Par 9-29 Page 9-427 technician refer to the troubleshooting information furnished by the manufacturer.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The importance of the proper torque applied to all nuts and fittings in a system cannot be over-emphasized. 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.
Disposal of Used Hydraulic Fluids. In the absence of organizational guidelines, the
Par 9-29 Page 9-428 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 esterbased fluid is incineration.
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.
Replacement of Metal Lines. When inspection shows a line to be damaged or def ective, 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) may also be used as a template. For forming of all tubing use an acceptable hand or power t ube-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 5 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.
Tube Connections. Many tube connections are made using flared tube ends with standard connection fitti ngs: 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 a nd 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. Over-tightening will damage the tube or fitting, which may cause a failure.
Under-tightening 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.
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 unacceptable and cause for rejection. A dent less than 10 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 replacem ent 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.
Par 9-30 Page 9-429 TABLE 9-2. Tube data.
Wrench torque for tightening AN-818 Nut (pound inch) Minimum bend radii Dash Nos. Tubing OD Aluminum-alloy tubing Steel tubing Aluminum-alloy tubing measured to tubing Ref. inches Minimum
Maximum
Minimum
Maximum (Flare MS33583) for use on oxygen lines only centerline. Dimension in inches.
Minimum Maximum Alum.
Alloy Steel
-2 -3 -4 -5 -6 -8 -10 -12 -16 -20 -24 -28 -32
1/8 3/16 1/4 5/16 3/8 1/2 5/8 3/4 1 1-1/4 1-1/2 1-3/4 2
20 25 50 70 110 230 330 460 500 800 800 -- 1800
30 35 65 90 130 260 360 500 700 900 900 -- 2000
75 95 135 170 270 450 650 900 1200 1520 1900 -- 2660
85 105 150 200 300 500 700 1000 1400 1680 2100 -- 2940
-- -- -- 100 200 300 -- -- -- -- -- -- --
-- -- -- 125 250 400 -- -- -- -- -- -- --
3/8 7/16 9/16 3/4 15/16 1-1/4 1-1/2 1-3/4 3 3-3/4 5 -- 8
-- 21/32 7/8 1-1/8 1-5/16 1-3/4 2-3/16 2-5/8 3-1/2 4-3/8 5-1/4 -- 7
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 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 se rvice 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 specifi cations 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.
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 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.
If the hose fittings are of the reusable type, a replacement hose may be
Par 9-30 Page 9-430 fabricated as described in figure 9-8. 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. Refer to figure 9-10 for minimum bend radii.
All flexible hose installations should be supported at least every 24 inches. Closer supports are preferred. They should be carefully routed and securely clamped to avoid abrasion, kinking, or excessive flexing.
Excessive flexing may cause weakening of the hose or loosening at the fittings.
O-Ring Seals. An understanding of O -ring seal applications is necessary to determine when replacement should be made. The simplest application is where the O-ring merely serves as a gasket when it is compressed within a recessed area by applying pressure with a packing nut or screw cap. Leakage is not normally acceptable in this type of installation. In other installations, the O -ring seals depend primarily upon their resiliency to accomplish their sealing action.
When moving parts are involved, minor seepage may be normal and acceptable. A moist surface found on moving parts of hydraulic units is an indication the seal is being properly lubricated. When systems are static, seepage past the seals is not normally acceptable.
During inspection, consider the following to determine whether seal replacement is necessary.
How much fluid is permitted to seep past the seals? In some installations minor seepage is normal. Refer to the manufacturer’s maintenance information.
What effect does the leak have on the operation of the system? Know the system.
Does the leak of fluid create a hazard or affect surrounding installations? A check of the system fluid and a knowledge of previous fluid replenishment is helpful.
Will the system function safely without depleting the reservoirs until the next inspection?
Hydraulic System Pressure Test. When a flexible hose has been repaired or overhauled using existing hard worn and new hose material, before the hose is installed on the aircraft it is recommended that the hose is tested to at least 1.5 system pressure. A new hose can be operationally checked after it is installed in the aircraft using system pressure.
