What happened
While an Fokker 100 was being deiced at YUL, the aircraft's auxiliary power unit (APU) ingested a mixture of snow and deicing fluid. This ingestion resulted in an uncontained turbine failure. The event caused minor damage to the airplane's pressure bulkhead. There were 40 passengers and 4 crew on board, all of whom were not injured.
At the time of the incident, weather conditions at YUL included winds from 050 degrees at 25 knots gusting to 33 knots, visibility of 15 statute miles, and drifting snow. The sky was broken at 2,800 feet and overcast at 7,600 feet, with a temperature of -1 degree C and a dew point of -5 degrees C.
The investigation
A teardown inspection of the Honeywell APU revealed evidence within the Electronic Control Unit (ECU) non-volatile memory of an engine overspeed event accompanied by a commanded auto-shutdown.
Investigation records noted that in February 2001, American Airlines had issued a 'Winterization Bulletin' for the F-100, which advised against spraying de-ice fluid into the APU inlet. Furthermore, the Fokker 100 maintenance manual (section 12-31-00, page 301) specifically warns that de-icing or anti-icing fluid and water mixtures should not be allowed to enter the APU inlet due to the risk of equipment damage or personal injury.