N516AS 737-800 MLG Failure on Landing
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On 20 Aug 2023, 737-800, N516AS, 39044/2751, age 15 Years, operated by Alaska Airlines suffered a failure of the left MLG on touchdown at John Wayne-Orange County Airport (SNA), Santa Ana, California at 23:15 LT.
The flight crew reported that there was moderate rain, shifting winds, light turbulence, and IMC until the aircraft broke out at about 800 feet AGL. Although the weather was improving, the captain and the first officer discussed that a firm touchdown was appropriate. As soon as they saw the runway, the captain, who was the pilot flying, disengaged the autopilot and autothrottle.
Preliminary DFDR data shows that the landing had a maximum vertical acceleration of 1.71g. The value is below the hard landing threshold of 2.2G per Alaska Airlines aircraft maintenance manual (AMM) Chapter 5 limitations.
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.
On 12 Sep 2023, the NTSB released their preliminary report. It states that Post accident 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.
The left MLG beam sustained damage as a result of the left MLG moving out of position and contacting the beam after the failure of the trunnion pin. The left-wing flap transmission drive unit, located in the left wheel well, separated from its mounting structure and was found on the runway near the airplane’s initial touchdown point.
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*** Updated 05 Aug 2023 ***
