On April 8, 2017, about 0750 central daylight time, a Grumman G-164A agricultural airplane, N971QC, conducted a forced landing near Nome, Texas. The pilot was not injured and the airplane was destroyed by fire during the accident. The airplane was registered to and operated by Twin County Air-AG, Inc. under the provisions of 14 Code of Federal Regulations Part 137 flight. Visual meteorological conditions prevailed at the time and no flight plan had been filed. The pilot reported that he was conducting spray runs, when he noticed a burnt wire smell, he then noticed a low oil pressure indication. He partly opened the cabin door and noticed flames. The smoke and fire increased. Due to smoke in the cockpit, he had difficulty in seeing, but was able to find the engine fuel shut-off valve and closed it. The smoke cleared enough for him to select a hay field for a forced landing. The responding Federal Aviation Administration (FAA) inspector noted that after landing, the fire consumed the majority of the airplane. A review of the airplane records on file with the FAA, revealed that the airplane's original radial engine was replaced by a Walter M601E-11 turboprop engine; the Walter engine was subsequently replaced by Pratt & Whitney turboprop PT6A-27 engine on February 1, 2010. The FAA inspector reported that the shop who owned the supplemental type certificate (STC) for the Pratt & Whitney did not perform, nor authorize the engine installation. He added the STC installation required special routing on the engine connections, including a 5-year replacement on the fuel lines. Investigators did not receive any records indicating the fuel lines were replaced in accordance with the STC. He also noted that the airplane's battery had recently been replaced. The fire damage prevented a detailed inspection of the engine installation.