Overview
A F-100 commercial passenger airplane sustained an uncontained failure of its auxiliary power unit (APU) during de-icing procedures. The incident occurred while the aircraft was stationary on the ground. The APU's turbine wheel ruptured, sending fragments through the compressor case and into the cabin.
Incident Details
Approximately ten minutes after de-icing operations began, the captain reported hearing and feeling a thud-like noise from outside the aircraft. Simultaneously, the APU shut down. A de-icing crewmember on the ground observed a flame from the APU exhaust and activated the ground APU fire bottle discharge switch. At the time, winds were from 320 degrees at 20 knots, gusting to 27 knots, as recorded by the Automated Surface Observing Station for DFW at 0700. The five de-icing crew members were trained and experienced; they had coordinated to begin de-icing at opposite ends of the airplane to avoid spraying each other in the windy conditions, and were aware not to direct fluid into the APU inlet area.
Examination and Findings
Inspection of the APU revealed two large holes in the compressor case aligned with the plane of rotation of the turbine wheel. The turbine wheel was fractured into multiple pieces. One fragment of the turbine wheel was found embedded in a first aid kit mounted inside the cabin on the aft wall, directly in front of the aft pressure bulkhead. Metallurgical examination of the fractured pieces confirmed that the APU had experienced an overspeed, with no evidence of fatigue on any fracture surfaces. The APU electronic control unit (ECU) was removed and its non-volatile memory was interrogated; it indicated an overspeed event—defined as rotor speed of 107 percent or greater—and that a commanded shutdown occurred. Diagnostic testing of the ECU found no discrepancies.