What happened
On August 20, 2023, at approximately 2315 PDT, an Alaska Airlines Boeing 737-800, registration N516AS, landed on runway 20R at John Wayne-Orange County Airport (SNA) in Santa Ana, California. The flight was a scheduled domestic service from Seattle, Washington.
During the approach, the flight crew encountered instrument meteorological conditions, moderate rain, light turbulence, and shifting winds. The captain, acting as pilot flying, disengaged the autopilot and autothrottle approximately 800 feet above ground level. Flight data recorder information indicated the left main landing gear touched down at 2314:57, followed by the nose gear about one second later. The pilot reported a "firm jolt" and a strong leftward pull, which was corrected with rudder input.
As the aircraft exited the runway onto a taxiway, the crew noticed the plane was leaning to the left. The captain stopped the aircraft and observed that the left engine cowling was resting on the ground. The left main landing gear position indicator light had extinguished. The crew evacuated all 112 passengers and crewmembers via airstairs; there were no injuries.
The investigation
Investigators examined the left main landing gear (MLG) and found that the aft trunnion pin had fractured in half vertically across its diameter. One half of the pin remained in the spherical bearing of the rear landing gear beam, while the other half remained in the outer cylinder of the MLG trunnion.
Detailed analysis of the pin's fracture surface revealed a thumbnail crack that had progressed via fatigue. Examination of the aft section of the pin showed a rough fracture surface with river patterns and chevron marks, indicating the fracture progressed from a small initial crack. The investigation identified an intergranular region where the crack initiated. This region exhibited a darker visual contrast and elevated Barkhausen noise readings, which were consistent with the metal being softened by heat exposure.
Further analysis determined that this heat exposure likely occurred during the maintenance overhaul of the landing gear assembly performed in July 2018. During that overhaul, the pin's surface was ground to remove the original chromium layer and later machined to achieve specific dimensions. The investigation concluded that the fatigue crack had been present for at least 797 landing cycles, meaning it developed after the 2018 overhaul.