# NTSB ENG22FA009: Boeing 737-8K2, N817SY, Las Vegas, NV, February 4, 2022 - Type: NTSB incident investigation - Classification: Incident · No injuries · Class 3 · Field · Final report - Date: February 4, 2022, 01:19 local (09:19 UTC) - Place: Las Vegas, NV · Runway 26R - Coordinates: 36.076362, -115.14199 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=36.0764&lon=-115.1420&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Defining event: [Landing-landing roll: Landing gear collapse](https://openairdocs.org/ntsb-events/094) - Highest injury: No injuries - Injuries: 56 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Aviation Engineering Division - Parties: ALPA, FAA AVP-100, Boeing, Sun Country Airlines - ADs cited in the narratives: [AD 2019-01-03](https://openairdocs.org/ad/2019-01-03) (OpenAirDocs) - NTSB case key (mKey): 104596 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ENG22FA009 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/104596/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=104596 - Page: https://openairdocs.org/ntsb/ENG22FA009 ## Probable cause 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. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Landing gear system: main landing gear](https://openairdocs.org/ntsb-findings/01023210): Aircraft › Aircraft systems · Fatigue/wear/corrosion (probable cause) - [Maintenance: scheduled/routine maintenance](https://openairdocs.org/ntsb-findings/02062010): Personnel issues › Task performance · Maintenance personnel (probable cause) ## Analysis 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. ## Factual narrative The RMLG assembly, P/N 161A1100-40, S/N MAL02520Y1240, was delivered new on a 737-800 on December 2, 2002. The RMLG was removed from the airplane in October 2007 after accruing 17,381 hours and 12,122 cycles since new and installed on a different airplane. The RMLG was removed in January 2012 for overhaul after accruing 32,780 hours and 20,995 cycles since new. The overhaul of the RMLG was performed by TAP Maintenance & Engineering, Brazil, and was completed in March 2012. The overhaul included the stripping of all paint, chrome, nickel, and cadmium plating, inspection of the outer cylinder assembly, repair, and the application of new plating and paint. There were no discrepancies noted around the tooling hole in the overhaul documentation. The NTSB attempted to obtain more information on the plating process used during the overhaul, but TAP stopped performing landing gear overhauls in 2019 and the facility was closed in early 2022. The RMLG was installed on a different airplane in July 2012 before being removed in August 2013 after accruing 36,199 hours and 23,896 cycles since new. It was then installed on a different airplane. The RMLG was removed in January 2020 after accruing 51,697 hours and 37,550 cycles since new. Aerospace Rotables, Inc. had the RMLG inspected and recertified per the latest Component Maintenance Manual (CMM) revision by Seman, Peru, in October 2020. The RMLG was cleaned, inspected, and repainted by Seman and compliance with all ADs and SBs was verified. The RMLG was installed on the accident airplane in November 2020. The RMLG had accrued 54,509 hours and 38,491 cycles since new (17,496 cycles since overhaul) at the time of the accident. The most recent lubrication of the RMLG in accordance with AD 2019-01-03 was performed on February 1, 2022. Overhaul procedures for the RMLG outer cylinder are published in the Boeing CMM, Chapter 32-11-12. Typically, an outer cylinder would be stripped of all paint, primer, and existing cadmium plating before undergoing inspection and repair. Nickel and chromium plated surfaces may remain on the part if they are of acceptable condition for return to service. Once the inspection and repair are complete, the outer cylinder would have nickel and/or chromium plating applied in specific seal areas if they had been previously removed, while the remainder of the part would have cadmium plating applied per CMM requirements for corrosion resistance. Finally, the outer cylinder would have primer and paint applied. Cadmium plating is applied using an electroplating process where the outer cylinder is submerged in a tank of electrolyte plating solution with an electrical current supplied through the outer cylinder that acts as the cathode and installed anodes in the tank. For parts with complex geometries such as the outer cylinder, auxiliary anodes must be designed and placed in such a way to ensure that the cadmium plating is uniform and of the desired thickness throughout the part.Alternatively, areas of complex geometry such as the tooling hole can be cadmium plated using a stylus or brush plating process. In this process, the part is still acting as the cathode while a technician inserts a portable handheld anode into the hole. The anode stylus, which is saturated with plating solution, would be manipulated around the hole to ensure plating is applied in all areas. Typically, the stylus would be wrapped in an electrically non-conductive sleeve, which prevents the anode from making direct physical contact with the