On June 5, 2004, about 1600 Pacific daylight time, a Schweizer 269C-1, N9421K, collided with terrain and rolled over near Brentwood, California. Helicopter Adventures, Inc., was operating the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and the student pilot were not injured; the helicopter sustained substantial damage. The local instructional flight departed Buchanan Field Airport, Concord, California, about 1500. Visual meteorological conditions prevailed, and a company visual flight rules flight plan had been filed. The wreckage was at 37 degrees 53 minutes north latitude and 121 degrees 45 minutes west longitude. In a written statement, the operator of the helicopter stated that the CFI had planned to take the student pilot to the Silva Ranch practice area, about 15 miles east of Concord. During the en route portion of the flight, the student practiced maneuvering the helicopter in straight and level flight. Upon arriving at the practice area (a large flat valley), the CFI instructed the student on hover techniques by demonstrating the use of control inputs. The student practiced maneuvering the helicopter via the use of a sole control input, starting with pedals, then collective, followed by cyclic. While the student was manipulating the cyclic, the CFI noted that the friction setting on the collective had been engaged, and he attempted to disengage it. As the CFI was focusing his attention on the friction setting for the collective, the student made a large forward cyclic input. The CFI input aft cyclic and increased the collective in an effort to counteract the student's abrupt maneuver and maintain ground separation. The helicopter impacted the ground and rolled over. In a written statement, the CFI reported that, while maneuvering about 3 to 4 feet above ground level, he noticed that the collective was difficult to adjust. He glanced down at the collective and realized that the collective friction had been activated. (According to the Schweitzer Pilot's Operating Handbook, the friction control is activated by adjustment of a lever.) While the CFI attempted to adjust the friction setting, the student made a large forward cyclic input. The helicopter responded in an extreme nose down position and began to move forward. The helicopter impacted terrain and rolled over the nose, coming to rest on the right side. The CFI reported no preimpact mechanical malfunctions or failures with the helicopter.