What happened
On 3 September 2007, an AS332 L2 Super Puma, registration G-REDO, was conducting an aerial work mission in the Sumburgh area of the Shetland Islands. The crew was tasked with recovering a casualty from the deck of a fishing vessel. Upon arriving at the location, the helicopter entered a hover at 50 feet, and the winchman was lowered to the vessel's deck.
Due to the pitching and rolling of the fishing boat, the vessel moved from beneath the helicopter, necessitating the payout of additional cable and the repositioning of the aircraft. During this period, the winch cable was observed lying across the bow of the vessel, though the crew reported no immediate snagging or tension on the line. Once the helicopter was repositioned and the cable became vertical, the winch operator began the lift. As load was applied, the primary winch cable parted approximately one metre from the hook. This left both the winchman and the casualty on the vessel's deck. The crew subsequently used the secondary winch to successfully recover both individuals.
The investigation
The AAIB examined the broken cable to determine the cause of the separation. Microscopic visual inspection revealed that a significant number of the cable strands had suffered mechanical damage, specifically characterized by scoring and cutting. The investigation found that the majority of the failed strands exhibited brittle tensile failure in the area of this damage. While approximately 15% of the strands showed signs of plastic deformation (necking) due to tensile overload, there was no evidence of corrosion or fatigue-related cracking.
Findings
- The failure of the primary winch cable was caused by mechanical damage to the majority of the cable strands.
- This damage likely occurred when the cable was resting across the bow of the fishing vessel as the swell moved the boat.
- There was no evidence of prior fatigue or corrosion on the cable strands.