Par 9-28 Page 9-19
e. Do not expose fluids to high temperature or open flames. Mineral-based fluids are highly flammable.
f. The hydrocarbon-based hydraulic fluids are, in general, safe to handle. To work with these fluids, reasonable handling procedures must always be followed. Take precaution to avoid fluid getting in the eyes. If fluid contacts the eye, wash immediately with water.
g. When handling Skydrol/Hyjet hydraulic fluids, gloves that are impervious to the fluid must be worn. If skin contact occurs, wash with soap and water.
h. When handling phosphate ester-based fluid use eye protection. If the eye is exposed to fluid, severe eye pain will occur.
i. When Skydrol/Hyjet mist or vapor exposure is possible, a respirator capable of removing organic vapors and mists must be worn.
j. Ingestion of any hydraulic fluid should be avoided. Although small amounts do not appear to be highly hazardous, any significant amount should be tested in accordance with manufacturer’s direction, followed with hospital supervised stomach treatment.
9-29. HYDRAULIC SYSTEM MAINTENANCE PRACTICES. The maintenance of hydraulic and pneumatic systems should be performed in accordance with the aircraft manufacturer’s instructions. The following is a summary of general practices followed when dealing with hydraulic and pneumatic systems.
a. Service. The servicing of hydraulic and pneumatic systems should be performed at the intervals specified by the manufacturer. Some components, such as hydraulic reservoirs, have servicing information adjacent to the component. When servicing a hydraulic reservoir, make certain to use the correct type of fluid. Hydraulic fluid type can be identified by color and smell;
however, it is good practice to take fluid from the original marked container and then to check the fluid by color and smell for verification. Fluid containers should always be closed, except when fluid is being removed.
b. Contamination Control. 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 terms of “micron,” and is an indication of the particle size that will be filtered out. The replacement interval of these filters is established by the manufacturer and is included in the maintenance manual. However, in the absence of specific replacement instructions, a recommended service life of the filter elements is:
• Pressure filters—3000 hrs.
• Return filters—1500 hrs.
• Case drain filters—600 hrs.
Par 9-29 Page 9-20
(1) When replacing filter elements, be sure that there is no pressure on the filter bowl.
Protective clothing and a face shield must be used to prevent fluid from contacting the eye.
Replace the element with one that has the proper rating. After the filter element has been replaced, the system must be pressure tested to ensure that the sealing element in the filter assembly is intact.
(2) In the event of a major component failure, such as a pump, consideration must be given to replacing the system filter elements, as well as the failed component. System filters may also be equipped with differential pressure (ΔP) indicators. These indicators are designed to “pop up” when the pressure drop across the element reaches a predetermined value caused by contamination held by the element. The indicators are designed to prevent false indications due to cold start, pump ripple, and shock loads. Consequently, a filter whose indicator has been activated must be replaced. In fact, some indicator designs are such that the indicator cannot be reset, unless the filter bowl is removed and the element replaced.
c. Flushing a Hydraulic System. 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.
d. Inspections. Hydraulic and pneumatic systems are inspected for leakage, worn or damaged tubing, worn or damaged hoses, wear of moving parts, security of mounting for all units, safetying, and any other condition specified by the maintenance manual. A complete inspection includes considering the age, cure date, stiffness of the hose, and an operational check of all subsystems.
(1) Leakage from any stationary connection in a system is not permitted, and if found, it should be repaired. A small amount of fluid seepage may be permitted on actuator piston rods and rotating shafts. In a hydraulic system, a thin film of fluid in these areas indicates that the seals are being properly lubricated. When a limited amount of leakage is allowed at any point, it is usually specified in the appropriate manual.
Par 9-29 Page 9-21
(2) Tubing should not be nicked, cut, dented, collapsed, or twisted beyond approved limits. The identification markings or lines on a flexible hose will show whether the hose has been twisted. (See Figure 9-9.)
(3) All connections and fittings associated with moving units must be examined for play evidencing wear. Such units should be in an unpressurized condition when they are checked for wear.
(4) Accumulators must be checked for leakage, air or gas preload, and position. If the accumulator is equipped with a pressure gauge, the preload can be read directly.
