Recirculation cleaning is a decontamination process in which the system to be cleaned is powered from a clean external power source. The system is cycled so it produces a maximum interchange of fluid between the powered system and the SE used to power it. When decontaminating a system, the contaminated fluid is circulated through the hydraulic filters in the aircraft system and in the portable hydraulic test stands.

Decontamination that uses the recirculation cleaning method is a filtration process. It can remove only that foreign matter that is retained by the filter elements normally found in the equipment. A key factor in recirculation cleaning is the use of high-efficiency, 3-micron (absolute) filter elements. Absolute filter elements have no fluid bypass when the filter clogs. The filters have a large dirt-holding capacity in the portable test stands used for this purpose. In a single fluid pass, these filters remove all particulate matter larger than 3 microns, and a high percentage of the other particles down to submicron size. Recirculation cleaning is effective in removing hard particulate matter from hydraulic fluid that is otherwise serviceable. It must be recognized that the filters are not capable of removing water, other foreign fluids, or dissolved solids. Therefore, recirculation cleaning is limited to decontamination of systems found to have a particulate level in excess of Navy Standard Class 5, whose fluid is considered otherwise acceptable. For specific procedures on recirculation cleaning, you should refer to the applicable MIM. Use recirculation cleaning to remove excessive particulate matter that results from normal component wear, limited component failure, or external sources. Clean the system by powering it with an external portable hydraulic test stand. Operate the aircraft systems so maximum interchange of fluid is produced between the aircraft and the test stand. View A of figure 4-6 shows a flow diagram for recirculation cleaning.

Figure 4-6.-Fluid flow during decontamination.

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.