On June 25, 2017, about 1400 central daylight time, a Bellanca 17-30A airplane, N14728, was substantially damaged during a forced landing to a field 1/2 mile south of the Grove Municipal Airport (KGMJ), Grove, Oklahoma. The private pilot and passenger were not injured. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The flight had departed Bollinger-Crass Memorial Airport (MO5), Van Buren, Missouri, about 1230, and was en route to KGMJ. According to the written statement submitted by the pilot, while en route at an altitude of 8,500 ft mean sea level, he leaned the fuel mixture and the engine started running rough with a partial loss of power and a "miss-fire." The pilot enrichened the fuel mixture and switched fuel tanks, and the rough-running engine cleared up. The flight continued without incident until the pilot was maneuvering for landing. While in a right turn from the base leg to the final leg, the engine lost partial power. The pilot switched from the right fuel tank to the auxiliary fuel tank but was unable to restore all the engine power. According to the FAA inspector who responded to the accident, both the left and right wing spars were substantially damaged during the forced landing. A post-accident examination of the fuel tanks reveled 5.75 gallons of fuel in the right fuel tank and 15 gallons of fuel (full) in the auxiliary fuel tank. There was visible fuel in the left tank, but it was not measured. The airplane was recovered from the accident site for further examination and testing. According to the airframe and powerplant mechanic who assisted with the investigation, 5 gallons of fuel were drained from the right tank, 5 gallons of fuel were drained from the left tank, and the auxiliary fuel tank was full. There was no evidence of fuel leaks around either fuel cap and the fuel screens were free of contamination. Additional fuel was added to the fuel tanks, the engine started without hesitation, and ran for 25 minutes at varying power settings. After the engine warmed up to normal operating temperatures, the mechanic cycled through all three fuel tanks, spending no less than 5 minutes on each tank. A check of both magnetos and the mixture control revealed no anomalies. The loss of engine power and engine roughness could not be duplicated. The airplane was equipped with a 34-gallon left and a 34-gallon right main fuel tank and a 15-gallon auxiliary tank. According to both the placards on the fuel filler caps and the Pilot Operating Handbook, 4 gallons on each side was unusable.