(5) An operational check of the system can be performed using the engine-driven pump, an electrically operated auxiliary pump (if such a pump is included in the system), or a ground test unit. The entire system and each subsystem should be checked for smooth operation, unusual noises, and speed of operation for each unit. The pressure section of the system should be checked with no subsystems to see that pressure holds for the required time without the pump supplying the system. System pressure should be observed during operation of each subsystem to ensure that the engine-driven pump maintains the required pressure.
e. Troubleshooting. Hydraulic system troubleshooting varies according to the complexity of the system and the components in the system. It is, therefore, important that the technician refer to the troubleshooting information furnished by the manufacturer.
(1) Lack of pressure in a system can be caused by a sheared pump shaft, defective relief valve, the pressure regulator, an unloading valve stuck in the “kicked-out” position, lack of fluid in the system, the check valve installed backward, or any condition that permits free flow back to the reservoir or overboard. If a system operates satisfactorily with a ground test unit but not with the system pump, the pump should be examined.
(2) If a system fails to hold pressure in the pressure section, the likely cause is the pressure regulator, an unloading valve, a leaking relief valve, or a leaking check valve.
(3) If the pump fails to keep pressure up during operation of the subsystem, the pump may be worn or one of the pressure-control units may be leaking.
(4) High pressure in a system may be caused by a defective or improperly adjusted pressure regulator, an unloading valve, or by an obstruction in a line or control unit.
(5) Unusual noise in a hydraulic system, such as banging and chattering, may be caused by air or contamination in the system. Such noises can also be caused by a faulty pressure regulator, another pressure-control unit, or a lack of proper accumulator action.
(6) Maintenance of hydraulic system components involves a number of standard practices together with specialized procedures set forth by manufacturers, such as the replacement of valves, actuators, and other units, including tubing and hoses. Care should be exercised to prevent system contamination damage to seals, packings, and other parts, and to apply proper torque in connecting fittings. When installing fittings, valves, etc. always lubricate the threads with hydraulic fluid.
Par 9-29 Page 9-22
(7) Overhaul of hydraulic and pneumatic units is usually accomplished in approved repair facilities; however, replacement of seals and packings may be done from time to time by technicians in the field. When a unit is disassembled, all O-ring and Chevron seals should be removed and replaced with new seals. The new seals must be of the same material as the original and must carry the correct manufacturer’s part number. No seal should be installed unless it is positively identified as the correct part and the shelf life has not expired.
(8) When installing seals, care should be exercised to ensure that the seal is not scratched, cut, or otherwise damaged. When it is necessary to install a seal over sharp edges, the edges must be covered with shim stock, plastic sheet, or electrical tape.
(9) The replacement of hydraulic units and tubing usually involves the spillage of some hydraulic fluid. Care should be taken to ensure that the spillage of fluid is kept to a minimum by closing valves, if available, and by plugging lines immediately after they are disconnected. All openings in hydraulic systems should be capped or plugged to prevent contamination of the system.
(10) The importance of the proper torque applied to all nuts and fittings in a system cannot be overemphasized. Too much torque will damage metal and seals, and too little torque will result in leaks and loose parts. The proper torque wrenches with the appropriate range should be used in assembling system units.
f. Disposal of Used Hydraulic Fluids. In the absence of organizational guidelines, the technician should be guided by local, state, and Federal regulations, with regard to means of disposal of used hydraulic fluid. Presently, the most universally accepted procedure for disposal of phosphate ester-based fluid is incineration.
9-30. HYDRAULIC LINES AND FITTINGS. Carefully inspect all lines and fittings at regular intervals to ensure airworthiness. Investigate any evidence of fluid loss or leaks. Check metal lines for leaks, loose anchorage, scratches, kinks, or other damage. Inspect fittings and connections for leakage, looseness, cracks, burrs, or other damage. Replace or repair defective elements. Make sure the lines and hoses do not chafe against one another and are correctly secured and clamped.
a. Replacement of Metal Lines. When inspection shows a line to be damaged or defective, replace the entire line or, if the damaged section is localized, a repair section may be inserted. In replacing lines, always use tubing of the same size and material as the original line.
Use the old tubing as a template in bending the new line, unless it is too greatly damaged, in which case a template can be made from soft iron wire. Soft aluminum tubing (1100, 3003, or 5052) under 1/4-inch outside diameter may be bent by hand. For all other tubing use an acceptable hand or power tube-bending tool. Bend tubing carefully to avoid excessive flattening, kinking, or wrinkling. Minimum bend radii values are shown in Table 9-2. A small amount of flattening in bends is acceptable, but do not exceed 75 percent of the original outside diameter.
