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NTSB Report
October 9, 1989 · Boeing 757-251 · N512US
San Francisco, CA
Overview
Report Status
Probable Cause
Event Type
Incident
Date
October 9, 1989
Injury Outcomes
No injuries (147 uninjured)
Location
San Francisco, CA
Aircraft
Boeing 757-251
Tail Number
N512US
Operator
Northwest Airlines
Injuries
None
Aircraft Damage
Minor
NTSB Number
CHI90IA006
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.
NTSB Probable Cause
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.
Full Narrative
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.
Flight
Event TypeIncident
Event Time11:45 PDT
Nearest Airport0nm N
Runway0
Runway Length0
Runway Width0
Flight Plan FiledIFR
Aircraft FireNo
Aircraft ExplosionNo
Aircraft
AircraftBoeing 757-251
Tail NumberN512US
Aircraft CategoryAir
DamageMinor
OperatorNorthwest Airlines
Operating RulePart 121
Engines2
Engine TypeTurbofan
Engine DetailsP&W · 2037 · Turbofan · 38200
Seats178
People & Injuries
Injury SummaryNo injuries (147 uninjured)
Fatal0
Serious0
Minor0
Uninjured147
Total147
Crew 1Age 49 · First Class
Conditions
ConditionsVMC
LightDaylight
Wind290° @ 6 kt
Visibility15 sm
CeilingNone
Temperature / Dew Point70 F / 0 C
Altimeter0
Weather SourcePILO