What happened
On the morning of July 12, 2023, a Bell 47G-2 helicopter, registered F-BIFN, was performing agricultural spraying operations over rice fields south of Arles, France. After completing several spraying rotations and a refueling stop, the pilot was flying the aircraft back to the supply area at approximately 40 to 50 knots and an altitude of 20 meters.
During this return leg, the helicopter suddenly began to yaw to the left. The pilot attempted to correct the movement by applying right rudder pedal input; however, the controls felt ineffective, and the aircraft entered a rapid leftward spin. The helicopter descended vertically, completing at least five full rotations before striking the ground in a partially submerged rice field. The pilot sustained severe injuries but managed to extricate himself from the wreckage before help arrived.
The investigation
The investigation focused on the mechanical condition of the aircraft and the sequence of the loss of control. Investigators examined the main gearbox (MGB), the clutch, and the tail rotor drive system. While the MGB and free wheel showed no anomalies, the tail rotor drive shaft was found to have ruptured in torsion. This rupture was consistent with the drive shaft still being powered by the engine at the moment of impact.
Examination of the clutch revealed discoloration on the drum consistent with local heating. The investigation also reviewed the pilot's flight history and the meteorological conditions, which were reported as calm with light winds. The wreckage analysis confirmed that the damage to the airframe and rotor blades was primarily caused by the impact with the ground.
Findings
- The primary cause of the accident was a loss of yaw control due to a reduction in reaction torque.
- A clutch slip likely caused a disconnection between the engine and the main gearbox, leading to a decrease in engine revolutions and a subsequent reduction in the torque required to counteract the main rotor's rotation.
- The pilot's attempt to correct the initial yaw with right rudder input may have inadvertently accelerated the rapid spinning motion.
- High centrifugal forces during the rapid rotation likely disoriented the pilot, making effective use of the rudder pedals difficult.
- The tail rotor drive shaft ruptured due to the impact, indicating that power was still being transmitted from the engine to the tail rotor at the time of the crash.