The NTSB published its final report on the incident.
Damaged Alaska 737-800 in Santa Ana (Photo: Orange County Fire Authority)
The National Transportation Safety Board has determined that excessive grinding during maintenance led to the collapse of a Boeing 737-800’s left main landing gear during landing at John Wayne/Orange County Airport in Santa Ana, California.
Alaska Airlines flight 1288 experienced “substantial” damage on Aug. 20, 2023, when its left main landing gear failed upon touchdown. None of the 112 passengers and crew members aboard were injured.
“The National Transportation Safety Board determines the probable cause of this accident to be maintenance personnel’s excessive grinding of the left main landing gear’s aft trunnion pin during machining, which imparted heat damage to the base metal and led to the fatigue cracking that caused the pin to fracture during landing,” the agency stated in its final report published Thursday.
According to the NTSB report, the flight from Seattle encountered moderate rain, shifting winds, and light turbulence during its approach to Santa Ana.
The flight crew reported that the aircraft was fully configured for landing and on a stabilized approach before the final approach fix. After visually acquiring the runway at about 800 feet above ground level, the captain, who was the pilot flying, disengaged the autopilot and autothrottle.
Flight data recorder information showed that the main landing gear touched down at 11:14:57 p.m., with the nose gear following about one second later. The maximum vertical acceleration recorded at touchdown was approximately 1.71 G.
The captain reported feeling a “firm jolt” and noted the aircraft was “pulling reasonably hard” to the left, which he initially attributed to a flat tire. After bringing the aircraft to taxi speed and exiting the runway, the crew noticed the airplane leaning to the left.
Upon inspection, they discovered the aircraft was resting on its left engine cowling. Photos show the landing gear strut piercing through the top of the wing.
NTSB investigators found that the aft trunnion pin in the left main landing gear had fractured during landing due to a fatigue crack that had grown to a depth of 0.144 inches. The crack was large enough to cause the remaining material to fracture during landing, resulting in the collapse of the gear.
Further examination revealed the fatigue crack had initiated from a small intergranular region below the external chromium-electroplated layer of the pin. This region showed an elevated reading from a Barkhausen noise inspection and displayed darker visual contrast following temper etch and metallographic inspections.
“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 report stated. “The heat exposure most likely occurred as part of the excessive grinding of the surface, performed during the maintenance overhaul of the pin that occurred on July 5, 2018.”
The investigation determined that the overtempered region and surrounding material created an area of intergranular fracture within the material where grinding had occurred. The grinding burn likely happened during machining after the new chromium layer electroplating process was applied.
A fatigue crack analysis performed on the aft trunnion pin fracture surface indicated the crack had been present for at least 797 landing cycles. The maintenance records showed that the left main landing gear assembly, including the aft trunnion pin, had been overhauled approximately 4,710 landing cycles before the accident.
“The crack was not present when the pin was overhauled in July 2018 but had likely developed later as a result of the grinding performed during the overhaul,” the report stated.
The NTSB noted that most nondestructive inspection techniques used to detect damage to plated trunnion pins rely on identifying cracks, making them ineffective before crack initiation. However, the investigation found that “even relatively mild heat exposure from grinding and/or machining during overhaul can lead to cracking, which can lead to fatigue crack growth and failed landing gear components.”
Damage to the 737 (Photo: NTSB)
At the time of the left main landing gear assembly overhaul in 2018, the landing gear had accumulated 11,116 landing cycles. The overhauled assembly was installed on the accident airplane on July 17, 2018, and accumulated an additional 4,710 landing cycles before the failure.
The aircraft – registered as N516AS – remained out of service until December 2023.
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