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 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, flush, and service the reservoirs or other fluid storage devices in the contaminated system before system flushing. If you know that the contamination originated at an aircraft pump, drain and flush the hoses and lines directly associated with the pump output. Case drains should be drained and flushed separately. If the aircraft filters are suspected of being loaded, install new or cleaned and tested filter elements in the aircraft hydraulic filters before flushing. Test stands must be set up and operated in according to the requirements of the specific aircraft and the system being flushed. Adjust the test output pressure and the flow volume for normal operation of the aircraft system being flushed. Monitor the reservoir level in the portable test stand continuously during the flushing operation. Use approved fluid-dispensing equipment to replenish the reservoir before the level decreases to the half-full point. Depletion of the SE reservoir fluid may result in cavitation or failure of the test stand pump. Operate all the circuits (actuators) on the system undergoing decontamination until the amount of fluid collected from the aircraft return line is equivalent to approximately three times the fluid capacity of the affected system. 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 the filter differential pressure indicators on the aircraft and in the SE. Replace any loaded filter elements. Sample and analyze the system after cycling of the components. If contaminant level shows improvement but is still unacceptable, continue the flushing operation. If no improvement is observed, try to find the source of contamination and correct it. If extensive system flushing fails to decontaminate the affected system adequately, request assistance from the cognizant engineering activity.

Upon successful completion of system flushing, recirculation clean it for a minimum period to eliminate possible residual debris and to ensure that the system is in acceptable condition. Sample the system after recirculation cleaning to verify that contaminant level is satisfactory. If an unsatisfactory condition is again indicated, repeat the flushing or recirculation cleaning operation as required. Upon successful completion of system decontamination, service the system to establish proper reservoir fluid level and to bleed entrapped air.

Purging is a decontamination process in which the aircraft hydraulic system is drained to the maximum extent practicable and the removed fluid discarded. Then, a suitable cleaning agent is introduced into the hydraulic system and circulated as effectively as possible to dislodge or dissolve contaminating sub-stances.

The cleaning operation is followed by complete removal of the cleaning agent, and then replace it with new hydraulic fluid. After purging the system, flushing and recirculation cleaning is performed to ensure adequate decontamination. Purging aircraft hydraulic systems is performed only upon recommendation from, and under the direct supervision of, the cognizant engineering activity. The cognizant engineering activity is responsible for selecting the required cleaning agents, providing detailed cleaning procedures, and performing tests upon completion of purging to ensure satisfactory removal of all cleaning agents. Whenever possible, purging operations should be accomplished at a naval aviation depot (NADEP).

NOTE: Organizational and intermediate maintenance activities are not authorized to perform system purging.

purification is the process of removing air, water, solid particles, and chlorinated solvents (MIL-C-81302 and MIL-T-81533) from hydraulic fluids. Con-taminated fluid going to the purifer tower is first filtered by a 25-micron (absolute) filter. The vacuum applied to the tower removes air, water, and chlorinated solvents from the contaminated fluid. As fluid comes out of the tower, it is filtered through a 3-micron (absolute) filter to remove solid particles. See figure 4-6. This cycle is repeated until a desired level of cleanliness is attained. For systems contaminated with air, water, and chlorinated solvents MIL-C-81302 and MIL-T-81533, you can use a purifier to clean the aircraft and support equipment (SE) to reduce the consumption of fluid and replace the need for flushing.