List one reason why a helicopter may experience "vortex ring state."

A helicopter may experience "vortex ring state" primarily due to descending rapidly with insufficient forward airspeed. This condition occurs when the helicopter descends through its own downwash, creating a phenomenon where the induced airflow above the rotor disc begins to rotate in a way that reduces lift, leading to a loss of control and difficulty in recovering.

In this state, the helicopter's descent rate can increase significantly, and the pilot may find it challenging to regain controlled flight without proper corrective actions, such as increasing forward airspeed. The dynamics of rotor aerodynamics play a significant role, and when combined with rapid descent, the helicopter essentially becomes trapped in its own vortex, hence the name "vortex ring state."

Other factors such as rapid altitude changes or turning do not directly lead to forming a vortex ring state as significantly as descending with insufficient forward airspeed does.