On August 17, 2011, at 1542 central daylight time, a Piper PA-31-310 multiengine airplane, N46TB, was substantially damaged after an explosion and fire prior to takeoff at Perryton Ochiltree County Airport (PYX), Perryton, Texas. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan was filed. The pilot sustained minor injuries. At the time of the accident the airplane was departing for a flight to Shawnee Regional Airport (SNL), Shawnee, Oklahoma. After taxiing the airplane to the end of the runway the pilot performed pretakeoff cockpit checks with the engines running at 1,200 rpms. He reported there was an explosion under his seat that propelled his head into the ceiling and he then saw flames inside the airplane. With both engines still operating he immediately left his cockpit seat and the unpiloted airplane began moving as the pilot exited through the cabin door. The airplane was completely engulfed in flames when it finally stopped moving about 400 feet away. Firefighters arrived in about 10 minutes and quickly put out the fire. The airplane was substantially damaged by the fire. The pilot reported that he used either his airplane or his vehicle in his duties as a sales manager for a company which provides chemicals to oil and gas drilling and production operations in the area. He also reported that he was not transporting chemicals at the time of the accident. An examination of the wreckage did not disclose any preaccident malfunctions or anomalies that would have prevented normal operation. Investigators were unable to determine the source of the fire and explosion.