Excessive flattening will cause fatigue failure of the tube. When installing the replacement tubing, line it up correctly with the mating part so that it is not forced into alignment by tightening of the coupling nuts.
Par 9-30 Page 9-23 TABLE 9-2. TUBE DATA Dash Nos.
Ref.
Tubing OD inches Wrench torque for tightening AN-818 Nut (pound inch) Minimum bend radii measured to tubing centerline.
Dimension in inches.
Aluminum-alloy tubing Steel tubing Aluminum-alloy tubing (Flare MS33583) for use on oxygen lines only Minimum Maximum Minimum Maximum Minimum Maximum Alum Alloy Steel -2 1/8 20 30 75 85 -- -- 3/8 -- -3 3/16 25 35 95 105 -- -- 7/16 21/32 -4 1/4 50 65 135 150 -- -- 9/16 7/8 -5 5/16 70 90 170 200 100 125 3/4 1-1/8 -6 3/8 110 130 270 300 200 250 15/16 1-5/16 -8 1/2 230 260 450 500 300 400 1-1/4 1-3/4 -10 5/8 330 360 650 700 -- -- 1-1/2 2-3/16 -12 3/4 460 500 900 1000 -- -- 1-3/4 2-5/8 -16 1 500 700 1200 1400 -- -- 3 3-1/2 -20 1-1/4 800 900 1520 1680 -- -- 3-3/4 4-3/8 -24 1-1/2 800 900 1900 2100 -- -- 5 5-1/4 -28 1-3/4 -- -- -- -- -- -- -- -- -32 2 1800 2000 2660 2940 -- -- 8 7
b. Tube Connections. Many tube connections are made using flared tube ends with standard connection fittings: AN-818 (MS 20818) nut and AN-819 (MS 20819) sleeve. In forming flares, cut the tube ends square, file smooth, remove all burrs and sharp edges, and thoroughly clean. The tubing is then flared using the correct 37-degree aviation flare forming tool for the size of tubing and type of fitting. A double flare is used on soft aluminum tubing 3/8-inch outside diameter and under, and a single flare on all other tubing. In making the connections, use hydraulic fluid as a lubricant and then tighten. Overtightening will damage the tube or fitting, which may cause a failure. Undertightening may cause leakage which could result in a system failure.
CAUTION: Mistaken use of 45-degree automotive flare forming tools may result in improper tubing flare shape and angle; causing misfit, stress and strain, and probable system failure.
c. Repair of Metal Tube Lines. Minor dents and scratches in tubing may be repaired.
Scratches or nicks not deeper than 10 percent of the wall thickness in aluminum alloy tubing, that are not in the heel of a bend, may be repaired by burnishing with hand tools. Replace lines with severe die marks, seams, or splits in the tube. Any crack or deformity in a flare is
Par 9-30 Page 9-24 unacceptable and cause for rejection. A dent less than 20 percent of the tube diameter is not objectionable unless it is in the heel of a bend. A severely damaged line should be replaced;
however, it may be repaired by cutting out the damaged section and inserting a tube section of the same size and material. Flare both ends of the undamaged and replacement tube sections and make the connection by using standard unions, sleeves, and tube nuts. If the damaged portion is short enough, omit the insert tube and repair by using one union and two sets of connection fittings.
d. Replacement of Flexible Lines. When replacement of a flexible line is necessary, use the same type, size, part number, and length of hose as the line to be replaced. Check Technical Standard Order (TSO) requirements. If the replacement of a hose with a swaged-end type fitting is necessary, obtain a new hose assembly of the correct size and composition. Certain synthetic oils require a specially compounded synthetic rubber hose, which is compatible. Refer to the aircraft manufacturer’s service information for the correct part number for the replacement hose.
If the fittings on each end are of the correct type or sleeve type, a replacement may be fabricated as shown in Figure 9-8. Before cutting new flexible wire braided hose to the proper size, tape the hose tightly with masking tape and cut in the center of the masking tape to prevent fraying. The use of a mandrel will prevent cutting the inside of the hose when inserting the fittings. Typical aircraft hose specifications and their uses are shown in Table 9-3. Install hose assemblies without twisting. (See Figure 9-9.) A hose should not be stretched tight between two fittings as this will result in overstressing and eventual failure. The length of hose should be sufficient to provide about 5 to 8 percent slack. Avoid tight bends in flex lines as they may result in failure. Never exceed the minimum bend radii as indicated in Figure 9-10.
