NTSB Identification: CHI90IA006.
The docket is stored on NTSB microfiche number 41595.
Scheduled 14 CFR operation of NORTHWEST AIRLINES
Incident occurred Monday, October 09, 1989 in SAN FRANCISCO, CA
Probable Cause Approval Date: 08/26/1992
Aircraft: BOEING 757-251, registration: N512US
Injuries: 147 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

THE FLIGHT CREW REPORTED THAT THEY RECEIVED A 'FLAP DISAGREE' MESSAGE DURING THE DESCENT AND APPROACH TO THEIR DESTINATION AIRPORT. WHEN CHECKLIST PROCEDURES WERE NOT SUCCESSFUL IN CORRECTING THE SITUATION, AN UNEVENTFUL 'ABNORMAL FLAP' LANDING WAS PERFORMED. POST-INCIDENT INSPECTION REVEALED A FAILED INBOARD TRAILING EDGE FLAP DRIVE TORQUE TUBE COUPLING. FURTHER INVESTIGATION REVEALED ONE SIMILAR FAILURE, AND EXAMINATION OF THE OPERATING 757 FLEET REVEALED NUMEROUS OCCURRENCES OF EXCESSIVE WEAR ON THE AFT COUPLING OF THE TORQUE TUBE ASSEMBLY. LAB ANALYSIS OF THE WEAR, AND A METALLURGICAL EXAMINATION RESULTED IN THE MANUFACTURER REDUCING THE RECOMMENDED MAINTENANCE INTERVAL AND RECOMMENDING MORE FREQUENT VISUAL INSPECTIONS OF THE COMPONENT.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

THE EXTREME WEAR AND EVENTUAL FAILURE OF A TORQUE TUBE IN THE INBOARD TRAILING EDGE FLAP SYSTEM. INSPECTION AND LUBRICATION INTERVALS WERE A FACTOR, AS WAS THE PHYSICAL LOCATION OF THE TORQUE TUBE WITH REFERENCE TO EXTERNAL FACTORS.

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