federal Aviation Administration seemed to know this, but was nowhere effective in

preventing the tragic chain of events. So N963AS began its fateful path in a C-check years

before falling to the ocean. Its maintainers found a jackscrew that needed to be pulled, but

no spare was found and, as the part was arguably acceptable, they pushed the plane back

into service, with no watch list, no trailers, or orders to keep track of its condition. There

were no specific procedures to do so, and no one thought enough to ask for one. The

aircraft was arguably, that is technically, legal, and it was probably safe, if it was carefully

greased. It was not, we know that without question. And the lengthened inspection

intervals were such that it was not to be looked at again, until it was in our laboratory.

When it finally failed, ground support from Alaska Airlines seemed to encourage the crew

to proceed with a broken plane on to their scheduled destination, for reasons perhaps of

convenience both to passengers and maintenance ñ we wonít really ever know. But the

impression is inescapable. An aircraft that had been hustled out the door three years earlier

for the convenience of scheduling was now encouraged to keep to its appointed routing. It

is less coincidence than culture. 

NTSB has made several specific maintenance recommendations, some already

accomplished, that will, if followed, prevent the recurrence of this particular accident. But

maintenance, poorly done, will find a way to bite somewhere else. Alaska needs to

re-constitute its will to performance and perfection on the shop floor. FAA needs to

revitalize its Alaska staff and become more intelligent about its efforts to oversee. And if,

while all this goes on, the manufacturers can also add greater margins for error, that is

icing on the cake. I am interested to see what system enhancements come from this, but I

am still left with a mechanicís perspective -- you either maintain it or it breaks. This is

universally applicable. Like the old adage says, ìyou schedule maintenance, or the

maintenance will schedule you.î

Acting Chairman Carmody and Members Hammerschmidt and Black concurred

with this statement.

191 Aircraft Accident Report

5. Appendixes

Appendix A

Investigation and Public Hearing

Investigation

The National Transportation Safety Board was initially notified of this accident

about 1945 eastern standard time on January 31, 2000. Investigators from the Safety

Boardís regional office in Los Angeles, California, went immediately to the scene of the

accident. A full go-team was assembled in Washington, D.C., and arrived on scene early

the next morning. The go-team was accompanied by Member John Hammerschmidt and

representatives from the Boardís Office of Government, Public, and Family Affairs. 

Parties to the investigation were the Federal Aviation Administration (FAA); the

Boeing Commercial Airplane Group; Alaska Airlines, Inc.; the Aircraft Mechanics

Fraternal Association (AMFA); the Air Line Pilots Association (ALPA); the Association

of Flight Attendants; the National Air Traffic Controllers Association; Pratt & Whitney,

and Shell Global Solutions.

Public Hearing

A public hearing was conducted for this accident from December 13 to 15, 2000,

in Washington, D.C. Member John Hammerschmidt presided over the hearing. Parties to

the public hearing were the FAA, Alaska Airlines, Boeing, ALPA, and AMFA.
Appendix B 192 Aircraft Accident Report

Appendix B

Cockpit Voice Recorder Transcript

The following is a transcript of the Fairchild A-100A cockpit voice recorder

(CVR) serial number 62892, installed on the accident airplane. The CVR transcript

reflects the 31 minutes before power was lost to the CVR. All times are Pacific standard

time, based on a 24-hour clock.

,

LEGEND

RDO Radio transmission from accident aircraft, Alaska 261 

CAM Cockpit area microphone voice or sound source 

PA Voice or sound heard on the public address system channel. 

HOT Hot microphone voice or sound source1

 For RDO, CAM, HOT, and PA comments: 

-1 Voice identified as the Captain 

-2 Voice identified as the First Officer 

-3 Voice identified as a Flight Attendant 

-? Voice unidentified 

MZT Radio transmission from Mazetlan Center 

LAX CTR1 Radio transmission from the Los Angeles Air Route Traffic Control 

Center sector 30 controller 

LAX CTR2 Radio transmission from the Los Angeles Air Route Traffic Control 

Center sector 25 controller 

 

LAX-MX Radio transmission from Alaska Airlines Maintenance facility in Los 

Angeles 

LAX-OPS Radio transmission from Alaska Airlines Operations facility in Los 

Angeles 

SEA-DIS Radio transmission from Alaska Airlines Dispatch facility in Seattle 

 

SEA-MX Radio transmission from Alaska Airlines maintenance facility in 

Seattle 

 -1 First voice 

 -2 Second voice 

 

1

 This recording contained some audio from one Hot microphone. The voices or sounds heard on this 

channel were also heard on the CAM channel, and are annotated as coming from the CAM channel in this 

transcript. The audio from the HOT microphone was used to clarify the CAM audio when possible. 
Appendix B 193 Aircraft Accident Report

ATIS Radio transmission from Los Angeles airport Automated Terminal 

Information System 

CAWS Mechanical voice or sound source from the Central Aural Warning 

System, as heard on the Cockpit Area Microphone channel. 

