On December 11, 2003, at 2115 eastern standard time, a Piper PA-23-250, N40259, operated by Northeast Aviation Inc., was substantially damaged by fire at the Buffalo Niagara International Airport (BUF), Buffalo, New York. The certificated airline transport pilot was not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the flight which originated at the Morristown Municipal Airport (MMU), Morristown, New Jersey. The on-demand cargo flight was conducted under 14 CFR Part 135. According to the pilot, after landing at BUF, while taxiing to the ramp, he smelled a burning smell coming from the airplane. He checked the cockpit gauges, and noticed no abnormalities. As the pilot pulled into the ramp area, he noticed an orange glow at his feet, and notified FBO personnel over the radio that he had a fire in the airplane. FBO personnel brought hand-held fire extinguishers to the airplane, and attempted to extinguish the fire until airport crash fire rescue arrived. A Federal Aviation Administration (FAA) inspector examined the airplane after the accident. According to the inspector, the fire burnt a hole through the fuselage skin in the area of the Janitrol heater, just aft of the left wing root. The entire area forward of the Janitrol heater, including the wiring and plumbing of the heater, as well as the cockpit and cabin area was consumed by fire. The pilot stated that his normal procedure for using the Janitrol heater was to turn it off at the same time he extended the landing gear while on approach. He stated he followed the same procedure during the accident flight. The pilot reported 2,800 hours of total flight experience, 500 of which were in make and model. According to the Piper PA-23-250 Pilot's Operating Handbook (POH), Description and Operations section: "...The three positions of the cabin heat control switch are 'HEAT,' 'FAN,' and 'OFF'...It is advisable to place the switch in the fan position for several minutes after the heater has been operating to allow the unit to cool down before it is turned off." The Normal Procedures section of the POH required that the switch be turned to the "FAN" position while completing the Before and After Landing checklists. The Shutdown checklist called for the switch to be switched to the "OFF" position. The expanded Normal Procedures section, BEFORE LANDING, stated: "...If the heater has been in operation, turn the heater control switch to 'FAN' to allow unit time to cool down." The AFTER LANDING section stated: "...If the heater has been in operation, check that the heater control switch is in the 'FAN' position. The heater should never be turned 'OFF' unless it has had time to cool down." The SHUT DOWN section stated: "...If the heater has been used and has been allowed to cool down, turn it to 'OFF'."