External Cargo Sling Failure During MI-8MTV-1 Flight in Colombia (HK3908)

Incident Overview and Location On November 15, 2017, at 10:00 local time (15:00 UTC), an MI-8MTV-1 helicopter with registration HK3908 was conducting a commercial external cargo transport operation in Antioquia, Colombia. The aircraft was operated by Vertical de Aviación S.A.S. and was carrying steel structures intended for the construction of electrical transmission towers. The cargo was released over Vereda Piedras, a populated area in the municipality of Tarazá, resulting in one serious injury to a person on the ground and damage to approximately eight houses. The helicopter itself sustained no damage, and its three occupants were unharmed. ## Operational Context and Flight Details The operation involved transporting steel structures from Tarazá to designated tower construction sites. On the day of the incident, the helicopter had already completed several cargo flights. The final flight, which departed Tarazá at 09:48 local time, carried a 2,882 kg load of steel angles. The cargo was secured using a choker technique with a nylon sling. The aircraft took off with 640 kg of fuel and was climbing to a cruise altitude of 3,500 feet MSL when the incident occurred. ## Sling Failure and Cargo Release Approximately four minutes after takeoff, while in cruise flight at 2.45 nautical miles from the departure point, the left leg of the nylon sling failed at the eye surrounding the cargo. This failure caused the helicopter to bank to the right, a maneuver that was controlled by the flight crew. The remaining right leg of the sling experienced a pendulum effect, which displaced the cargo laterally. The load, consisting of approximately 40 steel angles, fell at a steep angle of about 55 degrees, impacting structures 300 to 450 meters from the flight path. ## Investigation and Probable Causes The Colombian accident investigation authority, GRIAA, determined that the probable cause of the accident was the failure of the nylon sling during the cruise phase. The failure was attributed to a cutting action, likely caused by the movement of the steel structures during flight. Additionally, the investigation identified weaknesses in the operator's risk management regarding external cargo procedures and the preparation and securing of the load. The investigation noted that the aircraft's systems and powerplant were functioning normally and did not contribute to the event.