The NTSB determined Tuesday that the probable cause of the Dec. 5, 2005 landing overrun of a Southwest Airlines Boeing 737 at Chicago Midway Airport was the pilots' failure to use reverse thrust to slow the airplane after landing. This occurred because the pilots' lack of experience with the airplane's autobrake system distracted them from using the thrust reversers, the Safety Board said. After it overran the end of Runway 31C, the airliner penetrated blast and perimeter fences before coming to rest on a road, where it struck an automobile. One automobile occupant was killed, one was seriously injured, and three received minor injuries. Eighteen passengers of the 98 aboard the jet received only minor injuries, but the airplane was substantially damaged. According to the NTSB, contributing factors were Southwest Airlines' failure to clarify policies regarding landing distance calculations and the design of its on-board performance computer. Also contributing to the severity of the accident was the absence of an arresting system at the end of 6,522-foot Runway 31C, which offers only 5,826 feet of available landing distance.