On July 28, 1998, at 1500 hours Pacific daylight time, an Aero Commander 690, N349AC, sustained left wing spar damage following left wheel separation from its strut while taxing for takeoff at the Ely, Nevada, airport. The aircraft was operated as public-use by the Bureau of Land Management, U.S. Department of the Interior, for fire reconnaissance missions. Visual meteorological conditions prevailed and a company VFR flight plan was filed. The aircraft sustained substantial damage and the airline transport pilot and his observer were not injured. The flight was originating at the time for a local area fire-spotting mission. According to the pilot and observer's statements, they were taxing out for departure when the left main wheel came off its axle. As the strut grounded on the taxiway, rear wing spar and fuselage crown skin damage occurred. The aircraft was examined by investigators from the Office of Aircraft Services (OAS), U.S. Department of the Interior. They reported that the rear spar was fractured in the area of the wheel well. Left wing and fuselage crown skin was wrinkled and several wing ribs were damaged. The left wheel was found separated into its two halves and the tire had come off the wheel. All ten nuts were stripped off the bolts that hold the wheel halves together. The air pressure in the right tire was measured with a gage that only goes to 125 psi; the pressure in the tire pegged the gage at that value. According to the aircraft's flight manual, the maximum main landing gear tire pressure should be 60 psi. According to the OAS investigators, 1 week prior to the accident when the airplane was still in Fairbanks, Alaska, the pilot asked a mechanic to add air to the main tires because they looked low. The mechanic measured the pressure in the tires at 80 psi and informed the pilot that the tires were inflated correctly.