• Allow approximately 1 pint (250 ml) of fluid to drain. This purges any settled particles from the sampling port. 

• Insert a precleaned sample bottle under the fluid stream and fill, leaving an air space at the top. Withdraw the bottle and cap immediately. 

• Close drain valve. 

• Fill out sample identification label supplied in sample kit, making sure to include customer name, aircraft type, aircraft tail number, hydraulic system sampled, and date sampled. Indicate on the sample label under remarks if this is a routine sample or if it is being taken due to a suspected problem. 

• Service system reservoirs to replace the fluid that was removed. 

• Submit samples for analysis to laboratory. 

## Contamination Control 

Filters provide adequate control of the contamination problem during all normal hydraulic system operations. Control of the size and amount of contamination entering the system from any other source is the responsibility of the people who service and maintain the equipment. Therefore, precautions should be taken to minimize contamination during maintenance, repair, and service operations. If the system becomes contaminated, the filter element should be removed and cleaned or replaced. As an aid in controlling contamination, the following maintenance and servicing procedures should be followed at all times: 

• Maintain all tools and the work area (workbenches and test equipment) in a clean, dirt-free condition. 

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

• Before disconnecting hydraulic lines or fittings, clean the affected area with dry-cleaning solvent. 

• All hydraulic lines and fittings should be capped or plugged immediately after disconnecting. 

• Before assembly of any hydraulic components, wash all parts in an approved dry-cleaning solvent, i.e., Stoddard solvent. 

• After cleaning the parts in the dry-cleaning solution, dry the parts thoroughly and lubricate them with the recommended preservative or hydraulic liquid before assembly. Use only clean, lint-free cloths to wipe or dry the component parts. 

• All seals and gaskets should be replaced during the reassembly procedure. Use only those seals and gaskets recommended by the manufacturer. 

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

• All hydraulic servicing equipment should be kept clean and in good operating condition. 

Contamination, both particulate and chemical, is detrimental to the performance and life of components in the aircraft hydraulic system. Contamination enters the system through normal wear of components, by ingestion through external seals during servicing, or maintenance, when the system is opened to replace/repair components, etc. To control the particulate contamination in the system, filters are installed in the pressure line, in the return line, and in the pump case drain line of each system. The filter rating is given in microns as an indication of the smallest particle size that is filtered out. The replacement interval of these filters is established by the manufacturer and is included in the maintenance manual. In the absence of specific replacement instructions, a recommended service life of the filter elements is: 

• Pressure filters—3,000 hours 

• Return Filters—1,500 hours 

• Case drain filters—600 hours 

## Hydraulic System Flushing 

When inspection of hydraulic filters or hydraulic fluid evaluation indicates that the fluid is contaminated, flushing the system may be necessary. This must be done according to the manufacturer’s instructions; however, a typical procedure for flushing is as follows: 

1. Connect a ground hydraulic test stand to the inlet and outlet test ports of the system. Verify that the ground unit fluid is clean and contains the same fluid as the aircraft. 

2. Change the system filters. 

3. Pump clean, filtered fluid through the system, and operate all subsystems until no obvious signs of contamination are found during inspection of the filters. Dispose of contaminated fluid and filter. Note: A visual inspection of hydraulic filters is not always effective. 

4. Disconnect the test stand and cap the ports. 

5. Ensure that the reservoir is filled to the full line or proper service level. 

It is very important to check if the fluid in the hydraulic test stand, or mule, is clean before the flushing operation starts. A contaminated hydraulic test stand can quickly contaminate other aircraft if used for ground maintenance operations. 