* Unintelligible word 

@ Non-pertinent word 

# Expletive

- - - Break in continuity or interruption in comment

( ) Questionable insertion 

[ ] Editorial insertion

... Pause 

Note 1: Times are expressed in Pacific Standard Time (PST). 

Note 2: Generally, only radio transmissions to and from the accident aircraft were transcribed. 

Note 3: Words shown with excess vowels, letters, or drawn out syllables are a phonetic representation of the words 

as spoken. 

Note 4: A non-pertinent word, where noted, refers to a word not directly related to the operation, control or condition 

of the aircraft. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT Start of Transcript 1549:49.3 [start of recording] 1549:50 SEA-MX um beyond that I have verified no history on y

the past thirty days. 1549:54.8 CAM [sound of click] 1549:57.7 RDO-1 yea we didn't see anything in the logbook. 1550:14 CAM-2 why don't you pull your your seat forward and I'll just check this pedestal back there. I don't think there's anything be- yond that we haven't checked. 1550:22 CAM-1 see when he's saying pedestal... I believe he's talking about this--- 1550:25 CAM-2 oh. 1550:25 CAM-1 ---switch that's on the * that's on the pedestal. 1550:27 CAM-2 yea okay. 1550:31 CAM-1 do you see anything back there? 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1550:32 CAM [Sound of click] 1550:33 CAM-2 uh there's *. 1550:40 SEA-MX and two sixty one, maintenance. 1550:42.0 RDO-1 go ahead maintenance two six one. 1550:44 SEA-MX understand you're requesting uh diversion to 

discrepancy is there a specific reason you pSan Francisco? 1550:45 MZT Alaska two sixty one radar service terminate

Los Angeles center frequency one one nine five good day. 1550:54.4 RDO-1 well a lotta times its windy and rainy and we

cisco and uh, it seemed to me that a dry rthe wind is usually right down the runway more reasonable. 1550:55.0 RDO-2 one one nine ninety five Alaska two sixty one

1551:01.2 RDO-2 say again the frequency one one nine point e
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1551:05 MZT affirm one one nine decimal ninety five. 1551:09.3 RDO-2 roger. 1551:09.9 SEA-MX ok and uh... is this added fuel that you're gonna h

gonna be a complication or an advantage? 1551:18.1 RDO-1 well the way I'm reading it uh heavier airplanes laright now I got fifteen five on board, I'm thinking tabout twelve which is still uh an hour and forty mand those are the numbers I'm running up here. 1551:20.6 RDO-2 L A Alaska two sixty one three one zero. 1551:36 SEA-MX ok uh two sixty one standby for dispatch. 1551:38 RDO-2 Los Angeles Alaska two sixty one three one zero. 1551:40 RDO-1 OK the other thing you gotta know is that they

about holding and delays in San Francisco um maintenance facil- eh you know planning uh seemed like a smarter move from airworthy move

1551:42 LAX-CTR1 Alaska two sixty one L A center roger. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1551:50 RDO-2 * there's two people on the frequency I'm so

sixty one I didn't hear your response. 1551:58 LAX-CTR1 Alaska two six one squawk two zero one zero

1552:01 RDO-2 two zero one zero Alaska two sixty one. 1552:02 SEA-DIS two sixty one dispatch... uh current San Fran

one eight zero at six, nine miles, few at fifteeken twenty eight hundred overcast thirty fouif uh you want to land at L A of course for safewill do that uh wu we'll uh tell you though that A uh we'll be looking at probably an hour to half we have a major flow program going that's for ATC back in San Francisco. 1552:31 RDO-1 well uh yu you eh huh... boy you put me 

um.... 1552:41 RDO-1 I really didn't want to hear about the flow be

you're calling us cause I'm concerned about able airports. 1552:51 SEA-DIS well we wanna do what's safe so if that's wha

safe we just wanna make sure you have all the info. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1552:59 RDO-1 yea we we kinda assumed that we had... wh

wind again there in San Francisco? 1553:03 SEA-DIS wind at San Francisco currently zero uh on

six. 1553:08 CAM-2 what runway they landing... one zero? 1553:09 CAM-1 what's that? 1553:10 CAM-2 ask him what runway they're landing. 1553:11 RDO-1 and confirm they're landing runway one zero

