On January 19, 1994, at 1740 eastern standard time, a British Aerospace ATP, N857AW, operated as United Express Flight 5112, experienced an Auxiliary Power Unit (APU) fire while on the ramp at the Michiana Airport, South Bend, Indiana. The airplane was being operated as a scheduled passenger flight under 14 CFR Part 121. An emergency evacuation was initiated following notification of the fire. The airplane was not damaged. The flight crew of 2, cabin crew of 2, and 29 passengers were not injured. The intended destination of the flight was Chicago, Illinois. The captain reported that while boarding the passengers an auto shutdown of the APU was experienced. He reported that he checked the APU system and all indications were normal so he performed the restart procedures. He stated that "someone" behind him then yelled, "The APU is on fire" at which time he activated the APU fire bottle. He stated he then left the cockpit to investigate the situation and the airplane was evacuated. The 'A' flight attendant reported hearing a "large pop & hiss..." while in the cabin. The flight attendant reported going into the cockpit to find out what the problem was. According to the flight attendant, the Captain stated there was a problem with the APU and he was going to investigate. The flight attendant continued to report, "When I was by the door and I heard people shouting there's a fire, there's a fire. So I turned to the Captain and asked if I should evacuate, and he said 'Immediately' so I evacuated through the forward exit." The 'B' flight attendant in the rear of the airplane reported she heard the noise and looked out the right aft door which was already opened. She stated she saw smoke to the left and people to the right yelling that there was a fire and to get everyone off the airplane. She stated she also heard a P/A announcement from the front to perform an emergency evacuation. She stated she checked for fire and not seeing any, opened the door and engaged the slide. All passengers were evacuated through the right rear door slide and the two forward exits without injury. Post accident inspection of the APU by Garrett determined, "The malfunction appears to have occurred because of hot gas corrosion of the turbine nozzle. There was a hole through the nozzle vane platform which caused thermo distress of the turbine wheel blades."