Incident Overview
A freighter flight experienced a landing gear malfunction after the left main landing gear (LMLG) indicated "unsafe" when the gear was extended for landing at destination. The pilot reported that the flight had been normal in all aspects until that point.
Attempts to Extend Landing Gear
All attempts to extend the LMLG using emergency and abnormal checklists, as well as telephone and radio-relayed communications with company maintenance, were unsuccessful. The crew subsequently performed an emergency landing with the LMLG retracted. The aircraft sustained only minimal damage.
Postcrash Investigation
Investigation revealed that during prior maintenance, company maintenance personnel had installed a one-way check valve in the LMLG extend hydraulic lines instead of the required restricted flow valve. The wrongly installed valve had no factory specification or part number attached. The tag reportedly removed from the valve at installation bore the wrong factory specification number, although the correct vendor part number was present.
Maintenance Procedures and Testing
The company maintenance manual specifies that after valve installation, a leak and operational test of the MLG retract/extension system must be performed. Both the valve installation mechanic and the company inspector stated that the finished job was leak and operational tested.