1553:15 SEA-DIS and uh standby I'll confirm that. 1553:17 CAM-2 and see if the runways are dry or wet. 1553:19 RDO-1 and we need to know if they're dry or wet. 1553:21 SEA-DIS eh yup I'll uh find that out and uh correctio

one eight zero at six and standby. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATIO

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1553:28 CAM-1 one eight zero at six... so that's runway one six what we need is runway one nine, and they're not landing runway one nine. 1553:35 CAM-2 I don't think so. 1553:37 CAM-2 we might just ask if there's a ground school instructor there available and and discuss it with him... or a uh simulator in- structor. 1553:40 CAM-1 yea. 1553:46 RDO-1 and uh dispatch one sixty one... we're w

get some support out of the uh instructe

1553:53 RDO-1 ---instructors up there if they got any ide

1554:23 CAM-1 you're talkin to ATC huh? 1554:24 CAM-2 yea uh huh. 1554:26 CAM-2 well lets confirm the route of flight its uh, I wasn't totally sure but its uh direct Oceanside? 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMU

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1554:32 CAM-1 Tijuana Oceanside... Oceanside right... then Santa Cata- lina. 1554:47 CAM-1 ehh somebody was callin in about wheelchairs--- 1554:50 CAM-3 oh really? 1554:50 CAM-1 ---when I'm workin a problem. 1554:51 CAM-3 is that why it went static? 1554:53 CAM-1 ok yea now... I just that's something that oughta be in the computers, if they want it that bad they you guys oughta be able to pick up the phone--- 1555:00 CAM-3 mmm hmm. 1555:00 CAM-1 ---just... drives me nuts. not that I wanna go on about it... you know I it just blows me away they think we're gonna land, they're gonna fix it, now they're worried about the flow, I'm sorry this airplane's idn't gonna go anywhere for a while.... so you know. 1555:16 CAM-3 so they're trying to put the pressure on you--- 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1555:18 CAM-1 well no, yea. 1555:19 CAM-3 ---well get it to where it needs to be. 1555:20 CAM-1 and actually it doesn't matter that much to us. 1555:23 CAM-3 still not gonna go out on time to the next *. 1555:24 CAM-1 yea... yea... I thought they'd cover the people better from L A--- 1555:29 CAM-3 L A 1555:30 CAM-1 ---then San Francisco. 1555:32 RDO-2 L A Alaska two sixty one just confirm our r

Tijuana is, direct Oceanside? 1555:38 LAX-CTR1 Alaska two sixty one after Tijuana cleared to 

via direct San Marcos jet five zero one Big Stain flight level three one zero. 1555:47 RDO-2 OK uh San Francisco San Marcos J five ze

uh direct three one zero Alaska two sixty one
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1555:55 LAX-CTR1 * 1556:03 SEA-DIS Alaska two sixty one dispatch. 1556:06 RDO-1 dispatch Alaska two six one go ahead. 1556:08 SEA-DIS yea I called uh ATIS they're landing two eig

left and uh wasn't able to get the the runwlooking at past uh weather it hasn't rained I'm looking at uh probably a dry runway. 1556:21 RDO-1 ok uh. 1556:26 RDO-1 I have with the information I have available 

waitin on that CG update I'm looking at speed of a hundred and eighty knots, uh wind at L A X lax? 1556:50 SEA-DIS its two six zero at nine. 1556:56 RDO-1 ok two six at nine.... 1556:59 RDO-1 ...versus a direct crosswind which is effec

in groundspeed... I gotta tell you, when I safety point I think that something that lowspeed makes sense. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1557:16 SEA-DIS ok two sixty one that'll uh that'll mean L A X t

I was gonna get you if I could to call L A X wand they can probably whip out that CG for yo

1557:30 RDO-1 I suspect that uh that's what we'll have to do

my plan is we're gonna continue as if going cisco get all that data then begin our descenX, and at a lower altitude we will configure, handling uh envelope before we proceed with 

1558:05 SEA-DIS ok two sixty one dispatch copied that, if you 

uh L A ops updated on uh your ETA, that wand I'll be talking with them. 1558:15 RDO-1 ok well ah if you'll let them know we're comin 

they'll probably listen as we talk... were goin gonna stay up here and burn a little more ducks in a row, and then we'll uh be talking we start down to go in there. 1558:29 SEA-DIS ok and if you have any problems with them g

gimme a call back. 1558:34 RDO-1 ok. break, L A X do you read Alaska two six o

1558:39 LAX-OPS two sixty one I do copy do you have an ETA f
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1558:43 RDO-1 well.... 1558:45 RDO-1 ...yea I'm gonna put it at about thirty, thirty f

could actually, the longer the more fuel I burn am... but I wonder if you can compute oubased on the information we had at takeoff for 

