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9/8/98 AC 43.13-1B 9-25. GENERAL. Hydraulic systems in aircraft provide a means for the operation of aircraft components. The operation of landing gear, flaps, flight control surfaces and brakes is largely accomplished with hydraulic power systems. Hydraulic system complexity varies from small aircraft that require fluid only for manual operation of the wheel brakes to large transport aircraft where the systems are large and complex. To achieve the necessary redun dancy and reliability, the system may consist of several subsystems. Each subsystem has a power generating device (pump) reservoir, ac cumulator, heat exchanger, filtering system, etc. System operating pressure may vary from a couple hundred psi in small aircraft and ro torcraft to several thousand psi in large trans ports. Generally, the larger the aircraft, the more mechanical work is required to control the aircraft’s various functions. Consequently, the system operating pressure increases ac cordingly. Primarily, hydraulic power is gen erated by either engine driven or electric motor driven pumps. The majority of hydraulic pumps are pressure compensated to provide a constant output pressure at a flow-rate de manded by the system. Some constant dis placement pumps with a relief valve are used on the smaller aircraft. 9-26. PURPOSES OF HYDRAULIC SYSTEMS. Hydraulic systems make possible the transmission of pressure and energy at the best weight per horsepower ratio. 9-27. TYPES OF HYDRAULIC FLUID. There are three principal categories of hydrau lic fluids; mineral base fluids, polyalphaolefin base, and phosphate ester base fluids. When servicing a hydraulic system, the technician must be certain to use the correct category of replacement fluid. Hydraulic fluids are not necessarily compatible. For example, con tamination of the fire-resistant fluid MIL-H-83282 with MIL-H-5606 may render the MIL-H-83282 non fire-resistant. a. Mineral-Base Fluids. MIL-H-5606, mineral oil-based hydraulic fluid is the oldest, dating back to the 1940’s. It is used in many systems, especially where the fire hazard is comparatively low. MIL-H-6083 is simply a rust-inhibited version of MIL-H-5606. They are completely interchangeable. Suppliers generally ship hydraulic components with MIL-H-6083. b. Polyalphaolefin-Based Fluids. MIL-H-83282, is a fire-resistant hydrogenated polyalphaolefin-based fluid developed in the 1960’s to overcome the flammability charac teristics of MIL-H-5606. MIL-H-83282 is sig nificantly more flame resistant than MIL-H-5606, but a disadvantage is the high viscosity at low temperature. It is generally limited to -40 �F. However, it can be used in the same system and with the same seals, gas kets, and hoses as MIL-H-5606. MIL-H-46170 is the rust-inhibited version of MIL-H-83282. Small aircraft predominantly use MIL-H-5606 but some have switched to MIL-H-83282, if they can accommodate the high viscosity at low temperature. c. Phosphate Ester-Based Fluid (Sky drol/Hyjet). These fluids are used in most commercial transport category aircraft, and are extremely fire-resistant. However, they are not fireproof and under certain conditions, they will burn. The earliest generation of these fluids was developed after World War II as a result of the growing number of aircraft hy draulic brake fires which drew the collective concern of the commercial aviation industry. (1) Progressive development of these fluids occurred as a result of performance re quirements of newer aircraft designs. The Par 9-25 Page 9-13 SECTION 2. HYDRAULIC SYSTEMSPDF Image | AFS-640
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