What happened
While the F-100 was stationary during de-icing procedures at DFW, the aircraft's auxiliary power unit (APU) suffered an uncontained rupture. Approximately ten minutes after the de-icing process began, the captain reported hearing and feeling a thud-like noise originating from outside the aircraft, coinciding with an automatic APU shutdown. A member of the ground de-icing crew observed a flame exiting the APU exhaust and subsequently activated the ground APU fire bottle discharge switch.
At the time of the event, weather observations from the Automated Surface Observing Station at DFW reported winds from 320 degrees at 20 knots, gusting to 27 knots. The de-icing crew noted similar windy conditions. To prevent spray interference caused by the wind, the truck operators had coordinated to begin de-icing from opposite ends of the aircraft.
The investigation
Mechanical examination of the APU revealed two large holes in the compressor case, aligned with the plane of rotation of the turbine wheel. The turbine wheel had broken into several pieces, and one fragment was found embedded in a first aid kit mounted on the aft cabin wall near the aft pressure bulkhead.
Metallurgical analysis of the fractured turbine wheel fragments confirmed that the unit had experienced an overspeed. No evidence of fatigue was found on the fracture surfaces. Investigators also interrogated the non-volatile memory of the APU's electronic control unit (ECU). The data indicated that the APU had reached a rotor speed of 107 percent or greater, which triggered a commanded shutdown. A diagnostic test of the ECU revealed no functional discrepancies.