Contamination of an electrical circuit affecting operation of the horizontal stabiliser trim control system of a Bombardier Challenger 604 led to a “serious incident” in November in which the crew had difficulty controlling the aircraft, according to a special bulletin issued by the UK Air Accidents Investigation Branch (AAIB).

The AAIB has recommended that Transport Canada require Bombardier to eliminate the risk of contamination in the Challenger 604 and other aircraft with similar trim systems. Bombardier says it is investigating the incident, and examining other units removed from this aircraft after two previous pitch trim system malfunctions. The rest of the Challenger fleet is not affected, the manufacturer says.

The Challenger (VP-BJM) was en route from Lagos, Nigeria to Farnborough, UK on 11 November when the incident occurred. At flight level 400 (40,000ft/12,200m) the crew received an autopilot pitch trim caution. Some 30min later the stabiliser trim and Mach trim cautions illuminated. Trim modes were temporarily re-engaged by the crew, but shortly afterwards the cautions illuminated again.

While descending towards Farnborough, the AAIB says, several further attempts at re-engagement resulted in disconnection of the autopilot.

Application of nose-up stabiliser trim commands resulted in nose-down trimming of the horizontal stabiliser and the crew elected not to attempt further trim engagements. The autopilot was re-engaged, but almost full nose-down trim had been applied and could not be corrected.

Concerned at the physical effort required to fly the aircraft manually, the crew elected to divert to London Heathrow, where they made a successful flapless landing “achieved by the co-ordinated efforts of the commander and co-pilot operating the flight controls and an off-duty pilot operating the thrust levers”, says the bulletin.

Examination of the horizontal stabiliser trim control unit revealed contamination of a circuit board, causing shorting of the pins for the unit’s external connectors. The source of the contamination is still being investigated. The AAIB notes Bombardier issued a service letter in May 2003 telling operators to put tape over the top and sides of the unit after fluid contamination was found inside.


Source: Flight International