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Cases › ENG17IA005 · Part 121 airline

EMBRAER ERJ170 200LRSKYWEST AIRLINES INC

San Antonio, TX, USA · 2016-12-04·N161SY

0
fatal
0
serious
Minor injury
highest injury
Minor
damage
Replay the flight data391,875 samples · 37 parameters17:59:44.0 – 20:53:53.9AnalyseThe analystOne timeline · precedents · ask the docket · what the record saysGenerateInvestigation briefFacts, cause, recorder extremes, sequence, sourcesSourceNTSB docket4 documents on data.ntsb.gov

What happened

NTSB summary, verbatim

Based on the material laboratory examinations, the area around the failure point contained higher than allowable levels of retained austenite. The presence of too much retained austenite subsequently resulted in the formation of intergranular cracking. The intergranular cracking resulted in the formation of progressive cracking, which caused the separation and subsequent failure of the downlock spring. After the failure, the spring became lodged in the nose landing gear yoke, which prevented the nose gear from achieving a down and locked position. Without the NLG being fully down and locked, as the weight of the aircraft during landing was transferred to the nose and main landing gear, the NLG subsequently retracted back into the nose gear wheel.

Photographs

1the aircraft, and what the investigators found
The aircraft before the accident
N161SY - Embraer ERJ-175LR - United Express (48027460253)
Colin Brown from St Ann, MO, USA · CC BY 2.0 · Wikimedia Commons

Probable cause

the Board's determination
A failure of the nose landing gear down lock spring which precluded normal downlock operation of the nose landing gear.  The spring failure was due to the presence of too much retained austenite which led to the formation of progressive cracking and subsequent failure of the spring.

Occurrence sequence

1 stepsNTSB coding · CICTT taxonomy
  1. 1 · Landing-landing roll
    Landing gear collapse
    defining event

Findings

1 causal · 0 contributing
  • CAUSE
    Aircraft › Aircraft systems › Landing gear system › Gear extension and retract sys — Failure

Sequence of events

22 timed eventsfrom the FDR factual report · claude-sonnet-5
  1. 18:16Departure/Climbp.3
    Flight departed and climbed to cruise altitude of 36,000 ft
  2. 19:06Descent (first approach to MMTY)p.3
    Aircraft began to descend
  3. 19:30:08Approachp.3
    Landing gear commanded down with landing gear lever, aircraft descending through 3,677 ft; left, right, and nose landing gear status transitioned from Up and Locked to In Transit
  4. 19:30:20 (approx, 12 sec after transition)Approachp.3
    Left and right landing gear status became Down and Locked; nose gear status remained In Transit
  5. 19:30:29Approachp.3
    Master Warning annunciated with associated Gear Lever Disagree message (7 seconds after main gear became Down and Locked)
  6. ~19:31:00Missed approachp.3
    Aircraft stopped its descent at about 3,650 ft
  7. 19:31:34Missed approachp.3
    Gear handle moved to the up position; both main and nose gear transitioned to Up and Locked; Gear Lever Disagree message cleared
  8. ~19:36 (about 5 minutes after 19:31:34)Second gear extension attemptp.3
    Gear handle moved to down position again, with same result as first attempt (nose gear remained In Transit)
  9. 19:40:48Climb / diversion decisionp.3
    Gear handle moved back to up position; aircraft began to climb
  10. 20:02Climb/Descentp.3
    Aircraft climbed to 33,000 ft then began a second descent
  11. 20:18:40Descent toward KSATp.3
    Gear handle moved to down position with aircraft descending through 10,200 ft; all gear status became In Transit, followed by left and right gear becoming Down and Locked; nose gear status remained In Transit
  12. 20:19:12Descent toward KSATp.3
    Gear retracted
  13. 20:19:34Descent toward KSATp.3
    Gear commanded down again; left and right gear transitioned to Down and Locked, nose gear remained In Transit
  14. 20:29Low passp.3
    Aircraft descended to 500 feet AGL for one minute
  15. 20:29-20:41 (approx, climb then circle for ~12 minutes)Holdingp.3
    Aircraft climbed to 4,000 ft where it circled one time
  16. 20:49Final approachp.3
    Aircraft began its final descent
  17. prior to 20:53:28Final approachp.3
    Airspeed stable at 130-132 knots during approach
  18. 20:53:28Landingp.3
    Aircraft touched down on main gear at KSAT runway 4 with airspeed 113 knots and ground speed 95 knots
  19. 20:53:28 to 20:53:38 (approx 10 sec)Landing rolloutp.3
    Pitch decreased slowly to a value consistent with the nose gear resting on the ground
  20. 20:53:41Landing rolloutp.4
    Nose gear began to retract at a ground speed of 52 knots with no change in the recorded gear handle position
  21. ~20:53:51 (about 10 seconds later)Landing rollout / stopp.4
    Aircraft slowed to a stop; final resting pitch of the aircraft was -5.98 degrees
  22. 20:53:54Post-landingp.4
    Engines were shut down and recording stopped

NTSB analysis

from the final report

Based on the material laboratory examinations, the area around the failure point contained higher than allowable levels of retained austenite. The presence of too much retained austenite subsequently resulted in the formation of intergranular cracking. The intergranular cracking resulted in the formation of progressive cracking, which caused the separation and subsequent failure of the downlock spring. After the failure, the spring became lodged in the nose landing gear yoke, which prevented the nose gear from achieving a down and locked position. Without the NLG being fully down and locked, as the weight of the aircraft during landing was transferred to the nose and main landing gear, the NLG subsequently retracted back into the nose gear wheel.