What happened
On June 22, 2023, a Cessna C421, registration HK4964G, was performing a maintenance check flight at Olaya Herrera Airport in Medellín, Colombia. The flight included a series of maneuvers, including a touch-and-go. During the initial touch-and-go, the crew felt a heavy impact on the left side of the aircraft.
During the subsequent full landing on runway 02, the aircraft initially maintained its centerline. However, during the deceleration phase, the left main landing gear began to collapse gradually. This caused the aircraft to tilt and veer toward the left, eventually resulting in the left wing making contact with the runway surface and the number one engine propeller blades striking the pavement. The pilot managed to maintain control of the aircraft's direction until it came to a stop on the left edge of the runway. All four occupants (the pilot, co-pilot, and two technical passengers) evacuated the aircraft without injuries.
The investigation
The GRIAA investigation focused on the structural integrity of the landing gear components. Laboratory analysis, including scanning electron microscopy (SEM) and metallographic studies, examined the fractured pivot bolt (S/N: NAS464P4-26). The investigation found that the bolt had fractured into three distinct sections: the head, the nut, and the bushing.
Technical examinations revealed that the failure was a result of fretting fatigue. The analysis showed that the surfaces of the bushing and the bolt had experienced sliding contact, which led to the loss of the anti-corrosion coating and the formation of microscopic notches. These notches acted as stress concentrators, eventually initiating a fatigue crack that propagated through the component.
Findings
- The primary cause of the landing gear collapse was the sudden fracture of the left main landing gear pivot bolt due to material fatigue.
- The fatigue mechanism was identified as fretting fatigue, caused by low-amplitude sliding contact between the bolt and the bushing.
- The component failed before reaching its scheduled inspection interval, which was due in September 2023.
- There was evidence of cyclic structural overstress, likely resulting from variable loads applied during takeoffs and landings, particularly on unprepared surfaces.
- The protective coating on the bushing had deteriorated, leading to oxidation and surface wear.