A vortex ring state is when the helicopter’s downwash recirculates into the induced flow and the helicopter descends while under power.
A vortex ring state is a very dangerous situation but can be avoided. The condition occurs when the vortices from the blade tips recirculate into the induced flow of the rotor. For this to occur, there are several conditions that must be present:
1) The helicopter must be under power, generating lift
2) The helicopter must be descending at least 300 feet per minute
3) The helicopter must be below effective translational lift (ETL)
If these conditions are present, a vortex ring state could develop. As such, a pilot should be very careful to avoid these conditions. For example, when conducting a steep approach to a confined area, ensure not to descend more rapidly than 300 feet per minute.
A vortex ring state is very dangerous as the descent rate that can approach 6,000 feet per minute. Because the aircraft is descending, many pilots want to increase collective to stop the decent, but this only increases the problem as the increased collective increases the induced flow. The correct response is to lower collective and move the helicopter out of the downwash. If altitude permits, an autorotation can be conducted. Without power, the vortex ring state cannot occur.*
Some may refer to a vortex ring state as settling with power. However, the two should not be considered synonymous. Although the helicopter descends (or settles) while under power when in a vortex ring state, settling with power can occur under other circumstances. Anytime the power demands for the flight conditions exceed the power available, the helicopter will descend, resulting in settling with power. For clarity, it is best to use the term vortex ring state when referring to the aerodynamic phenomenon where the vortices are recirculated into the rotor system. When people refer to a vortex ring state, they generally are referring to the main rotor system. However, a vortex ring state can also occur with the tail rotor, potentially leading to loss of tail rotor effectiveness.
* The Vuichard Recovery technique is a new method for recovery from settling with power the requires increasing collective, left pedal, and right cyclic.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-9
Principles of Helicopter Flight, 2nd Edition, pg. 157
FM 3-04.203-2007 Fundamentals of Flight pg. 1-61