Flexible Hose Lines: Inspection and Installation

9-16 Cable Bullet Dent or plastic plugs or caps. When repairing a low-pressure line using a flexible fluid connection assembly, position the hose clamps carefully to prevent overhang of the clamp bands or chafing of the tightening screws on adjacent parts. If chafing can occur, the hose clamps should be repositioned on the hose. assembly and gives the maximum allowable angular and dimensional offset. When replacing rigid tubing, ensure that the layout of the new line is the same as that of the line being replaced. Remove the damaged or worn assembly, taking care not to further damage or distort it, and use it as a forming template for the new part.

If the old length of tubing cannot be used as a pattern, make a wire template, bending the pattern by hand as required for the new assembly. Then bend the tubing to match the wire pattern. Never select a path that does not require bends in the tubing. A tube cannot be cut or flared accurately enough so that it can be installed without bending and still be free from mechanical strain. Bends are also necessary to permit the tubing to expand or contract under temperature changes and to absorb vibration. If the tube is small (under 1⁄4") and can be hand formed, casual bends may be made to allow for this. If the tube must be machine formed, definite bends must be made to avoid a straight assembly. Start all bends a reasonable distance from the fittings because the sleeves and nuts must be slipped back during the fabrication of flares and during inspections. In all cases, the new tube assembly should be so formed prior to installation that it is not necessary to pull or deflect the assembly into alignment by means of the coupling nuts.

Flexible Hose Fluid Lines

Flexible hose is used in aircraft fluid systems to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems. Hose Materials & Construction Pure rubber is never used in the construction of flexible fluid lines. To meet the requirements of strength, durability, and workability, among other factors, synthetics are used in place of pure rubber. Synthetic materials most commonly used in the manufacture of flexible hose are Buna-N, neoprene, butyl, ethylene propylene diene rubber (EPDM) and Teflon™.

While Teflon™ is in a category of its own, the others are synthetic rubber. Buna-N Buna-N is a synthetic rubber compound that has excellent resistance to petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester base hydraulic fluid (Skydrol™). Neoprene Neoprene is a synthetic rubber compound that has an acetylene base. Its resistance to petroleum products is not as good as Buna-N, but it has better abrasive resistance. Do not use for phosphate ester base hydraulic fluid (Skydrol™). Butyl Butyl is a synthetic rubber compound made from petroleum raw materials. It is an excellent material to use with phosphate ester base hydraulic fluid (Skydrol™). Do not use with petroleum products.

Flexible rubber hose consists of a seamless synthetic rubber inner tube covered with layers of cotton braid and wire braid and an outer layer of rubber-impregnated cotton braid. This type of hose is suitable for use in fuel, oil, coolant, and hydraulic systems. The types of hose are normally classified by the amount of pressure they are designed to withstand under normal operating conditions: low, medium, and high. • Low pressure—below 250 psi. Fabric braid reinforcement. • Medium pressure—up to 3,000 psi. One wire braid reinforcement. Smaller sizes carry up to 3,000 psi. Larger sizes carry pressure up to 1,500 psi.

• High pressure—all sizes up to 3,000 psi operating pressures. Flexible hoses used for brake systems have sometimes a stainless steel wire braid installed over the hose to protect the hose from damage. [Figure 9-32] Hose Identification Lay lines and identification markings consisting of lines, letters, and numbers are printed on the hose. [Figure 9-33] Most hydraulic hose is marked to identify its type, the quarter and year of manufacture, and a 5-digit code identifying the manufacturer. These markings are in contrasting colored letters and numerals that indicate the 9-17 Type of Failure Repair Method a. Make 1 or 2 cuts, as necessary, to remove damaged section. If 2 cuts are required, the distance between them shall not exceed 0.30". If distance is more than 0.30", go to repair method 2.

b. Swage 1 tube-to-tube union in tube section under repair. a. Make 2 cuts to enable removal of damaged section. b. Remove damaged section and duplicate. c. Swage replacement section into tubing under repair using 2 tube-to-tube unions. a. Cut out defective tee or elbow. b. Duplicate tubing sections for each branch. c. Swage splice sections to tee or elbow. d. Connect each splice section to tubing under repair using a tube-to-tube union. a. Cut tubing to remove defective fitting. b. Swage appropriate end fitting to tube end. c. Connect new end fitting to mating connection, torquing nut as required.

