NTSB Issues Preliminary Report On Alaska Airlines Boeing 737 Landing Gear Failure

After conducting a firm landing in the midst of a tropical storm last month, an Alaska Airlines Boeing 737-800 suffered significant damage. After preliminary investigations, the US National Transportation Safety Board (NTSB) released a report highlighting that the left landing gear of the aircraft had come to rest on one of the engines after landing.

Landing amid Tropical Storm Hilary

The flight took place on August 20th, when Alaska Airlines flight 1288 was scheduled to fly from Seattle–Tacoma International Airport to John Wayne-Orange County Airport. Operating the day’s flight was an almost 15-year-old Boeing 737-800 registered N516AS, with 106 passengers and six crew members onboard. While this would have usually been a routine flight, Tropical Storm Hilary was in play.

After evaluating that flight 1288 could still proceed as scheduled, the flight departed from Seattle-Tacoma without incident at approximately 20:26. The flight crew reported that there was moderate rain, shifting winds, light turbulence, and instrument meteorological conditions until the aircraft broke out at approximately 800 feet above ground level.

Photo: FlightRadar24.com

Although the weather was improving, the NTSB report suggested the flight crew discussed that a firm touchdown was most appropriate with the aircraft fully configured and the before-landing checklist completed. They were also making position reports via the common traffic advisory frequency as the tower was closed, and as soon as they saw the runway, the captain – who was the pilot flying – disengaged the autopilot and autothrottle.

The first officer monitored the flight’s status during the approach and reported that the aircraft was on perfect speed and path. All systems were operational during the descent – including all green lights for the landing gear position indicators. The aircraft crossed the end of the runway on the glide path and touched down just past the 1,000-foot marker at John Wayne Airport at approximately 23:15.

Favored to the left

According to the captain, upon landing, the touchdown had a firm, jolted feeling to it. The captain also noted the aircraft was listing reasonably hard to the left, which he could correct with rudder pedal input and keep the aircraft on the center line while slowing to taxi speed and exiting the runway onto taxiway E. The first officer also noted that it felt like the aircraft had a flat tire on the left main landing gear, especially since the left landing gear position indicator was not green anymore.

During the turn onto the taxiway, the flight crew confirmed the aircraft favored too heavily to the left, prompting the captain to set the parking brake on the taxiway and open his window for a better look. It was then noticed the airplane was resting on the left engine cowling, and although it appeared to be still running normally, the captain took no chances and shut down the engine. With the auxiliary power unit online, the captain also shut down the right engine.

While these were from the flight crew’s perspective, the passengers had a slightly scarier view when they saw bright white sparks emitted as the engine scraped across the ground. Panic set in for some passengers as they feared the aircraft would catch fire, given the sparks, but fortunately, the landing was safe. There were no reported injuries from this incident as passengers were evacuated on the taxiway.

Possibly caused by a broken pin

Postaccident examinations and preliminary investigations of the aircraft by the NTSB revealed that the left main landing gear trunnion pin had fractured and that the aft end of the left main landing gear had traveled up. This resulted in the left main landing gear puncturing the composite panels and protruding out and above the wing’s surface, along with the main landing gear walking beam protruding above the wing’s surface.

Photo: NTSB

The agency also noted that 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 found in the main landing gear beam spherical bearing. The left-wing flap transmission drive unit, located in the left wheel well, was also separated from its mounting structure and was found on the runway near the airplane’s initial touchdown point.

Bottom line

However, data showed that the Boeing 737-800 touched down with a maximum vertical acceleration of 1.71G, well below the hard landing threshold of 2.2G, per Alaska Airlines’ aircraft maintenance manual Chapter 5 limitations. As such, the left main landing gear trunnion pin and the structures were sent to the Boeing equipment quality analysis laboratory in Seattle for further analysis of what caused the fracture, and the NTSB investigation into this incident continues.