FIGURE 9-8. HOSE ASSEMBLY INSTRUCTIONS (CAN BE USED FOR
LOW-PRESSURE HYDRAULIC FLUID, AND OIL LINE APPLICATIONS)
LOCATE AND SLIT
COYER WITH KINFE
TO WHE BALLO
TWIST OFF
COVER WITH PLIERS
1. Place hose in vise fine tooth 2. Locate length of hose to be cut desired length using fine off and slit cover with knife to wire hacksaw or cut off wheel. braid. After slitting cover, twist off with pair of pliers. (see note below)
3. Place hose in vise and screw 4. *Lubricate inside of hose and socket on hose counterclockwise. nipple threads liberally.
NOTE:
Hose assemblies fabricated per MIL-H-8790 must have the exposed wire braid coated with a special sealant.
5. Screw nipple into socket using wrench on hex of nipple and leave .005 inches to .031 inches clearance between nipple hex and socket.
NOTE:
Step 2 applies to high pressure hose only.
*CAUTION:
Do not use any petroleum product with hose designed for synthetic fluids,iSKYDROL and /or HYJET product. For a lubricant during assembly use a vegetable soap liquid.
DISASSEMBLE IN REVERSE ORDER Par 9-30 Page 9-25
Par 9-30 Page 9-26 TABLE 9-3. AIRCRAFT HOSE SPECIFICATIONS
SINGLE WIRE BRAID FABRIC COVERED MIL. PART NO.
TUBE SIZE OD HOSE SIZE ID HOSE SIZE OD RECOMM OPER.
PRESS.
MIN.
BURST PRESS.
MAX.
PROOF PRESS.
MIN BEND RADIUS MIL-H-8794-3-L 3/16 1/8 .45 3,000 12,000 6,000 3.00 MIL-H-8794-4-L 1/4 3/16 .52 3,000 12,000 6,000 3.00 MIL-H-8794-5-L 5/16 1/4 .58 3,000 10,000 5,000 3.38 MIL-H-8794-6-L 38 5/16 .67 2,000 9,000 4,500 4.00 MIL-H-8794-8-L 1/2 13/32 .77 2,000 8,000 4,000 4.63 MIL-H-8794-10-L 5/8 1/2 .92 1,750 7,000 3,500 5.50 MIL-H-8794-12-L 3/4 5/8 1.08 1,750 6,000 3,000 6.50 MIL-H-8794-16-L 1 7/8 1.23 800 3,200 1,600 7.38 MIL-H-8794-20-L 1 1/4 1 1/8 1.50 600 2,500 1,250 9.00 MIL-H-8794-24-L 1 1/2 1 3/8 1.75 500 2,000 1,000 11.00 MIL-H-8794-32-L 2 1 13/16 2.22 350 1,400 700 13.25 MIL-H-8794-40-L 2 1/2 2 3/8 2.88 200 1,000 300 24.00 MIL-H-8794-48-L 3 3 3.56 200 800 300 33.00 Construction: Seamless synthetic rubber inner tube reinforced with one fiber braid, one braid of high tensile steel wire and covered with an oil-resistant rubber impregnated fiber braid.
Uses: Hose is approved for use in aircraft hydraulic, pneumatic, coolant, fuel and oil systems.
Identification: Hose is identified by specification number, size number, quarter year and year, hose manufacturer’s identification.
Operating Temperatures:
Sizes—3 through 12:
Minus 65 °F to plus 250 °F Sizes—16 through 48:
Minus 40 °F to plus 275 °F Note: Maximum temperatures and pressures should not be used simultaneously.
Par 9-30 Page 9-27 TABLE 9-3. AIRCRAFT HOSE SPECIFICATIONS (CONTINUED)
MULTIPLE WIRE BRAID RUBBER COVERED MIL PAR NO.
TUBE
SIZE OD HOSE
SIZE ID HOSE SIZE OD RECOMM OPER.
PRESS.
MIN.
BURST PRESS.
MIN.
PROOF PRESS.
MIN.
