Heir to US retail giant Wal-Mart and one of the world's richest men, John Walton, has died in a light plane crash. The accident happened in Walton's home state of Wyoming. He was flying in a homemade, experimental ultralight craft, which crashed shortly after taking off from Jackson Hole Airport in Grand Teton National Park, said the Wal-Mart company in a statement. Walton, 58, was the pilot and sole occupant of the aircraft, and was pronounced dead at the scene. The cause of the accident is not yet known.