Eurocopter AS350 B3 accident near Beluga, Alaska, December 15, 2012

The NTSB's narrative final · quoted from the NTSB record

The commercial helicopter pilot was transporting a 1,000-pound external sling load attached to a lightweight, synthetic 100-foot long-line when the load began to spin and swing underneath the helicopter. As the helicopter neared its destination, it suddenly yawed, and then started to spin uncontrollably, and the pilot immediately jettisoned the external load. As the helicopter continued to spin uncontrollably, it descended into an area of tall trees and then came to rest on its left side.  A postaccident inspection of the helicopter revealed that portions of the severed lightweight synthetic long-line had become entangled in the tail rotor gearbox output shaft and that the tail rotor gearbox assembly was torn free of the tail boom. Examination of the long-line revealed that its entire length was tightly wrapped, knotted, and kinked. The pilot reported that he had not received training in procedures to adequately stabilize external sling loads. Given the postaccident condition of the lightweight synthetic long-line, the pilot's statements, and the lack of a rotating swivel, it is likely that, once the external load began to spin, the long-line continued to tangle and tightly knot and ultimately reached its breaking point. Once the synthetic long-line separated, the portion still attached to the belly of the helicopter recoiled upward and eventually became entangled with the tail rotor gearbox output shaft. In addition, it is likely the pilot did not adequately monitor the condition of the external sling load while en route, and he failed to recognize the need to stabilize the load.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.