settling with power
a flight condition in which helicopter main rotor lift is suddenly lost even though it is powered and loss of control results.
Skybrary (2013), (electronic repository of safety data related to ATM and aviation safety) URL: http://www.skybrary.aero/index.php/Vortex_Ring
The cause of what might be termed secondary vortex ring is the recycling of the normal main rotor tip vortices into the induced airflow of inner blade sections of the rotor. This condition is possible when the rate of descent of a helicopter is greater than half the air speed induced by the rotor and results in airflow of the inner blade sections changing from downward relative to the rotor disk to upwards. The boundary along the rotor disc at which the upward motion of the inner blade sections meets the normal downward motion of the outer blade sections produces the severe airflow instability of the ‘vortex ring’ and the resultant aerodynamic inefficiencies. The vortex ring condition is an unstable one. If it is allowed to continue, loss of control in rough air conditions will occur as the ring moves outwards along the blade span until there is no lift at all. (same source)