LOCATE AND SLIT
COYER WITH KNIFE
TO WIRE BRAD
TWIST OFF
COVER WITH PLIERS
1. Place hose in vise and cut to 2. Locate length of hose to be cut desired length using fine tooth 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, "SKYDROL and /or HYJET product." For a lubricant during assembly use a vegetable soap liquid.
DISASSEMBLE IN REVERSE ORDER FIGURE 9-8. Hose assembly instructions (can be used for low pressure hydraulic fluid, and oil line applications).
Par 9-30 Page 9-431
Par 9-30 Page 9-432 TABLE 9-3. Aircraft hose specifications.
SINGLE WIRE BRAID FABRIC COVERED MIL.
PART NO.
TUBE SIZE O.D.
HOSE SIZE I.D.
HOSE SIZE O.D.
RECOMM.
OPER.
PRESS.
MIN.
BURST PRESS.
MAX.
PROOF PRESS.
MIN BEND RADIUS
MIL-H-8794- 3-L MIL-H-8794- 4-L MIL-H-8794- 5-L MIL-H-8794- 6-L MIL-H-8794- 8-L MIL-H-8794-10-L MIL-H-8794-12-L MIL-H-8794-16-L MIL-H-8794-20-L MIL-H-8794-24-L MIL-H-8794-32-L MIL-H-8794-40-L MIL-H-8794-48-L 3/16 1/4 5/16 38 1/2 5/8 3/4 1 1 1/4 1 1/2 2 2 1/2 3 1/8 3/16 1/4 5/16 13/32 1/2 5/8 7/8 1 1/8 1 3/8 1 13/16 2 3/8 3 .45 .52 .58 .67 .77 .92
1.08
1.23
1.50
1.75
2.22
2.88
3.56 3,000 3,000 3,000 2,000 2,000 1,750 1,750 800 600 500 350 200 200 12,000 12,000 10,000 9,000 8,000 7,000 6,000 3,200 2,500 2,000 1,400 1,000 800 6,000 6,000 5,000 4,500 4,000 3,500 3,000 1,600 1,250 1,000 700 300 300
3.00
3.00
3.38
4.00
4.63
5.50
6.50
7.38
9.00
11.00
13.25
24.00
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.
Identification: Hose is identified by specification number, size number, quarter year and year, hose manufacturer’s identification.
Uses: Hose is approved for use in aircraft hydraulic, pneumatic, coolant, fuel and oil systems.
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.
MULTIPLE WIRE BRAID RUBBER COVERED
MIL PAR NO.
TUBE SIZE O.D.
HOSE SIZE I.D.
HOSE SIZE O.D.
RECOMM.
OPER.
PRESS.
MIN.
BURST PRESS.
MIN.
PROOF PRESS.
MIN.
BEND RADIUS
MIL-H-8788- 4-L MIL-H-8788- 5-L MIL-H-8788- 6-L MIL-H-8788- 8-L MIL-H-8788-10-L MIL-H-8788-12-L MIL-H-8788-16-L 1/4 5/16 3/8 1/2 5/8 3/4 1 7/32 9/32 11/32 7/16 9/16 11/16 7/8
0.63
0.70
0.77
0.86
1.03
1.22
1.50 3,000 3,000 3,000 3,000 3,000 3,000 3,000 16,000 14,000 14,000 14,000 12,000 12,000 10,000 8,000 7,000 7,000 7,000 6,000 6,000 5,000
3.00
3.38
5.00
5.75
6.50
7.75
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).
Identification: Hose is identified by specification number, size number, quarter year and year, hose manufacturer’s identification.
Uses: High pressure hydraulic, pneumatic, coolant, fuel and oil.
Operating Temperatures:
Minus 65 °F. to plus 200 °F.
Par 9-30 Page 9-433
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.
FIGURE 9-9. Proper hose installations.
