Bombardier has completed modifications to the horizontal stabiliser trim control units (HSTCU) of all 366 Challenger 604 series business jets to the satisfaction of the Transportation Safety Board of Canada.

This action, signed off in December 2007, was carried out in response to recommendations by the UK Air Accident Investigation Branch in a serious incident report published in January this year describing an 11 November 2005 event in which the horizontal stabiliser pitch trim system failed.

The crew landed the aircraft safely at London Heathrow but, because of the position in which the stabiliser trim failed, the pilots had to struggle with heavy manual control forces on the elevators.

Meanwhile, Bombardier has been concerned that the publication of the AAIB report after the approved system modification had been incorporated across the entire Challenger fleet has confused operators.

The AAIB report identified the reason for the HSTCU failure as the ingress of moisture into the unit, where it caused short-circuiting in the two separate printed circuit boards that contained the independent channels controlling the horizontal stabiliser in its pitch trim function.

The AAIB said this was one of several examples of moisture ingress into electronic control units in several aircraft types which had resulted in multi-channel common-cause failures, and it has recommended that the European Aviation Safety Agency should advise system designers of the need to prevent moisture contamination of electronic controls, and that EASA should work with other regulators, particularly the US Federal Aviation Administration and the TSB, to this end.

Bombardier's director of customer support engineering, David Field, says the TSB-approved fix for the Challengers' HSTCU was to apply a moisture-proof conformal coating to the units' printed circuit boards, and that this has satisfied the AAIB recommendations. He says EASA has not approached Bombardier with any further requirements.

Source: Flight International