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.
Par 9-30 Page 9-31 FIGURE 9-11. SUGGESTED HANDLING OF PREFORMED HOSE
Par 9-30 Page 9-32 FIGURE 9-12. MINIMUM BEND RADII–TEFLON HOSE (
3) 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.
e. 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
Par 9-30 Page 9-33 surface found on moving parts of hydraulic units is an indication the seal is being properly lubricated. In pneumatic systems, seal lubrication is provided by the installation of a grease-impregnated felt wiper ring. When systems are static, seepage past the seals is not normally acceptable.
f. Storage of Replacement Seals.
(1) Store O-ring seals where temperature does not exceed 120 °F.
(2) Keep seals packaged to avoid exposure to ambient air and light, particularly sunlight.
g. During inspection, consider the following to determine whether seal replacement is necessary.
(1) How much fluid is permitted to seep past the seals? In some installations minor seepage is normal. Refer to the manufacturer’s maintenance information.
(2) What effect does the leak have on the operation of the system? Know the system.
(3) 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.
(4) Will the system function safely without depleting the reservoirs until the next inspection?
h. Dos and Don’ts that Apply to O-Ring Seals.
(1) Correct all leaks from static seal installations.
(2) Don’t retighten packing gland nuts; retightening will, in most cases, increase rather than decrease the leak.
(3) Never reuse O-ring seals because they tend to swell from exposure to fluids and become set from being under pressure. They may have minor cuts or abrasions that are not readily discernible by visual inspection.
(4) Avoid using tools that might damage the seal or the sealing surface.
(5) Do not depend upon color-coding. Coding may vary with manufacturer.
(6) Be sure that part number is correct.
(7) Retain replacement seals in their package until ready for use. This provides proper identification and protects the seal from damage and contamination.
(8) Assure that the sealing surfaces are clean and free of nicks or scratches before installing seal.
Par 9-30 Page 9-34
(9) Protect the seal from any sharp surfaces that it may pass over during installation.
Use an installation bullet or cover the sharp surfaces with tape.
(10) Lubricate the seal so it will slide into place smoothly.
(11) Be sure the seal has not twisted during installation.
i. Hydraulic System Pressure Test. When a flexible hose has been repaired or overhauled using existing hardware 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.
j. Hydraulic Components. Hydraulic components such as pumps, actuating cylinders, selector valves, relief valves, etc., should be repaired or adjusted following the airplane and component manufacturer’s instructions. Inspect hydraulic filter elements at frequent intervals and replace as necessary.
9-31. –9-36 [RESERVED.]