designed to remove most foreign particles that are visible  to the naked eye. Hydraulic liquid that appears clean to the  naked eye may be contaminated to the point that it is unfit for  use. Thus, visual inspection of the hydraulic liquid does not  determine the total amount of contamination in the system.  Large particles of impurities in the hydraulic system are  indications that one or more components are being subjected  to excessive wear. Isolating the defective component requires  a systematic process of elimination. Fluid returned to the  reservoir may contain impurities from any part of the system.  To determine which component is defective, liquid samples  should be taken from the reservoir and various other locations  in the system. Samples should be taken in accordance with  the applicable manufacturer’s instructions for a particular  hydraulic system. Some hydraulic systems are equipped with  permanently installed bleed valves for taking liquid samples,  whereas on other systems, lines must be disconnected to  provide a place to take a sample.  Aircraft hydraulic systems designed around phosphate  ester-based fluids should be virtually trouble-free if  properly serviced. Phosphate ester-based fluids do not  appreciably affect common aircraft metals—aluminum,  silver, zinc, magnesium, cadmium, iron, stainless steel,  bronze, chromium, and others—as long as the fluids are  kept free of contamination. Thermoplastic resins, including  vinyl compositions, nitrocellulose lacquers, oil-based paints,  linoleum, and asphalt may be softened chemically due to  phosphate ester-based fluids. However, this chemical action  usually requires longer than just momentary exposure, and  spills that are wiped up with soap and water do not harm  most of these materials. Paints that are resistant to phosphate  ester-based fluids include epoxies and polyurethanes. Today,  polyurethanes are the standard of the aircraft industry because  of their ability to keep a bright, shiny finish for long periods  of time and for the ease with which they can be removed.  Hydraulic systems require the use of special accessories that  are compatible with the hydraulic fluid. Appropriate seals,  gaskets, and hoses must be specifically designated for the  type of fluid in use. Care must be taken to ensure that the  components installed in the system are compatible with the  fluid. When gaskets, seals, and hoses are replaced, positive  identification should be made to ensure that they are made of  the appropriate material. Phosphate ester-based type V fluid is  compatible with natural fibers and with a number of synthetics,  including nylon and polyester, which are used extensively in  most aircraft. Petroleum oil hydraulic system seals of neoprene  or Buna-N are not compatible with phosphate ester-based  fluids and must be replaced with seals of butyl rubber or  ethylene-propylene elastomers.  Experience has shown that trouble in a hydraulic system  is inevitable whenever the liquid is allowed to become  contaminated. The nature of the trouble, whether a simple  malfunction or the complete destruction of a component,  depends to some extent on the type of contaminant. Two  general contaminants are: Whenever it is suspected that a hydraulic system has become  contaminated or the system has been operated at temperatures  in excess of the specified maximum, a check of the system  should be made. The filters in most hydraulic systems are  12-4 •  Nonabrasives, including those resulting from oil  oxidation and soft particles worn or shredded from  seals and other organic components. •  Abrasives, including such particles as core sand, weld  spatter, machining chips, and rust. •  Place a waste container under the reservoir drain  valve and open valve so that a steady, but not forceful,  stream is running. •  Wipe off sampling port or tube with a lint-free cloth.  Do not use shop towels or paper products that could  produce lint. Generally speaking, the human eye can  see particles down to about 40 microns in size. Since  we are concerned with particles down to 5 microns in  size, it is easy to contaminate a sample without ever  knowing it. •  Before taking samples, always be sure to wear the  proper personal protective equipment that should  include, at the minimum, safety glasses and gloves. •  Shut down and depressurize the system.  •  Pressurize and operate hydraulic system for 10–15  minutes. During this period, operate various flight  controls to activate valves and thoroughly mix  hydraulic fluid. •  Routine sampling—each system should be sampled  at least once a year, or every 3,000 flight hours, or  whenever the airframe manufacturer suggests. •  Unscheduled maintenance—when malfunctions may  have a fluid related cause, samples should be taken. •  Suspicion of contamination—if contamination is  suspected, fluids should be drained and replaced,  with samples taken before and after the maintenance  procedure. Hydraulic Sampling Schedule Sampling Procedure Hydraulic Fluid Contamination Contamination Check  Compatibility with Aircraft Materials