Aviation Safety Letter 1/2003

Blown Auxiliary Power Unit (APU) During De-Icing Illustrates Need to Brush-up on Ground Icing Program

On March 6, 2001, in Dorval, Quebec, a F28 aircraft was being de-iced in strong, gusty wind conditions with engines and the APU running.

De-Icing

Despite all precautions taken by the crew applying the fluids, some fluid entered the APU inlet. The fluid passed through the compressor and entered the combustion chamber as hot, compressed additional fuel that had not been processed by the APU's fuel control unit. The APU reacted to the extra fuel as it was designed to do - more fuel, more fire, faster rotation - by auto-accelerating, the design limits were exceeded and a rotorburst occurred. The aircraft sustained substantial damage: the centrifugal compressor turbine was broken in five distinctive sections, with extensive blade/aerofoil damage and shaft failure. The containment ring failed to retain the turbine components and turbine debris punctured the firewall shroud mostly towards the ground and punctured the aircraft skin.

Air operators and service providers should ensure that all personnel involved in the application of de-icing/anti-icing fluids are aware of this incident. Additional precautions must be taken when strong winds make control of fluid application difficult, and consideration should be given to asking the flight crew to shut down the APU if there is any doubt that fluid cannot be prevented from entering the APU inlet.

Safety action taken as a result includes the release on November 28, 2001, of Commercial and Business Aviation Advisory Circular (CBAAC) 0194, titled Aircraft Ground Icing Update. This circular was re-issued as CBAAC 194R on September 20, 2002, to include holdover time tables for winter 2002-2003. This comprehensive circular is intended to inform air operator personnel of recent developments and issues pertaining to aircraft ground icing operations. The following statement is found in CBAAC 0194R:

Lesson Learned - Anti-icing Fluid Causes APU Rotorburst 

Air Carrier Advisory Circular 072R, issued on January 20, 1997, contains the following statement regarding the danger of spraying de-icing/anti-icing fluids into the inlet of an APU: "Particular care should be exercised for the APU inlet, because fluid ingestion could cause an APU runway condition or, in an extreme case, an APU rotorburst."

The validity of this statement was proven with the F28 incident described above, and also as a result of a similar incident to another F28 in the United States on March 2, 2002. We encourage you to review both TC circulars 0072R and 0194R, at http://www.tc.gc.ca/eng/civilaviation/standards/commerce-circulars-menu-284.htm

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