1558:58 LAX-OPS ok you're transmission is coming in broken 

ahead. 1559:02 RDO-1 you know what I'll wait a minute we'll be a l

and that'll help everything. 1559:06 LAX-OPS ok also uh two sixty one just be advised uh be

an international arrival we have to get landing know how long that's gonna take me... but clear it all through customs first. 1559:19 RDO-1 ok I unders... I remember this is complicated, 

ter start that now cause we are comin to you. 1559:26 LAX-OPS copy. 1559:29 CAM-1 we'll call em back over as we get closer to Catalina. 1559:34 CAM-2 as we get what? 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1559:34 CAM-1 closer to L A she's got to get landing rights. 1559:37 CAM-2 were ninety four miles from L A now. 1559:38 CAM-1 oh ok. you wanna listen to the ATIS you can. 1559:42 CAM-2 in fact I switched it once already just kinda late. 1559:44 CAM-1 you got the jet. 1559:44 CAM-2 I got it. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1559:50 ATIS charlie five and charlie six is restricted * tax

is restricted to MD eleven and smaller. read hold short instructions. upon receipt of your read back only your callsign and transpondyou have a question. advise on initial contaformation mike. Los Angeles international tion mike. two two five zero zulu. wind tweight. visibility eight. few clouds at two thoudred. one two thousand scattered. ceiling sand overcast. temperature one six dewpotimeter three zero one seven. simultaneous in progress runway two four right and two fifor visual approach will be provided. simuapproaches to all runways are in progress. calizer approaches are in progress betweeinternational and Hawthorne airports. simument departure in progress runway two founotices to airmen. 1601:01 CAM-2 so he wanted us to go to San Fran initially? 1601:06 CAM-1 to keep the schedule alive. I mean it was just... it was I mean he had all the reasons to do it, I stated concern about flying overflying a suitable airport--- 1601:15 CAM-2 yea. 1601:16 CAM-1 ---but I was listening, then when he gives me the wind, its it's... the wind was a ninety degree cross at ten knots. two eight and we'd be landing on--- 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1601:30 CAM-2 and they are using one nine? 1601:33 CAM-1 you know I don't know... I wrote it down there... the winds were... one eighty at six... I don't know. 1601:49 CAM-2 I don't know. 1601:49 CAM-1 I don't care... you know what? I expect him to figure all that # --- 1601:53 CAM-2 right. 1601:53 CAM-1 ---he's got it on the screen--- 1601:54 CAM-2 that's why I was thinking that an instructor would really uh--- 1601:58 CAM-1 yea. 1601:58 CAM-2 ---cut through the crap there. 1601:59 CAM-2 they... not available? 1602:00 CAM-1 well they just don't talk to each other. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1602:02 CAM-2 oh. 1602:02 CAM-1 I mean I * --- 1602:04 CAM-2 * they've always told us they were available you know--- 1602:06 CAM-1 yea yea. 1602:07 CAM-2 ---anytime you have a problem. 1602:09 CAM-2 if they get one down there. 1602:12.6 RDO-1 Los Angeles one sixty one do you read m

1602:29 CAM-1 I got the track goin over there. 1602:31 LAX-OPS go ahead two six one. 1602:33.6 RDO-1 two sixty one, I... I know you're busy on 

discussing it up here could you give us Francisco if you could just pull em up on y

1602:57 CAM-2 I thought they.... 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1603:00 LAX-OPS ok ahhh San Francisco, ok we've got uh.

seventy at six knots. 1603:15.6 RDO-1 ok thank you that's what I needed. we are 

you... and I've misplaced the paperwork he

1603:23 CAM-2 there it is. 1603:35 CAM-1 I can't read your writing... can you read her the uh zero fuel weight--- 1603:40 CAM-2 yea. 1603:41 CAM-1 ---and all those numbers and CG. 1603:43 LAX-OPS L A operations from two six to two six one. 1603:48 CAM-2 I got it. 1603:48.5 RDO-2 uhhh two sixty one... do you need our uh, o

1603:52 LAX-OPS yea we just wanna advise that we do n

rights as yet. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1603:56 RDO-2 here's our numbers we had uh ten in first 

coach, zero fuel weight one zero two one one fuel on board thirty four point niner takthirty six five one one point eight, CG eleve

