Las Vegas, NV, USA
N817SY
BOEING 737-8K2
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.
HISTORY OF FLIGHTOn 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 indicator lights 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. AIRCRAFT INFORMATIONThe 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. AIRPORT INFORMATIONThe 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. WRECKAGE AND IMPACT INFORMATIONExamination 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 treat condition.
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.
Source: NTSB Aviation Accident Database
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