At 1708 central time, an Aviat S-2C crashed following an aerobatic flight near Sheridan, killing the flight instructor and pilot. A witness reported observing the airplane perform aerobatic maneuvers adjacent to the accident site for approximately five minutes prior to the accident. He stated that the airplane climbed to approximately 1,500 – 2,000 feet above the ground in a pure vertical maneuver that ended in a tail slide. The airplane then fell backwards toward the ground for approximately 200 feet and began to spin inverted for approximately 6-10 turns.