On October 3, 2000, at 1430 hours Pacific daylight time, an Aerospatiale AS350BA, N189ND, made a hard landing after experiencing a loss of directional control at Gillespie Field Airport, El Cajon, California. Vortex Helicopters, LLC, operated the helicopter under the provisions of 14 CFR Part 91. The helicopter sustained substantial damage when the tail boom contacted the ground. The airline transport pilot and one passenger were not injured. Visual meteorological conditions prevailed for the positioning flight, and no flight plan had been filed. The helicopter had been returned from another operator after 15 hours of training flights in which a series of full autorotations to the ground were conducted. The purpose of the accident flight was to reposition the helicopter from the ground onto a landing cart for storage. While over the landing cart, the pilot experienced difficulty maintaining directional control. He suspected an antitorque failure and initiated an autorotation. The pilot stated at the time the landing cart was partially beneath the helicopter. The pilot maneuvered away from the cart in order to land on the ground and prevent a partial landing on the cart. As he increased power to maneuver away from the cart, there was a further loss of directional control. After clearing the landing cart, the subsequent hard landing resulted in the tail boom contacting the ground. In an interview with a Federal Aviation Administration (FAA) inspector, the pilot stated that while he was attempting to land on the cart, he saw something fly off the helicopter prior to losing directional control. During the on-scene investigation conducted by the FAA inspector, the aft tail rotor drive shaft coupling was found remotely located from the main wreckage. The FAA inspector noted that three bolts retain the coupling. One bolt was found attached to the coupling with no nut. He stated that there was no evidence of the nut having stripped the bolt. The second bolt was found on the ground with the threads intact. He was not able to find the third bolt or the other two nuts. The FAA inspector reviewed the maintenance logbooks and noted the University of North Dakota, Grand Forks, North Dakota, performed the last annual on March 9, 2000. The University of North Dakota performed a track and balance inspection on the tail rotor system about 48 hours prior to the accident. According to maintenance instructions provided by the manufacturer, the balancing is accomplished by placing washers under the nuts used to secure the bolts that attach to the aft drive shaft coupling. These were the same nuts that were not located at the scene of the accident. A caution to the maintenance instructions specifies that the bolts in this area should be checked for proper torque and safety after an inspection. The FAA San Diego, California, Flight Standards District Office reported the accident to the Safety Board on October 12, 2000.