1604:19 LAX-OPS OK I got ten and seventy Z fuel weight o

one zero point one, fuel on board thirty fotake off weight five one one decimal eigeleven decimal eight. 1604:32 RDO-2 yea uh take off one three six five one one pCG one one point eight. and we currenthousand six hundred pounds of fuel on boa

1604:43 CAM-1 estimate ten thousand on landing. 1604:45 RDO-2 estimating ten thousand pounds on landing

1604:53 LAX-OPS ok you said your takeoff weight was... one 

one one decimal eight? 1604:58 RDO-2 one three six five one one point eight. 1605:05 LAX-OPS one three six five one one point eight thank 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1605:07 RDO-2 and we're currently a hundred and fiftee

weight, and we'll burn another three thousa

1605:19 CAM [sound of two clicks] 1605:27 CAM-2 I'm back on the uh I'm off of the uh company. 1606:26 CAM-1 no... that's what I was expecting them to do. duh. 1606:47 CAM-2 so our... actually our landing speed will be one forty eight plus... some additive right? 1607:06 CAM-1 lets guess... lets guess one twelve. 1607:10 CAM-2 ok. 1607:10 CAM-1 one forty six... plus... I get a minus two, worst case... twenty four knots... fifty sixty seventy... *. 1607:33 LAX-OPS Alaska two sixty one from operations can 

tail number? 1607:38 RDO-1 uh two sixty one, it was ship number nine s
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1607:43 LAX-OPS copy that two... uh your aircraft number is nin

1607:47 RDO-1 affirmative thank you. 1607:51 LAX-MX-1 and two sixty one maintenance. 1607:53 RDO-1 two sixty one go. 1607:54 LAX-MX-1 yea are you guys with the uh, horizontal situat1607:58 RDO-1 affirmative. 1607:59 LAX-MX-1 yea did you try the suitcase handles and the p

right? 1608:03 RDO-1 yea we tried everything together, uh.... 1608:08 RDO-1 ...we've run just about everything if you've g

circuit breakers we'd love to know about 'em. 1608:14 LAX-MX-1 I'm off I'll look at the uh circuit breaker uh g

double check and um yea I just wanted to knthe pickle switches and the suitcase handles movin in with any of the uh other switches othsuitcase handles alone or nothing. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1608:29.9 RDO-1 yea we tried just about every iteration. 1608:32 LAX-MX-1 and alternate's inop too huh? 1608:35.1 RDO-1 yup its just it appears to be jammed the uh t

it spikes out when we use the primary, we gtells me the motor's tryin to run but the brake when we use the alternate, nothing happens. 1608:50 LAX-MX-1 ok and you you you say you get a spike whe

up there in the cockpit when you uh try to muh um with the primary right? 1608:59 CAM-1 I'm gonna click it off you got it. 1609:00 CAM-2 ok. 1609:01.5 RDO-1 affirmative we get a spike when we do the p

there's no appreciable uh change in the uwhen we do the alternate. 1609:09 LAX-MX-1 ok thank you sir see you here. 1609:11 RDO-1 ok. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1609:13 CAM-1 lets do that. 1609:14.8 CAM [sound of click] 1609:14.8 CAM-1 this'll click it off. 1609:16 CAM [sound of clunk] 1609:16.9 CAM [sound of two faint thumps in short succession] 1609:17.0 CAWS [sound similar to horizontal stabilizer-in-motion audible tone] 1609:18 CAM-1 holy #. 1609:19.6 CAWS [sound similar to horizontal stabilizer- in-motion audible tone] 1609:21 CAM-1 you got it?... # me. 1609:24 CAM-2 what are you doin? 1609:25 CAM-1 I it clicked off--- 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1609:25.4 CAWS [sound of chime] Altitude 1609:26 CAM-1 ---it * got worse... ok. 1609:30 CAM [sound similar to airframe vibration begins] 1609:31 CAM-1 you're stalled. 1609:32 CAM [sound similar to airframe vibration becomes louder] 1609:33 CAM-1 no no you gotta release it ya gotta release it. 1609:34 CAM [sound of click] 1609:34 CAM [sound similar to airframe vibration ends] 1609:42.4 CAM-1 lets * speedbrake. 1609:46 CAM-1 gimme a high pressure pumps. 1609:52 CAM-2 ok. 1609:52 CAM-1 help me back help me back. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1609:54 CAM-2 ok. 1609:55 RDO-1 center Alaska two sixty one we are uh in a 

