What happened
On October 7, 2023, a Cessna 340A, registration HK4967, was operating a non-scheduled medical evacuation flight from Barranquilla to Medellín. The aircraft was transporting a patient and an accompanying person, along with a crew of two, when it approached runway 02 at Olaya Herrera Airport.
Following standard landing procedures, the aircraft made contact with the runway. Immediately upon touchdown, the crew felt a vibration in the right rudder pedal and noted a directional deviation. Despite efforts to maintain the centerline, the right main landing gear retracted, causing the aircraft to veer toward the right safety strip. The crew responded by feathering the propeller and cutting the fuel mixture to the right engine. The aircraft subsequently drifted into the right safety area at low speed before coming to a stop. All 6 occupants were unharmed.
Significant damage was sustained to the right side of the aircraft, including fractures to the bellcrank assembly, pivot bolt, and trunnion coupling sections. The right engine and propeller suffered damage from direct contact with the asphalt, including plastic deformation of all three propeller blades. Additional damage was noted to the right wing tip fuel tank, flaps, and fuselage structure. \n## The investigation
The GRIAA investigation focused on the structural integrity of the landing gear components, specifically the bellcrank, pivot bolt, and bushing. Laboratory analysis, including Scanning Electron Microscopy (SEM) and optical microscopy, was performed to identify the failure mechanism.
Investigators examined the pivot bolt, which showed a significant shear fracture approximately 1.4 cm from the bolt head. The analysis revealed that the bolt was off-center within the bushing, and the surface of the bolt had been notched by the protective bushing, creating a site for potential crack initiation. Metallographic studies confirmed that the fracture was a ductile failure resulting from high shear loads and overstress.
Findings
- The primary cause of the incident was the retraction of the right main landing gear due to a ductile fracture of the pivot bolt.
- The pivot bolt failed because it was subjected to excessive shear loads and overstress during landing.
- Variable meteorological conditions, including shifting winds and nearby thunderstorms, likely contributed to an unstabilized approach or a hard landing.
- The use of a Supplemental Type Certificate (STC) to increase the aircraft's maximum operating weight may have contributed to the pivot bolt being subjected to loads exceeding its design limits.
- Corrosion pitting and wear on the bolt surface were observed, which may have facilitated crack propagation.