Helicopter accident at Bormes-les-Mimosas

Incident Overview and Aircraft Details On July 31, 2010, at 10:51 local time, a privately operated Eurocopter AS350-B3 helicopter, registered F-GKBF, was involved in a fatal accident near Bormes-les-Mimosas (83). The aircraft was destroyed, and the incident resulted in one passenger death and severe injuries to the pilot. The helicopter departed from La Môle aerodrome with five passengers, heading toward a helipad in Bormes-les-Mimosas. After approximately ten minutes of flight, the pilot was on final approach facing east, characterized by a steep angle and low speed. At a height of about 15 meters, the pilot observed a lack of power, strong vibrations, and abnormal sinking. The main rotor blades struck a cypress tree. The pilot reduced the collective pitch to maintain rotor speed and then pulled the collective pitch control at approximately three meters to cushion the impact with the ground. The passenger seated in the rear center-right position was ejected from the right side of the helicopter during the impact. The helicopter rolled onto its right side above the ejected passenger. The other four passengers evacuated the aircraft. The pilot shut off the fuel supply and electrical power before evacuating. The pilot held a private helicopter pilot license, a professional helicopter pilot license, and a helicopter flight instructor qualification, with a total of 12,700 flight hours, including 12,000 as commander and 50 in the preceding three months. The pilot also held a permanent authorization to use helipads. The landing area used consisted of a dirt and rock surface, approximately 60 meters long and 30 meters wide, bordered by cypress trees up to 10 to 15 meters tall, oriented roughly northwest/southeast. No ground system was available to assess wind direction and speed. A nearby alternative area had clearer surroundings but was farther from the passengers' destination. Site examination indicated the helicopter severed a cypress tree at a height of about 10 to 12 meters during the final approach. This tree was located 17 meters from the wreckage. A 25-centimeter-long piece of the cypress trunk was recovered, with blade-cut marks at both ends, allowing an estimated descent rate of approximately 1,000 ft/min at 15 meters above the ground. Damage to the front of the fuselage indicated the helicopter struck the ground with a nose-down attitude and slight right bank before bouncing and rolling over. Examinations of the helicopter, including the engine, freewheeling unit, anticipator, computers, servocommands, and fuselage, revealed no pre-impact malfunction that could have contributed to the accident. No audible or visual alarms were recorded during the flight or reported by the pilot or passengers. Cabin examination showed that the seatbelt installation performed by the maintenance workshop the week before the accident did not conform to the manufacturer's documentation. The seatbelt arrangement was reversed on the left outer seat and the right inner seat. Additionally, on the right side, different types of seatbelts were installed, making it impossible to fasten the lap belt of the passenger seated in the rear center-right position. No functional check of the seatbelts on the inner seats was performed after installation. The vortex phenomenon, or powered sinking, in a helicopter is similar to an aerodynamic stall of the main rotor blades. Favorable conditions include low airspeeds (forward or backward) and descent rates between -700 ft/min and -2,500 ft/min, with a tailwind or steep approach. The phenomenon is characterized by vertical sinking with rapid altitude loss, a rapid and significant increase in fall rate, variometer lag, soft flight controls with reduced control moment, increased vibration levels, power variations at constant collective pitch, and a sensation of power loss not correlated with actual indications. Testimonies indicated the arrival was from the north in descent after crossing a hill, with the pilot turning left to align for the final approach. The pilot chose the helipad he usually used, where he had landed the previous day. He stated he adopted a steep profile due to the height of the trees bordering the area to ensure sufficient margin for obstacle clearance. He estimated the indicated airspeed during the final approach was about 30 knots and the descent rate was 500 ft/min. He used the meteorological data he had estimated during takeoff from the same area one hour earlier. Meteorological conditions estimated by Météo France at the site at the time of the accident were: clear sky, visibility greater than 10 km, no turbulence, wind 100° to 140° at about 2 kt, temperature 26 °C. The pilot's testimony on the approach and the description of the symptoms: appearance of vibrations, apparent lack of power, low estimated airspeed of 30 knots, steep descent profile, suggest the helicopter probably entered a vortex state. The fact that this phenomenon occurred in the immediate vicinity of the ground did not allow the pilot to recover control of the helicopter. The pilot paid particular attention during boarding to passengers seated near the doors. The incorrect seatbelt installation could have been reported to the pilot by the passenger or detected by the pilot during the pre-takeoff visual inspection. It was not possible to determine the reasons why the passenger did not report the anomaly.