What happened
Following a landing at Detroit, an APU fire occurred shortly after the aircraft crew attempted to start the auxiliary power unit (APU). Prior to the flight, the aircraft had undergone deicing procedures at its departure point using a mixture of glycol and water.
The investigation
The investigation determined that the aircraft's APU exhaust outlet is located on the upper surface of the right wing, near the fuselage. This positioning allows deicing fluid to enter the exhaust opening if directed toward it during the deicing process. While company policy required the flight crew to shut off the APU during deicing operations, residual fluid remained in the system.
Upon starting the APU after landing, the heat from the engine ignited the residual glycol that had been trapped in the combustion chamber and exhaust stack. Once the water component of the deicing mixture evaporated, the highly flammable nature of the glycol led to the fire in the exhaust stack.