INSIDE BEND RADII V.S. OPERATING PRESSURE MIL-H-8794 HOSE WITH NO FLEXING 10 BEND RADIUS INCHES 20
- 16 5 4 200 400 600 800 1000 1500 2000 2500 3000 OPERATING PRESSURE - POUNDS/SQ. IN. (-3 TO -24 INCLUSIVE) MINIMUM BEND RADII FOR . - 40, AND 48 AT ALL PRESSURES ARE AS FOLLOWS: -32 13.25" MIL-H-8788 HOSE WITH - 40 24" NO FLEXING -48 33" DASH HO. BEND RADII
3.000 BEND FACTOR 3.375
5.000
5.750 10 6.500 20 40 60 80 100 120 140 160 180 12 7.750 16 9.625
TOTAL FLEXING RANGE OF INSTALLED HOSE (DEGREES) MINIMUM BEND RADIUS OF HOSE UNDER FLEXING CONDITIONS = "N" × NO FLEXING BEND RADIUS OF EITHER MIL-H-8794 OR MIL-H-8788 HOSE EXAMPLE: FOR MIL-H-8794 HOSE ONCE OF STIMUM BEND US AND MAN INS (MEASURED AT INSIDE OF BEND).
MINIMUM BEND RADII MEASURED AT INSIDE OF BEND DIMENSIONS IN INCHES.
FIGURE 9-10. Minimum bend radii.
Par 9-30 Page 9-434
Par 9-30 Page 9-435 (and 9-26) TABLE 9-4. Ball diameters for testing hose restrictions or kinking.
HOSE SIZE -4 -5 -6 -8 -10 -12 -16 -20
BALL SIZE 5/64 9/64 13/64 9/32 3/8 1/2 47/64 61/64
FIGURE 9-11. Suggested handling of preformed hose.
9-31.9-36. [RESERVED.]
Par 9-37 Page 9-436
SECTION 3. EMERGENCY EQUIPMENT
LIFE RAFTS. Inflatable life rafts are subject to general deterioration due to aging. Experience has indicated that such equipment may be in need of replacement at the end of 5 years due to porosity of the rubber-coated material. Wear of such equipment is accelerated when stowed on board aircraft because of vibration which causes chafing of the rubberized fabric. This ultimately results in localized leakage. Leakage is also likely to occur where the fabric is folded because sharp corners are formed. When these corners are in contact with the carrying cases or with adjacent parts of the rubberized fabric, they tend to wear through due to vibration (Ref: TSO-C70a).
When accomplishing maintenance, repair, and inspection of unpacked rafts, personnel should not step on any part of the raft or flotation tubes while wearing shoes. Rafts should not be thrown or dropped, since damage to the raft or accessories may result. Particular care should be exercised at all times to prevent snagging, cutting, and contact with gasoline, acids, oils, and grease. High standards of performance for proper maintenance, inspection, and repair cannot be overemphasized, since the lives of passengers could be involved.
Inspection and inflation tests, when applicable, will be accomplished during storage and after installation in an aircraft in accordance w ith the manufacturer’s specifications and/or FAA-approved procedures.
Accessory items will be installed during these inspections. A raft knife will be attached by a 24-inch nylon lanyard to the mooring eye located above the CO2 cylinder case to enable rapid cutting of the mooring line.
LIFE RAFT INSPECTIONS. Inspection of life rafts should be performed in accordance with the manufacturer’s specifications. General inspection procedures to be performed on most life rafts are as follows.
CAUTION: Areas where life rafts are inspected or tested must be smooth, free of splinters, sharp projections, and oil stains.
Floors with abrasive characteristics, such as concrete or rough wood, will be covered with untreated tarpaulins or heavy clean paper.
Inspect life rafts for cuts, tears, or other damage to the rubberized material. If the raft is found to be in good condition, remove the CO 2 bottle(s) and inflate the raft with air to a pressure of 2 psi. The air should be introduced at the fitting nor mally connected to the CO 2 bottle(s). After at least 1 hour, to allow for the air within the raft to adjust itself to the ambient temperature, check pressure and adjust, if necessary, to 2 psi and allow the raft to stand for 24 hours. If, after 24 hours, the pressure is less than 1 psi, examine the raft for leakage by using soapy wate r. In order to eliminate pressure variations due to temperature differences at the time the initial and final reading are taken, test the raft in a room where the temperature is fairly constant. If the pressure drop is satisfactory, the raft should be considered as being in an airworthy condition and returned to service after being fitted with correctly charged CO 2 bottles as determined by weighing them.
