On March 5, 1996, about 1510 central standard time, a Bellanca 8GCBC, N86928, registered to and operated by Aerial Ads, Inc., of the Southeast, landed hard then nosed over at a private airstrip called the Buchanan STOLPORT, Panama City Beach, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 banner towing flight. The airplane was substantially damaged and the commercial-rated pilot-in-command (PIC) and commercial-rated trainee were not injured. The flight originated about 1405, from the Buchanan STOL PORT. The PIC was in the rear seat and was training the front seat pilot regarding banner towing operations. The front seat pilot who was flying the airplane reported that gusty wind conditions existed and after dropping the banner, he turned base to final with the flaps fully extended. After a three point landing during the landing roll, a gust of wind occurred and the airplane then became airborne. He applied only partial power and the airplane landed hard causing the right main landing gear wheel/axle assembly to separate. The airplane then nosed over and came to rest inverted. The PIC stated that there was no preimpact failure or malfunction of the engine or flight controls. Postaccident inspection of the right main landing gear axle attach hardware by an FAA inspector revealed that the threads of all four of the AN365-524 nuts were stripped. The nuts attach to bolts that secure the axle to the gear leg.