What happened
On February 21, 2023, a Cessna 421B, registration HK4837, was performing a scheduled air taxi flight from Corozal to Cartagena, Colombia. The flight proceeded normally until the aircraft arrived at Rafael Núñez International Airport.
Upon landing on runway 01 at approximately 11:50 HL, the aircraft began its deceleration roll. As the pilot applied the brakes, the left main landing gear unexpectedly retracted. This caused the left wing to drop and strike the runway surface, leading the aircraft to slide approximately 13 and 130 meters along the pavement while veering toward the left edge of the runway. The aircraft eventually came to a stop with its left wing positioned outside the runway boundary.
All five occupants (the pilot, co-pilot, and three passengers) evacuated the aircraft via the main door and were uninjured. The aircraft sustained significant damage, including structural damage to the left wing and landing gear, as well as damage to the left propeller blades due to an engine stoppage.
The investigation
The GRIAA investigation focused on the mechanical failure of the landing gear assembly. Investigators examined the state of the landing gear components and found that the upper and lower linkage bars were not in their normal, secured position for an extended gear configuration.
Technical analysis revealed that the mechanical alignment of the overcenter guide was incorrect. This misalignment, combined with existing mechanical tolerances (play) in the mechanism, led to an improper distribution of forces during the landing impact and subsequent braking. The investigation specifically looked at the condition of the pivot bolt and the stability of the gear's locking mechanism.
Findings
- The primary cause of the incident was the fracture of the pivot bolt (barra de acoplamiento), which is responsible for securing the gear arm under compression. This fracture created a discontinuity in the landing gear safety mechanism, allowing the left main gear to retract once it was subjected to the weight of the aircraft and the inertial forces of braking.
- A contributing factor was the redistribution of stresses within the extension mechanism. This overstress was likely caused by mechanical tolerances and clearances that, when amplified by the mechanism's geometry, caused the forces to deviate from the manufacturer's design specifications.
- The investigation noted that the manufacturer had previously issued Supplemental Inspection Documents (SID) regarding similar issues, specifically addressing inspections for excessive wear in the retraction system and potential deformation of the pivot bolt.