History of Flight
On August 20, 2023, about 2315 Pacific daylight time, Alaska Airlines flight 1288, a Boeing 737-800 (N516AS), landed on runway 20R at John Wayne-Orange County Airport (SNA) in Santa Ana, California. The flight was a scheduled domestic passenger service from Seattle, Washington, operating under 14 CFR Part 121. The 112 passengers and crew evacuated via stairs onto a taxiway with no injuries reported.
The flight crew reported a stabilized approach with moderate rain, shifting wind, light turbulence, and instrument meteorological conditions. About 800 ft above ground level, the captain, who was pilot flying, disengaged the autopilot and autothrottle after visually acquiring the runway. All landing gear position indicator lights were green before touchdown.
Flight data recorder data showed the main landing gear (MLG) touched down at 2314:57, with the nose gear touching down about 1 second later. Maximum vertical acceleration was about 1.71 G. Thrust reverser and spoiler deployment occurred normally.
The captain described a "firm jolt feeling" and the airplane pulling left. He corrected with rudder input. After slowing to taxi speed, the airplane exited onto a taxiway, where the crew noticed a left lean. The airplane came to rest with a roll attitude about 5° left. The captain observed the airplane resting on its left engine cowling and shut down the left engine, then the right engine after confirming the APU was running.
Aircraft Information
The left MLG assembly, including the aft trunnion pin, had been overhauled by Sunvair Inc. on July 5, 2018, and installed on the accident airplane on July 17, 2018. At the time of the accident, the airplane had accumulated 53,207 flight hours and 15,854 flight cycles; the landing gear had been installed for 4,710 flight cycles since overhaul. The overhaul procedure included stripping the original chromium layer, temper etch and magnetic particle inspections, shot peening, chromium electroplating, grinding to diameter, and another magnetic particle inspection.
Wreckage and Impact Information
Post-accident inspection found the left MLG forward trunnion pin and spherical bearing in place with no visible wing spar damage. The left MLG aft trunnion pin had fractured in half vertically across its diameter. One half remained in the spherical bearing of the rear landing gear beam, protruding aft; the other half remained in the outer cylinder trunnion.
Examination of the aft section revealed a fracture surface with river patterns, chevron marks, and a thumbnail crack with fatigue features. The thumbnail crack measured 0.144 inches deep and 0.391 inches long, with a subsurface intergranular region of 0.016 inches deep by 0.290 inches long. Barkhausen noise inspection indicated elevated residual stress near the fracture. Fluorescent and magnetic particle inspections found no abnormal crack network but showed small cracks in the chromium layer near fracture origins.
The forward section mirrored the aft section, with a thumbnail crack showing ratchet marks and intergranular fracture. Fatigue striations indicated crack propagation, and dimpled rupture indicated overstress fracture during the landing.
Additional Information
The National Transportation Safety Board reviewed four similar Boeing 737 MLG trunnion pin failure accidents: Jet Airways (April 2015), Jet Airways (March 2016), Gol Airlines (July 2018), and United Airlines (December 2019). All involved fatigue cracking of the aft trunnion pin.
Flight Recorders
The aircraft was equipped with a flight data recorder (FDR) and cockpit voice recorder (CVR). FDR data recorded 91 parameters, with left and right brake pressure parameters noted as not in compliance with federal requirements. The CVR recorded good quality audio.
Tests and Research
A fatigue crack growth rate analysis counted striations at 32 locations, finding that 797 flight cycles occurred from the intergranular initiation site to the crack end. The crack was not present at or before the overhaul 4,710 flight cycles earlier.