fier. The final stage of the amplifier controls the hermetically sealed relay which provides a.c. power to the windshield heating current bus bars. When the windshield temperature is above calibrated value, the sensing elements will have a higher resistance value than that needed to balance the bridge.
This decreases the flow of current through the amplifiers and the relays of the control unit are de-energized. As the temperature of the windshield drops, the resistance value of the sensing elements also drops and the current through the amplifiers will again reach sufficient magnitude to operate the relays in the control unit, thus energizing the circuit.
There are several problems associated with electrically heated windshields. These include delamination, scratches, arcing, and discoloration.
Delamination (separation of the plies) although undesirable, is not harmful structurally provided it is within the limits established by the aircraft manufacturer, and is not in an area where it affects the optical qualities of the panel.
Arcing in a windshield panel usually indicates that there is a breakdown in the conductive coating.
Where chips or minute surface cracks are formed in the glass plies, simultaneous release of surface compression and internal tension stresses in the highly tempered glass can result in the edges of the crack and the conductive coating parting slightly. Arcing is produced where the current jumps this gap, particularly where these cracks are parallel to the window bus bars. Where arcing exists, there is invariably a certain amount of local overheating which, depending upon its severity and location, can cause further damage to the panel. Arcing in the vicinity of a temperature-sensing element is a particular problem since it could upset the heat control system.
Electrically heated windshields are transparent to directly transmitted light but they have a distinctive color when viewed by reflected light. This color will vary from light blue, yellow tints, and light pink depending upon the manufacturer of the window panel. Normally, discoloration is not a problem unless it affects the optical qualities.
Windshield scratches are more prevalent on the outer glass ply where the windshield wipers are indirectly the cause of this problem. Any grit trapped by a wiper blade can convert it into an extremely effective glass cutter when the wiper is set in motion. The best solution to scratches on the windshield is a preventive one; clean the windshield wiper blades as frequently as possible. Incidentally, windshield wipers should never be operated on a dry panel, since this increases the chances of damaging the surface.
Assuming that visibility is not adversely affected, scratches or nicks in the glass plies are allowed within the limitations set forth in the appropriate service or maintenance manuals. Attempting to improve visibility by polishing out nicks and scratches is not recommended. This is because of the unpredictable nature of the residual stress concentrations set up during the manufacture of tempered glass.
Tempered glass is stronger than ordinary annealed glass due to the compression stresses in the glass surface which have to be overcome before failure can occur from tension stresses in the core. Polishing away any appreciable surface layer can destroy this balance of internal stresses and can even result in immediate failure of the glass.
Determining the depth of scratches has always presented some difficulty. An optical micrometer can be used for this purpose. It is essentially a microscope supported on small legs rather than the more familiar solid base mounting. When focused on any particular spot, the focal length of the lens (distance from the lens to the object) can be read from a micrometer scale on the barrel of the instrument.
The depth of a scratch or fissure in a windshield panel, for example, can thus be determined by ob taining focal length readings to the surface of the glass and to the bottom of the scratch. The difference between these two readings gives the depth of the scratch. The optical micrometer can be used on flat, convex, and concave surfaces of panels whether they are installed on the aircraft or not.
Window Defrost System The window defrost system directs heated air from the cabin heating system (or from an auxiliary heater, depending on the aircraft) to the pilot's and copilot's windshield and side windows by means of a series of ducts and outlets. In warm weather when heated air is not needed for defrosting, the system can be used to defog the windows.
This is done by blowing ambient air on the windows using the blowers.
Windshield and Carburetor Alcohol Deicing Systems An alcohol deicing system is provided on some aircraft to remove ice from the windshield and the carburetor. Figure 7-15 illustrates a typical two-engine system in which three deicing pumps (one for each carburetor and one for the windshield) are used. Fluid from the alcohol supply tank is con- 302
trolled by a solenoid valve which is energized when any of the alcohol pumps are on. Alcohol flow from the solenoid valve is filtered and directed to the alcohol pumps and distributed through a system of plumbing lines to the carburetors and windshield.
Left carburetor Windshield alcohol alcohol pump pump Anti-ice fluid Right carburetor solenoid valve alcohol pump 300 50W 5A
d.c. bus FreURE 7-15. Carburetor and windshield deicing system.