1610:01.6 RDO-1 and I've lost control, vertical pitch. 1610:01.9 CAWS [sound of clacker] Overspeed. (begins and repeats for approx 33 seconds) 1610:05 LAX-CTR1 Alaska two sixty one say again sir. 1610:06.6 RDO-1 yea were out of twenty six thousand feet

cal dive... not a dive yet... but uh we've of our airplane. 1610:15 CAM [sound of click] 1610:20 CAM-1 just help me. 1610:22 CAM-1 once we get the speed slowed maybe... we'll be ok. 1610:28.2 RDO-1 we're at twenty three seven request uh. 1610:33 RDO-1 yea we got it back under control here. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATI

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1610:34 RDO-2 no we don't, ok. 1610:37 CAM-1 ok. 1610:37 LAX-CTR1 the altitude you'd like to uh to remain a

1610:40 CAM [sound of click] 1610:45 CAM-2 lets take the speedbrakes off I'm * --- 1610:46 CAM-1 no no leave them there. it seems to be helping. 1610:51 CAM-1 # me. 1610:53 CAWS [sound of chime] Altitude 1610:55 CAM-1 ok it really wants to pitch down. 1610:58 CAM-2 ok. 1610:59 CAM-1 don't mess with that. 1611:04 CAM-2 I agree with you. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1611:04 LAX-CTR1 Alaska two sixty one say your condition. 1611:06.6 RDO-1 two sixty one we are at twenty four thousa

stabilized. 1611:10 RDO-1 we're slowing here, and uh, we're gonna uh. 1611:15 RDO-1 do a little troubleshooting, can you gimme a 

uh, twenty and twenty five? 1611:21 LAX-CTR1 Alaska two sixty one maintain block altitude 

zero zero through flight level two five zero. 1611:27 RDO-1 Alaska two sixty one we'll take that block we

the freq. 1611:31 CAM-2 you have the airplane let me just try it. 1611:33 CAM-1 ok. 1611:33 CAM-2 uh how hard is it? 1611:33 CAM-1 I don't know my adrenaline's goin... it was really tough there for a while. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1611:38 CAM-2 yea it is. 1611:39 CAM-1 ok. 1611:43 CAM-2 whatever we did is no good, don't do that again. 1611:44 CAM-1 yea, no it went down it went to full nose down. 1611:48 CAM-2 uh it's a lot worse than it was? 1611:50 CAM-1 yea yea we're in much worse shape now. 1611:59 CAM-1 I think its at the stop, full stop... and I'm thinking, we can- can it go any worse... but it probably can... but when we slowed down, lets slow it lets get down to two hundred knots and see what happens. 1612:16 CAM-2 ok? 1612:16 CAM [sound of click] 1612:17 CAM-2 we have to put the slats out and everything... flaps and slats. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1612:20 CAM-1 yea... well we'll wait ok you got it for a second? 1612:23 CAM-2 yea. 1612:25.3 RDO-1 maintenance two sixty one are you on? 1612:30 LAX-MX-2 yea two sixty one this is maintenance. 1612:32.0 RDO-1 ok we did--- 1612:33.2 RDO-1 ---we did both the pickle switch and the s

and it ran away full nose trim down. 1612:39 LAX-MX-2 oh it ran away trim down. 1612:42 RDO-1 and now we're in a * pinch so we're holding 

than we were. 1612:50 LAX-MX-2 ok uh... geez. 1612:52 LAX-MX-1 you want me to talk to em? (in the backgro

vious transmission) 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1612:55 LAX-MX-1 yea two sixty one maintenance uh uh yo

trim down but are you getting any you donup is that correct? 1613:04 RDO-1 that's affirm we went to full nose down an

again to see if we can get it to go in the ot

1613:10 LAX-MX-1 ok well your discretion uh if you want to t

me if not that's fine. um we'll see you at th

1613:20 CAM-2 did it happen went in reverse? when you pulled back it went forward? 1613:22 CAM-1 I went tab down... right, and it should have come back in- stead it went the other way. 1613:29 CAM-2 uh huh. 1613:30 CAM-1 what do you think? 1613:32 CAM-2 uhhh. 1613:32 CAM-1 you wanna try it or not? 1613:32 CAM-2 uhh no. boy I don't know. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1613:33 CAM-1 its up to you man. 1613:35 CAM-2 lets head back toward uh here lets see... well we're--- 1613:39 CAM-1 I like where were goin out over the water myself... I don't like goin this fast though. 1613:50 CAM [sound of click] 1613:57 CAM-1 ok you got * [sound similar to short interruption in recording] second? 1613:58 CAM-2 yea. 1613:59 CAM-2 we better... talk to the people in the back there. 1614:03 CAM-1 yea I know. 1614:04 LAX-CTR1 Alaska two sixty one let me know if you nee

