On September 30, 1996, at 1130 central daylight time, a Piper PA-28R-201, N6008H, operated by D&A Aviation, sustained substantial damage during a hard landing in Chesterfield, Missouri. The commercial rated flight instructor and commercial rated dual student reported no injuries. The 14 CFR Part 91 instructional flight was operating in visual meteorological conditions. No flight plan was on file. The local flight departed Chesterfield, Missouri, about 1015. The flight instructor reported that the student was training to renew his flight instructor certificate. During the second landing, the student was five to ten knots slow on final approach. The instructor made a verbal recommendation. During a telephone interview, the instructor reported that he might have said "airspeed." Two or three seconds before touchdown, the instructor put his hands on the yolk and "assisted" the student. The instructor reported that the landing was a "hard landing.... There was no indication after this landing that the airplane was not safe for another circuit and we proceeded to do another takeoff and another landing." After the plane was parked, he observed dented skin on the left wing. The Federal Aviation Administration Inspector who examined the airplane reported that the left wing was substantially damaged.