Improper MRO Work Led To 2023 Alaska Airlines 737 Gear Collapse

Improper procedures during a routine overhaul left a main landing gear (MLG) component damaged and set the stage for a fatigue crack to form and cause the gear to collapse when an Alaska Airlines Boeing 737-800 touched down at John Wayne-Orange County Airport (SNA) in August 2023, the NTSB found.

The final report on the accident, issued May 27, found “excessive grinding” of the left MLG aft trunnion pin during a July 2018 overhaul damaged the part’s base metal. That led to fatigue cracking, “which caused the pin to fail during landing.”

None of the 106 passengers or six crew were hurt, but the aircraft suffered significant damage to its left wing. It has been repaired and is back in service.

Weather on the night of the incident, combined with SNA’s 5,700 ft. main runway and noise abatement procedures, led to immediate speculation that a hard landing may have contributed to the accident. The NTSB’s analysis determined that the aircraft’s maximum vertical rate at touchdown was about 1.7g, which is below the aircraft’s hard-landing threshold.

Post-accident analysis focused on the broken pin. Investigators determined the crack was not present when the gear was overhauled in July 2018 at Valencia, California-based landing gear MRO specialist Sunvair. The damage “had likely developed later as a result of the grinding performed during the overhaul,” the report said.

The area where the pin cracked “was located along an area with a darker visual contrast following temper etch and metallographic inspections,” the report said. “This region also showed an elevated reading” from another inspection method. “The elevated readings and area of visual contrast were consistent with the area being exposed to higher temperatures becoming softer than the surrounding material,” the NTSB added.

The heat exposure “most likely” came from the grinding—part of a multi-step overhaul process, the NTSB added.

Following the accident, Sunvair reviewed its trunnion pin overhaul process and added another inspection to the repair procedure, an NTSB group report prepared as part of the investigation said.