BOEING 737-890

Full Narrative

HISTORY OF FLIGHTOn 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. AIRCRAFT INFORMATIONThe 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. AIRPORT INFORMATIONThe 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. WRECKAGE AND IMPACT INFORMATIONPost 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 linear length on the outer diameter). This subsurface intergranular region measured 0.016 inches deep by 0.290 inches long inside the thumbnail crack.

A Barkhausen noise inspection analysis detected elevated readings near the fracture surface. The highest readings were in areas with visible physical damage to the pin surface and near the thumbnail crack. Boeing indicated that these readings appeared to indicate residual stress in the base metal.

A fluorescent penetrant inspection and a magnetic particle inspection showed no indications of an abnormal network of cracks were observed on the surfaces of the part. Small cracks parallel and adjacent to the fracture surface were found in the chromium electroplated layer. These cracks were observed near the fracture origins. Additional indications were observed on the surface in areas that exhibited impact damage.

Trunnion Pin – Forward Section

The forward section of the aft trunnion pin had features on its fracture surface that mirrored those on the aft section of the aft trunnion pin, including a thumbnail crack along the edge.

A closer examination of the initiation region of the thumbnail crack, including the chromium electroplating surface layer and the darker subsurface intergranular layer, was conducted. The examination showed that small ratchet marks were present on the thumbnail crack adjacent to the intergranular area and below an area of the fractured external chromium electroplating layer. The plating layer exhibited layered fanshaped characteristics with radiating features. The faceted morphology of the area was consistent with fracture between grain boundaries. Much of the intergranular surfaces exhibited ridged or smooth surface features with clear triple points and some secondary cracking.

Beyond the intergranular region, the thumbnail crack exhibited fatigue striations consistent with fatigue crack propagation. These striations originated along the edge of the intergranular region of the fracture surface. The ratchet marks, which were consistent with multiple initiation sites, indicated that the fatigue crack started along stress concentrations created by the morphology of the initial intergranular zone. An abrupt change in fracture features was observed at the end of the fatigue thumbnail crack. Outside the striations comprising the fatigue, the fracture surface exhibited dimpled rupture, which was consistent with subsequent overstress fracture of the pin during the accident landing.

Overall, the fracture features of the thumbnail crack were consistent with multiple crack initiation sites along the facets of the intergranular region. These smaller cracks had coalesced during early crack propagation, as evidenced by the ratchet marks in that portion of the fatigue crack. ADDITIONAL INFORMATIONIn a review of National Transportation Safety Board investigations and assisted international investigations, there were four similar main landing gear trunnion pin failure accidents involving Boeing 737 aircraft. Those investigations were:

April 13, 2015 - Jet Airways Boeing 737-800, VT-JGA:
Jet Airways flight 9W-2423 (Delhi -Varanasi-Khajuraho–Delhi) was involved in an accident at Khajuraho Airport at 0751 UTC. During landing Roll, 15 seconds after touchdown, at speed 36 knots, left main landing gear aft trunnion pin broke and aircraft started deviating to the left of center line. The aircraft came to stop on
runway with damage to the LH landing gear and resting on LH engine.

March 3, 2016 - Jet Airways Boeing 737-900, VT-JGD:
While operating scheduled domestic passenger flight 9W-354 from Delhi to Mumbai with 120 passengers and 8 crew on 03-03-2016 was involved in an accident at Chhatrapati Shivaji International (CSI) Airport, Mumbai. The aircraft had safely landed on runway 27 at 16:22 UTC and was about to vacate the runway via taxiway N9 when the right main landing gear collapsed.

July 16, 2018 - Gol Airlines Boeing 737-800, PR-GGD:
During landing, at the moment of touch, a loud noise was heard by the cabin crew. The aircraft taxied by its own means to the apron, where passengers disembarked normally. When conducting the post-flight inspection, the maintenance team identified a leakage of hydraulic fluid and the breakdown of a component of the right main landing gear.

December 22, 2019 - United Airlines Boeing 737-800, N87513
This accident occurred when the left main landing gear of a United Airlines Boeing 737-800 collapsed during the landing roll. Both flight crewmembers reported that the airplane had touched down smoothly on the runway centerline, but the first officer reported that he then felt the airplane “shudder” and tilt left wing down. Postaccident examination found that the aft trunnion pin in the left main landing gear failed during the landing due to a fatigue crack. FLIGHT RECORDERSThe event airplane was manufactured in 2008 and was operating such that it was required to be equipped with a flight data recorder (FDR) that recorded, at a minimum, 91 parameters, as cited in 14 CFR Part 121.344(f). The parameters evaluated for the purpose of this investigation appeared to be in accordance with federal FDR carriage requirements, except left brake pressure (Brake Press-LT) and right brake pressure (Brake Press-RT). For more detailed information, see the Flight Data Recorder - Specialist’s Factual Report located in the docket.

The aircraft began its descent about 2245. During final approach, the airplane encountered a 60-degree crosswind with a recorded windspeed reaching 43 knots before decreasing closer to touchdown. At 2314:57, the MLG touched down, and the nose gear touched down 1 second later. The maximum vertical acceleration recorded at touchdown was about 1.71 g. The thrust reverser and spoiler deployment occurred normally, and the airplane came to rest with its left wing rolled about 5 degrees to the left.

The aircraft was required to be equipped with a cockpit voice recorder (CVR) that recorded at a minimum the last 2 hours of aircraft operation. The L-3/Fairchild FA2100 model CVR records a minimum of 120 minutes of digital audio stored on solid state memory modules. Four channels are recorded: one channel for each flight crew member, one channel for a cockpit observer, and one channel for the cockpit area microphone. All used channels had good to excellent quality audio and a summary was created of the audio associated with the descent, approach, and landing, up to the time the aircraft exited the runway for taxiway echo. TESTS AND RESEARCHA fatigue crack growth rate analysis was performed by counting striations at intervals along the crack depth to better understand how long the fatigue crack had been present. The fatigue striation counts were taken at 32 random and unique places on the fracture surface, with the distances from the initiation site recorded.

The fatigue crack growth rate analysis found that a total of 797 flight cycles had occurred from the intergranular initiation site used as the origin to the end of the crack. The crack depth steadily increased over time, corresponding with an expected increase in crack growth rate. The number of landings since the aft trunnion pin’s last overhaul was 4,710 flight cycles. These data indicate that the fatigue crack had not been present at, or before, the part overhaul.