HYDRAULIC CONTAMINATION AND RELATED SERVICING/TEST EQUIPMENT 

Chapter Objective: equipment used in servicing, troubleshooting, repairing, and maintaining aircraft hydraulic systems.

All modern naval aircraft contain hydraulic systems that operate various mechanisms. The number of hydraulically operated units depends upon the model of aircraft. The average operational aircraft has about a dozen hydraulically operated units. Aircraft hydraulic systems are designed to produce and maintain a given pressure over the entire range of required fluid flow rates. The pressure used in most Navy high-performance aircraft is 3,000 psi. The primary use of hydraulic fluids in aircraft hydraulic systems is to transmit power, but hydraulic systems perform other functions. Hydraulic fluid acts as a lubricant to reduce friction and wear. Hydraulic fluid serves as a coolant to maintain operating temp-eratures within limits of critical sealant materials, and it serves as a corrosion and rust inhibitor. Critical functions of hydraulic systems maybe impaired if the hydraulic system fluid is allowed to become contaminated beyond acceptable limits.

Hydraulic fluid contamination is defined as any foreign material or substance whose presence in the fluid is capable of adversely affecting the system performance or reliability. control consists of requirements, techniques, and practices that minimize and control fluid contami-nation. Remember the proverb, "An ounce of prevention is worth a pound of cure."

FLUID SAMPLING

Contamination measurement standards and acceptability limits define and control hydraulic contamination levels. The maximum acceptable hydraulic fluid particulate level is Navy Standard Class 5 for naval aircraft, and Navy Standard Class 3 for related SE. The contamination level of a particular system is determined by analysis of a fluid sample drawn from the system. Analysis isaccomplished at all levels of maintenance through the use of Contamination Analysis Kit 57L414. Hydraulic system fluid sampling is accomplished on a periodic basis according to the applicable maintenance instruction manual (MIM), maintenance requirement cards (MRC), and rework specification. Figure 4-1 shows the requirements for periodic fluid surveillance.

You should perform analysis of hydraulic systems if extensive maintenance and/or crash/battle damage occurs. You should perform the analysis when a metal-generating component fails, an erratic flight control function or a hydraulic pressure drop is noted, or there are repeated and/or extensive system malfunctions. Analysis is performed when there is a loss of system fluid, or when the system is subjected to excessive temperature. Analysis is also performed when an aircraft is removed from storage    accordance with NAVAIR 15-05-500. You should perform analysis of the hydraulic system anyti   hydraulic contamination is suspected.

MAINTENANCE PROCEDURES
The general contamination control procedures hydraulic systems, subsytems, and testing of components, and fluids are requirements for each maintenance level. Hydraulic fluid contamination controls ensure the cleanliness and purity of fluid in the hydraulic system. Fluid sampling and analysis is performed periodically. Checks are made sufficiently before the scheduled aircraft induction date so that if fluid decontamination is required, it may be accomplished at that time. The condition of the fluid depends, to a large degree, on the condition of the components in the system. If a system requires frequent component replacement and servicing, the condition of the fluid deteriorates proportionately. Replacement of aircraft hydraulic system filter elements takes place on a scheduled or conditional basis, depending upon the requirements of the specific system. A differential pressure flow check and bubble point test are performed to properly evaluate the condition of a cleanable filter element. These two checks are done to verify that the element is good before it is installed in a system or component. Many filter elements look identical, but all of them are not compatible with flow requirements of the system.

If the hydraulic system fluid is lost to the point that the hydraulic pumps run dry or cavitate, you

Figure 4-1.Periodic fluid surveillance requirements.

should change the defective pumps, check filter elements, and decontaminate the system as required. Check the applicable MIM for corrective action to be taken regarding decontamination of the system. If this action is not taken, the complete system could be contaminated. Hydraulic systems and components are serviced by using approved fluid dispensing equipment only. Unfiltered hydraulic fluid should NEVER be introduced into systems or components. All portable hydraulic test stands must receive the required periodic maintenance checks. Make certain that each unit is approved, and the applicable MIM is readily accessible and up to date. When the portable hydraulic test stand is not in use, it should be protected against contaminants such as dust and water. You should ensure that correct hoses are used on each stand, and that they are approved for the type of fluid being used. Properly cap hoses when they are not being used. Hoses must be serialized and must remain with the equipment. Make sure the hoses are coiled, kept free of kinks, and properly stowed. Make sure they are in satisfactory condition and are checked periodically. Replace any hose that exhibits fluid seepage from the outer cover or separation between the inner tube and the outer cover. Portable hydraulic test stands that show indications of contamination or that have loaded (clogged) filters are removed from service immediately and returned to the supporting activity for maintenance.

Use only approved lubricants for O-ring seals; incorrect lubricants will contaminate a system. Many lubricants look alike, but few are compatible with hydraulic fluids. The only approved O-ring seal lubricants are hydraulic fluid MIL-H-5606, hydraulic fluid MIL-H-83282, hydraulic fluid MIL-H-46170, or a thin film of grease, MIL-G-81322.