This SAE Aerospace Recommended Practice (ARP) establishes the processes to achieve and maintain the required cleanliness levels in flight vehicle hydraulic systems during fabrication, assembly and pre-flight functional tests. This recommended practice covers exclusion and removal primarily of solid contaminants that occur or are created during these successive steps. The flushing procedure for installed tubing is detailed. This ARP does not address contamination levels of hydraulic fluids as purchased, operation and maintenance of ground carts, details of component cleanliness or of contamination measurement. This ARP applies to military aircraft and helicopters designed to SAE AS 5440, commercial aircraft hydraulic systems designed to SAE ARP 4752 and commercial helicopter hydraulic systems designed to SAE ARP 4925.