On April 30, 1999, at 0917 hours Pacific daylight time, a Schweizer 269C-1 helicopter, N6148V, impacted the ground and severed the tail boom during a practice autorotation landing at the Concord, California, airport. The helicopter, operated by Helicopter Adventures, Inc., Concord, sustained substantial damage. The commercial licensed flight instructor and student pilot were not injured. The local instructional flight was conducted under 14 CFR Part 91 and originated at the Concord airport about 0830. Visual meteorological conditions prevailed and no flight plan was filed. The flight instructor reported that he and the student were practicing straight-in autorotations with power recoveries. During the first four autorotations, the student was the primary manipulator of the controls. The instructor reported that the student was initiating the flare too high, so he told the student that they would do the next approach together with both of them on the controls. During the flare portion of the autorotation, the stinger struck the runway surface. The tail rotor then contacted the surface and the helicopter began to yaw to the right. The instructor rolled the throttle off after about 90 degrees of right yaw and entered a hovering autorotation. The helicopter ultimately yawed 360 degrees to the right and the landing gear skids contacted the ground with the aircraft coming to rest approximately 100 feet from the point where the tail rotor struck the runway. The main rotor severed the tail boom. The flight instructor stated that he had intended for the student to initiate the flare, and he planned just to follow through on the controls to guide him to flare a little lower than on the previous attempts. He reported that he believes that the student interpreted his instructions to mean that he (the instructor) would be making the primary inputs. The flight instructor reported that the accident could have been prevented through "better communication between student and instructor as to who is responsible for initiating a control input during all phases of flight and especially during critical phases."