On March 12, 2005, about 1730 Pacific standard time, a Maule M-7-235C, N5509M, sustained substantial damage to the wing assembly following a main gear collapse during landing near Graham, Washington. The airplane is owned by the pilot, and was being was being operated as a cross-county flight under the provisions of Title 14, CFR 91, when the accident occurred. The commercial pilot, the sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight originated from Deer Park, Washington, with a planned destination of Vashon Island, Washington. During a telephone conversation with the NTSB IIC on March 22, the pilot reported that the airplane developed engine problems during the cross-country flight and he elected to make a precautionary landing to a private airstrip near Graham, Washington. The pilot stated the landing touchdown was normal, but during the rollout he heard a "loud pop sound from under the seat" 4-5 seconds before the right main gear collapsed. Post accident examination of the airplane’s landing gear, by representatives from the Federal Aviation Administration (FAA) and the operator, revealed that the center gear attach bolt and corresponding locking nut for the right main landing gear leg was missing. The bolt was later recovered, however, the locking nut was not found. The bolt was intact and nominal wear was noted. There was no evidence of thread failure or significant mechanical damage. The operator reported that the absence of the nut and bolt allowed the landing gear leg to separate from its corresponding attach assembly. It was not determined why the missing nut separated from the bolt and landing gear assembly. The operator provided no information as to the cause of the rough running engine.