Maintaining hydraulic fluid within allowable contamination limits for both water and particulate matter is crucial to the care and protection of hydraulic equipment.

Filters (discussed in chapter 9) will provide adequate control of the particular contamination problem during all normal hydraulic system operations if the filtration system is installed properly and filter maintenance is performed properly. Filter maintenance includes changing elements at proper intervals. Control of the size and amount of contamination entering the system from any other source is the responsibility of the personnel who service and maintain the equipment. During installation, maintenance, and repair of hydraulic equipment, the retention of cleanliness of the system is of paramount importance for subsequent satisfactory performance. The following maintenance and servicing procedures should be adhered to at all times to provide proper contamination control:

1. All tools and the work area (workbenches and test equipment) should be kept in a clean, dirt-free condition.

2. A suitable container should always be provided to receive the hydraulic liquid that is spilled during component removal or disassembly.

NOTE: The reuse of drained hydraulic liquid is prohibited in most hydraulic systems. In some large-capacity systems the reuse of fluid is permitted. When liquid is drained from these systems for reuse, it must be stored in a clean and suitable container. The liquid must be strained and/or filtered when it is returned to the system reservoir.

3. Before hydraulic lines or fittings are disconnected, the affected area should be cleaned with an approved dry-cleaning solvent.

4. All hydraulic lines and fittings should be capped or plugged immediately after disconnection.

5. Before any hydraulic components are assembled, their parts should be washed with an approved dry-cleaning solvent.

6. After the parts have been cleaned in dry-cleaning solvent, they should be dried thoroughly with clean, low-lint cloths and lubricated with the recommended preservative or hydraulic liquid before assembly.

NOTE: Only clean, low lint type I or II cloths as appropriate should be used to wipe or dry component parts.

7. All packings and gaskets should be replaced during the assembly procedures.

8. All parts should be connected with care to avoid stripping metal slivers from threaded areas. All fittings and lines should be installed and torqued according to applicable technical instructions.

9. All hydraulic servicing equipment should be kept clean and in good operating condition. Some hydraulic fluid specifications, such as MIL-H-6083, MIL-H-46170, and MIL-H-83282, contain particle contamination limits that are so low that the products are packaged under clean room conditions. Very slight amounts of dirt, rust, and metal particles will cause them to fail the specification limit for contamination. Since these fluids are usually all packaged in hermetically sealed containers, the act of opening a container may allow more contaminants into the fluid than the specification allows. Therefore, extreme care should be taken in the handling of these fluids. In opening the container for use, observation, or tests, it is extremely important that the can be opened and handled in a clean environment. The area of the container to be opened should be flushed with filtered solvent (petroleum ether or isopropyl alcohol), and the device used for opening the container should be thoroughly rinsed with filtered solvent. After the container is opened, a small amount of the material should be poured from the container and disposed of prior to pouring the sample for analysis. Once a container is opened, if the contents are not totally used, the unused portion should be discarded. Since the level of contamination of a system containing these fluids must be kept low, maintenance on the systems components must be performed in a clean environment commonly known as a controlled environment work center. Specific information about the controlled environment work center can be found in the Aviation Hydraulics Manual, NAVAIR 01-1A-17.

The condition of a hydraulic system, as well as its probable future performance, can best be determined by analyzing the operating fluid. Of particular interest are any changes in the physical and chemical properties of the fluid and excessive particulate or water contamination, either of which indicates impending trouble. Excessive particulate contamination of the fluid indicates that the filters are not keeping the system clean. This can result from improper filter maintenance, inadequate filters, or excessive ongoing corrosion and wear. Operating equipment should be sampled according to instructions given in the operating and maintenance manual for the particular equipment or as directed by the MRCs.

1. All samples should be taken from circulating systems, or immediately upon shutdown, while the hydraulic fluid is within 5C (9F) of normal system operating temperature. Systems not up to temperature may provide nonrepresentative samples of system dirt and water content, and such samples should either be avoided or so indicated on the analysis report. The first oil coming from the sampling point should be discarded, since it can be very dirty and does not represent the system. As a general rule, a volume of oil equivalent to one to two times the volume of oil contained in the sampling line and valve should be drained before the sample is taken.

2. Ideally, the sample should be taken from a valve installed specifically for sampling. When sampling valves are not installed, the taking of samples from locations where sediment or water can collect, such as dead ends of piping, tank drains, and low points of large pipes and filter bowls, should be avoided if possible. If samples are taken from pipe drains, sufficient fluid should be drained before the sample is taken to ensure that the sample actually represents the system. Samples are not to be taken from the tops of reservoirs or other locations where the contamination levels are normally low.

3. Unless otherwise specified, a minimum of one sample should be taken for each system located wholly within one compartment. For ships systems extending into two or more compartments, a second sample is required. An exception to this requirement is submarine external hydraulic systems, which require only one sample. Original sample points should be labeled and the same sample points used for successive sampling. If possible, the following sampling locations should be selected:

a. A location that provides a sample representative of fluid being supplied to system components
b. A return line as close to the supply tank as practical but upstream of any return line filter
c. For systems requiring a second sample, a location as far from the pump as practical

Operation of the sampling point should not introduce any significant amount of external contaminants into the collected fluid. Additional information on hydraulic fluid sampling can be found in NAVAIR 01-1A-17.

Most fluid samples are submitted to shore laboratories for analysis. NAVAIR 17-15-50-1 and NSTM, chapter 556, contain details on collecting, labeling, and shipping samples. NAVAIR 01-1A-17 contains procedures for unit level, both aboard ship and ashore, testing of aviation hydraulic fluids for water, particulate, and chlorinated solvent contamination.