Accident to the Bell 47G-2 registered F-BIFN on 12/07/2023 at Arles

At the end of a helicopter spraying sequence over a rice field, the pilot flew to the supply area to refill the helicopter with fertiliser. Flying forward at a speed of around 40 to 50 kt, with an intake pressure of 21 in and at a height of approximately 20 m, the helicopter suddenly began to yaw to the left. Initially, the angular velocity was relatively slow, but as soon as the pilot made an input on the right rudder pedal, the helicopter suddenly began to spin very quickly to the left and to lose height. The pilot was unable to maintain yaw control of the helicopter, which ended up in a partially submerged rice field after making at least five
full spins.

The rupture in the tail rotor drive indicates that the MGB was delivering energy when the T/R made contact with the ground. The motion transferred from the engine to the MGB and rotor drives may have been effective, although the examinations carried out were unable to quantify its power.

When the rotor receives engine power, it generates a reaction torque which tends to make the helicopter spin in the opposite direction, i.e. to the right in the case of the Bell 47 . To counter this effect, the T/R produces an opposite force applied to the aft end of the fuselage. In hover flight, the pilot therefore applies pressure on the left rudder pedal to stabilise the helicopter in yaw. In the event of an engine power reduction in forward flight, if the pilot maintains the rudder pedals’ position, the helicopter will begin to spin more or less rapidly to the left about the yaw axis, depending on the extent of the reduction.

The pilot reported that he felt a sudden yaw movement to the left, described as “a short sharp shock”, while flying in a straight line at a stable altitude, approximately two rotations before the accident. He did not report any other similar events during the 25 spraying sequences that preceded the accident. Apart from a discolouration of the clutch drum, nothing unusual was observed on the helicopter prior to the collision with the ground. A clutch slip may cause disconnection between the engine and the main gearbox. As the rotor is no longer receiving full power from the engine, the reaction torque decreases and the helicopter begins to yaw to the left. A clutch slip may result from a substantial reduction in engine revolutions, combined with an increase in the main rotor collective pitch.

In rapid yawing rotation situations, pilots may be subjected to high centrifugal forces, which may disorientate them and lead to a loss of control. In this event, when the helicopter yawed rapidly to the left, the pilot was centrifuged to the left, making it difficult to use the right rudder pedal. Using the rudder when the helicopter started to yaw would have made it possible to slow down and stop the rotation.