Transport Canada issued an emergency airworthiness directive, ordering of the DHC-8-400 MLG alternative extension system.
Transport Canada reported that two cases of main landing gear (MLG) alternate extension system (AES) cam mechanism failure were found during line checks.
The cam mechanism operates the cable to open the MLG door and releases the MLG uplock in sequence. In the case where it is necessary to deploy the MLG using the AES, the failure of the MLG AES cam mechanism on one side will lead to an unsafe asymmetrical landing configuration. Preliminary investigation indicates that the cam mechanism failure may have occurred and remained dormant after a previous AES operation. The cam mechanism may not have fully returned to the normal rested position. With the cam mechanism out of normal rested position, normal powered landing gear door operation could introduce sufficient loads to fracture the cam mechanism or rupture the door release cable.
The AD mandates the initial and subsequent inspections for proper operation of the MLG AES cam mechanism on DHC-8-400 Series aircraft, and rectify as necessary.