FlarePath
Cases › ENG22FA009 · Part 121 airline · Class 3

BOEING 737-8K2Sun Country Airlines, Inc.

Las Vegas, NV, USA · 2022-02-04·N817SY

0
fatal
0
serious
No injury
highest injury
Minor
damage
Replay the flight data270,149 samples · 103 parameters00:40:00.0 – 01:19:56.0AnalyseThe analystOne timeline · precedents · ask the docket · what the record saysGenerateInvestigation briefFacts, cause, recorder extremes, sequence, sourcesSourceNTSB docket9 documents on data.ntsb.gov

What happened

NTSB summary, verbatim

Upon retracting the landing gear after takeoff, the flight crew received a right main landing gear (RMLG) red position indicator light, indicating that the RMLG was not in the up and locked position. The crew elected to return to the departure airport and the RMLG red indicator light remained illuminated until the crew selected the gear DOWN position for landing. During the landing roll, the RMLG collapsed, resulting in minor damage when the right engine impacted the runway. It is likely that the RMLG was partially fractured after takeoff which prevented it from retracting to the up and locked position. Examination of the RMLG revealed that the outer cylinder fractured through a tooling hole. Examination of the fracture revealed the presence of fatigue cracking that initiated at the intersection of the tooling hole inner diameter and the outer cylinder inner diameter. Four areas of arc burn damage were identified in the tooling hole area, two of which were at the crack initiation sites. Cadmium identified at the intergranular separation initiation sites indicated the crack initiated due to liquid metal embrittlement by cadmium at the arc burn locations before propagating in fatigue. The RMLG was last overhauled about 10 years before the incident, during which the outer cylinder was replated with cadmium after repair. It is likely that the arc burning in the tooling hole area was caused by a hand-held stylus cadmium plating operation performed in the tooling hole when the stylus (anode) made contact with the outer cylinder (cathode), initiating an arc. A nonconductive sleeve normally installed on the anode was either not present or deteriorated, allowing physical contact between the anode and cathode.

Photographs

1the aircraft, and what the investigators found
The aircraft before the accident
Sun Country Airlines B737-8K2, N817SY, as SY 8810 MIA-CFG (21371619975)
Charles from Port Chester, New York · CC BY 2.0 · Wikimedia Commons

Probable cause

the Board's determination
The fatigue failure of the right main landing gear initiated by liquid metal embrittlement by cadmium at arc burn locations in the outer cylinder tooling hole area. The arc burns were likely the result of operator error during stylus cadmium plating operations during overhaul.

Occurrence sequence

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

Findings

2 causal · 0 contributing
  • CAUSE
    Aircraft › Aircraft systems › Landing gear system › Main landing gear — Fatigue/wear/corrosion
  • CAUSE
    Personnel issues › Task performance › Maintenance › Scheduled/routine maintenance — Maintenance personnel

Sequence of events

19 timed eventsfrom the FDR factual report · claude-sonnet-5
  1. 00:40:28Departurep.4
    The airplane departed Harry Reid International Airport (KLAS), Las Vegas, Nevada.
  2. 00:40:31Departurep.4
    Nose and main landing gear warning parameters indicated "Warn", consistent with the crew moving the landing gear lever to the 'Up' position.
  3. 00:40:32Departurep.4
    The nose and left main 'Gear Down' parameters simultaneously indicated 'Up'. The right main 'Gear Down' parameter remained in 'Down' status.
  4. 00:40:38Departurep.4
    The right main 'Gear Down' parameter indicated 'Up'.
  5. 00:40:39Climbp.5
    The nose and left main 'Gear Warn' parameters transitioned from 'Warn' to 'No Warn'. The right 'Gear Warn' parameter remained in 'Warn' status. The airplane continued to climb.
  6. About 00:43:30Climb/Level-offp.5
    The airplane leveled off at about 9,500 feet MSL.
  7. About 00:53:00Descent/Level-offp.5
    The airplane descended and leveled off at 7,500 feet MSL.
  8. About 01:10:40Approachp.5
    The airplane started to descend and maneuver back for an approach at KLAS.
  9. 01:14:37Approachp.5
    The airplane was on approach for KLAS at an altitude of about 5,300 feet MSL. The right main 'Gear Warn' parameter continued to indicate 'Warn'. The nose and left main 'Gear Warn' parameters transitioned to 'Warn', consistent with the crew moving the landing gear lever to the 'Down' position. The airplane continued to descend.
  10. 01:14:44Approachp.5
    The nose 'Gear Down' parameter transitioned from 'Up' to 'Down'. Simultaneously, the 'Gear Warn-N' parameter transitioned to 'No Warn'.
  11. 01:14:47Approachp.5
    The left main 'Gear Down' parameter transitioned to 'Down' and simultaneously, the 'Gear Warn-L' parameter transitioned to 'No Warn'.
  12. 01:14:48Approachp.5
    The right main 'Gear Down' parameter transitioned to 'Down' and simultaneously, the 'Gear Warn-R' parameter transitioned to 'No Warn'. The airplane continued to descend.
  13. 01:18:59Landingp.5
    The nose, left main, and right main landing gear WOW all indicated 'Grnd (Ground)'.
  14. 01:19:00Landing rollp.5
    The right main 'Gear Down' and right main 'Gear Warn' parameters transitioned to 'Up' and 'Warn', respectively. The recorded roll angle began to increase (right wing down).
  15. 01:19:04Landing rollp.5
    Both engine's thrust reversers indicated full deployment. Air and ground speed continued to decrease.
  16. 01:19:19Landing rollp.5
    The 'Eng2 Fire' parameter briefly indicated 'Fire'. One second later, the parameter transitioned back to 'No Fire' for about 26 seconds.
  17. 01:19:22Landing rollp.5
    Both engine thrust reversers indicated they were no longer deployed. The recorded groundspeed was about 17 knots.
  18. 01:19:27Landing rollp.5
    The recorded groundspeed was 0 knots.
  19. 01:19:47Post-landingp.5
    The 'Eng2 Fire' parameter indicated 'Fire'. This indication persisted for the duration of the recording.

NTSB analysis

from the final report

Upon retracting the landing gear after takeoff, the flight crew received a right main landing gear (RMLG) red position indicator light, indicating that the RMLG was not in the up and locked position. The crew elected to return to the departure airport and the RMLG red indicator light remained illuminated until the crew selected the gear DOWN position for landing. During the landing roll, the RMLG collapsed, resulting in minor damage when the right engine impacted the runway. It is likely that the RMLG was partially fractured after takeoff which prevented it from retracting to the up and locked position. Examination of the RMLG revealed that the outer cylinder fractured through a tooling hole. Examination of the fracture revealed the presence of fatigue cracking that initiated at the intersection of the tooling hole inner diameter and the outer cylinder inner diameter. Four areas of arc burn damage were identified in the tooling hole area, two of which were at the crack initiation sites. Cadmium identified at the intergranular separation initiation sites indicated the crack initiated due to liquid metal embrittlement by cadmium at the arc burn locations before propagating in fatigue. The RMLG was last overhauled about 10 years before the incident, during which the outer cylinder was replated with cadmium after repair. It is likely that the arc burning in the tooling hole area was caused by a hand-held stylus cadmium plating operation performed in the tooling hole when the stylus (anode) made contact with the outer cylinder (cathode), initiating an arc. A nonconductive sleeve normally installed on the anode was either not present or deteriorated, allowing physical contact between the anode and cathode.