On May 29, 2003, about 1900 Alaska daylight time, a Cessna 320 airplane, N5712X, sustained substantial damage when the left main landing gear collapsed during the landing roll at the McCarthy NR2 Airport, McCarthy, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country business flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by Wards Aero Service, Anchorage, Alaska. The private certificated pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. The flight originated at Merrill Field, Anchorage, about 1750. No flight plan was filed, nor was one required. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on May 30, the pilot reported that he was flying to McCarthy as part of his aircraft maintenance business. He said he landed on runway 01, and during the landing roll, the left main gear suddenly collapsed. The airplane received structural damage to the underside of the fuselage. The pilot indicated that the airplane is equipped with one landing gear position light. He said prior to landing, he observed the right main landing gear and the nose gear position via a mirror positioned on the right engine nacelle, and they appeared to be extended. The landing gear position light was green. The pilot noted the landing gear drive mechanism consists of an actuator rod, driven by an electric motor. In another telephone conversation with an NTSB investigator on July 27, 2004, the pilot said that the airplane's landing gear had been subjected to side loading during the flight just prior to the accident flight, and he suspected the left main actuator rod, or supporting structure, may have been damaged. At the time of this report, the pilot, an aviation mechanic, had not had an opportunity to exam the landing gear mechanism to see if he could determine why the landing gear collapsed.