On final approach approximately 10 miles from the airport, a fire alarm was triggered in the aft cargo bay of an aircraft. The flight crew immediately initiated checklist procedures, activated fire extinguishers, and declared an emergency. The fire indicator was detected some 20-30 seconds after activation, but a slight smell of smoke was noticed by the cabin crew and flight crew. Flight 116 landed and stopped on the runway to allow airport firefighters to inspect the aircraft for fire. Firefighters, utilizing infrared sensing equipment, did not detect any sign of fire. The flight crew taxied the aircraft to the terminal but stopped approximately 40 feet back from the gate to allow firefighters to open the aft cargo compartment for a detailed inspection. When the cargo door was opened, a significant amount of smoke was observed. Firefighters entered the cargo compartment and confirmed that the fire had been extinguished. TSB investigators discovered that an intense but relatively small fire had occurred, causing significant structural damage in the floor area of the aft cargo compartment. Before it was extinguished, the fire had progressed approximately 18 inches up the right side wall of the aircraft, outside the aft cargo compartment. The cargo bay fire-extinguishing system, a Halon-based system, effectively suppressed the fire before it could spread further. The fire appears to have been a direct result of an electrical failure of a heater tape used to prevent water lines from freezing. The electrical failure of the heater ribbon ignited the covering of the insulation blankets installed below the open cargo floor and some debris found in the area. The B110 heater ribbon attached to the water supply line failed at the site of a recent water line repair, allowing the elements of the heater ribbon to electrically arc, providing a source of ignition to surrounding materials.On final approach approximately 10 miles from the airport, a fire alarm was triggered in the aft cargo bay of an aircraft. The flight crew immediately initiated checklist procedures, activated fire extinguishers, and declared an emergency. The fire indicator was detected some 20-30 seconds after activation, but a slight smell of smoke was noticed by the cabin crew and flight crew. Flight 116 landed and stopped on the runway to allow airport firefighters to inspect the aircraft for fire. Firefighters, utilizing infrared sensing equipment, did not detect any sign of fire. The flight crew taxied the aircraft to the terminal but stopped approximately 40 feet back from the gate to allow firefighters to open the aft cargo compartment for a detailed inspection. When the cargo door was opened, a significant amount of smoke was observed. Firefighters entered the cargo compartment and confirmed that the fire had been extinguished. TSB investigators discovered that an intense but relatively small fire had occurred, causing significant structural damage in the floor area of the aft cargo compartment. Before it was extinguished, the fire had progressed approximately 18 inches up the right side wall of the aircraft, outside the aft cargo compartment. The cargo bay fire-extinguishing system, a Halon-based system, effectively suppressed the fire before it could spread further. The fire appears to have been a direct result of an electrical failure of a heater tape used to prevent water lines from freezing. The electrical failure of the heater ribbon ignited the covering of the insulation blankets installed below the open cargo floor and some debris found in the area. The B110 heater ribbon attached to the water supply line failed at the site of a recent water line repair, allowing the elements of the heater ribbon to electrically arc, providing a source of ignition to surrounding materials.