Vortex ring state

In this condition, the rotor falls into a new topological state of the surrounding flow field, induced by its own downwash, and suddenly loses lift.

Vortex ring state caused the loss of a heavily modified MH-60 helicopter during Operation Neptune Spear, the 2011 raid in which Osama bin Laden was killed.

As forward airspeed decreases and vertical descent rates increase, an upflow begins simply because there are no airfoil surfaces in the area of the hub, mast and blade-grip assembly.

[1] A helicopter normally encounters this condition when attempting to hover out-of-ground-effect (OGE) without maintaining precise altitude control, and while making downwind or steep, powered approaches when the airspeed is below Effective Translational Lift (ETL).

[7] The signs of VRS are a vibration in the main rotor system[8] followed by an increasing sink rate and possibly a decrease of cyclic authority.

In some cases, vortex ring state is encountered and allowed to advance to the point that the pilot may severely lose cyclic authority due to the disrupted airflow.

[citation needed] Radio controlled multirotors (common on drones) are subject to normal rotorcraft aerodynamics, including vortex ring state.

Multirotors that do not have altitude hold are also more likely to succumb to operator error, where the pilot drops the craft too fast resulting in the upwash at the rotor hubs that can lead to vortex ring state.