Toggle switches control the operation of the carburetor alcohol pumps. When the switches are placed in the "on" position, the alcohol pumps are turned on and the solenoid-operated alcohol shutoff valve is opened. Operation of the windshield deicer pump and the solenoid-operated alcohol shutoff valve is controlled by a rheostat-type switch, located in the pilot's station. When the rheostat is moved away from the "off" position, the shutoff valve is opened and the alcohol pump will pump fluid to the windshield at the rate selected by the rheostat.
When the theostat is returned to the "off" position, the shutoff valve closes and the pump stops operating.
Pitot Tube Anti-Icing To prevent the formation of ice over the opening in the pitot tube, a built-in electric heating element is provided. A switch, located in the cockpit, controls power to the heater. Use caution when ground checking the pitot tube since the heater must not be operated for long periods unless the aircraft is in flight. Additional information concerning pitot tubes is found in Chapter 2 of this handbook, Aircraft Instrument Systems.
Heating elements should be checked for functioning by ensuring that the pitot head begins to warm up when power is applied. If an ammeter or loadmeter is installed in the circuit, the heater operation can be verified by noting the current consumption when the heater is turned on.
WATER AND TOILET DRAIN HEATERS Heaters are provided for toilet drain lines, water lines, drain masts, and waste water drains when they are located in an area that is subjected to feezing temperatures in flight. The types of heaters used are integrally heated hoses, ribbon, blanket, or patch heaters that wrap around the lines, and gasket heaters (see figure 7-16). Thermostats are provided in heater circuits where excessive heating is undesirable or to reduce power consumption. The heaters have a low voltage output and continuous operation will not cause overheating.
RAIN ELIMINATING SYSTEMS When rain forms on a windshield during flight, it becomes a hazard and must be eliminated. To provide a clear windshield, rain is eliminated by wiping it off or blowing it off. A third method of rain removal involves chemical rain repellants. Rain is blown from the windshield of some aircraft by air from jet nozzles located beneath the windshield. On other aircraft, windshield wipers are used to eliminate the rain. The windshield wipers of an aircraft accomplish the same function as those of an automobile. In each instance, rubber blades wipe across the windshield to remove rain and slushy ice.
Electrical Windshield Wiper Systems In an electrical windshield wiper system, the wiper blades are driven by an electric motor (s) which receive (s) power from the aircraft's electrical system. On some aircraft the pilot's and copilot's windshield wipers are operated by separate systems to ensure that clear vision is maintained through one of the windows should one system fail.
Figure 7-17 shows a typical electrical windshield wiper installation. An electrically operated wiper is installed on each windshield panel. Each wiper is driven by a motor-converter assembly. The converters change the rotary motion of the motor to reciprocating motion to operate the wiper arms. A shaft protruding from the assembly provides an attachment for the wiper arm.
• Blanket type heater Gasket heater Typical integrally heated hose Ribbon heater Heated drain mast Waterline heater patch type FIGURE 7-16. Typical water and drain line heaters.
The windshield wiper is controlled by setting the wiper control switch to the desired wiper speed.
When the "high" position is selected (figure 7-18), relays 1 and 2 are energized. With both relays energized, field 1 and field 2 are energized in parallel. The circuit is completed and the motors operate at an approximate speed of 250 strokes per minute.
When the "low" position is selected, relay 1 is energized. This causes field 1 and field 2 to be energized in series. The motor then operates at approximately 160 strokes per minute. Setting the switch to the "off" position, allows the relay contacts to return to their normal positions. However, the wiper motor will continue to run until the wiper arm reaches the "park" position. When both relays are open and the park switch is closed, the excitation to the motor is reversed. This causes the wiper to move off the lower edge of the windshield, opening the cam-operated park switch. This de-energizes the motor and releases the brake solenoid applying the brake. This ensures that the motor will not coast and re-close the park switch.
Hydraulic Windshield Wiper Systems Hydraulic windshield wipers are driven by pressure from the aircraft's main hydraulic system. Figure 7-19 shows the components making up a representative hydraulic windshield wiper system.
WASHER WIPER
PILOT'S OVERHEAD PANEL
- Wiper blade Wiper arm Motor converter
• Parked position FICURE 7-17. Electrical windshield wiper system.
