HISTORY OF FLIGHT On July 22, 2011, at 1225 mountain daylight time, a Cessna 210-5, N8427Z, sustained substantial damage following a loss of engine power and subsequent forced landing near Darby, Montana. The airplane was registered to the pilot and operated under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot and passenger sustained minor injuries. The flight departed Lemhi County Airport, Salmon, Idaho, at 1155, with a planned destination of Chamberlin, Idaho. Visual meteorological conditions (VMC) prevailed and no flight plan was filed for the cross-country flight. The pilot reported that the airplane was in a cruise flight configuration when the engine began to run rough and eventually lost power. The pilot maneuvered the airplane towards a nearby open valley and gravel road. The pilot positioned the airplane to land downwind to the road and prepared for a short/soft field landing. He reported that the airplane touched down on the gravel road about 200 feet beyond a cattle guard and began to skid. The pilot reduced braking pressure and crossed a second cattle guard. During the landing rollout, the airplane's left wing impacted a tree and the airplane pivoted approximately 90-degrees to the left before it came to an abrupt stop. The airplane sustained substantial damage to the left wing and forward fuselage. AIRCRAFT INFORMATION The accident airplane (serial number 205-0427) was manufactured in 1963. The single engine airplane was equipped with a Continental IO-470 S engine and constant-speed propeller manufactured by McCauley. The most recent annual inspection of the airplane, engine, and propeller was completed on January 9, 2011. At the time of the accident, the airplane had accumulated 7,343 total hours and the engine had accumulated 475 total hours since new. The airplane had accumulated approximately 56 hours between the annual inspection and the accident. TESTS AND RESEARCH The NTSB conducted a postaccident examination of the engine and airframe after the airplane was transported to a storage facility in Phoenix, Arizona. The examination revealed a dark residue that covered the left side of the engine accessory section (aft) and associated baffling and hoses. The residue was concentrated in the area of the outboard engine air intake duct assembly, just aft of the bracket air filter. The air filter, bracket assembly and aluminum air intake duct were removed. A large hole (approximately 4.25 inches by 1.75 inches) and material loss was noted on the bottom side of the duct. The circumference of the hole was an irregular shape with both jagged and smooth edges/fracture surfaces. A dark residue was noted to the face of the fracture surfaces. A weld repair was noted adjacent to the hole. A section of fuel pump baffle tubing was routed adjacent to the hole in the outboard intake duct. Rub marks and dark residue was noted along the baffle tubing, concentrated in the area of the hole in the duct. Material loss and deformation was noted to the baffle tubing in the area adjacent to the hole in the intake duct. Internal borescope examination of the piston cylinders revealed mechanical etching to the No. 4 and No. 6 combustion chambers and associated valve heads. The corresponding spark plugs were removed and mechanical damage was noted. The ground electrodes were bent down onto the insulators and center electrodes. The examination of the airframe and engine revealed no additional findings or anomalies. Additional examination notes are located in the public docket associated with this case file. A senior NTSB metallurgist examined the engine air intake duct and reported that the fractures were consistent with fatigue and possibly the result of vibration. Although there was evidence that a weld repair had been performed near this area, it was not determined if it contributed to the hole and loss of material. A review of the airplane’s maintenance records showed no recent modifications or repairs to the engine air intake duct.