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AAIB issues Special Bulletin on Challenger pitch trim system contamination
16 December 2005

The U.K. AAIB reports that on November 11 a Challenger 604 crew experienced cautions and malfunctions concerning the stabiliser trim system. Nose-up trim commands resulted in almost full nose-down trim and the crew diverted to London Heathrow for a successful flapless landing. Later examination showed contamination within the Horizontal Stabiliser Trim Control Unit (HSTCU) where the two channels of trim command are physically close, with no mechanical back-up. There appear to have been previous occurrences with a different HSTCU on this aircraft. An AAIB Safety Recommendation is made to Transport Canada relating to the effect of the contamination on the pitch trim system. (AAIB)