The speed control valve is used to start, stop, and fow of fluid to the control unit, and therefore the control the operating speed of the windshield wip- speed of the windshield wipers.
ers. The speed control valve is a type of variable The control unit directs the flow of hydraulic restrictor. Turning the handle of this valve counter- fluid to the wiper actuators and returns fluid disclockwise increases the size of the fluid opening, the charged from the actuators to the main hydraulic 305
Field High Field 2 Windshield control module | Relay 1 Relay 2 High Low Left windshield wiper motor FIcURE 7-18. Windshield wiper circuit diagram.
Actuator of aero Speed control valve From main hydraulic Control Unit Retur line system FIcURE 7-19. Hydraulic windshield wiper schematic.
system. The control unit also alternates the direction of hydraulic fluid flow to each of the two wiper actuators. The wiper actuators convert hydraulic energy into reciprocating motion to drive the wiper arms back and forth.
Figure 7-20 shows the construction and the plumbing of the actuators. Notice that each actuator consists of a two-port housing, a piston rack, and a pinion gear. The teeth of the pinion gear mesh with those on the piston rack. Thus, whenever pressurized fluid enters the actuator and moves the piston rack, the pinion gear is rotated a fraction of a turn.
Since the pinion gear connects through a shaft to the wiper blade, the blade rotates through an arc.
Notice that one line from the control unit connects to port No. 1 of actuator A, while the other line connects to port No. 4 of actuator B. Notice, toc that a line connects ports No. 2 and No. 3 of th..
actuators.
Turning on the speed control valve allows fluid to flow from the main hydraulic system into the control unit, which directs pressure first into one line and then into the other. When line No. 1 is placed under pressure, fluid flows into port No. 1 and into the chamber at the left of actuator A. This drives the piston rack to the right, causing the pinion and wiper blade to rotate through a counterclockwise arc. As the piston rack moves to the right, it forces fluid in the right chamber of actuator A to move out port No. 2, through the connecting line to port No. 3 and into actuator B. This causes the piston rack in actuator B to move to the right, causing its pinion and wiper blade to rotate counterclockwise.
As the piston rack moves to the right, it forces fluid in the right chamber of actuator B to move out of 306
012N96 Actuator-A Actuator-B Ma Line 1 Line 2 Code Pressure Return FIGURE 7-20. Windshield wiper actuators.
port No. 4, into line No. 2, through the control unit, and into the hydraulic system return line.
When line No. 2 is pressurized by fluid from the control unit, the fow of fluid and the action of the actuators are reversed.
Pneumatic Rain Removal Systems Windshield wipers characteristically have two basic problem areas. One is the tendency of the slipstream aerodynamic forces to reduce the wiper blade loading pressure on the window, causing ineffective wiping or streaking. The other is in achieving fast enough wiper oscillation to keep up with high rain impingement rates during heavy rain falls. As a result, most aircraft wiper systems fail to provide satisfactory vision in heavy rain.
With the advent of turbine-powered aircraft, the pneumatic rain removal system became feasible.
This method uses high pressure, high temperature engine compressor bleed air which is blown across the windshields (figure 7-21). The air blast forms a barrier that prevents raindrops from striking the windshield surface.
Windshield Rain Repellant When water is poured onto clean glass, it spreads out evenly. Even when the glass is held at a steep angle or subjected to air velocity, the glass remains wetted by a thin film of water.
However, when glass is treated with certain chemicals, a transparent film is formed which causes the water to behave very much like mercury on glass. The water draws up into beads which cover only a portion of the glass and the area between beads is dry. The water is readily removed from the glass.
This principle lends itself quite naturally to removing rain from aircraft windshields. The high velocity slipstream continually removes the water beads, leaving a large part of the window dry.
Jet blast rain removal system -Fairing Plenum -Duct assembly From bleed air system FIcURE 7-21. Typical pneumatic rain removal system.
A rain repellant system permits application of the chemical repellant by a switch or push button in the cockpit. The proper amount of repellant is applied regardless of how long the switch is held. The repellant is marketed in pressurized disposable cans which screw into the aircraft system and provide the propelling force for application. Actuating the control switch opens an electrically-operated solenoid valve which allows repellant to flow to the discharge nozzles. The liquid repellant is squirted 307
onto the exterior of the windshield and uses the rain itself as the carrying agent to distribute the chemicals over the windshield surface.
The rain repellant system should not be operated on dry windows because heavy undiluted repellant will restrict window visibility. Should the system be operated inadvertently, do not operate the windshield wipers or rain clearing system as this tends to increase smearing. Also the rain repellant residues caused by application in dry weather or very light rain can cause staining or minor corrosion of the aircraft skin. To prevent this, any concentrated repellant or residue should be removed by a thorough fresh-water rinse at the earliest opportunity.
