What happened
On June 13, 2015, a Cessna 421, registration HK4793, was performing a medical evacuation flight from Cúcuta to Paipa, Colombia. The aircraft, operated by Ambulancias Aéreas de Colombia Ltda., was carrying two crew members and four passengers.
During the landing roll on runway 22 at Juan Jose Rendón Aerodrome (SKPA), the crew applied braking after the wheels made contact with the centerline. Suddenly, a violent lateral vibration occurred in the left main landing gear, causing the pilot to lose directional control. The aircraft drifted toward the left side of the runway, traveling approximately 30 meters before exiting the paved surface. The aircraft eventually struck a drainage canal located within the safety area, which caused the nose gear and both main landing gears to detach. All six occupants evacuated the aircraft safely and sustained no injuries.
The investigation
The GRIAA investigation focused on the mechanical failure of the landing gear and the condition of the airfield. Laboratory analysis of the components revealed that the left main gear's alignment link assembly had separated. Investigators found that the bolt securing the link had been improperly assembled during a previous maintenance task. Specifically, the washer distribution was unbalanced: three washers were placed on the nut side, while only one washer was placed on the bolt head side. This imbalance led to the shearing of the single washer under load.
Furthermore, the investigation examined the airfield surface at SKPA, noting that the asphalt was in poor condition, characterized by potholes and depressions. The presence of a drainage canal within the runway safety area was also identified as a significant hazard that contributed to the structural damage of the aircraft upon excursion. \n## Findings
- The primary cause was the failure of the left main landing gear assembly due to the improper arrangement of washers on the alignment link bolt.
- A contributing factor was the continuous loosening of the bolt, likely exacerbated by previous hard landings or operations on irregular surfaces.
- The operator's maintenance processes were insufficient, as they failed to implement specialized inspections for components subjected to high stress from operating on unpaved or poorly maintained surfaces.
- The degraded state of the runway surface and the presence of an obstacle (the drainage canal) in the safety zone increased the severity of the incident.