What happened
On February 8, 2023, at approximately 00:15 local time, a Boeing 737-400SF, registration 5N-OTT, operated by Allied Air Limited, was landing on runway 05 at Maya-Maya International Airport in Brazzaville. The aircraft was performing a cargo flight from Libreville, Gabon.
The crew reported that the approach was stabilized with the autopilot engaged until approximately 200 feet above the ground, at which point manual control was taken by the co-pilot. Shortly after touchdown and during the deceleration phase following thrust reversal, the crew heard a loud detonation originating from the left main landing gear, followed by intense vibrations. A second, less intense noise was also noted.
The structural failure caused the left main landing gear components—including the shimmy damper, side braces, and the axle-wheel assembly—to detach from the fractured gear leg. Additionally, tire number 2 burst. The dragging of the detached gear components and the engine nacelle against the runway surface caused visible damage to the pavement, as well as significant damage to the aircraft's left wing, engine, and fuselage. There were no fatalities among the three crew members on board.
The investigation
The Congolese BEA examined the wreckage, flight recorders, and maintenance records. Investigators analyzed the impact site, noting that debris from the landing gear was found approximately 98 meters from the aircraft's final position. The investigation also included an examination of the tire condition and components of the left main gear.
Technical analysis of the flight data recorder (FDR) and cockpit voice recorder (CVR) was complicated by a lack of updated decoding documentation for the aircraft's specific configuration, which limited the depth of the post-event analysis.
Findings
The investigation identified that the most probable cause of the accident was a landing gear failure triggered by a shimmy event occurring shortly after touchdown. This phenomenon generated dynamic loads that led to the successive breakage of mechanical components, specifically the shimmu damper, side braces, and an internal cylinder which failed due to a pre-existing intergranular crack.
Contributing factors included:
- Excessive wear on apex joint components (washers and bushings), resulting in abnormal play.
- Reduced effectiveness of the shimmy damper due to this excessive play.
- Advanced wear on the tires, which had been used beyond recommended limits.
- The presence of a latent structural defect (intergranular crack) that was not detected during maintenance inspections.