What happened
On March 5, 2025, a Bell 206L-3 helicopter, registration HK4717, was performing a non-scheduled flight from Olaya Herrera Airport in Medellín to Carmen de Atrato, Colombia. During the takeoff phase, as the aircraft reached approximately one meter of altitude, a sudden structural failure occurred within the tail rotor drive shaft hanger assembly.
The failure produced a loud noise and caused the separation of the drive shaft cover. This mechanical disconnection immediately neutralized the aircraft' and's anti-torque capability, resulting in an uncontrolled yaw to the right. The pilot was forced into a hard landing on the taxiway. There were no fatalities or injuries among the four occupants on board.
The investigation
The GRIAA investigation focused on the structural integrity of the tail rotor components. Metallurgical analysis of the fractured hanger assembly (made of A356 aluminum alloy) revealed that the components suffered from intergranular brittle fracture.
Laboratory testing, including chemical composition and metallography, determined that the alloy contained concentrations of Silicon and Magnesium that exceeded the maximum limits established by the ASTM B108 standard. Furthermore, the investigation found that the heat treatment process during manufacturing was inadequate, leading to the formation of brittle intermetallic compounds (Mg2Si and Al5FeSi) at the grain boundaries.
Additionally, the inspection of the aircraft revealed several maintenance-related findings, including missing torque marks on various bolts in the engine and control linkages, and corrosion on the internal fuselage supports for the landing skids.