On September 15, 2017, at 1615 central daylight time, a Beech A36 airplane, N18403, collided with a hay bale during a forced landing following a loss of engine power while on approach to the McAlester Regional Airport (MLC), McAlester, Oklahoma. The private pilot received minor injuries. The airplane was substantially damaged. The airplane was registered to Travel Air LLC and was being operated by the pilot as a 14 Code of Federal Regulations Part 91 personal flight. Visual flight rules conditions existed near the accident site at the time of the accident. The pilot had canceled his instrument flight rules flight plan when he had MLC in sight. The flight departed from the Garner file Airport (UVA), Uvalde, Texas, at 1357.The pilot reported he purchased fuel prior to takeoff and that both main fuel tanks and both tip tanks were full. The pilot stated he selected the right fuel tank for takeoff and used a stop watch to switch the fuel tanks every 25 minutes during the flight. About 8 minutes after takeoff, while climbing through 5,000 ft, the pilot noticed fuel leaking from the left fuel tank filler cap for a short period. He continued the flight which was uneventful until he prepared to enter a downwind leg at MLC which was when the engine lost power. The pilot switched fuel tanks and attempted to restart the engine to no avail. The pilot stated he touched down hard during the off airport forced landing. Both main landing gear were pushed up through the wings and the right wing contacted a hay bale. The left wing remained attached to the fuselage. The right tip tank was separated from the right wing which remained connected to the fuselage by the control cables and wiring. The fuel system was compromised due to impact damage. A postaccident examination of the wreckage was conducted on October 24, 2017. Both wings were detached from the airframe and were stored outside. Both fuel caps were removed, inspected, and found to have cracks in the main O-ring seal. The left fuel cap was tested in place, by applying air pressure to the fuel outlet line and plugging the vent system and tip tank fuel inlet. A soap and water solution placed around the cap bubbled up indicating an air leak. Air could be felt escaping from around the fuel cap outer seal and the center seal. The right wing was not tested due to impact damage. The aircraft logbook shows that the fuel cap O-rings were replaced with new O-rings during the annual inspection on April 1, 2016. The engine, which was not damaged, had been converted from an IO-520 per STC SE10746SC. A borescope was used to examine the piston domes, cylinder walls, and valve faces which exhibited normal wear and combustion deposits. The propeller was damaged during the accident. The extent of the damage precluded using the accident propeller during the engine run. The propeller was removed, and slave propeller was installed on the airplane. The top spark plugs appeared to be normal in wear and color. The plugs were then reinstalled in the engine. The airframe was secured on a flatbed trailer. An alternate fuel system was rigged to the left fuel wing fuel inlet line at the fuselage. The fuel line going into the JPI fuel flow transducer was disconnected, power was applied to the airplane and the fuel system boost pump was turned on until positive fuel flow at the disconnected line was verified. The line was then reconnected. The engine started on the first attempt and was idled between 600 – 700 rpm to confirm oil pressure and fuel flow. The rpm was increased to 1,000 rpm. The engine power was then increased to 1,500 rpm and a magneto check was performed. The rpm drop for each magneto during the check was 60 to 80 rpm. The engine run was limited to 1,500 rpm due to the security of the fuselage on the trailer. The above process was repeated with an alternate fuel supply rigged to the right-wing fuel inlet line. The engine run results were the same. With the engine running 1,500 rpm, the fuel selector was switched to the off position. The engine ran for about 25 seconds prior to it shutting down. Examination of the engine revealed the engine started and ran smoothly as tested up to 1,500 rpm. The airplane was equipped with a JPI FS-450 which monitored fuel flow, fuel used, and fuel remaining. The unit was removed from the airplane and shipped to the National Transportation Vehicle Recorder Division for examination. The device stores the last remaining record of fuel used and fuel remaining in gallons. It does not show which fuel tank contains the remaining fuel. A power supply was attached to the unit and the unit display showed 45.4 gallons of fuel used, and 66.6 gallons of fuel remaining. These amounts do not take into consideration the unknown amount of fuel that leaked from the left fuel tank filler cap. The pilot reported that at 1524, both fuel tanks were indicating about ¾ full.