BOMBARDIER CL600 2B19SKYWEST AIRLINES, Inc.

While the flight crew was configuring the airplane for landing and shortly after placing the landing gear selector in the "down" position, they noted a "gear disagree" warning message displayed on the engine indication and crew alerting system that showed that the nose gear and the right main landing gear (MLG) were down and locked but that the left MLG was in transit. The flight crew followed the quick reference handbook directions to troubleshoot the landing gear issue without success. The flight crewmembers then tried to extend the landing gear by pulling the alternate landing gear release handle; however, the left MLG failed to extend, and they subsequently landed the airplane with the left MLG retracted. During postincident activities, the airplane was lifted, and an examination revealed that the left MLG remained in its full-up position within the wheel well. The alternate landing gear release handle was found in its fully extended position. The incident airplane's left MLG uplock pin exhibited signs of slight wear and flat spotting, consistent with in-service usage, and the uplock mechanism latch had wear marks that were within the in-service maintenance wear limits. After the incident, functional ground testing of the airplane's landing gear system found that, although wear was observed on these system components, the left MLG extended as designed when the landing gear was selected down normally and manually.  Additionally, the incident airliner's left MLG uplock pin was found properly rigged within its mating uplock mechanism.  Therefore, it is unlikely that the worn components alone would have prevented the left MLG from extending during the incident flight. Functional performance testing of the left MLG sidestay actuator showed that it operated within all test specifications. Although an examination of the actuator's restrictor assembly found that it contained two small pieces of aluminum and several particles of debris this contamination did not prevent the actuator from operating during ground tests after the incident.  Therefore, it is unlikely that the contamination within the left MLG sidestay actuator would have prevented the left MLG from extending during the incident flight. The force to operate the lever on the bypass valve was measured and found to be 60 lbs, which exceeded the acceptance test procedure -specified force requirement of 40 lbs. However, during the incident flight, the nose and both MLG uplock assemblies did unlock when the alternate landing gear release handle was pulled indicating that the bypass valve functioned during the flight. As previously discussed, neither hydraulic contamination within the left MLG actuator nor any single MLG system component was identified as preventing the left MLG from extending during the incident flight.  However, because the left MLG failed to extend when the crew used either the normal or alternate extension systems, it is likely that a combination of several factors contributed to the system malfunction. Some potential factors are: uplock pin rigging and/or wear, uplock mechanism latch wear, hydraulic system pressure and contamination, and bypass valve operation.  Even though these items may be within maintenance limits when considered individually, there may be combinations of these factors that result in failure of the gear to extend.  When combined with environmental and flight related conditions such as temperature, humidity, and landing gear component deflection due to in-flight loading, the interaction results of all of these factors are difficult to predict and to demonstrate during testing.