If the bank angle starts to increase to an angle of approximately 5 to 8°, and full corrective cyclic does not reduce the angle, the collective should be reduced to diminish the unstable rolling condition.

During slope operations, excessive application of cyclic control into the slope, together with excessive collective pitch control, can result in the downslope skid rising sufficiently to exceed lateral cyclic control limits, and an upslope rolling motion can occur. [Figure 11-7] When performing slope takeoff and landing maneuvers, follow the published procedures and keep the roll rates small. Slowly raise the downslope skid or wheel to bring the helicopter level, and then lift off. During landing, first touch down on the upslope skid or wheel, then slowly lower the downslope skid or wheel using combined movements of cyclic and collective. If the helicopter rolls approximately 5 to 8° to the upslope side, decrease collective to correct the bank angle and return to level attitude, then start the landing procedure again.

The collective is more effective in controlling the rolling motion than lateral cyclic, because it reduces the main rotor thrust (lift). A smooth, moderate collective reduction, at a rate less than approximately full up to full down in two seconds, is adequate to stop the rolling motion.

Take care, however, not to dump collective at too high a rate, as this may cause a main rotor blade to strike the fuselage. Additionally, if the helicopter is on a slope and the roll starts to the upslope side, reducing collective too fast may create a high roll rate in the opposite direction.

When the upslope skid/wheel hits the ground, the dynamics of the motion can cause the helicopter to bounce off the upslope skid/wheel, and the inertia can cause the helicopter to roll about the downslope ground contact point and over on its side. [Figure 11-8]

The collective should not be pulled suddenly to get airborne, as a large and abrupt rolling moment in the opposite direction could occur. Excessive application of collective can result in the upslope skid rising sufficiently to exceed lateral cyclic control limits. This movement may be uncontrollable. If the helicopter develops a roll rate with one skid/wheel on the ground, the helicopter can roll over on its side.

The following lists several areas to help you avoid dynamic rollover.

For cyclic control, small helicopters depend primarily on tilting the main rotor thrust vector to produce control moments about the aircraft center of gravity (CG), causing the helicopter to roll or pitch in the desired direction. Pushing the cyclic control forward abruptly from either straight-and-level flight or after a climb can put the helicopter into a low G (weightless) flight condition. In forward flight, when a push-over is performed, the angle of attack and thrust of the rotor is reduced, causing a low G or weightless flight condition. During the low G condition, the lateral cyclic has little, if any, effect because the rotor thrust has been reduced. Also, in a counter-clockwise rotor system (a clockwise system would be the reverse), there is no main rotor thrust component to the left to counteract the tail rotor thrust to the right, and since the tail rotor is above the CG, the tail rotor thrust causes the helicopter to roll rapidly to the right, If you attempt to stop the right roll by applying full left cyclic before regaining main rotor thrust, the rotor can exceed its flapping limits and cause structural failure of the rotor shaft due to mast bumping, or it may allow a blade to contact the airframe. [Figure 11-9]

Since a low G condition could have disastrous results, the best way to prevent it from happening is to avoid the conditions where it might occur. This means avoiding turbulence as much as possible. If you do encounter turbulence, slow your forward airspeed and make small control inputs. If turbulence becomes excessive, consider making a precautionary landing. To help prevent turbulence induced inputs, make sure your cyclic arm is properly supported. One way to accomplish this is to brace your arm against your leg. Even if you are not in turbulent conditions, you should avoid abrupt movement of the cyclic and collective.

If you do find yourself in a low G condition, which can be recognized by a feeling of weightlessness and an uncontrolled roll to the right, you should immediately and smoothly apply aft cyclic. Do not attempt to correct the rolling action with lateral cyclic. By applying aft cyclic, you will load the rotor system, which in turn produces thrust. Once thrust is restored, left cyclic control becomes effective, and you can roll the helicopter to a level attitude.

As mentioned earlier, low rotor r.p.m. during an autorotation might result in a less than successful maneuver. However, if you let rotor r.p.m. decay to the point where all the rotor blades stall, the result is usually fatal, especially when it occurs at altitude. The danger of low rotor r.p.m. and blade stall is greatest in small helicopters with low blade inertia. It can occur in a number of ways, such as simply rolling the throttle the wrong way, pulling more collective pitch than power available, or when operating at a high density altitude.

When the rotor r.p.m. drops, the blades try to maintain the same amount of lift by increasing pitch. As the pitch increases, drag increases, which requires more power to keep the blades turning at the proper r.p.m.