Created Monday, Aug 21st 2023 19:30Z, last updated Friday, May 23rd 2025 10:29Z
An Alaska Airlines Boeing 737-800, registration N516AS performing flight
AS-1288 from Seattle,WA to Santa Ana,CA (USA) with 106 passengers and 6 crew, landed on Santa Ana's runway 20R at 23:14L (06:14Z Aug 21st) but touched down hard causing the left main gear strut being punched through the left wing. The aircraft came to a stop on the runway and was disabled. There were no injuries, the aircraft sustained substantial damage however.
On Aug 25th 2023 The Aviation Herald learned, that other than initial reports suggested, the aircraft did not suffer a hard landing. The aft trunnion shear pin failed causing the damage to the upper wing panels.
On Sep 11th 2023 the NTSB released their preliminary report into the occurrence rated an accident stating that according to the crew approach and final approach were normal. The NTSB then wrote:
According to the captain, upon landing, the touchdown had a “firm jolt feeling” to it, and the aircraft was pulling reasonably hard to the left. The captain was able to overcome this with rudder pedal input and was able to keep the plane on the center line. He stated to the FO that it felt like the airplane had a flat tire on the left MLG. The FO completed the after-landing procedures, which included bringing the auxiliary power unit (APU) online. The captain slowed the aircraft to taxi speed and exited the runway onto taxiway E. After the aircraft made the turn onto the taxiway, the crew noticed that the aircraft was listing to the left.
The captain stopped the airplane on the taxiway and set the parking brake. He opened his window, looked outside, and noticed that the aircraft was resting on the left engine cowling, and it appeared to be still running normally. He immediately shut down the left engine. The captain also noted that the left landing gear position indicator light was not green anymore; it had gone completely dark, whereas the nose and right MLG position indicator lights were still green. After confirming the APU was running, he shut down the right engine. He made announcements to the passenger cabin and briefed the flight attendants. No injuries were reported.
The NTSB summarized the damage:
Postaccident examination of the airplane revealed that the MLG aft trunnion pin had fractured. The aft end of the left MLG had traveled up, puncturing the composite panels, and protruded out and above the wing’s surface. The MLG walking beam was also protruding above the surface of the wing. The forward section of the aft trunnion pin was located inside the trunnion and was held in place by a damaged but intact cross bolt. The aft section of the pin had separated and was located in the MLG beam spherical bearing.
On May 23rd 2025 the NTSB released their final report and the investigation docket concluding the probable cause of the accident was:
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 NTSB analysed:
This accident occurred when the left main landing gear (MLG) of an Alaska Airlines Boeing 737-800 collapsed during landing. Postaccident examination revealed that the aft trunnion pin in the left MLG failed during landing due to a fatigue fracture. The fracture initiated from a small intergranular region below the external chromium electroplated layer. The fatigue crack, which had propagated to a depth of 0.144 inches, was large enough to cause the remaining material to fracture in tensile or upward-bending overstress during landing, resulting in the collapse of the left MLG.
Further examination of the aft trunnion pin revealed that the intergranular region where the fatigue crack initiated was located along an area with a darker visual contrast following temper etch and metallographic inspections. This region also showed an elevated reading from a Barkhausen noise inspection. 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 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, as part of the overhaul of the left MLG assembly. This area was subject to grinding to first remove the original chromium layer, and then a second grinding after the new chromium layer was applied to bring the trunnion pin into specified dimensions.
The over-tempered region and the surrounding material created an area of intergranular fracture within the material in the area where grinding had occurred. The investigation considered whether the formation of this area could have occurred during the baking and stress relief processes following aggressive grinding during the chromium removal step of the overhaul.
However, after removing the chromium electroplating layer, the temper etch inspections did not detect any grinding burns. Because a temper etch inspection cannot be used after the chromium electroplating layer was reapplied, the grinding burn likely occurred during machining after the new chromium layer electroplating process.
At the time of the left MLG assembly overhaul, the landing gear assembly had accumulated 11,116 landing cycles. The overhauled left MLG assembly was installed on the accident airplane on July 17, 2018. Afterward, the left MLG, including the trunnion pin, accumulated an additional 4,710 landing cycles.
A fatigue crack analysis performed on the aft trunnion pin fracture surface showed that the fatigue crack had been present for at least 797 landing cycles. Therefore, 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. Because most of the nondestructive inspection techniques used to detect damage to plated trunnion pins typically rely on identifying cracks, techniques such as magnetic particle inspection and fluorescent penetrant inspection would have been ineffective before crack initiation. However, results of this examination and previous NTSB investigations demonstrate 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, as occurred in this accident.
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