On September 7, 1999, at 1130 central daylight time, a Hughes OH6A, N584SD, collided with the ground while on approach to a helipad in Hernando, Mississippi. The helicopter was operated by the Desoto County Sheriff's Department under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The helicopter sustained substantial damage. The private pilot was not injured. The public use flight departed Hernando, Mississippi, at 1110. The pilot, a volunteer with the sheriff's department, was conducting proficiency training when the accident occurred. According to the pilot, he had completed three approaches and was on the fourth approach when he encountered a dust devil along the approach path. After encountering the dust devil, several feet above the ground, the pilot reported that the helicopter yawed to the right. In an effort to recover from the yaw condition, the pilot instantly rolled off the throttle control, and the helicopter collided with the ground. No mechanical problems were reported with the helicopter by the pilot. The post-accident examination of the helicopter also failed to disclose a mechanical problem.