Vortex ring state

The vortex ring state, also known as settling with power, is a dangerous condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor causing severe loss of lift. The FAA sees these terms as synonymous, whereas Transport Canada sees them as two different phenomena.

Essentially, the helicopter descends into its own downwash. When the condition arises, increasing the rotor power merely feeds the vortex motion without generating additional lift.

This condition also occurs with tiltrotors, and was responsible for an accident involving a V-22 Osprey. 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.

In forward flight, there is no upward flow (upflow) of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow (air pulled or "induced" down through the rotor system) of the inner blade sections is overcome and the blades begin to stall near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system. The inner set of vortices decreases the amount of lift being produced and causes an increase in sink rate. In an accelerated condition, the inner and outer vortices begin to feed each other to the point where any increase in rotor blade pitch angle increases the interaction between the vortices and increases the rate of descent. In this state, the helicopter is operating in its own downwash, descending through descending air. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and impact with terrain.

...
Wikipedia