History of Flight
On February 4, 2022, about 0119 Pacific standard time, Sun Country Airlines flight 110, a Boeing 737-8K2 (registration N817SY), sustained minor damage when the right main landing gear (RMLG) collapsed during landing at Harry Reid International Airport (LAS), Las Vegas, Nevada. The flight, a scheduled domestic passenger service under 14 CFR Part 121, had departed LAS at about 0040 en route to Minneapolis, Minnesota. According to crew statements, after takeoff the landing gear handle was moved to the UP position, and a red position indicator light appeared for the RMLG. The crew consulted the GEAR DISAGREE checklist, placing the handle in the OFF position, but the autopilot began retrimming, so they returned the handle to UP. A loud sound from aft of the cockpit prompted the crew to declare an emergency and return to LAS. During approach, the landing gear handle was moved to DOWN, and three green lights indicated all gear were down and locked. Shortly after touchdown, the RMLG collapsed, causing the airplane to roll right and the right engine to impact the runway. The crew stopped on the runway, and passengers and crew deplaned via airstairs to a bus. The airplane was later moved to the ramp.
Flight data recorder (FDR) data showed lift-off at 0040:28, with gear commanded UP about 3 seconds later. The left main and nose landing gear began retracting about 1 second later, while the RMLG began retracting about 6 seconds after command. The red lights for all three gear illuminated during transit. The left main and nose gear indicated green (up and locked) about 8 seconds later, but the RMLG light remained red throughout the flight. The landing gear were commanded DOWN at 0114:37, and all three were down and locked about 11 seconds later with green lights. Touchdown of the left main gear occurred at 0118:56, followed by the RMLG less than one second later. The RMLG collapsed about 4 seconds after touchdown, with the red light illuminating. The airplane stopped at 0119:27.
Aircraft Information
The RMLG assembly (part number 161A1100-40, serial number MAL02520Y1240) was delivered new on a Boeing 737-800 on December 2, 2002. It was removed and reinstalled on different airplanes over its service life. The RMLG underwent an overhaul by TAP Maintenance & Engineering, Brazil, completed in March 2012, which included stripping and reapplying plating. No discrepancies around the tooling hole were noted in the overhaul documentation. The NTSB attempted to obtain plating process details, but TAP stopped performing landing gear overhauls in 2019 and its facility closed in early 2022. In October 2020, Aerospace Rotables, Inc. arranged for inspection and recertification by Seman, Peru, which included cleaning, inspection, and painting. The RMLG was installed on the accident airplane in November 2020. At the time of the accident, the RMLG had accrued 54,509 hours and 38,491 cycles since new (17,496 cycles since overhaul). The most recent lubrication per AD 2019-01-03 was performed on February 1, 2022.
Overhaul procedures for the RMLG outer cylinder are described in Boeing CMM Chapter 32-11-12. The process involves stripping paint, primer, and cadmium plating, inspection and repair, then reapplying nickel, chromium, or cadmium plating as required. Cadmium plating is applied via electroplating or brush plating. During brush plating, a handheld anode stylus wrapped in a non-conductive sleeve is used. If the sleeve is deteriorated or absent, direct contact between anode and cathode can cause electrical arcing, leading to localized melting or degradation.
Wreckage and Impact Information
Post-accident examination revealed that the RMLG outer cylinder was fractured circumferentially between the forward and aft trunnions through the bung hole, a tooling locator hole. The fractured portion was examined at Boeing's laboratory under NTSB supervision. Primary and secondary fracture origins were found on opposite sides of the tooling hole, where the inner diameters of the hole and outer cylinder intersect. Scanning electron microscopy showed intergranular separation with pure cadmium on grain faces and boundaries extending about 0.02 inch, transitioning to transgranular fatigue cracking (0.42 inch on the primary side, 0.07 inch on the secondary side), and then ductile separation.
Stripping of paint, primer, and plating from the aft fracture piece revealed four spherical anomalies, each about 0.1 inch in diameter, at the intersection of the tooling hole and outer cylinder inner diameters. Two anomalies coincided with the fracture origins. These anomalies were consistent with base metal heat damage from electrical arcing. Microstructure examination, hardness testing, and nital etching confirmed undertempered and overtempered martensite in these areas, indicating heat damage. Away from the arc burns, protective finishes met Boeing requirements, the material was 4340M stainless steel, and hardness was consistent with the 275-300 ksi heat treat condition.