On March 21, 2008, at 2206 central daylight time, a Piper PA-34-200T, N66AH, was destroyed in an on-ground fire while taxiing at Covington Municipal Airport (M04), Covington, Tennessee. The certificated airline transport pilot was not injured. Night visual meteorological conditions prevailed, and no flight plan was filed for the personal flight, which was destined for Charles W. Baker Airport (2M8), Millington, Tennessee. The flight was conducted under 14 Code of Federal Regulations Part 91. During a telephone interview, the pilot stated that the purpose of the flight was to practice takeoffs and landings at night. The pilot departed his home base at 2M8 around 2230, flew to and landed at an intermediate airport, and then flew to and landed at M04. He was taxiing the airplane to the hold short line of runway 01, when he saw a flicker of light in his peripheral vision. He looked aft, and saw a flame behind the aft cabin seats. He immediately shut down both engines and exited the airplane. He looked into the cabin through the aft-most, co-pilot side window and saw flames reaching up to about the bottom of the window. He then retreated away from the airplane and telephoned for assistance. The airplane was subsequently consumed by the fire. The airplane was equipped with a combustion type heater, of the type originally installed in the airplane during manufacture, which was located in the area where the pilot reported seeing the fire and was operating at the time of the accident. The heater was forwarded to the Safety Board Materials Laboratory for further examination. According to the report prepared by the laboratory, the combustion heater was manufactured by Janitrol and was of the B-series type. The fire destroyed the identification tag containing the model and serial number information. The combustion heater was severely thermally damaged throughout the entire assembly. Examination of the heater did not identify any fire damage that could have been causal or associated with the initiation of a fire. The combustion heater was completely disassembled and each component was examined for signs of a failure that could have initiated the fire. The fuel drain line was found intact and unobstructed. The combustion chamber was intact with no visible cracks. The combustion air blower rotated freely and the brushes of the blower’s motor were intact. The ventilation air blower also rotated freely and its motor’s brushes were also intact. The combustion air pressure switch was found to have a cracked housing and the housing was partially open. The combustion air pressure switch had three connection terminals. The terminals with wires attached were the COMMON and NORMALLY OPEN. The NORMALLY CLOSED terminal was not being used. Conductivity was present between all the connection terminals. The fire destroyed the mechanism of the switch’s rocker arm, therefore movement of the rocker arm would not change the state of the switch. The pressure-sensing portion of the combustion air pressure switch was severely damaged by the fire and its pre fire condition could not be ascertained. The fuel pump that fed the combustion heater was not recovered and could not be examined. According to the pilot, the airplane’s maintenance records were onboard the airplane when the fire occurred, and were not recovered. He also stated that the airplane’s most recent annual inspection was completed on May 1, 2007. A review of Federal Aviation Administration aircraft registration data revealed that the pilot purchased the airplane on September 9, 2007.