The pilot in command and aircraft owner was restricted to flying with a co-pilot because of his age (79). The pilots were carrying out practice for a spot-landing competition to take place later that day. The pilot reported that his first approach was lower than normal and he had to apply power in an attempt to reach the runway threshold. Although the wind was blowing directly down the runway, at about 5 kt, there was some turbulence and he believed there was a downdraft on late, final approach. The pilot was concentrating his attention on touching down on the threshold markers, as practice for landing on his selected spot, when the co-pilot called "up, up". Immediately after the call, the aircraft collided with the top rail of a cattle holding yard, crashed to the ground, slid across a road and through the airfield boundary fence and onto the runway, where it came to a stop. The major factor in the accident was that the pilot was concentrating all his attention on achieving his selected touchdown point (tunnel vision) and was not situationally aware in that he was not observing the other required parameters, height above the ground, glide-path angle and obstacle clearance, as he allowed the aircraft to descend well below a safe glidepath. It is possible that a downdraft may have increases the rate of descent in the last seconds of flight.