BEND RADIUS MIL-H-8788-4-L 1/4 7/32 0.63 3,000 16,000 8,000 3.00 MIL-H-8788-5-L 5/16 9/32 0.70 3,000 14,000 7,000 3.38 MIL-H-8788-6-L 3/8 11/32 0.77 3,000 14,000 7,000 5.00 MIL-H-8788-8-L 1/2 7/16 0.86 3,000 14,000 7,000 5.75 MIL-H-8788-10-L 5/8 9/16 1.03 3,000 12,000 6,000 6.50 MIL-H-8788-12-L 3/4 11/16 1.22 3,000 12,000 6,000 7.75 MIL-H-8788-16-L 1 7/8 1.50 3,000 10,000 5,000 9.63 Hose Construction: Seamless synthetic rubber inner tube reinforced with one fabric braid, two or more steel wire braids, and covered with a synthetic rubber cover (for gas applications request perforated cover).
Uses: High pressure hydraulic, pneumatic, coolant, fuel and oil.
Operating Temperatures:
Minus 65 °F to plus 200 °F Identification: Hose is identified by specification number, size number, quarter year and year, hose manufacturer’s identification.
FIGURE 9-9. PROPER HOSE INSTALLATIONS
RIGHT WAY WRONG WAY Do not bend or twist the hose as illustrated.
Allow enough slack in the hose line to provide for changes in length when pressure is applied. The hose will change in length from + 2% to - 4%.
Metal end fittings cannot be considered as part of the flexible portion of the assembly.
The use of elbows and adapters will ensure easier installation and in many installations will remove the strain from the hose line and greatly increase service life.
At all times keep the minimum bend radii of the hose as large as possible to avoid tube collapsing.
Par 9-30 Page 9-28
FIGURE 9-10. MINIMUM BEND RADII
- 3 INSIDE BEND RADII V.S. OPERATING PRESSURE MIL-H-8794 HOSE WITH NO FLEXING 12 - 10 20 200 600 800 1000 1500 2000 2500 3000 OPERATING PRESSURE - POUNDS/SQ. IN. (-3 TO -24 INCLUSIVE) IRES ARE AS FOLLOWS:
MIL-H-STER HOLE WITH 40 NO FLEXING - 48 DASH NO. SEND RADI
3.000
3.375
5.000 3750
4.500 0 #D 100 120 140 160 180 7350
9.415
TOTAL FLEXING RANCE OF INSTALLED HOSE (DEGREES) MINIMUM BEND RADIUS OF HOSE UNDER FLEXING CONDITIONS = "N" X NO FLEXING BEND RADIUS OF EITHER MIL-H-8794 OR MIL-H-8788 HOSE EXAMPLE: FOR MIL-H-8794 HOSE (MEASURED AT INSIDE OF BEAD). RADIUS
MININUM BEND RADI MEASURED AT INSIDE OF BEND DIMENSIONS IN INCHES.
Par 9-30 Page 9-29
Par 9-30 Page 9-30
(1) Teflon hose is used in many aircraft systems because it has superior qualities for certain applications. Teflon is compounded from tetrafluoroethylene resin which is unaffected by fluids normally used in aircraft. It has an operating range of -65 °F to 450 °F. For these reasons, Teflon is used in hydraulic and engine lubricating systems where temperatures and pressures preclude the use of rubber hose. Although Teflon hose has excellent performance qualities, it also has peculiar characteristics that require extra care in handling. It tends to assume a permanent set when exposed to high pressure or temperature. Do not attempt to straighten a hose that has been in service. Any excessive bending or twisting may cause kinking or weakening of the tubing wall. Replace any hose that shows signs of leakage, abrasion, or kinking. Any hose suspected of kinking may be checked with a steel ball of proper size. Table 9-4 shows hose and ball sizes. The ball will not pass through if the hose is distorted beyond limits.
TABLE 9-4. BALL DIAMETERS FOR TESTING HOSE RESTRICTIONS OR KINKING
HOSE SIZE BALL SIZE -4 5/64 -5 9/64 -6 13/64 -8 9/32 -10 3/8 -12 1/2 -16 47/64 -20 61/64
(2) If the hose fittings are of the reusable type, a replacement hose may be fabricated as described in Figure 9-8. See Figure 9-10 for minimum bend radii. When a hose assembly is removed, the ends should be tied as shown in Figure 9-11, so that the preformed shape will be maintained. See Figure 9-12 for minimum bend radii for Teflon hose.