What happened
On March 29, 2017, an Airbus Helicopters AS332 L1 helicopter, registered as OE-XSP, was performing a logging operation near Soazza, Switzerland. Operated by Heli Austria GmbH, the aircraft was engaged in transporting timber using a 50-meter long line. During the mission, the pilot had completed approximately 100 rotations.
While attempting to release two large logs with a combined mass of roughly 3.5 tonnes, the safety hook of the shock absorber opened unexpectedly. This caused the long line, including the secondary hook, to detach and fall onto a pile of wood. The pilot reported that the two logs had landed on the pile with a slight time delay, and as the pilot was preparing to move the load forward, the rigging dropped to the ground. No injuries were reported among the crew or bystanders, though one person on the ground was noted as slightly injured.
The investigation
SUST examined the lifting equipment, which consisted of a shock absorber attached to the helicopter's primary hook, a safety hook (type LHW 13-10), and the long line. Technical inspection of the safety hook revealed that while the spring and trigger (latch) were functional, there was plastic deformation in the seat where the trigger engages, which was attributed to normal wear.
The investigation focused on how the hook could open under load. It was determined that the hook is designed by the manufacturer for straight-line, shock-free loading only. The investigation analyzed scenarios where the trigger could be manually or mechanically manipulated. It was found that the lower fitting of the shock absorber or the fitting of the long line could potentially strike and disengage the trigger.
Findings
Logging operations involve significant dynamic stresses. The investigation concluded that the sudden release of energy during the unloading process likely caused the components of the lifting gear to move violently. In this specific instance, the hard landing of the load likely caused the lower fitting of the shock absorber to strike the safety hook's trigger, causing the hook to open and the line to detach.
Furthermore, the investigation found that the specific safety hook used is not suitable for the dynamic loads inherent in logging operations, as it lacks the necessary safeguards to prevent accidental opening under impact.