# NTSB CHI90IA006: Boeing 757-251, N512US, San Francisco, CA, October 9, 1989

- Type: NTSB incident investigation
- Classification: Incident · No injuries · Incident investigation · Final report
- Date: October 9, 1989, 11:45 local (18:45 UTC)
- Place: San Francisco, CA
- Coordinates: 37.799781, -122.399513 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=37.7998&lon=-122.3995&mi=10
- Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping)
- Highest injury: No injuries
- Injuries: 147 uninjured
- Status: Completed, report August 26, 1992
- Report: Approved by Office Director
- NTSB case key (mKey): 14560
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CHI90IA006
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/14560/pdf
- Page: https://openairdocs.org/ntsb/CHI90IA006

## 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.

## Sequence of events

In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf).

- Airframe/component/system failure/malfunction, descent - normal
  - flt control syst,wing flap control cable/rod - worn (contributing factor)
  - maintenance,lubrication - inadequate (contributing factor)
  - insufficient standards/requirements,manufacturer, manufacturer (contributing factor)
  - flt control syst,wing flap control cable/rod - failure,total (cause)
  - flight control,flap - inoperative (cause)

## Analysis

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.

## Aircraft

- Registration: [N512US](https://openairdocs.org/deregistered/N512US-2013-07-16), registration cancelled 2013-07-16, S/N 23200, built 1985 (OpenAirDocs)
- Reported make and model: BOEING 757-251
- Registry model: [Boeing 757-251](https://openairdocs.org/model/boeing-757-251) (registry code 1384970) (OpenAirDocs)
- Registry engine: [Amateur-built UNKNOWN ENG](https://openairdocs.org/engine/amateur-built-unknown-eng) (registry code 99999) (OpenAirDocs)
- Category: Airplane
- Serial number: 23201
- Airframe hours: 11582
- Engines: P&W 2037 38200 lbs thrust turbo fan
- Damage: Minor damage
- Flight: Part 121
- Operator: Northwest Airlines
- Registered owner: First Nat'L Bank Of Boston Tst
- Airworthiness certificate: Standard, Transport
- Departed: DTW, DETROIT, MI
- Flight plan: IFR

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 49; airline transport, commercial; airplane ratings: multi-engine land, single-engine sea, single-engine land; instrument ratings: airplane; medical: class 1, valid medical--w/ waivers/lim; seat: left; flight hours: 11,700 total, 150 in the last 90 days, 50 in the last 30 days

## Weather

- Conditions: Visual (VMC), Day
- Wind: 290° at 6 knots
- Visibility: 15 statute miles
- Lowest ceiling: none
- Temperature: 70 °F

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

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