What happened
On July 31, 2010, an Eurocopter AS3/350-B3, registration F-GKBF, departed from La Môle aerodrome carrying five passengers for a flight to a landing site in Bormes-les-Mimosas. During the final approach, the pilot executed a steep descent at a low airspeed of approximately 30 knots. At an altitude of roughly 15 meters, the aircraft experienced intense vibrations, a perceived loss of power, and an abnormal sink rate. During this instability, the main rotor blades struck a cypress tree.
In an attempt to maintain rotor RPM, the pilot reduced collective pitch. As the aircraft descended to approximately three meters, the pilot applied collective pitch to cushion the ground impact. During the collision, a passenger seated in the rear right position was ejected from the aircraft. The helicopter subsequently rolled onto its right side. The pilot successfully shut off the fuel and electrical supplies before evacuating the wreckage. The accident resulted in one fatality and left the pilot with serious injuries.
The investigation
Investigators examined the wreckage and the flight profile, noting that the landing area was a dirt and rock surface bordered by cypress trees reaching 10 to 15 meters in height. Analysis of the site confirmed the rotor blades had severed a portion of a tree, allowing investigators to estimate a descent rate of approximately 1,000 ft/min at 15 meters altitude.
Technical inspections of the engine, flight controls, and electrical systems revealed no mechanical malfunctions prior to the impact. However, an examination of the cabin revealed that seatbelts had been installed incorrectly by a maintenance workshop the previous week. The configuration was non-compliant with manufacturer documentation, featuring reversed mounting on the left exterior and right interior seats. Furthermore, the right-side belts were of incompatible types, making it impossible for the passenger in the rear right seat to fasten their lap belt.
Findings
- The primary cause of the accident was the aircraft entering Vortex Ring State during the final approach.
- The pilot's choice of a landing site requiring a steep approach and low airspeed created the aerodynamic conditions conducive to this phenomenon.
- The passenger's inability to secure their seatbelt was due to improper installation and the use of incompatible belt types.
- Contributing factors included the lack of a post-maintenance inspection of the seatbelts and the pilot's failure to verify that all passengers were properly restrained.