On July 2, 1997, about 0830 central daylight time, a Grumman G-164, N10273, registered to a private owner, operating as a 14 CFR Part 91 positioning flight, experienced a failure of the left main landing gear on taxi and nosed down on the ground. Visual meteorological conditions prevailed and no flight plan was filed. The airplane was consumed by a postcrash fire. The commercial pilot reported no injuries. The flight was originating at the time of the accident. The pilot stated he was taxing out to conduct an engine run-up before departing on a positioning flight to pick up a load of chemical before departing on an aerial application flight. As he started to turn around the left main gear separated. The airplane came to rest on the nose and left wing. Fuel started leaking out of the fuel tank and the airplane was consumed by fire. A portion of the left main landing gear spring was forwarded to the NTSB Materials Laboratory for analysis. A fracture was located about 2 to 3 inches inboard of the fuselage skin. A visual magnified examination of the fracture revealed a small preexisting fatigue crack emanating from the lower surface of the spring about 3.5 inches from the leading edge. Closer examination established that the fatigue area was made up of four fatigue cracks, two of the larger had joined to form a single front. See Metallurgist's Factual Report No. 97-134. The left main landing gear spring was released to Mr. Donald T. Johnson, on August 18 , 1997, and signed for by A. Golden, on August 20, 1997.