Researchers have hypothesized that a helicopter pilot's semicircular canal (SCC) system can become degraded due to normal in-flight activity and this may lead to accidents in low visibility conditions. This report describes an investigation of the effects of SCC suppression on a helicopter pilot's ability to maintain control in a yaw disturbance rejection task under 3 different visual settings of varying quality. For each visual setting, 4 motion levels were used to emulate the suppressed vestibular system by scaling the physical motion cue given to trained non-pilot subjects.Statistical analyses of the experimental data indicated that scaling down the physical yaw rate by 15 percent had no significant effect on pilots' performance; and the motion level only became significant when it was scaled down by 50 percent or more. It was also found that there was a significant interaction between the visual and motion level settings used.