1614:08 RDO-2 yea we're still workin this. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1614:12 PA-1 folks we have had a flight control problem up front here we're workin it uh that's Los Angeles off to the right there that's where we're intending to go. we're pretty busy up here workin this situation I don't anticipate any big problems once we get a couple of sub systems on the line. but we will be going into L A X and I'd anticipate us parking there in about twenty to thirty minutes. 1614:39 CAM-1 ok... did the, first of all, speedbrakes. did they have any effect? 1614:49 CAM-1 lets put the power where it'll be for one point two, for land- ing. you buy that? 1614:53 CAM-1 slow it down and see what happens. 1614:54 LAX-CTR1 Alaska two sixty one contact L A center o

five two they are aware of your situation. 1615:00.0 RDO-2 ok Alaska two sixty one say again the freq

zero five two? 1615:02 CAM-1 I got the yoke. 1615:04 LAX-CTR1 Alaska two sixty one, twenty six fifty two. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATIO

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1615:06 RDO-2 thank you. 1615:07 LAX-CTR1 you're welcome have a good day. 1615:19.7 RDO-2 L A Alaska two sixty one we're with you 

five, we have a jammed stabilizer analtitude with difficulty. uh but uh we can think... and our intention is to land at Lo

1615:36 LAX-CTR2 Alaska two sixty one L A center roger u

Los Angeles airport via present positMonica, direct Los Angeles and uh, yowhat do you want to do sir? 1615:54 CAM-1 let me get let me have it. 1615:56 RDO-1 center uh Alaska two sixty one. I need 

ten, change my configuration, make sujet and I'd like to do that out here over th

1616:07 LAX-CTR2 ok Alaska two sixty one roger that stand

1616:11 CAM-2 lets do it at this altitude instead--- 1616:11 CAM-1 what? 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1616:12 CAM-2 ---of goin to ten lets do it at this altitude. 1616:14 CAM-1 cause the airflow's that much difference down at ten this air's thin enough that that you know what I'm sayin? 1616:20 CAM-2 yea uh I'll tell em to uh--- 1616:22 CAM-1 I just made a PA to everyone to get everybody--- 1616:24 CAM-2 ok. 1616:26 CAM-1 ---down you might call the flight attendants. 1616:27 CAM [sound similar to cockpit door operating] 1616:32 CAM-3 I was just comin up this way. 1616:32 LAX-CTR2 Alaska two sixty one fly a heading of t

descend and maintain one seven thousan

1616:34 CAM-2 uhh. 1616:36 CAM [sound similar to cockpit door operating] 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1616:39.0 RDO-1 two eight zero and one seven seventeen 

two sixty one. and we generally need a bloc

1616:45 LAX-CTR2 ok and just um I tell you what do that for n

tact L A center on one three five point five ther uhh instructions for you sir. 1616:56.9 RDO-2 ok thirty five five say the altimeter setting? 1616:59 LAX-CTR2 the L A altimeter is three zero one eight. 1617:01 CAM-1 I need everything picked up--- 1617:02 RDO-2 thank you. 1617:02 CAM-1 ---and everybody strapped down--- 1617:04 CAM-3 ok. 1617:04 CAM-1 ---cause I'm gonna unload the airplane and see if we can--- 1617:06 CAM-3 ok. 1617:07 CAM-1 ---we can regain control of it that way. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1617:09 CAM-3 ok we had like a big bang back there--- 1617:11 CAM-1 yea I heard it--- 1617:12 CAM-3 ok. 1617:12 CAM-1 ---the stab trim I think it--- 1617:13 CAM-2 you heard it in the back? 1617:13 CAM-3 yea. 1617:14 CAM-2 yea. 1617:15 CAM-3 so--- 1617:15 CAM-1 I think the stab trim thing is broke--- 1617:17 CAM-3 ---I didn't wanna call you guys... but--- 1617:18 CAM-1 no no that's good. 1617:20 CAM-3 ---that girl, they're like you better go up there--- 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATIO

