Incident Overview
An official investigation report details a dynamic rollover occurrence involving a Robinson R-44 helicopter, registration RA-06338. The report, issued by the Interstate Aviation Committee, describes the mechanics of dynamic rollover and the recommended recovery procedure.
Dynamic Rollover Mechanics
According to the report, dynamic rollover may initiate when the landing gear contacts a stationary or secured object. This contact forces the helicopter to rotate around the point of contact rather than its own center of gravity. Any obstacle or surface that prevents the skid from sliding sideways can serve as the pivot point. Once initiated, dynamic rollover cannot be arrested solely by applying cyclic control in the opposite direction of rotation. For example, if the right skid contacts an object and becomes the pivot point while the helicopter begins to roll (rotate about the longitudinal axis) to the right, even full left cyclic input will not stop the roll. The thrust vector of the main rotor still passes to the right of the pivot point and produces an additional rightward moment, rather than a leftward one.
Prevention and Recovery
The report references the Robinson R-44 Pilot's Operating Handbook (POH), Section 10, Safety Notice SN-9, which states that the most effective method to prevent the development of dynamic rollover is to reduce collective pitch. Rapidly lowering the collective pitch lever is described as the most effective way to stop the event.
Visual Illustrations
The report includes diagrams (Figures 8 and 9) illustrating the development of dynamic rollover. Figure 8 shows a schematic of the forces involved: lift, rotor thrust, horizontal component of thrust, and rolling moment. Figure 9 depicts a similar case of dynamic rollover in actual flight.
Conclusion
The investigation report provides factual information on the dynamics of the incident but does not assign a probable cause. The analysis focuses on the physical principles of dynamic rollover and the prescribed corrective action per the aircraft's flight manual.