1. Pin hole leak or circumferential crack in tubing. 2. Longitudinal crack in tubing (crack length in excess of 0.30"). 3. Leaking tee or elbow (permanent tube connection type). 4. Leaking flared, flareless, or lipseal end fittings. Not to exceed 0.30" Original tubing New section natural lay (no twist) of the hose and are repeated at intervals of not more than 9 inches along the length of the hose. Code markings assist in replacing a hose with one of the same specifications or a recommended substitute. Hose suitable for use with phosphate ester base hydraulic fluid is marked Skydrol™ use. In some instances, several types of hose may be suitable for the same use. Therefore, to make the correct hose selection, always refer to the applicable aircraft maintenance or parts manual.

Teflon™ is the DuPont trade name for tetrafluoroethylene resin. It has a broad operating temperature range (−65 °F to +450 °F). It is compatible with nearly every substance or agent used. It offers little resistance to flow; sticky, viscous materials do not adhere to it. It has less volumetric expansion than rubber, and the shelf and service life is practically limitless. Teflon™ hose is flexible and designed to meet the requirements of higher operating temperatures and pressures in present aircraft systems. Generally, it may be used in the same manner as rubber hose. Teflon™ hose is processed and extruded into tube shape to a desired size. It is covered with stainless steel wire, which is braided over the tube for strength and protection. Teflon™ hose is unaffected by any known fuel, petroleum, or synthetic base oils, alcohol, coolants, or solvents commonly used in aircraft. Teflon™ hose has the distinct advantages of a practically unlimited storage time, greater operating temperature range, and broad usage (hydraulic, fuel, oil, coolant, water, alcohol, and pneumatic systems). Medium-pressure Teflon™ hose assemblies are sometimes preformed to clear obstructions and to make connections using the shortest possible hose length. Since preforming permits tighter bends that eliminate the need for 9-18 Clamps Fitting Hose Minimum gap “G” shall be ½" or Tube OD/4 Tubing G. ref ¼" Min.

" Max. offset 3° Max. angular deviation MIL-H-8794: SIZE-6-2/90 Mfg Symbol Two cotton braids—impregnated with synthetic compound Hose number Hose size Synthetic inner tube Single wire braid Quarter and year of manufacturer (A) MIL-H-8794 Type Hose (D) MIL-H-27267 Type Hose (E) MIL-H-83797 Type Hose Mfg SymbolMIL-H-6000-SIZE-3-4/90 Quarter and year of manufacturer Hose size (B) MIL-H-6000 Type Hose (Views showing opposite side of hose) MIL-H-5593-6-4/90 Mfg Symbol Lay lines Quarter and year of manufacturer (C) MIL-H-5593 Type Hose Hose size special elbows, preformed hose assemblies save space and weight. Never straighten a preformed hose assembly. Use a support wire if the hose is to be removed for maintenance.

[Figure 9-34]

Flexible Hose Inspection

Check the hose and hose assemblies for deterioration at each inspection period. Leakage, separation of the cover or braid from the inner tube, cracks, hardening, lack of flexibility, or excessive “cold flow” are apparent signs of deterioration and reason for replacement. The term “cold flow” describes the deep, permanent impressions in the hose produced by the pressure of hose clamps or supports. When failure occurs in a flexible hose equipped with swaged end fittings, the entire assembly must be replaced. Obtain a new hose assembly of the correct size and length, complete with factory installed end fittings. When failure occurs in hose equipped with reusable end fittings, a replacement line can be fabricated with the use of such tooling as may be necessary to comply with the assembly instructions of the manufacturer.

Fabrication & Replacement of Flexible Hose To make a hose assembly, select the proper size hose and end fitting. [Figure 9-35] MS-type end fittings for flexible hose are detachable and may be reused if determined to be serviceable. The inside diameter of the fitting is the same as the inside diameter of the hose to which it is attached. [Figure 9-36] 9-19 Support wire 1. Place hose in vise and cut to desired length using fine tooth hacksaw or cut off wheel. 2. Locate length of hose to be cut off and slit cover with knife to wire braid, taking care to not damage underlying materials. After slitting cover, twist off with pair of pliers. (See note below.) 3. Place hose in vise and screw socket on hose counterclockwise.

