What happened
On January 3, 2016, an Airbus Helicopters AS 350 B2, registered F-GKYG, was performing an external load flight over the forests of French Guiana. The aircraft was transporting a 700 kg load, consisting of a hydraulic reducer and flexible hoses, secured in a net. The flight was traveling between Tonnegrande and Sofn at a stabilized speed of approximately 75 knots.
Midway through the 6 Nm transit, the pilot and assistant heard a loud noise from the rear of the aircraft. Upon checking the mirrors, they observed the external load falling away from the helicopter. The pilot reduced speed and continued to the destination, where the aircraft landed safely. During the engine shutdown sequence, the crew noted vibrations coming from the rear of the helicopter. Upon ground inspection, the crew found that the load, net, and slings were missing, and the helicopter's right stabilizer was pierced and damaged, with visible impact marks on the tail rotor.
The investigation
The BEA examined the aircraft's tail section and found evidence of nylon sling friction on the right side of the tail boom. The right stabilizer had been pierced through by a metallic object, and the tail rotor showed numerous impact marks from a metallic object. The investigation also looked into the external load attachment system, specifically the INDRAERO SIREN release hook. Technical tests conducted on the hook and its components in mainland France revealed no malfunctions, and the release mechanism was found to be operating normally.
Discrepancies were noted in the pilot's statements regarding the configuration of the slings. Initial reports suggested two 6-meter slings, while subsequent communications mentioned a 15-meter long-line setup and even a configuration involving four different slings. To stabilize the asymmetrical load, the crew had utilized "bois canon" (hollow tree trunks) as stabilizers within the net to prevent the load from rotating during flight.
Findings
- The damage to the tail boom, stabilizer, and tail rotor was caused by the sling moving violently toward the rear of the aircraft following the sudden loss of the load.
- While the exact cause of the initial load loss could not be determined, it is possible that the loss of the stabilizing tree trunks caused the load to destabilize and rotate, leading to the failure of the net.
- The release hook and electrical release system were functioning correctly and did not cause an accidental release.