BEECH 1900DGREAT LAKES AVIATION LTD

During approach to landing the crew selected landing gear down and observed a problem with the left main landing gear (MLG). The landing gear hydraulic motor pump automatically shut down when the landing gear was not locked down after 16 seconds and an internal mechanical lock intended to hold the landing gear actuator in the selected position, did not engage. The crew carried out the abnormal procedures checklist and attempted to extend the left MLG using the manual landing gear extension procedures, but was unsuccessful. The crew then prepared for landing in accordance with the procedures contained in the quick reference handbook for landing with main gear up or unsafe. As the airplane slowed through about 80 knots on the landing roll, the left main landing gear collapsed. Post-accident inspection indicated hydraulic fluid had leaked from a hole, which was normally plugged with a Lee plug, on the left main actuator between the primary and secondary extend ports. The Lee plug was missing and was not found. Examination of the actuator and of the hydraulic fluid quantity remaining indicated the Lee plug was in place during the in-flight attempts to extend the landing gear. The airplane landed with the gear not locked down and as the landing gear collapsed, the piston in the actuator forced hydraulic fluid back through the system increasing the landing gear actuator internal hydraulic pressure sufficiently to force the Lee plug out of position. The remaining fluid in the actuator leaked out of the hole until system pressure decreased to ambient pressure.