A dynamic rollover is a catastrophic event where rotor thrust pulls the helicopter sideways around a pivot point, such as from catching a skid on an object.
There are three conditions needed for a dynamic rollover: lift, pivot point, and sideways movement. The cyclic has only so much ability to tilt, usually 5-8 degrees. Should the helicopter landing gear/skid become caught on an object, that skid may become a pivot point for the helicopter to tip or roll over. Once airborne, the pendular action of a helicopter makes it quite easy for the angle to exceed the cyclic’s compensation capability.
Past this point, the rotor thrust compounds the problem as the thrust basically pulls the helicopter around the pivot point. The risk of a dynamic rollover is one of the reasons a pilot must be extremely vigilant whenever there is any sideways movement of the helicopter. Obstacles such as taxi lights, aircraft tugs, and signs can pose significant problems at airports. When performing off airport landings, having a landing gear stuck under a tree or hooked on a foreign object is a serious concern. A pilot should ensure the aircraft is free of any obstacles before ascending.
Slope operations are another area of flight where the possibility of a dynamic rollover exists. When ascending or moving laterally, the upslope skid is a potential pivot point. If too much collective is applied before obtaining a level flight attitude, a dynamic rollover may occur. Should the collective be lowered too aggressively, the helicopter motion may initiate a roll towards the downslope skid. Due to translating tendency, the conditions for a dynamic rollover are increased with a right skid/landing gear touching. In addition, a crosswind from the left, left yaw inputs, and a right center of gravity also increase the potential for a dynamic rollover.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-12
Principles of Helicopter Flight, 2nd Edition, pg. 165
FM 3-04.203-2007 Fundamentals of Flight pg. 1-63