2010 Eurocopter AS 350 B3 accident at Gotzenalm

Overview and flight details On 6 October 2010, a Eurocopter AS 350 B3 helicopter operated by an Austrian air transport company crashed at Gotzenalm near Schönau am Königssee. The aircraft was conducting external-load flights, with the final flight involving the transport of a tree trunk approximately 20 meters long to a drop-off point about 500 meters away. The pilot, a 48-year-old German professional helicopter pilot, was lightly injured and was able to leave the aircraft under his own power. The helicopter sustained heavy damage, and there was minor damage to the ground. ## Aircraft and load configuration The single-engine Eurocopter AS 350 B3, manufactured in 2007 with serial number 4364, was certified for up to seven passengers and had a maximum takeoff weight of 2,250 kg, or 2,800 kg with an external load. It was equipped with a Turbomeca Arriel 2B1 engine, a three-blade main rotor, skid landing gear, and a tail rotor. The external load was secured using a sling system from a specialized aerial work company, consisting of a damper, a 10 m x 16 mm textile Dyneema core rope, a tension load element, a coupling hook with a twist stopper, and a 5 m lifting strap. The total mass of the load attachment equipment was approximately 30 kg. The manufacturer's instructions stated that a safety factor of seven was considered, and the transport of tree trunks (logging) was not permitted with this sling. ## Accident sequence During the final approach to the drop-off point, the pilot reported unexplained vibrations in the helicopter around 12:10 local time. Unable to see the external load in the external load mirrors, the pilot immediately disconnected it. However, the vibrations intensified, and the helicopter began to rotate to the left around the vertical axis. The pilot attempted to stop the rotation by reducing power and increasing speed, but the helicopter landed while rotating and overturned. A witness observed the tree trunk making ground contact two to three times before a loud bang, after which the tail of the helicopter folded upward and the aircraft began to rotate before crashing. A short video sequence showed a bent tail boom shortly before the impact. ## Investigation findings The German Federal Bureau of Aircraft Accident Investigation (BFU) investigated the accident. The external load sling was examined by the Wehrwissenschaftliches Institut für Werk- und Betriebsstoffe (WIWeB), which concluded that the break in the load rope was due to overload from an impact load during the transport of the tree trunk. No evidence of manufacturing defects or prior damage to the rope was found. The two break points were located in the transition area from the splice to the free rope length, a typical break point for this type of rope construction under overload. A comparison rope provided by the helicopter owner was subjected to a tensile test by the Institute for Material Handling and Logistics at the University of Stuttgart, which found a maximum tensile force of 64.43 kN, with the break point in the transition to the splice zone. This maximum tensile force was 4.6 times (460%) above the allowable working load limit. The investigation also included physical approximation calculations to estimate possible load peaks in an external load sling, finding that short-term load peaks could occur far above the allowable payload of the external load rope (14 kN) and even above the breaking load of the comparison rope (64.43 kN).