Test stands used for recirculation cleaning must be equipped with 3-micron (absolute) filtration. Before connecting the test stand to the aircraft, the stand itself must be recirculation cleaned and deaerated, and its contamination level verified to meet the Navy Standard Class 3 cleanliness level. If the system has a makeup reservoir, drain and reservice the system reservoir prior to recirculation cleaning. Makeup reservoirs have a single fluid port similar to an accumulator; therefore, little or no fluid exchange takes place during recirculation cleaning. If contamination is severe, or if aircraft filters are suspected of being loaded or damaged, or if differential pressure indicators have been activated, install new (or cleaned and tested) filter elements in the aircraft before you begin cleaning. Set up and operate the test stand in a manner compatible with the requirements of the specific aircraft and system being powered. Adjust the test stand output pressure and low volume for normal operation of the aircraft system being recirculation cleaned. Operate all circuits (actuators) on the system undergoing decontamination a minimum of 15 complete cycles, or according to procedures in the specific MIM or MRCs. Give particular emphasis to the operation of large displacement actuators, such as those associated with landing gear and wingfold, when powered by the affected system. Continuously monitor all filter differential pressure indicators, both on the aircraft and on the portable hydraulic test stand, during the cleaning process. Replace any loaded fiber elements. Sample and analyze the system after the cycling of components. If the contaminant level shows improvement but is still unacceptable, repeat the recirculation cleaning process. If no improvement is observed, attempt to determine the source of contamination. System flushing may be required. When successful recirculation cleaning is complete, service the system, as required, to establish the proper reservoir fluid level and to eliminate entrapped air. Flushing Flushing is a decontamination method in which contaminated system fluid is removed to the maximum extent practicable and then discarded. It is a draining process that is generally accomplished by powering the aircraft system with a portable hydraulic test stand. See figure 4-6. The contaminated return-line fluid from the aircraft is then allowed to flow overboard into a suitable receptacle for disposal. In effect, filtered fluid from the portable hydraulic test stand is used to displace contaminated fluid in the system and to replenish it with clean serviceable fluid. The amount of fluid removed and replaced during system flushing varies. It depends upon such factors as the nature of the contaminant, layout of the system, and the ability to produce flow in all affected circuits. Portions of operating systems are often dead ended. Fluid found in these portions is static and not affected by the normal system fluid flow. Remove contaminated fluid in these circuits and associated components by partially disassembling the unit. Drain and totally flush the unit. Generally, system flushing continues until anal ysis of the return line fluid from the system being decontaminated indicates that the fluid is acceptable. If there is severe contamination, considerable quantities of hydraulic fluid may be expended, making it important to closely monitor the portable hydraulic test stand reservoir level, and replenish it as required. Flushing effectively decontaminates systems containing water, large amounts of gelatinous-type materials, or fluid that is chemically unacceptable (containing chlorinated or other solvents). This type of fluid contamination or degradation cannot be remedied by conventional filtration. In severe cases of particulate contami- nation, such as those that result from major component failure, flushing techniques may more easily correct the problem than will recirculation cleaning. Detailed procedures for flushing hydraulic systems are found in the aircraft MIMs. The basic procedures are discussed in the following text, and will give you some idea of the procedures used when flushing aircraft hydraulic systems. Remember, use the MIM for the specific procedures to use when flushing hydraulic systems. Use flushing to decontaminate systems that cannot be cleaned by recirculation cleaning or purifying. Normally, flushing requires you to remove fluids that are found to be chemically or physically unacceptable, or fluids contaminated with water, other foreign fluids, or particulate matter not readily filterable because of its nature or the quantity involved. Use an external portable hydraulic test stand to power the contaminate system and accomplish flushing. Allow return fluid from the aircraft to flow overboard into a waste container for disposal. Aircraft subsystems should be operated to produce maximum displacement of aircraft fluids by cleaned, filtered fluid from the portable test stand. View B of figure 4-6 shows fluid flow during system flushing. Test stands used for system flushing must be equipped with 3-micron (absolute) filtration and must have a minimum internal reservoir of 16 gallons. The stand itself should be recirculation cleaned and deaerated before it is connected to the aircraft. Drain, 4-16