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Cases › ENG08IA042 · Part 121 airline

BOEING 737Southwest Airlines, Co.

Chicago, IL, USA · 2008-10-01·N551WN

0
fatal
0
serious
No injury
highest injury
Minor
damage
Replay the flight data95,020 samples · 16 parameters00:00:00.0 – 26:23:39.0AnalyseThe analystOne timeline · precedents · ask the docket · what the record saysGenerateInvestigation briefFacts, cause, recorder extremes, sequence, sourcesSourceNTSB docket3 documents on data.ntsb.gov

What happened

NTSB summary, verbatim

The airplane experienced separation of the right nose landing gear (NLG)wheel assembly as it exited runway after landing. Metallurgical examination revealed that the NLG axle fractured in a transverse plane, and that the failure of the axle was due to intergranular cracking that acted as stress concentrators from which fatigue cracks emanated. The fracture of the right side inboard journal on the NLG inner cylinder assembly was most likely due to the failure of a prior tapered roller bearing. Eventually the size of the fatigue crack and the forces generated during landing were sufficient to drive the crack in overstress, leading to fracture of the inboard journal. The change in ownership of the plane and the monthly swapping of bearings and wheels make it difficult to determine when the bearing failure occurred or why.

Photographs

1the aircraft, and what the investigators found
The aircraft before the accident
N551WN (7896598372)
redlegsfan21 from Vandalia, OH, United States · CC BY-SA 2.0 · Wikimedia Commons

Probable cause

the Board's determination
The right nose landing gear axle's failure from intergranular and fatigue cracking due to an earlier bearing failure.

Occurrence sequence

2 stepsNTSB coding · CICTT taxonomy
  1. 1 · Taxi-from runway
    Aircraft structural failure
  2. 2 · Taxi-from runway
    Landing gear collapse
    defining event

Findings

2 causal · 0 contributing
  • CAUSE
    Aircraft › Aircraft systems › Landing gear system › Nose/tail landing gear — Failure
  • CAUSE
    Aircraft › Aircraft systems › Landing gear system › Nose/tail landing gear — Fatigue/wear/corrosion

Sequence of events

2 timed eventsfrom the FDR factual report · claude-sonnet-5
  1. 1055 CDTLanding rollout/exitp.1
    Southwest Airlines Boeing 737-700, registration N551WN, experienced a separation of the right nose landing wheel assembly as it exited runway 31C after landing at the Midway Airport, Chicago, Illinois.
  2. N/APost-landingp.2
    The right nose landing wheel assembly separation event was recorded after the last recorded landing.

NTSB analysis

from the final report

The airplane experienced separation of the right nose landing gear (NLG)wheel assembly as it exited runway after landing. Metallurgical examination revealed that the NLG axle fractured in a transverse plane, and that the failure of the axle was due to intergranular cracking that acted as stress concentrators from which fatigue cracks emanated. The fracture of the right side inboard journal on the NLG inner cylinder assembly was most likely due to the failure of a prior tapered roller bearing. Eventually the size of the fatigue crack and the forces generated during landing were sufficient to drive the crack in overstress, leading to fracture of the inboard journal. The change in ownership of the plane and the monthly swapping of bearings and wheels make it difficult to determine when the bearing failure occurred or why.