What happened
The flight, operating as a freighter, proceeded normally until the crew attempted to extend the landing gear for arrival at their destination. During this process, the left main landing gear (LMLG) indicated an "unsafe" condition. The crew followed all emergency and abnormal checklists and communicated with company maintenance via telephone and radio-relayed instructions, but all attempts to extend the gear failed.
As a result, the pilot performed an emergency landing with the LMLG in the retracted position. The aircraft sustained minimal damage during the landing.
The investigation
Post-landing investigation determined that company maintenance had installed a one-way check valve in the LMLG extension hydraulic lines instead of the required restricted flow valve. The incorrectly installed valve lacked an attached factory specification or part number. Furthermore, the tag removed from the valve during installation contained the correct vendor part number but the incorrect factory specification number.
Although the company maintenance manual requires a leak and operational test of the main landing gear retract/extension system following such an installation, both the mechanic who performed the work and the company inspector stated that the completed job had undergone both leak and operational testing.