surface of the part being plated (cathode). In cases where the sleeve is deteriorated or not installed, physical contact between the anode and cathode can result in an electrical arc between the stylus and outer cylinder. An electric arc can produce high currents and heat sufficient to cause localized melting or other degradation of the base metal material properties. On February 4, 2022, about 0119 Pacific standard time (PST), Sun Country Airlines flight 110, a Boeing 737-8K2, N817SY, sustained minor damage when it suffered a right main landing gear (RMLG) collapse during landing at Harry Reid International Airport (LAS), Las Vegas, Nevada. The 50 passengers and 6 flight crew members were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 121 scheduled domestic passenger flight. Sun Country flight 110 departed LAS about 0040 PST enroute to Minneapolis, Minnesota. According to crew statements, after takeoff the crew moved the landing gear handle to the UP position to retract the landing gear, and subsequently received a red position indicator light for the RMLG. The crew engaged the autopilot while they consulted the GEAR DISAGREE checklist. They placed the landing gear handle in the OFF position and the autopilot began retrimming the airplane, so they placed the handle back in the UP position. Subsequently, the crew reported hearing a loud sound from aft of the cockpit, so they declared an emergency and returned to LAS. During the approach, the crew moved the landing gear handle to the DOWN position and received three green position indicator lights for the landing gear. Shortly after touching down on the runway, the RMLG collapsed. The airplane rolled right, and the right engine impacted the runway. The crew stopped the airplane on the runway and the passengers and crew deplaned using airstairs to a waiting bus. The airplane was then moved to the ramp. Flight data recorder (FDR) data showed that the airplane lifted off about 0040:28 and the landing gear were commanded to the UP position about 3 seconds later. The left main landing gear (LMLG) and nose landing gear (NLG) began retracting about 1 second later. The RMLG began retracting about 6 seconds after being commanded. The red landing gear position indicator lights for all three landing gear illuminated when they were commanded to the up position indicating the landing gear were in transit. The LMLG and NLG indicatorlights changed to green about 8 seconds later, when they were in the up and locked position. The RMLG indicator light remained red, indicating that the RMLG was not in the up and locked position during the flight. The landing gear were commanded to the DOWN position about 0114:37 and all three were in the down and locked position about 11 seconds later with green indicator lights illuminated for each. The airplane touched down with the LMLG about 0118:56, followed by the RMLG less than one second later. The RMLG collapsed about 4 seconds after it touched down and the red position indicator light illuminated. The airplane came to a stop on the runway about 0119:27. Examination of the airplane after the event revealed that the RMLG outer cylinder was fractured around the circumference between the forward and aft trunnions through the bung hole. (see Figure 1.) The bung hole is a tooling locator hole used during the manufacture of the outer cylinder. Figure 1. Fractured RMLG installed on incident airplane (photo provided by Boeing) The fractured portion of the RMLG was removed from the airplane and examined at the Boeing Equipment Quality Analysis Laboratory under NTSB supervision. The examination revealed the presence of primary and secondary origin areas on opposite sides of the tooling hole where the inner diameter of the tooling hole and the inner diameter of the outer cylinder intersect. (see Figure 2.) Scanning electron microscope (SEM) examination of the origin areas showed fracture features consistent with intergranular separation and the presence of pure cadmium on the grain faces and grain boundaries that extended to a depth of about 0.02 inch. Beyond this depth, the fractures transitioned to transgranular slow growth fatigue cracking that propagated to a depth of about 0.42-inch on the primary side and 0.07-inch on the secondary side. The remaining area of fracture exhibited features consistent with ductile separation. Figure 2. RMLG tooling hole fracture origins (photo provided by Boeing) The aft fracture piece was stripped of all paint, primer, and plating, revealing the presence of four spherical shaped anomalies, each about 0.1-inch in diameter, at the intersection of the tooling hole inner diameter and the outer cylinder inner diameter. (see Figure 3.) Two of these anomalies were coincident with the primary and secondary origins. The spherical anomalies were consistent with areas of base metal heat damage due to electrical arcing. Microstructure examination, hardness testing, and nital etching of the spherical areas showed the presence of undertempered martensite and overtempered martensite, which confirmed the base metal heat damage. Figure 3. Arc burns on RMLG tooling hole (photo provided by Boeing) In the areas away from the arc burns, the protective finishes were in accordance with Boeing requirements, the material was consistent with 4340M stainless steel, and the hardness was consistent with the 275-300 ksi heat treatcondition. ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Landing-landing roll: Landing gear collapse](https://openairdocs.org/ntsb-events/094) (defining event) ## Aircraft - Registration: [N817SY](https://openairdocs.org/deregistered/N817SY-2022-12-21), registration cancelled 2022-12-21, S/N 30392, built 2001 (OpenAirDocs) - Reported make and model: BOEING 737-8K2 - Registry model: [Boeing 737-8K2](https://openairdocs.org/model/boeing-737-8k2) (registry code 13844CX) (OpenAirDocs) - Registry engine: [CFM International CFM56 series](https://openairdocs.org/engine/cfm-international-cfm56-series) (registry code 13802, turbo-fan) (OpenAirDocs) - Category: Airplane - Serial number: 30392 - Year built: 2001 - Airframe hours: 68382 - Engines: CFM INTL. CFM56 SERIES 27300 lbs thrust turbo fan; CFM INTL. CFM56 SERIES 27300 lbs thrust turbo fan - Damage: Minor damage - Flight: Part 121 - Operator: Sun Country Airlines, Inc. - Registered owner: Sun Country Airlines, Inc. - Airworthiness certificate: Standard, Transport - Last inspection: Continuous airworthiness, February 3, 2022 - Departed: LAS, Las Vegas, NV - Destination: MSP, Minneapolis, MN - Flight plan: IFR - Airport: LAS, Harry Reid International Airport, runway 26R, 14,515 × 150 ft, concrete - Equipment: Auto-pilot, Data recorder, Angle of attack indicator, Electronic flight bag or handheld device, Electronic multifunction display, Electronic primary flight display, Onboard weather, Stall warning system, ADS-B ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 38; injuries: none; airline transport; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 1, without waivers/limitations; last flight review May 31, 2021; seat: left; restraint: 5-point; flight hours: 4,869 total, 1,471 in this make and model, 3,195 as pilot in command, 87 in the last 90 days, 23 in the last 30 days, 0 in the last 24 hours - Co-pilot; age 44; injuries: none; airline transport; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 1, without waivers/limitations; last flight review January 17, 2022; seat: right; restraint: 5-point; flight hours: 13,141 total, 74 in this make and model, 1,243 as pilot in command, 74 in the last 90 days, 52 in the last 30 days, 0 in the last 24 hours ## Weather - Conditions: Visual (VMC), Night - Wind: 20° at 9 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 39 °F, dew point 12 °F - Altimeter: 30.46 inHg - Observed at: KLAS, 1 nm away - METAR: KLAS 040856Z 02009KT 10SM CLR 04/M11 A3046 RMK AO2 SLP319 T00391106 50011 $ ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 20220204000279I](https://openairdocs.org/incident/20220204000279I), Feb 4, 2022: Boeing 737-8K2, Las Vegas, NV, Minor damage. The aircraft had departed from klas at approximately 1241 PST, with 56 persons on board, headed to kmsp and reportedly experienced a gear retraction problem… ## Docket (9 files) The files NTSB released with the investigation on March 13, 2023, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=104596. 61.0 MB in all. - Flight data recorder - specialist's factual report (item 1, PDF, 13 pages, 5 photos, 2.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14511347&FileExtension=pdf&FileName=FINAL_ENG22FA009_FDR-Rel.pdf - Flight data recorder - attachment 1 (compressed tabular data for incident flight) (item 2, ZIP, 1.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14511492&FileExtension=zip&FileName=Attachment1_ENG22FA009_FDR-Rel.zip - Release of aircraft wreckage, NTSB form 6120.15 (item 3, PDF, 1 page, 105 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14594573&FileExtension=pdf&FileName=ENG22FA009%20Wreckage%20Release-Rel.pdf - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 4, PDF, 14 pages, 2.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14594368&FileExtension=pdf&FileName=NTB%206120%20Form%20SCNA%20Flt%20110-Rel.pdf - Statement of party representatives to NTSB investigation (item 5, PDF, 9 pages, 4.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14594624&FileExtension=pdf&FileName=Party%20Forms_Redacted-Rel.pdf - Rmlg maintenance records (item 6, PDF, 64 pages, 30.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14594758&FileExtension=pdf&FileName=RMLG%20Mx%20Records_Redacted-Rel.pdf - Boeing equipment quality analysis lab report (item 7, PDF, 41 pages, 70 photos, 20.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14596768&FileExtension=pdf&FileName=EQA%20Report%20-2022-02-04%20SCA%20737-800%20N817SY_Redacted-Rel.pdf - Pilot's statement-captain (item 8, PDF, 2 pages, 145 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14781036&FileExtension=pdf&FileName=Statement%20Captain%20Flight%20110-Rel.pdf - Pilot's statement-first officer (item 9, PDF, 2 pages, 144 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14780659&FileExtension=pdf&FileName=Statement%20FO%20Flight%20110-Rel.pdf From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.