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Shortly after World War II, as aircraft became more sophisticated and power-assist, flight-control functions became a requirement, hydraulic system operating pressures rose from the 1000 psi level to the 3000 psi level found on most aircraft today. Since then, 4000 psi systems have been developed for the U.S. Air Force XB-70 and B-1 bombers and a number of European aircraft including the tornado multirole combat aircraft and the Concorde supersonic transport. The V-22 Osprey incorporates a 5000 psi hydraulic system. The power levels of military aircraft hydraulic systems have continued to rise. This is primarily due to higher aerodynamic loading, combined with the increased hydraulic functions and operations of each new aircraft. At the same time, aircraft structures and wings have been getting smaller and thinner as mission requirements expand. Thus, internal physical space available for plumbing and components continues to decrease.

Shortly after World War II, as aircraft became more sophisticated and power-assist, flight-control functions became a requirement, hydraulic system operating pressures rose from the 1000 psi level to the 3000 psi level found on most aircraft today. Since then, 4000 psi systems have been developed for the U.S. Air Force XB-70 and B-1 bombers and a number of European aircraft including the tornado multirole combat aircraft and the Concorde supersonic transport. The V-22 Osprey incorporates a 5000 psi hydraulic system. The power levels of military aircraft hydraulic systems have continued to rise. This is primarily due to higher aerodynamic loading, combined with the increased hydraulic functions and operations of each new aircraft. At the same time, aircraft structures and wings have been getting smaller and thinner as mission requirements expand. Thus, internal physical space available for plumbing and components continues to decrease.

This SAE Aerospace Information Report (AIR) has been compiled to provide information on hydraulic systems fitted to the following categories of military vehicles. Attack Airplanes Fighter Airplanes Bombers Anti-Sub, Fixed Wing Airplanes Transport Airplanes Helicopters Boats

This report supplies data on various survivability design approaches and techniques to provide a broad frame of reference for future fluid power designers. Before the designer embarks on the overall system design, a comprehensive understanding of the total hostile environment in which the air vehicle is to operate is mandatory. The overall approach is heavily dependent upon the level of threat, small arms versus medium, or heavy caliber antiaircraft projectiles, missile, single or multiple hit survivability and the projected angle of fire. Overall aircraft stability is a factor which dictates recovery speed from any given failure and limits the magnitude of transient disturbances in the control system. The designer should strive to achieve at a minimum a system which allows high performance type aircraft to obtain a quasi-stable period following total control loss to allow safe ejection of the air crew.

This SAE Aerospace Information Report (AIR) provides the hydraulic system designer with the various design options and techniques currently available to enhance the survivability of hydraulic systems. A comprehensive knowledge of the hostile environment to which the air vehicle will be exposed will form the basis upon which the overall design philosophy is formulated. The designer should strive to achieve at the absolute minimum a system which provides the actuation and control capability to meet the minimum acceptable flying quality level to complete the operational mission for which the aircraft is designed; i.e., the aircraft can be controlled and the mission terminated safely, including landing. This AIR will attempt to address the following threats: a Typical Small Arms Fire (5.56, 7.62, 12.7 and 14.5 mm AP) b Cannon (20, 30, and 40 mm API/HEI) c NBC/EMI/EMP/Beamed Particle d Chemical/Biological Protection against missiles is beyond the scope of this AIR.

This SAE Aerospace Recommended Practice (ARP) provides processes for achieving the required cleanliness standards during the fabrication, assembly, and functional test of aircraft hydraulic systems. It covers exclusion and removal of solid and liquid contaminants from tubing during manufacture and final assembly, flushing of the installed system, and final checks to ensure cleanliness requirements are met.

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 AS5440, commercial aircraft hydraulic systems designed to ARP4752 and commercial helicopter hydraulic systems designed to ARP4925.