What happened
On 3 March 2020, a Eurocopter AS350B2, registration G-PDGF, was performing aerial work near Glencoe, Argyll, to assist in the refurbishment of an electricity line. The helicopter was tasked with transporting wooden poles from a storage facility to various work sites. During the mission, the aircraft was carrying a 700 kg wooden pole as an underslung load.
As the pilot transitioned into forward flight and increased airspeed, the pole began to spin with increasing intensity. The pilot felt the motion through the airframe and attempted to bring the helicopter into a rapid hover by lowering the collective and applying rear cyclic. However, before the maneuver could be completed, the pole detached from the aircraft. The load struck a steep hillside approximately 200 m from a minor public road, breaking into two pieces. There were no injuries to the crew, and the helicopter and lifting equipment sustained no damage.
The investigation
The investigation examined the circumstances surrounding the attachment of the load. At the time of the incident, the ground handler responsible for overseeing the loading process was away from the storage facility to refuel the aircraft. Consequently, a client's employee, who had previously attended training for such operations, was performing the hooking-on task without direct supervision.
The investigation also analyzed the mechanical state of the hook, which featured a spring-loaded keeper design. While the operator's engineering department found no faults during electrical checks of the release system, the possibility of an intermittent electrical malfunction was considered. The investigation focused on the positioning of the sling within the hook's load-bearing beam.
Findings
- The primary cause of the incident was that the sling was not correctly positioned within the semi-circular recess of the hook's load-bearing beam.
- This improper positioning allowed the sling to move during flight; as the load began to spin, the movement was sufficient to overcome the spring-loaded keeper and release the load.
- The client's employee was performing the loading operation without direct supervision from the ground handler.
- An intermittent electrical fault in the release system could not be entirely ruled out as an alternative cause.