Vortex ring state

The vortex ring state is a flight condition that occurs in helicopters and other rotorcraft when the rotor blades encounter a disturbance in airflow, typically happening when the helicopter descends vertically or hovers in its own downwash. In this state, the rotor system operates in an inefficient way, leading to a loss of lift and control. When a helicopter hovers or descends slowly, the rotor blades generate a downwash of air, which creates a column of descending air beneath the rotor.