Some aircraft use fin tubing for cooling hydraulic system fluid. Hydraulic fluid coolers are mounted internally in the wing inboard fuel tanks. As shown in figure 7-29, each cooler is an assembly of fin-walled tubing, two unions, and mounting supports. Fluid enters the inlet coupling and is passed through the fin-walled tubing, which acts as a heat exchanger, and is directed to the outlet coupling for return to the system reservoir. The heat of the fluid passing through the coolers is absorbed by both the fin-walled tubing and the fuel.

NOTE: The fuel level in the inboard tanks must be maintained at a specific level to ensure adequate cooling of the fluid.

A manifold is a hydraulic component used to conserve space and permit ease of unit removal and replacement. It also provides a means where common fluid lines may come together and be distributed to other subsystems. Manifolds are used in various types of installations, depending upon the needs of the system.

Figure 7-29.Fin tubing assembly Installation.

 Figure 7-30 shows two views of a manifold. This manifold joins both the pressure and suction lines from the No. 1 system pumps and the suction line from the emergency system pump. The assembly includes integral check valves to direct the flow of fluid through the manifold, filters to clean the fluid prior to its entry into the main system, and quick-disconnect fittings for the connection of ground test hydraulic equipment.

Hydraulic fluid will hold in suspension tiny particles generated during normal wear of selector valves, pumps, and other system components. These minute particles may damage or impair the function of the units and parts through which they pass if they are not removed by a filter. Because close tolerances exist within a hydraulic system, the performance and

Figure 7-30.Hydraulic manifold assembly.

reliability of the entire system depend upon adequate filtration. 

Continuous filtration of hydraulic fluid during system operation is necessary to maintain system cleanliness. You should use filters that have fine pores or openings to allow hydraulic fluid to pass but that are small enough to trap contaminant particles. Hydraulic filter elements are rated in several ways. The absolute filtration rating is the diameter in microns of the largest spherical particle that will pass through the filter under a certain test condition. This rating is an indication of the largest opening in the filter element. The mean filtration rating is the measurement of the average size of the openings in the filter element. The nominal filtration rating is usually interpreted to mean the size of the smallest particles of which 90 percent will be trapped in the filter at each pass through the filter. Figure 7-31 shows a typical filter arrangement in a hydraulic system.

Filters may be located within the reservoir, the pressure line, the return line, or any other location where they are needed to safeguard the hydraulic system against contaminants. Their location in the system and other design criteria determine their shape and size.