A condition in helicopters in which when the rotor RPM decreases, the pilot is unable to restore the thrust even by pulling up the collective. The more the pilot raises the collective, the lower is the rotor thrust. This condition is more likely to occur when the helicopter commences hovering or during the hover itself. It may also occur when approaching a high-altitude landing site if the power required to hover is not available. Under these conditions, if the rotor RPM decays and the collective is pulled up, it will result in the total reaction tilting away from the axis of rotation and thus worsening the situation. Also, decaying RPM causes the coning angle to increase, resulting in a reduced thrust and a smaller component for overcoming the helicopter’s weight. The rotor RPM may reach a stage where it is so low that even full power is unable to restore the situation. The solution lies in lowering the collective to increase the rotor RPM.
Greater coning angle produces insufficient component force to oppose gross weight if rotor thrust is constant.

Aviation dictionary. 2014.

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