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1617:21 CAM-1 I need you everybody strapped in now, dear. 1617:22 CAM-3 ---and tell them. 1617:23 CAM-3 ok. 1617:24 CAM-1 cause I'm gonna I'm going to release the back pressure and see if I can get it... back. 1617:30 CAM [sound similar to cockpit door operating] 1617:33 CAM-2 three zero one eight. 1617:37 CAM-1 I'll get it here. 1617:40 CAM-2 I don't think you want any more speedbrakes do you? 1617:42 CAM-1 uhh no. actually. 1617:46 CAM-2 he wants us to maintain seventeen. 1617:51 CAM-1 ok I need help with this here. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1617:52 CAM-1 slats ext... lets--- 1617:54 CAM-2 ok slats--- 1617:54 CAM-1 gimme slats extend. 1617:55 CAM-2 got it. 1617:56.6 CAM [sound similar to slat/flap handle movement] 1617:58 CAM-1 I'm test flyin now--- 1617:59 CAM-2 how does it feel? 1618:00 CAM-1 it's wantin to pitch over more on you. 1618:02 CAM-2 really? 1618:03 CAM-1 yea. 1618:04 CAM-2 try flaps?... fifteen, eleven? 1618:05 CAM-1 ahh lets go to eleven. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATIO

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1618:07.3 CAM [sound similar to slat/flap handle movement] 1618:09 CAM-2 ok... get some power on. 1618:10 CAM-1 I'm at two hundred and fifty knots, so I'm lookin.... 1618:17 CAM-2 real hard? 1618:17 CAM-1 no actually its pretty stable right here... see but we got to get down to a hundred an eighty. 1618:26 CAM-1 OK... bring bring the flaps and slats back up for me. 1618:32 CAM-2 slats too? 1618:33 CAM-1 yea. 1618:36.8 CAM [sound similar to slat/flap handle movement] 1618:37 CAM-2 that gives us... twelve thousand pounds of fuel, don't over boost them. 1618:47 CAM-1 what I'm what I wanna do... 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATIO

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1618:48 CAM [sound similar to slat/flap handle movement] 1618:49 CAM-1 is get the nose up... and then let the nose fall through and see if we can stab it when it's unloaded. 1618:54 CAWS [sound of chime] Altitude (repeats for approximately 34 sec- onds) 1618:56 CAM-2 you mean use this again? I don't think we should... if it can fly, its like--- 1619:01 CAM-1 it's on the stop now, its on the stop. 1619:04 CAM-2 well not according to that its not. 1619:07 CAM-2 the trim might be, and then it might be uh, if something's popped back there--- 1619:11 CAM-1 yea. 1619:11 CAM-2 ---it might be * mechanical damage too. 1619:14 CAM-2 I think if it's controllable, we oughta just try to land it--- 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1619:16 CAM-1 you think so? ok lets head for L A. 1619:21.1 CAM [sound of faint thump] 1619:24 CAM-2 you feel that? 1619:25 CAM-1 yea. 1619:29 CAM-1 ok gimme sl--- see, this is a bitch. 1619:31 CAM-2 is it? 1619:31 CAM-1 yea. 1619:32.8 CAM [sound of two clicks similar to slat/flap handle movement] 1619:36 CAM-? * 1619:36.6 CAM [sound of extremely loud noise] [increase in background noise begins and continues to end of recording] [sound similar to loose articles moving around in cockpit] 1619:37 CAM-? * 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICATION TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1619:37.6 PA [sound similar to CVR startup tone] 1619:43 CAM-2 mayday. 1619:49 CAM-1 push and roll, push and roll. 1619:54 CAM-1 ok, we are inverted... and now we gotta get it.... 1619:59 CAM [sound of chime] 1620:03 CAM-1 kick * 1620:04 CAM-1 push push push... push the blue side up. 1620:14 CAM-1 push. 1620:14 CAM-2 I'm pushing. 1620:16 CAM-1 ok now lets kick rudder... left rudder left rudder. 1620:18 CAM-2 I can't reach it. 1620:20 CAM-1 ok right rudder... right rudder. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNI

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1620:25 CAM-1 are we flyin?... we're flyin... we're flyin... tell 'em what we're doin. 1620:33 CAM-2 oh yea let me get * 1620:35 CAM-1 * 1620:38 CAM-1 gotta get it over again... at least upside down we're flyin. 1620:40.6 PA [sound similar to CVR startup tone] 1620:42 CAM-? * 1620:44 CAM-? * 1620:49 CAM [sounds similar to compressor stalls begin and continue to end of recording] 1620:49 CAM [sound similar to engine spool down] 1620:54 CAM-1 speedbrakes. 1620:55.1 CAM-2 got it. 
INTRA-COCKPIT COMMUNICATION AIR-GROUND COMMUNICAT

TIME and TIME and SOURCE CONTENT SOURCE CONTENT 1620:56.2 CAM-1 ah here we go. 1620:57.1 [end of recording] End of transcript