The pilot stated that he attempted to start the engine by "hand-propping." The engine started briefly and then stopped. The pilot added that he then primed the engine but "forgot the throttle in the full open position." He hand-propped the engine again, and it started. The airplane began to accelerate forward and became airborne. The airplane flew down a ravine and collided into trees. The pilot stated that the accident could have been prevented if the airplane had been tied down during the engine start.