On March 6, 2001, about 0640 eastern standard time, a Fokker F28 MK 0100, N1426A, operated as American Airlines flight #581 from Montreal/Dorval International (YUL), near Montreal, Quebec, Canada to Chicago O'Hare International Airport (ORD), Chicago, Illinois, piloted by an airline transport rated captain and copilot, sustained minor bulkhead damage while standing for deicing at YUL. The auxiliary power unit (APU) ingested deicing fluid and snow and sustained an uncontained turbine failure. The scheduled international 14 CFR Part 121 passenger flight was operating on an IFR flight plan. Visual meteorological conditions prevailed. The 2 flight crewmembers, 2 cabin crewmembers, and 40 passengers were uninjured. The flight was being deiced in preparation for departure to ORD at the time of the incident.

The operator stated, "On March 6, 2001, N2BE suffered a APU failure during deicing at Montreal/Dorval International Airport. The weather was cold with the temperature at -2C, and blowing snow. The APU sustained a uncontained turbine failure after ingesting snow, ice, and deicing fluid. The aircraft also sustained damage to the pressure bulkhead after the APU failure."

At 0700, the YUL weather was: Wind 050 degrees at 25 knots gusting to 33 knots; visibility 15 statute miles; present weather drifting snow; sky condition broken 2,800 feet overcast 7,600 feet; temperature -1 degree C; dew point -5 degrees C; altimeter 29.57 inches of mercury.

The APU was removed from the airplane, shipped to, and stored in a bonding room at the Honeywell Services Repair and Overhaul Facility, Phoenix, Arizona. On April 10, 2001, Federal Aviation Administration inspectors oversaw the teardown and inspection of the APU. Excerpts of Honeywell’s inspection report stated the following:

"In February 2001 American Airlines issued a 'Winterization Bulletin' for the F-100 advising that de-ice spray should not be sprayed into the APU inlet. Additionally, the Fokker 100 maintenance manual section 12-31-00, page 301specifically advises 'Do not let de-icing and/or anti-icing fluid/water mixture go into the APU inlet. Injury to persons and/or damage to equipment can occur.' ...

Evidence of an engine overspeed event with a commanded auto-shutdown was present in the Electronic Control Unit (ECU) non-volatile memory.

An engine overspeed condition is a known phenomenon when de-ice fluid is ingested. Since de-ice fluid is flammable when compressed, it will add to the combustion process of the engine. If enough is ingested, it will sustain combustion even though there is no metered fuel flow from the engine fuel control. Because the engine fuel control no longer has command of the engine speed, it cannot limit the preset maximum speed to its 107 percent limit. With the de-ice fluid maintaining combustion, the engine speed will be uncontrolled and can accelerate to the burst limit of the rotating components. This acceleration can occur extremely quickly because of the low inertia of the APU rotating components."

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