What happened
On April 11, 2018, a Bell 206 L3 helicopter, registration HK4776, operated by Helifly Colombia SAS, was performing a non-scheduled cargo and passenger transport mission. The flight departed from Popayán (SKPP) and landed at a designated landing site on Cerro El Pinche, in the Argelia municipality of Cauca, Colombia.
After landing, the passenger disembarked to begin cargo extraction. During this period, the pilot experienced sudden and extreme flight characteristics, including intense vibrations and oscillations throughout the airframe. The vibration became so severe that it compromised the controllability of the aircraft. As the vibrations intensified, the helicopter suffered massive structural damage, including the separation of the main rotor from the mast, a failure of the tail boom, and the displacement of cockpit instruments and windows. The pilot successfully performed an emergency shutdown and evacuated the aircraft without injury.
The investigation
The GRIAA investigation examined the aircraft's maintenance records, the flight history, and the physical wreckage. Technical inspections conducted by representatives from Bell Helicopter and Rolls Royce confirmed that the aircraft's systems, including the engine and hydraulic lines, were fully operational and maintained according to schedule prior to the event. The investigation focused on the mechanics of the landing and the interaction between the aircraft and the terrain.
Findings
- The primary cause of the accident was ground resonance induced by the pilot's landing technique.
- During the landing on the unprepared terrain, the pilot applied a slow and moderate descent with a gradual application of collective pitch. This caused the aircraft to remain partially supported by the landing gear in a manner that prevented the struts from providing necessary damping.
- This lack of damping led to an uncommanded, self-sustaining oscillatory movement.
- The resulting intense vibrations caused the rotor blades to move out of phase, shifting the center of gravity and creating a divergent spiral. This led to the rotor blades striking the mast, eventually causing the main rotor to separate from the fuselage.
- The pilot failed to use sufficient precaution or appropriate techniques to prevent the onset of resonance during the landing phase.