Investigators rule out hard landing in WestJet Boeing 737 gear collapse in St. Maarten

The Canadian Transportation Safety Board (TSB) has provided an update to its investigation into the WestJet Boeing 737-800 right main landing gear (MLG) collapse in September, saying that preliminary data showed that a hard landing did not cause it to collapse upon landing at Sint Maarten Princess Juliana International Airport (SXM).

In a short update, the TSB said that an initial review of the flight data recorder (FDR) indicated that “this was not a hard landing event.”

When investigators examined the aircraft, they found a fractured right gear aft trunnion pin, and the two portions of the aft trunnion pin were sent to TSB’s Engineering Laboratory in Ottawa, Canada, for further examination, it said.

According to the TSB, WestJet overhauled the right MLG of the 737-800 in 2016, around seven years after it took delivery of the aircraft, registered as C-GWSR, per planespotters.net. The Canadian investigators added that passengers and flight crew evacuated on the left-hand side of the aircraft, with only one person sustaining a minor injury during the evacuation.

“No injuries were reported due to the gear collapse.”

The TSB highlighted that it has been reviewing previous incidents involving aft trunnion pin fractures on 737s.

On August 20, 2023, an Alaska Airlines 737-800, registered as N516AS, suffered an almost identical left-hand MLG collapse at Orange County John Wayne Airport (SNA). The National Transportation Safety Board’s (NTSB) final report detailed that during the post-crash examination of the aircraft, it discovered that the “aft trunnion pin in the left MLG failed during landing due to a fatigue fracture.”

The NTSB concluded that the probable cause of the incident 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.”

In 2016, the Federal Aviation Administration (FAA) finalized an airworthiness directive for the 737 Next Generation (NG), which includes the 737-800, mandating operators to repetitively lubricate “the forward and aft trunnion pin assemblies of the right and left MLGs,” inspect the assemblies for corrosion and chrome damage, and install new or modified pin assembly components. 

The latter action would have terminated the requirement of repetitive lubrication and inspections, the FAA noted.

However, the US regulator issued a new directive that superseded the AD from 2016. In August 2019, the FAA, warning that it had “determined that rotable parts were not addressed in that AD [from 2016 – ed. note],” and had to “include all airplanes of the affected models in the applicability,” issued the AD.

The 2019 directive retained the mandated actions of the AD from 2016, added more aircraft to which the AD applied, and prohibited the installation of an MLG or MLG trunnion pin assemblies unless they were modified with the new assemblies.

“We are issuing this AD to address heavy corrosion and chrome damage on the forward and aft trunnion pin assemblies of the right and left MLGs, which could result in cracking of these assemblies and collapse of the MLGs.”

Meanwhile, the WestJet 737-800, which suffered the right-hand MLG collapse at SXM, has been grounded at the airport since the incident on September 7. The incident resulted in the temporary closure of the single-runway airport.

SXM reopened on September 8, after crews cleared the 737-800 from the airport's runway.

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The Engine CowlRytis Beresnevicius