CEN18LA158

The flight instructor and pilot receiving instruction were maneuvering to begin practice hovering when the helicopter experienced a lateral vibration. The instructor assumed control of the helicopter and attempted to land; however, the helicopter yawed to the left and both the instructor and pilot applied full right pedal to align the helicopter's nose with its ground track. Shortly thereafter, they heard the low rotor rpm horn and saw the associated warning light illuminate. The helicopter continued to the left, and as they lowered to the ground, the right skid caught on the grass and the helicopter experienced a dynamic rollover, resulting in substantial damage to the main rotor blades and fuselage. Examination of the airframe and engine revealed no preimpact mechanical malfunctions or failures that would have precluded normal operation, and the helicopter had adequate fuel onboard for the flight. Although the weather conditions at the time of the accident were conducive to the formation of carburetor icing at glide and cruise power, the helicopter was equipped with a carburetor heat assist device, which automatically applied or reduced carburetor heat with changes in collective input. The reason for the partial loss of engine power could not be determined based on the available information.

On May 4, 2018, about 1155 central daylight time, a Robinson R44 helicopter, N444VH, was substantially damaged when it was involved in an accident near East Troy, Wisconsin. The flight instructor and pilot receiving instruction were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 training flight for Part 137 aerial application operations. The flight instructor directed the pilot to hover-taxi the helicopter to the grass runway to practice hovering. The pilot reported that once over the grass runway the helicopter experienced a lateral vibration which the instructor described as loud. The instructor took the flight controls to land. The instructor stated that the helicopter was about 5 to 7 feet above the ground, and as it started to settle, it yawed to the left 30° to 40°. The instructor and pilot both added full right pedal to align the nose of the helicopter with their ground track after the left yaw. About 2 feet above the ground, the low rotor rpm warning system light illuminated, and the associated horn sounded. The instructor raised the collective to “cushion the landing,” but the helicopter continued to the left and the right skid caught the ground. The helicopter entered a dynamic rollover and came to rest on its right side. The instructor reported that he had been completing refresher training for the “spray pilots” for about 2.5 hours before the accident flight. Neither the pilot nor the instructor reported using the carburetor heat during the flight. The helicopter sustained substantial damage to the main rotor system and fuselage. Examination of the airframe and engine revealed no preimpact mechanical malfunctions or failures that would have precluded normal operation. The instructor reported that the helicopter had an adequate amount of fuel onboard to complete the flight. The helicopter was equipped with a carburetor heat assist device, which automatically correlated application of carburetor heat with changes in collective setting to reduce pilot workload; lowering collective applied carburetor heat and raising collective reduced carburetor heat. The temperature and dewpoint at the time of the accident were conducive to the formation of serious carburetor icing at cruise power settings, per the Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35, Carburetor Icing Prevention.