On November 27, 2010, at 1030 Pacific standard time, an American Champion 7KCAB tailwheel equipped airplane, N5101X, sustained substantial damage to the fuselage and left wing assembly following a main gear collapse during landing at the Rialto Municipal Airport, Rialto, California. The certified flight instructor (CFI) and private pilot receiving tail wheel instruction were not injured. The airplane was operated by Foothill Flying Club as a visual flight rules (VFR) instructional flight under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed for the instructional flight that originated from Upland, California, at 0930. In a written report to the National Transportation Safety Board (NTSB) investigator-in-charge, the CFI stated that during the wheel landing, with the private pilot manipulating the flight controls, the airplane's left main landing gear collapsed. He stated that the airplane skidded to the left, and contacted a taxiway sign before it came to rest. The instructor reported that the wind was calm during the timeframe of the accident. The fractured landing gear leg was shipped to the NTSB Materials Laboratory in Washington, D.C. for further examination. An NTSB senior metallurgist reported that examination of the left landing gear leg revealed that it fractured at the inboard end, approximately where the spring passes through the fillet plate and seal. The inboard fracture face had a dark crescent-shaped fracture on the aft edge. The smooth dark appearance, the presence of a crack arrest mark, and the presence of ratchet marks of this crescent-shaped region is consistent with the features of fatigue cracking from multiple origins. The remainder of the fracture face exhibited a coarse, grainy appearance, clear chevron markings originating adjacent to the fatigue crack, and a shear lip. All features were found consistent with overstress fracture. The paint on the underside of the spring had been worn away in the same region as the fracture, where the support wedges contact the surface of the spring. Corrosion damage is also present on the worn surfaces.