Settling with Power, also known as Vortex Ring State , is a hazardous condition of helicopter flight where the helicopter settles (descends) in spite of being in a high power setting. Three conditions are required for it to occur: (1) a descent rate of at least 300 fpm (feet per minute), 2) an airspeed slower than Effective Translational Lift, and 3) 20% or greater power applied. Since all of these conditions must be present, removing any one of them escapes the condition. Whether or not this condition actually occurs will be dependent upon a number of other influences such as gross weight, winds, etc.
When the above conditions are present, the blades begin to stall (airflow separation) near the hub. A second set of vortices, similar to the rotor tip vortices, then form in the center of rotor system, decreasing lift and accelerating sink rate. This is exacerbated by the lack of airfoil surface in the mast and blade grip area which can be from a couple of feet in size on small helicopters to several feet on larger helicopters. In forward flight, normal airflow patterns prevent an upflow in the hub area; however, when settling begins, an upflow forms. If not immediately corrected, vortices can develop in the blade root (mast) area—lift is lost because there is less rotor disk area producing lift. If not corrected, a vicious cycle of decreasing lift and increasing sink rate grows. In an accelerated condition, the inner and outer vortices actually begin to feed each other. If the pilot increases rotor lift (raises collective and increases power) in an attempt to reduce descent rate, the vortices will actually increase in size and settling will worsen.
Signs of settling with power are a vibration in the main rotor system followed by an increasing sink rate . Failure to recognize and react to the condition can allow high descent rates and impact with terrain, a frequently fatal accident.
The corrective action for Settling with Power is to apply forward cyclic to fly out of the condition and/or lowering collective pitch . Flying forward alone will eliminate the condition however reducing the power (lowering collective) will decrease the size of the vortices and consequently reduce the amount of time required to be free of the condition. Since the condition often occurs near the ground, lowering the collective may not be an option. A loss of altitude will occur while flying free of the condition which will be proportional to the rate of descent developed before beginning the recovery. A pilot must always know when the conditions conducive to settling with power are or will be present, and his/her personal limitations where corrective action must be applied.
It should also be noted that in a tandem rotor helicopter, forward cyclic will not arrest the rate of descent caused by settling with power. In such a helicopter, which utilizes differential collective pitch in order to gain airspeed, latteral cyclic inputs must be made accompanied by pedal inputs in order to slide horizontally out of the vortex ring state's disturbed air.
,  Basic Helicopter Handbook, FAA AC 51-13B, U.S. Department of Transportation, Federal Aviation Administration. 1978