After application, the repellant film slowly deteriorates with continuing rain impingement. This makes periodic re-application necessary. The length of time between applications depends upon rain intensity, the type of repellant used, and whether windshield wipers are in use.
MAINTENANCE OF RAIN ELIMINATING SYSTEMS Windshield Wiper Systems Maintenance performed on windshield wiper systems consists of operational checks, adjustments, and troubleshooting.
An operational check should be performed whenever a system component is replaced or whenever the system is suspected of not working properly.
During the check, make sure that the windshield area covered by the wipers is free of foreign matter and is kept wet with water.
Adjustment of a windshield wiper system consists of adjusting the wiper blade tension, the angle at which the blade sweeps across the windshield, and proper parking of the wiper blades. Figure 7-22 illustrates the adjustment points on a typical wiper blade installation.
One adjustment is that of the tie rod length. The tie rod length adjustment nut is shown in figure
7-22. The tie rod connects the wiper blade holder to a pivot bolt next to the drive shaft. With the drive arm and the tie rod connected to the wiper blade holder, a parallelogram linkage is formed between the wiper blade holder and the wiper converter. This linkage permits the wiper blade to remain parallel to the windshield posts during its travel from one side of the windshield to the other.
Wiper blade Tie rod • Tie rod length adjustment nut Drive arm Pressure adjustment Pivot bolt nut Mounting plate FIcURE 7-22. Adjustment of windshield wiper components.
The length of the tie rod may be adjusted to vary the angle at which the wiper blade sweeps across the windshield.
Another adjustment is that which is required for proper parking of the windshield wiper blades.
When they are not operating, the wiper blades should move to a position where they will not interfere with vision. If the wipers do not park as they should, the cam which actuates the microswitch on the converter can be adjusted.
The other adjustment to be made is that of the windshield wiper spring tension. To make the adjustment, place a lightweight spring scale under the drive arm at its point of attachment to the wiper blade, and lift the scale up at a 90° angle to the drive, to a point at which the blade is just ready to leave the glass. (If tension is properly adjusted, the spring scale should indicate between 5 and 6 Ibs.)
If the scale reading does not fall within this limit, adjust the pressure adjustment nut shown in figure 7-22 until the proper tension is indicated on the scale.
Pneumatic (Jet Blast) Systems Maintenance of a jet blast system includes the replacement of defective components, the checking (by hand) of duct-to-valve connections for leakage, and an operational checkout.
CHAPTER 8
HYDRAULIC AND PNEUMATIC POWER SYSTEMS
AIRCRAFT HYDRAULIC SYSTEMS The word hydraulics is based on the Greek word for water, and originally meant the study of the physical behavior of water at rest and in motion.
Today the meaning has been expanded to include the physical behavior of all liquids, including hy- Hydraulic systems are not new to aviation. Early aircraft had hydraulic brake systems. As aircraft became more sophisticated newer systems with hydraulic power were developed.
Although some aircraft manufacturers make greater use of hydraulic systems than others, the hydraulic system of the average modern aircraft performs many functions. Among the units commonly operated by hydraulic systems are landing gear, wing flaps, speed and wheel brakes, and flight control surfaces.
Hydraulic systems have many advantages as a power source for operating various aircraft units.
Hydraulic systems combine the advantages of light weight, ease of installation, simplification of inspection, and minimum maintenance requirements. Hydraulic operations are also almost 100% efficient, with only a negligible loss due to fluid friction.
All hydraulic systems are essentially the same, regardless of their function. Regardless of application, each hydraulic system has a minimum number of components, and some type of hydraulic fluid.
HYDRAULIC FLUID Hydraulic system liquids are used primarily to transmit and distribute forces to various units to be actuated. Liquids are able to do this because they are almost incompressible. Pascal's Law states that pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part. Thus, if a number of passages exist in a system, pressure can be distributed through all of them by means of the liquid.
Manufacturers of hydraulic devices usually specify the type of liquid best suited for use with their equipment, in view of the working conditions, the service required, temperatures expected inside and outside the systems, pressures the liquid must withstand, the possibilities of corrosion, and other conditions that must be considered.
If incompressibility and fluidity were the only qualities required, any liquid not too thick might be used in a hydraulic system. But a satisfactory liquid for a particular installation must possess a number of other properties. Some of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular system are discussed in the following paragraphs.