Rafts more than 5 years old are likely to be unairworthy due to deterioration. It is suggested that serviceable rafts be marked to indicate the date of inspection and that soapstone be used when folding them preparatory to insertion into the carrying case. Take care to see that all of the raft’s required equipment is on board and properly stowed. If the raft lanyard, used to prevent the raft from floating away from the airplane, is in need of
Par 9-37 Page 9-437 replacement, use a lanyard not less than 20 feet long and having a breaking strength of about 75 pounds.
It is recommended that the aforementioned procedure be repeated every 18 months using the CO 2 bottle(s) for inflation. If a single bottle is used for inflating both compartments, it should be noted whether the inflation is proceeding equally to both compartments.
Occasionally, the formati on of “carbon-dioxide snow” may occur in one passage of the distribution manifold and divert a larger volume of gas to one compartment, which may burst if the mattress valve is not open to relieve the pressure. If the pressure is satisfactory, return the raft to serv ice in accordance with the procedure outlined.
Inspect the CO 2 cylinder for evidence of crossthreading or stripping.
Inspect the CO 2 bottle inflation valve cable rigging as follows.
Remove the screws that attach the cover plate to the valve and remove the cover plate.
Inspect the firing line cable ball swage for engagement in the correct recess for either “Upward Pull” or “Downward Pull.” The cable will be wrapped around the sheave approximately 270 degrees.
Reposition the cable ball swage as required.
(See figure 9-12.)
Replace the cover plate. The green dot on the sheave should be visible through the window in the cover plate, indicating a charged cylinder.
Check the CO 2 cylinder release cable and housing for condition and security.
Make sure the safety deflector is removed from the cylinder outlet before connecting the cylinder to the raft. (See figure 9-12.)
Stencil the life raft’s inspection date on the raft.
SURVIVAL KIT INSPECTION.
Survival Kit Contents . Each raft accommodating passengers or crew members should contain, as a minimum, the following:
Hand Pump (if required) Desalting Kit, First-Aid Kit Mirror/Reflector Emergency Rations Tarpaulins Fishing Kit Raft Knife Compass Protective Ointment (Sunburn) Oars Emergency Water Containers Repair Kits Signal Flares Carrying Case Locator Beacon and Battery Lines and Anchor Police Whistle Flashlight Space Blankets (if required) Light-sticks Solar Still Kit Survival Manual Duct Tape Plastic Trash Bags Accessory Containers
- Bailing Bucket
- Sponge Dye Marker Shark Chaser
Par 9-39 Page 9-438
FIGURE 9-12. Inflation valve.
Exposure Suits. Quick-donning exposure suits should be provided in sufficient quantity to accommodate the passengers and crew on extended over-water missions whenever any of the following conditions exist.
The water temperature is 59 ºF or below, or
The Outside Air Temperature (OAT) is 32 ºF or below.
Physical Inspection. Make a physical inspection of the life raft’s accessories and/or contents, in accordance with manufacturer’s specificati ons, to ascertain that all items required are in a serviceable condition.
Pumps and Hoses.
Check the air pump for condition and security.
Check the air pump hose and hose fittings for ease of attachment to the pump and mattress valves.
Operate the pump to ensure that it delivers air.
Close the outlet and check the seal of the piston.
Blow into the outlet to determine if the pump check valve will seal.
Desalting Kit.
Check the desalting kit expiration date, if applicable.
Replace the severely dented or punctured cans.
NOTE: Type MK-2 desalter kits have an indefinite shelf and service life and do not have to be age-controlled.
First-Aid Kit. Inspect each kit prior to flight to ensure that the seal is intact; the kits have not been tampered with or opened; and check the date when the kit contents should be inspected (120-day interval), and containing the following: