BOMBARDIER INC CL-600-2B19ATLANTIC SOUTHEAST AIRLINES INC

While configuring the airplane for landing, the flight crew observed a landing gear disagree message and an indication that the left main landing gear was not down and locked. The flight crewmembers followed procedures to address the landing gear disagree message; however, they were unable to get the left landing gear to extend. The captain declared an emergency and chose to land the airplane with only the right main landing gear and nose landing gear extended. The airplane came to rest in an upright, left-wing-low position. Postincident examination of the left main landing gear system revealed that the upper attachment bolt for the left main landing gear uplock assembly, which is designed to be attached to both the uplock mechanism and the structure, was attached only to the airplane structure. The left main landing gear uplock assembly had been removed and reinstalled the day before the incident flight. The mechanic who performed the maintenance did not have training on nor prior experience performing the installation of the uplock assembly, and the maintenance inspector who inspected its installation did not have training on inspecting the uplock assembly. Neither the mechanic nor the maintenance inspector observed any problems during the postinstallation inspection. Thus, it is likely that the uplock assembly was installed incorrectly due to the lack of training in both the installation by the mechanic and the inspection by the inspector.