On May 14, 2014, about 1400 Alaska daylight time, a Piper PA 28-181 airplane, N30541, was involved in a ground fire at the South Naknek Airport, South Naknek, Alaska. The commercial pilot and two passengers had evacuated the airplane prior to the fire, and were not injured. The airplane was being operated by King Air, Inc., Naknek, Alaska, as a visual flight rules (VFR) non-scheduled commuter flight under the provisions of 14 Code of Federal Regulations Part 135 when the incident occurred. Day, visual meteorological conditions prevailed, and a company flight plan was filed for the two mile flight to Naknek. In a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot stated that he was taxiing out for departure when he smelled smoke in the cabin. He verified with a passenger that indeed there was a smoke smell in the cabin, and then shut the airplane's engine down, and evacuated the airplane. After evacuating the airplane and escorting the passengers to a safe location, he saw smoke coming from the engine cowling area, and attempted to put out any fire with the airplane's fire extinguisher. His attempt to put out the fire was unsuccessful, and the fire continued to burn a majority of the airplane's fuselage. The airplane was equipped with a Lycoming O-360-A4M carburated, reciprocating engine. The engine was examined on scene by a Federal Aviation Administration safety inspector from the Anchorage Flight Standards District Office. Due to the extent of the fire damage, the inspector was unable to determine the initiation point of the fire. One of the recovered fuel fittings was sent to the NTSB Materials Laboratory for examination, and the examination revealed that the fracture surfaces were consistent with an overstress fracture while the fitting was at elevated temperatures. No indications of preexisting cracking were noted.