DCA23FA417

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.

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.

The left and right MLG absorb landing forces through the tire assembly and support most of the airplane's weight when on the ground. Each MLG also transmits the wheel braking forces to the airplane structure. A main gear shock strut provides the primary support for each MLG (see figure 2). The shock strut consists of an integral drag strut, an outer cylinder, and an inner cylinder. When the MLG extends or retracts, the shock strut rotates about two trunnion bearings and two pins (forward and aft) at the top of the outer shock strut. The forward and aft trunnion pins extend through a trunnion bearing and support assembly in the wing and landing gear beam. Trunnion bolts keep the trunnion pins from backing out during landing gear operation. Figure 2. Illustration of the MLG components. (Source: Boeing. Image Copyright Boeing. Reproduced with permission.) Alaska Airlines' maintenance program indicated that the left and right MLG assemblies must be overhauled every 21,000 flight cycles or 10 years. Alaska Airlines’ maintenance records showed that the left MLG assembly, including the aft trunnion pin, had been overhauled by Sunvair Inc. in Valencia, California, on July 5, 2018. At that time, the landing gear assembly had accumulated 11,116 flight cycles. According to documentation of the overhaul procedure, during the overhaul of the aft trunnion pin the original chromium layer had been stripped from the trunnion pin by grinding its surface. After the removal of the chromium layer, a temper etch inspection and a magnetic particle inspection were completed. After these inspections and shot peening were completed, a new layer of chromium electroplating was applied to the surface of the trunnion pin. The outer diameter of the pin was then machined (by grinding) to achieve a specific diameter, and another magnetic particle inspection was completed. The overhauled left MLG assembly was installed on the accident airplane on July 17, 2018. At that time, the airplane had accumulated 11,144 flight cycles. 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 on the airplane for 4,710 flight cycles since its installation in July 2018. On August 20, 2023, about 2315 Pacific daylight time (PDT), Alaska Airlines flight 1288, a Boeing 737-800, N516AS, sustained substantial damage when the left MLG collapsed after landing on runway 20R at John Wayne-Orange County Airport (SNA), Santa Ana, California. The 112 passengers and crewmembers evacuated the airplane via stairs onto a taxiway with no injuries reported. The flight was operating under the provisions of Title 14 Code of Federal Regulations Part 121 as a scheduled domestic passenger flight from Seattle, Washington, to SNA. The flight crewmembers reported that, before the final approach fix, the airplane was fully configured for landing and on a stabilized approach and that the before landing checklist was completed. Moderate rain, shifting wind, light turbulence, and instrument meteorological conditions were encountered as the airplane descended toward the airport. About 800 ft above ground level, the flight crew visually acquired the runway, and the captain, who was the pilot flying, disengaged the autopilot and autothrottle. The airplane was aligned with the glidepath when crossing the runway threshold. The flight crew noted that all the landing gear position indicator lights were green, indicating both MLG and the nose gear were in their down and locked position. Flight data recorder (FDR) data showed that, at 2314:57, the MLG touched down, and the nose gear touched down about 1 second later. The maximum vertical acceleration recorded at touchdown was about 1.71 G. The thrust reverser and spoiler deployment occurred normally. According to the captain, the touchdown had a “firm jolt feeling,” and the airplane was “pulling reasonably hard” to the left. The captain overcame the left veer with rudder pedal input, and the airplane tracked the runway centerline. The captain thought that the tire on the left MLG was flat. The first officer (FO), who was the pilot monitoring, completed the after landing procedures, which included bringing the auxiliary power unit (APU) online. The captain slowed the airplane to taxi speed, and the airplane exited the runway onto a taxiway. After the airplane turned onto the taxiway, the crew noticed that the airplane was leaning to the left. FDR data indicated that the airplane came to rest at a roll attitude that was about 5° left. The captain stopped the airplane on the taxiway, set the parking brake, and opened his cockpit window to observe the airplane. He saw that the airplane was resting on its left engine cowling (see figure 1); as a result, he immediately shut down the left engine. Inside the cockpit, the captain noted that the left MLG green position indicator light had extinguished, whereas the nose and right MLG position indicator lights were still illuminated in green. The captain shut down the right engine after confirming the APU was running. He then made announcements to the passengers and briefed the flight attendants about the situation. Figure 1. Collapsed left MLG and engine resting on the ground.   The FO announced, via the common traffic advisory frequency, the crew’s intention for the airplane to remain on the taxiway, and SNA airport rescue and firefighting (ARFF) responded to the scene. After ARFF personnel determined that no fuel was leaking and the flight crew confirmed that no one aboard the airplane was injured, an evacuation was performed from airstairs positioned at the left forward door. Post accident inspection found that the left MLG forward trunnion pin and its spherical bearing were in place with no visible damage to the wing spar. The inspection also found that the left MLG aft trunnion pin had fractured in half vertically across its diameter. Onehalf of the aft trunnion pin remained within the spherical bearing of the rear landing gear beam, and the other half remained in the outer cylinder of the MLG trunnion. The trunnion pin half that remained in the landing gear beam spherical bearing had moved aft and was found protruding from the aft side of the bearing. The left MLG aft trunnion pin had fractured approximately perpendicular to the length of the pin at its midspan. The trunnion pin was extracted from the airplane for further examination. The aft section of the pin was between 4.5 and 5.0 inches long, and the forward section of pin was between 5.25 and 6.0 inches long. The aft section of the trunnion pin was examined at Boeing’s Equipment Quality Analysis (EQA) laboratory in Seattle, Washington with NTSB personnel present. The forward section of the trunnion pin was removed from the aircraft over a month later, and was sent to the NTSB Material Laboratory, where it was examined before bringing it to the Boeing EQA lab for additional Barkhausen and temper etch inspections. The results of the examinations at the NTSB Materials Lab and Boeing EQA lab are discussed below. Trunnion Pin – Aft Section When examined, the aft section of the trunnion pin was placed in approximately the same orientation as it would be installed on the airplane. Visual examination showed that most of the fracture surface was rough with a dull gray luster, and many of its edges had small shear lips. The fracture surface exhibited river patterns and chevron marks consistent with a fracture progressing from a small thumbnail crack. The damage was located at approximately the 12:00 position when looking at the fracture surface. This thumbnail crack exhibited diverse colors with crack arrest marks. These features were consistent with pre-existing fatigue cracking in this pin section. The thumbnail crack dimensions, which were measured with a digital optical microscope, were 0.144 inches deep and 0.391 inches long (the