4. *Lubricate inside of hose and nipple threads liberally. 5. Screw nipple into socket using wrench on hex of nipple and leave .005" to .031" clearance between nipple hex and socket. NOTE: Hose assemblies fabricated per MIL-H-8790 must have the exposed wire braid coated with a special sealant. 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. Flexible Hose Testing All flexible hose must be proof-tested after assembly and applying pressure to the inside of the hose assembly. The proof-test medium may be a liquid or gas. For example, hydraulic, fuel, and oil lines are generally tested using hydraulic oil or water, whereas air or instrument lines are tested with dry, oil-free air or nitrogen. When testing with a liquid, all trapped air is bled from the assembly prior to tightening the cap or plug. Hose tests, using a gas, are conducted underwater. In all cases, follow the hose manufacturer’s instructions for proof-test pressure and fluid to be used when testing a specific hose assembly.

[Figure 9-37] When a flexible hose has been repaired or overhauled using existing hardware and new hose material, and before the hose is installed on the aircraft, it is recommended that the hose be tested to at least 1.5 system pressure. A hydraulic hose burst test stand is used for testing flexible hose. [Figure 9-38] A new hose can be operationally checked after it is installed in the aircraft using system pressure. Size Designations Hose is also designated by a dash number according to its size. The dash number is stenciled on the side of the hose and indicates the size tubing with which the hose is compatible.

It does not denote inside or outside diameter. When the dash number of the hose corresponds with the dash number of the tubing, the proper size hose is being used. [Figure 9-33] 9-20 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 ³⁄8 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 MIL. Part No.

Tube Size OD (inches) Hose Size ID (inches) Hose Size OD (inches) Recomm. Operating Pressure (PSI) Min. Burst

Pressure

(PSI) Max. Proof

Pressure

(PSI) Min. Bend Radius (inches) MIL. Part No. Tube Size OD (inches) Hose Size ID (inches) Hose Size OD (inches) Recomm. Operating Pressure (PSI) Min. Burst

Pressure

(PSI) Max. Proof

Pressure

(PSI) Min. Bend Radius (inches) Single Wire Braid Fabric Covered MIL-H-8788- 4-L ¹⁄4 ⁷⁄32 .63 3,000 16,000 8,000 3.00 MIL-H-8788- 5-L ⁵⁄16 ⁹⁄32 .70 3,000 14,000 7,000 3.38 MIL-H-8788- 6-L ³⁄8 ¹¹⁄32 .77 3,000 14,000 7,000 5.00 MIL-H-8788- 8-L ¹⁄2 ⁷⁄16 .86 3,000 14,000 7,500 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 Multiple Wire Braid Rubber Covered 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: − 65 °F to + 250 °F Sizes 16 through 48: − 40 °F to + 275 °F Note: Maximum temperatures and pressures should not be used simultaneously. Construction: Seamless synthetic rubber inner tube reinforced with one fiber braid, two or more steel wire braids, and covered with 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: − 65 °F to + 200 °F 9-21 Socket Nipple Spur Reinforcement Sheath stripe running along its length. This stripe should not spiral around the hose. Bending To avoid sharp bends in the hose assembly, use elbow fittings, hose with elbow-type end fittings, or the appropriate bend radii. Bends that are too sharp reduce the bursting pressure of flexible hose considerably below its rated value.

[Figure 9-40] Clearance The hose assembly must clear all other lines, equipment, and adjacent structure under every operating condition. Flexible hose should be installed so that it is subject to a minimum of flexing during operation. Although hose must be supported at least every 24 inches, closer supports are desirable. Flexible hose must never be stretched tightly between two fittings. If clamps do not seal at specified tightening, examine hose connections and replace parts as necessary. The above is for initial installation and should not be used for loose clamps. For retightening loose hose clamps in service, proceed as follows: • Non-self-sealing hose—if the clamp screw cannot be tightened with the fingers, do not disturb unless leakage is evident. If leakage is present, tighten one- fourth turn.

