What happened
On October 10, 2007, an Eurocopter AS 350 B3 (registration EC-JRJ) was performing external load operations to transport logs near Forada del Toscar, Huesca. The aircraft was carrying a log weighing approximately 630 kg, suspended by a 7-meter sling and a 3-meter shock absorber rope.
While the pilot was attempting to deposit the load, the lower end of the log struck the soft ground and became embedded approximately 20 cm deep. The log also became entangled with another log already on the ground. As the helicopter continued its forward momentum, the log flexed and eventually snapped. This sudden event caused the helicopter to enter an uncontrolled anti-clockwise spin. Despite the pilot's attempts to counter the rotation using the pedals, the aircraft completed more than one full rotation and drifted laterally before impacting the terrain. The aircraft remained upright on its right side, and the log remained attached to the hook.
The investigation
The investigation focused on the functionality of the external cargo hook system and the mechanical failure of the load. Investigators examined the aircraft's engine management display, which recorded a torque excursion of 137% for two seconds during the event.
Technical analysis of the cargo release system revealed that the electrical connection for the remote hook had been modified using non-standard, non-aviation grade hardware, specifically a conventional male-female plug. This modification was not documented in the aircraft's records. Testing after the accident showed that the release system was highly unreliable, with failures occurring frequently unless the connector was manually manipulated. Physical inspection of the connector showed a deficient electrical connection and evidence of oxidation on a screw, indicating significant overheating.
Findings
- The primary cause of the accident was the failure of the cargo hook release system to function when the pilot attempted to jettison the load.
- The external cargo hook system had been modified with non-standard electrical components that were not certified for aviation use.
- A defective electrical connection within the modified plug caused poor conductivity and overheating, leading to intermittent operation of the remote release.
- The sudden mechanical stress caused by the log snagging in the ground and subsequently breaking created an unmanageable situation for the pilot.
- The attempt to stabilize the aircraft against the sudden forces of the snagged load resulted in a transient engine torque excursion of 137%.