HISTORY OF FLIGHTOn April 21, 2012, at 1717 Pacific daylight time, N3862C, a Beechcraft Corporation A36TC, registered to Hermiston Aviation Inc. and operated by the pilot collided with terrain short of the runway at Eastern Oregon Regional Airport, Pendleton, Oregon. The commercial pilot sustained serious injuries and the two passengers were not injured. The airplane was substantially damaged. The pilot was operating the airplane under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed. According to emergency response personnel, the pilot had departed Pendleton and radioed to the air traffic control tower controller that one of the airplane's fuel caps was not secured. The pilot was returning to the airport when the airplane impacted terrain short of runway 16. The pilot stated that the airplane had been refueled prior to the flight. It was a warm day with the tank full of fuel and the heat resulted in fuel draining from the vent. The pilot loosened the fuel cap to aid in ridding the tank of the excess fuel. Upon departure, the left wing fuel cap departed from the fuel tank. As the pilot returned to land, he did not maintain an appropriate glide path during final approach and the airplane collided with terrain on the approach end of runway 16. The pilot indicated there were no mechanical malfunctions or failures during the flight, and the engine continued to operate until impact with the terrain. Post accident examination of the airplane revealed that the pilot's shoulder harness separated during the accident sequence resulting in serious injuries. The shoulder harness was dated September of 1980 and was installed when the airplane was manufactured. ADDITIONAL INFORMATIONThe NTSB's safety alert "Check Your Restraints" notes the importance of restraint systems in preventing injuries and provides resources for inspecting restraint systems for wear and damage. The safety alert can be found at http://www.ntsb.gov/doclib/safetyalerts/SA_027.pdf. TESTS AND RESEARCHThe pilot's shoulder harness and lap belt were examined by a metallurgist from the NTSB Materials Laboratory. The separation point on the shoulder harness belt showed that some filaments contained mushroomed ends, consistent with a relatively high loading rate. Under such conditions, the loading of the filament produces heat which softens the filament and causes it to fracture. The shoulder harness was also examined using a Fourier Transform Infrared (FTIR) spectrometer. The test was indicative of negligible photodegradation of the webbing. Additionally, the restraint system was examined at Beechcraft with a representative from the Federal Aviation Administration. Sections of the shoulder harness were tensile tested and exceeded the proof load strength of 1,666 pounds, with an ultimate static load strength of 2,500 pounds.