Dynamic Roll Over

A helicopter is susceptible to a lateral rolling tendency, called dynamic rollover, when lifting off the surface.

Dynamic rollover begins when the helicopter starts to pivot around its skid or wheel. This can occur for a variety of reasons, including the failure to remove a tiedown or skid securing device, or if the skid or wheel contacts a fixed object while hovering sideward, or if the gear is stuck in ice, soft asphalt, or mud. Dynamic rollover may also occur if the pilot does not use the proper landing or takeoff technique or while performing slope operations. Whatever the cause, if the gear or skid becomes a pivot point, dynamic rollover is possible if the pilot does not use the proper corrective technique.

Once started, dynamic rollover cannot be stopped by application of opposite cyclic control alone. For example, if the left skid contacts an object and becomes the pivot point while the helicopter starts rolling to the left, even with full right cyclic applied the main rotor thrust vector and its moment follows the aircraft as it continues rolling to the left. Quickly applying down collective is the most effective way to stop dynamic rollover from developing. Dynamic rollover can occur in both skid and wheel equipped helicopters, and with all types of rotor systems.

To help avoid dynamic rollover:
1. Use extreme caution when hovering close to fences,sprinklers, bushes, runway/taxi lights, tiedown cables,deck nets, or other obstacles that could catch a skid or wheel. Aircraft parked on hot asphalt over night might find the landing gear sunk in and stuck as the ramp cooled during the evening.
2. Always use a two-step lift-off. Pull in just enough collective pitch control to be light on the skids or landing wheels and feel for equilibrium, then gently lift the helicopter into the air.
3. Hover high enough to have adequate skid or landing wheel clearance with any obstacles when practicing hovering maneuvers close to the ground, especially when practicing sideways or rearward flight.


Source: https://en.wikipedia.org/wiki/Dynamic_rollover

FAA Publication Helicopter Flying Handbook Chapter 11