What happened
On January 30, 2004, a BELL 206L-4 helicopter, registration EC-FTM, was performing specialized aerial work in the Collado de Bujaruelo, Huesca. The mission, originally intended for transporting personnel and equipment, had evolved into an attempt to unearth a broken high-voltage electrical line buried under approximately 140 cm of snow.
The crew utilized a long sling attached to the aircraft's cargo hook to loop around the buried cable. By lifting vertically, the crew intended to gradually pull the cable out of the snow. During one such maneuver, a section of approximately 20 meters of cable was suddenly unearthed. This sudden movement caused the helicopter to experience a sharp jerk, leading to an immediate loss of control, characterized by a vertical descent and a left yaw.
Although the pilot attempted to activate the electrical release for the cargo hook, the sling remained attached to the aircraft. As the pilot maneuvered the helicopter away from the line to regain airspeed, the tension from the still-attached sling caused a second, severe destabilization. The aircraft'ed nose sharply, entered a violent oscillation, and ultimately struck the ground, resulting in the destruction of the tail boom and main rotor blades.
The investigation
The CIAIAC investigation focused on the mechanical functionality of the cargo hook, the engine performance, and the crew's operational procedures. Investigators found that the cargo hook's electrical and manual release mechanisms were fully functional. They also examined the engine's performance, noting that the anti-ice system was not engaged despite the freezing temperatures and high humidity caused by the snow.
Furthermore, the investigation reviewed the operator's manual, finding that the specific task of unearthing a cable was not covered in their approved procedures. The investigation also looked into the crew's workload and task distribution during the emergency.
Findings
- The sudden unearthing of the electrical cable caused the initial destabilization of the aircraft.
- The sling failed to release from the cargo hook, likely because the sudden upward movement of the cable created a