An emergency Airworthiness Directive issued by Transport Canada yesterday calls for further inspection of Bombardier Q400 main landing gear (MLG) after line checks uncovered two cases of cam mechanism failure in the gear's alternate extension system (AES). The cam mechanism operates the cable to open the main landing gear door and releases the MLG uplock in sequence. In cases where an operator must use the alternate extension system to deploy the MLG, the failure of the cam mechanism on one side will lead to an asymmetrical landing configuration, said the AD.

Preliminary investigation revealed that the cam mechanism might have failed and remained dormant after a previous AES operation. The mechanism might not have fully returned to the normal position, in which case normal powered landing gear operation could introduce enough load to fracture the cam mechanism or rupture the door release cable.

The directive calls for inspection and “corrective action,” if necessary, within 50 hours flying time or 10 calendar days from the AD’s January 17 effective date. EASA today issued the same directive and the FAA will likely follow suit.

Specifically, the AD refers operators to a Bombardier repair drawing (RD), issued January 14, for inspection procedures, or a later revision approved by Transport Canada. If the cam mechanism is reset to the normal rested position without any sticking or binding, operators need only to repeat the inspection every 50 flight hours or 10 calendar days. If the cam mechanism has not reset to its normal rested position, or if any evidence of sticking or binding exists, operators must remove or repair the assembly as indicated by the AD. The AD directs operators to contact Bombardier if they find they cannot accomplish the repair of it the cam mechanism proves damaged or inoperative.

Last August Qantas removed from service five of the 21 Bombardier Q400s operated by regional affiliate QantasLink after the airline found a defect in what it described as a main landing-gear component. A Bombardier spokesman told AIN that the “issue” then in question involved a fitting in the rear spar nacelle area, not the landing gear itself. A Bombardier Service Bulletin issued in April and again in July called for visual inspection and tests of the area.

Neither the Qantas action nor this most recent AD appears related to the landing-gear difficulties that grounded SAS Commuter’s Q400 fleet in 2007. That year SAS removed from service all 27 of its Q400s, including five at its Wideroe subsidiary, after defects related to their landing gear led to three landing mishaps within two months. Investigators attributed the last accident, which occurred on Oct. 27, 2007, as an SAS Q400’s right main landing gear failed to extend fully upon landing at Copenhagen International Airport, to a loose o-ring that blocked an orifice within the gear’s actuator assembly. Authorities blamed each of the first two accidents, in Aalborg, Denmark, on September 9 and in Vilnius, Lithuania, on September 12, on corrosion inside the landing gear’s actuator piston that caused it to separate from its rod end.

SAS said it found a defect in a solenoid valve in a majority of its 27 airplanes that could explain the October 27 incident.

Bombardier compensated SAS with $163.5 million in cash and credit for future aircraft purchases. As part of the agreement SAS later placed a new firm order for 14 Q400s and 13 CRJ900s, along with an option for a further 24 aircraft.