• Self-sealing hose—if looser than finger-tight, tighten to finger-tight and add one-fourth turn. [Figure 9-41] Hose Fittings Flexible hose may be equipped with either swaged fittings or detachable fittings, or they may be used with beads and hose clamps. Hoses equipped with swaged fittings are ordered by correct length from the manufacturer and ordinarily cannot be assembled by the mechanic. They are swaged and tested at the factory and are equipped with standard fittings. The detachable fittings used on flexible hoses may be detached and reused if they are not damaged; otherwise, new fittings must be used. [Figure 9-39]

Installation of Flexible Hose Assemblies

Slack Hose assemblies must not be installed in a manner that causes a mechanical load on the hose. When installing flexible hose, provide slack or bend in the hose line from 5 to 8 percent of its total length to provide for changes in length that occurs when pressure is applied. Flexible hose contracts in length and expands in diameter when pressurized. Protect all flexible hoses from excessive temperatures, either by locating the lines so they are not affected or by installing shrouds around them. Flex When hose assemblies are subject to considerable vibration or flexing, sufficient slack must be left between rigid fittings.

Install the hose so that flexure does not occur at the end fittings. The hose must remain straight for at least two hose diameters from the end fittings. Avoid clamp locations that restrict or prevent hose flexure. Twisting Hoses must be installed without twisting to avoid possible rupture of the hose or loosening of the attaching nuts. Use of swivel connections at one or both ends relieve twist stresses. Twisting of the hose can be determined from the identification 9-22 Wrong Right Planning Hose Line Installations 1. Provide slack or bend in the hose line to provide for changes in length that will occur when pressure is applied.

2. Observe linear stripe. The hose must not be twisted. High pressures applied to a twisted hose may cause failure or loosen the nut. 3. Relieve sharp bends, avoid strain or hose collapse, and make cleaner installations by using Aeroquip elbows or other adapter fittings. Provide as large a bend radius as possible. Never use less than the recommended minimum bend radius specified for the hose. 4. Provide additional bend radius when lines are subject to flexing and remember that the metal end fittings are not flexible. Place line support clamps so as not to restrict hose flexing. Self-sealing hose approximately 15 in-lb All other aircraft hose approximately 25 in-lb Finger-tight plus 2 complete turns Finger-tight plus 1¼ complete turns Finger-tight plus 2½ complete turns Finger-tight plus 2 complete turns Initial Installation Only Worm screw type clamp (10 threads per inch) Clamps— radial and other type (28 threads per inch) Hose Clamps To ensure proper sealing of hose connections and to prevent breaking hose clamps or damaging the hose, follow the hose clamp tightening instructions carefully. When available, use the hose clamp torque-limiting wrench. These wrenches are available in calibrations of 15 and 25 in-lb limits. In the absence of torque-limiting wrenches, follow the finger-tight- plus-turns method. Because of the variations in hose clamp design and hose structure, the values given in Figure 9-41 are approximate. Therefore, use good judgment when tightening hose clamps by this method. Since hose connections are subject to “cold flow” or a setting process, a follow-up tightening check should be made for several days after installation.

Support clamps are used to secure the various lines to the airframe or powerplant assemblies. Several types of support clamps are used for this purpose. The most commonly used clamps are the rubber-cushioned and plain. The rubber-cushioned clamp is used to secure lines subject to vibration; the cushioning prevents chafing of the tubing. [Figure 9-42] The plain clamp is used to secure lines in areas not subject to vibration. A Teflon™-cushioned clamp is used in areas where the deteriorating effect of Skydrol™, hydraulic fluid, or fuel is expected. However, because it is less resilient, it does not provide as good a vibration-damping effect as other cushion materials.

9-23 Grommet Support tube at least 1/4" from edge of hole Tube OD (in.) Distance Between Supports (in.) Aluminum Alloy Steel ¹⁄8 9¹⁄2 11¹⁄2 ³⁄16 12 14 ¹⁄4 13¹⁄2 16 ⁵⁄16 15 18 ³⁄8 16¹⁄2 20 ¹⁄2 19 23 ⁵⁄8 22 25¹⁄2 ³⁄4 24 27¹⁄2 1 26¹⁄2 30 Use bonded clamps to secure metal hydraulic, fuel, or oil lines in place. Unbonded clamps should be used only for securing wiring. Remove any paint or anodizing from the portion of the tube at the bonding clamp location. Make certain that clamps are of the correct size. Clamps or supporting clips smaller than the outside diameter of the hose may restrict the flow of fluid through the hose. All fluid lines must be secured at specified intervals. The maximum distance between supports for rigid tubing is shown in Figure 9-43.