Viscosity One of the most important properties of any hydraulic fluid is its viscosity. Viscosity is internal resistance to flow. A liquid such as gasoline flows easily (has a low viscosity) while a liquid such as tar flows slowly (has a high viscosity). Viscosity increases with temperature decreases.
A satisfactory liquid for a given hydraulic system must have enough body to give a good seal at pumps, valves, and pistons; but it must not be so thick that it offers resistance to flow, leading to power loss and higher operating temperatures.
These factors will add to the load and to excessive wear of parts. A fluid that is too thin will also lead to rapid wear of moving parts, or of parts which have heavy loads.
The viscosity of a liquid is measured with a viscosimeter or viscometer. There are several types, but the instrument most often used by engineers in the U.S. is the Saybolt universal viscosimeter (figure 8-1). This instrument measures the number of seconds it takes for a fixed quantity of liquid (60 cc. (cubic centimeters)) to flow through a small orifice of standard length and diameter at a specific temperature. This time of flow is taken in seconds, and the viscosity reading is expressed as SSU (seconds, Saybolt universal). For example, a certain liquid might have a viscosity of 80 SSU at 130° F.
Heating unit Thermometer Liquid bath Oil Container Cork 60 c.c. - Reservoir FIGURE 8-1. Saybolt viscosimeter.
Chemical Stability Chemical stability is another property which is exceedingly important in selecting a hydraulic liquid. It is the liquid's ability to resist oxidation and deterioration for long periods. All liquids tend to undergo unfavorable chemical changes under severe operating conditions. This is the case, for example, when a system operates for a considerable period of time at high temperatures.
Excessive temperatures have a great effect on the life of a liquid. It should be noted that the temperature of the liquid in the reservoir of an operating hydraulic system does not always represent a true state of operating conditions. Localized hot spots occur on bearings, gear teeth, or at the point where liquid under pressure is forced through a small orifice. Continuous passage of a liquid through these points may produce local temperatures high enough to carbonize or sludge the liquid, yet the liquid in the reservoir may not indicate an excessively high temperature. Liquids with a high viscosity have a greater resistance to heat than light or low viscosity liquids which have been derived from the same source. The average hydraulic liquid has a low viscosity. Fortunately, there is a wide choice of liquids available for use within the viscosity range required of hydraulic liquids.
Liquids may break down if exposed to air, water.
salt, or other impurities, especially if they are in constant motion or subject to heat, Some metals, such as zinc, lead, brass, and copper, have an undesirable chemical reaction on certain liquids.
These chemical processes result in the formation of sludge, gums, and carbon or other deposits which clog openings, cause valves and pistons to stick or leak, and give poor lubrication to moving parts. As soon as small amounts of sludge or other deposits are formed, the rate of formation generally increases more rapidly. As they are formed, certain changes in the physical and chemical properties of the liquid take place. The liquid usually becomes darker in color, higher in viscosity, and acids are formed.
Flash Point Flash point is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily or flash when a flame is applied. A high flash point is desirable for hydraulic liquids because it indicates good resistance to combustion and a low degree of evaporation at normal temperatures.
Fire Point Fire point is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame. Like flash point, a high fire point is required of desirable hydraulic liquids.
TYPES OF HYDRAULIC FLUIDS To assure proper system operation and to avoid damage to non-metallic components of the hydraulic system, the correct fluid must be used.
When adding fluid to a system, use the type specified in the aircraft manufacturer's maintenance manual or on the instruction plate affixed to the reservoir or unit being serviced.
There are three types of hydraulic fluids currently being used in civil aircraft.
Vegetable Base Hydraulic Fluid Vegetable base hydraulic fluid (MIL-H-7644) is composed essentially of caster oil and alcohol:
It has a pungent alcoholic odor and is generally dyed blue. Although it has a similar composition to automotive type hydraulic fluid, it is not interchangeable. This fuid is used primarily in older type aircraft. Natural rubber seals are used with vegetable base hydraulic fluid. If it is contaminated with petroleum base or phosphate ester base fluids, the seals will swell, break down and block the system. This type fluid is flammable.
Mineral Base Hydraulic Fluid Mineral base hydraulic fluid (MIL-H-5606) is processed from petroleum. It has an odor similar to penetrating oil and is dyed red. Synthetic rubber seals are used with petroleum base fluids. Do not mix with vegetable base or phosphate ester base hydraulic fluids. This type fluid is flammable and turboprop aircraft.
operating factor.
Intermixing of Fluids Auids.
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 which are Skydrol ® resistant 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.
Skydrol ® is a registered trademark of Monsanto Company. Skydrol ® 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 Skydrol B and must be replaced with seals of butyl rubber or ethylene-propylene elastomers. These seals are readily available from any suppliers.
Health and Handling Skydrol ® fuid does not present any particular health hazard in its recommended use. Skydrol ® fluid has a very low order of toxicity when taken orally or applied to the skin in liquid form. It causes pain on contact with eye tissue, but animal studies and human experience indicate Skydrol ® fluid causes no permanent damage. First aid treatment for eye contact includes flushing the eyes immediately with large volumes of water and the application of any anesthetic eye solution. It pain persists, the individual should be referred to a physician.
In mist or fog form, Skydrol ® is quite irritating to nasal or respiratory passages and gen erally produces coughing and sneezing. Such irritation does not persist following cessation of exposure.
Silicone ointments, rubber gloves, and careful washing procedures should be utilized to avoid excessive repeated contact with Skydrol ® order to avoid solvent effect on skin.
Hydraulic Fluid Contamination Experience has shown that trouble in a hydraulic does not appre- 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:
(1) Abrasives, including such particles as core sand, weld spatter, machining chips, and rust.
(2) Nonabrasives, including those resulting from oil oxidation, and soft particles worn or shredded from seals and other organic components.
Contamination Check 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 designed to remove most foreign particles that are visible to the naked eye. Hydraulic liquid which 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
- To vacuum pump the hydraulic system are indications that one or more components in the system 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. In either case, while the fluid is being taken, a small amount of pressure should be applied to the system. This ensures that the liquid will flow out of the sampling point and thus prevent Filter paper Gooch crucible Rubber crucible holder Filter flask Polyethylene washing bottle Fluid sample bottle Reference filter patch FICURE 8-2. Contamination test kit.
dirt from entering the hydraulic system. Some contamination test kits have hypodermic syringes for taking samples.
Various test procedures are used to determine the contamination level in hydraulic liquids. The filter patch test provides a reasonable idea of the condition of the fluid. This test consists basically of filtration of a sample of hydraulic system liquid through a special filter paper. This filter paper darkens in degree in relation to the amount of contamination present in the sample, and is compared to a series of standardized filter disks which, by degree of darkening, indicate the various contamination levels. The equipment provided with one type of contamination test kit is illustrated in hgure
8-2.
When using this type of contamination test kit, the liquid samples should be poured through the filter paper, and the test filter paper should be compared with the test patches supplied with the test kit. More expensive test kits have a microscope for making this comparison.
To check liquid for decomposition, pour new hydraulic liquid into a sample bottle of the same size and color as the bottle containing the liquid to be checked. Visually compare the color of the two bottles. Liquid which is decomposed will be darker in color.
At the same time the contamination check is made, it may be necessary to make a chemical test.
This test consists of a viscosity check, a moisture check, and a flash point check. However, since special equipment is required for these checks, the liquid samples must be sent to a laboratory, where a technician will perform the test.
(1) Maintain all tools and the work area (workbenches and test equipment) 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 procedures.
(3) Before disconnecting hydraulic lines or fittings, clean the affected area with dry cleaning solvent.
(4) All hydraulic lines and fittings should be capped or plugged immediately after disconnecting.
(5) Before assembly of any hydraulic components, wash all parts in an approved dry cleaning solvent.
(6) 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.
(7) All seals and gaskets should be replaced during the re-assembly procedure. Use only those seals and gaskets recommended by the manufacturer.
(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 in accordance with applicable technical instructions.
(9) All hydraulic servicing equipment should be kept clean and in good operating condition.
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.
Should the system become 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:
FILTERS A filter is a screening or straining device used to clean the hydraulic fluid, thus preventing foreign particles and contaminating substances from remaining in the system. If such objectionable material is not removed, it may cause the entire hydraulic system of the aircraft to tail through the breakdown or malfunctioning of a single unit of the system.
The hydraulic fluid holds in suspension tiny particles of metal that are deposited during the normal wear of selector valves, pumps, and other system components. Such minute particles of metal may injure the units and parts through which they pass if they are not removed by a filter. Since tolerances within the hydraulic system components are 313