Aviation: Fluid Lines and Fittings
Aircraft fluid lines are usually made of metal tubing or flexible hose. Metal tubing (also called rigid fluid lines) is used in stationary applications and where long, relatively straight runs are possible. They are widely used in aircraft for fuel, oil, coolant, oxygen, instrument, and hydraulic lines. Flexible hose is generally used with moving parts or where the hose is subject to considerable vibration.
Occasionally, it may be necessary to repair or replace damaged aircraft fluid lines. Very often the repair can be made simply by replacing the tubing. However, if replacements are not available, the needed parts may have to be fabricated. Replacement tubing should be of the same size and material as the original tubing. All tubing is pressure tested prior to initial installation and is designed to withstand several times the normal operating pressure to which it is subjected. If a tube bursts or cracks, it is generally the result of excessive vibration, improper installation, or damage caused by collision with an object. All tubing failures should be carefully studied and the cause of the failure determined.
Rigid Fluid Lines
Tubing Materials
Copper
In the early days of aviation, copper tubing was used extensively in aviation fluid applications. In modern aircraft, aluminum alloy, corrosion-resistant steel, or titanium tubing have generally replaced copper tubing.
Aluminum Alloy Tubing
Tubing made from 1100 H14 (1/2-hard) or 3003 H14 (1/2-hard) is used for general purpose lines of low or negligible fluid pressures, such as instrument lines and ventilating conduits. Tubing made from 2024-T3, 5052-O, and 6061-T6 aluminum alloy materials is used in general purpose systems of low and medium pressures, such as hydraulic and pneumatic 1,000 to 1,500 psi systems, and fuel and oil lines.
Steel
Corrosion-resistant steel tubing, either annealed CRES 304, CRES 321, or CRES 304-1/8-hard, is used extensively in high-pressure hydraulic systems (3,000 psi or more) for the operation of landing gear, flaps, brakes, and in fire zones. Its higher tensile strength permits the use of tubing with thinner walls; consequently, the final installation weight is not much greater than that of the thicker wall aluminum alloy tubing. Steel lines are used where there is a risk of foreign object damage (FOD) (i.e., the landing gear and wheel well areas). Swaged or MS flareless fittings are used with corrosionresistant tubing. Although identification markings for steel tubing differ, each usually includes the manufacturer’s name or trademark, the Society of Automotive Engineers (SAE) number, and the physical condition of the metal.
Titanium 3AL–2.5V
Titanium 3AL–2.5V tubing and fitting is used extensively in transport category and high-performance aircraft hydraulic systems for pressures above 1,500 psi. Titanium is 30 percent stronger than steel and 50 percent lighter than steel. Cryofit fittings or swaged fittings are used with titanium tubing. Do not use titanium tubing and fittings in any oxygen system assembly. Titanium and titanium alloys are oxygen reactive. If a freshly formed titanium surface is exposed in gaseous oxygen, spontaneous combustion could occur at low pressures.
Material Identification
Before making repairs to any aircraft tubing, it is important to make accurate identification of tubing materials. Aluminum alloy, steel, or titanium tubing can be identified readily by sight where it is used as the basic tubing material. However, it is difficult to determine whether a material is carbon steel or stainless steel, or whether it is 1100, 3003, 5052-O, 6061-T6, or 2024-T3 aluminum alloy. To positively identify the material used in the original installation, compare code markings of the replacement tubing with the original markings on the tubing being replaced.
On large aluminum alloy tubing, the alloy designation is stamped on the surface. On small aluminum tubing, the designation may be stamped on the surface; but more often it is shown by a color code, not more than 4" in width, painted at the two ends and approximately midway between the ends of some tubing. When the band consists of two colors, onehalf the width is used for each color.
If the code markings are hard or impossible to read, it may be necessary to test samples of the material for hardness by hardness testing.
Sizes
Metal tubing is sized by outside diameter (OD), which is measured fractionally in sixteenths of an inch. For example, number 6 tubing is 6/16" (or 3/8") and number 8 tubing is 8/16" (or 1/2") and so forth. The tube diameter is printed on all rigid tubing. In addition to other classifications or means of identification, tubing is manufactured in various wall thicknesses. Thus, it is important when installing tubing to know not only the material and outside diameter, but also the thickness of the wall. The wall thickness is printed on the tubing in thousandths of an inch. To determine the inside diameter (ID) of the tube, subtract twice the wall thickness from the outside diameter. For example, a number 10 piece of tubing with a wall thickness of 0.063" has an inside diameter of 0.625" – 2(0.063") = 0.499".
Fabrication of Metal Tube Lines
Damaged tubing and fluid lines should be repaired with new parts whenever possible. Unfortunately, sometimes replacement is impractical and repair is necessary. Scratches, abrasions, or minor corrosion on the outside of fluid lines may be considered negligible and can be smoothed out with a burnishing tool or aluminum wool. Limitations on the amount of damage that can be repaired in this manner are discussed in this chapter under “Rigid Tubing Inspection and Repair.” If a fluid line assembly is to be replaced, the fittings can often be salvaged; then the repair involves only tube forming and replacement.
Tube forming consists of four processes: cutting, bending, flaring, and beading. If the tubing is small and made of soft material, the assembly can be formed by hand bending during installation. If the tube is 1/4" diameter or larger, hand bending without the aid of tools is impractical.
Fluid Line Identification
Fluid lines in aircraft are often identified by markers made up of color codes, words, and geometric symbols. These markers identify each line’s function, content, and primary hazard. Figure illustrates the various color codes and symbols used to designate the type of system and its contents.
Fluid lines are marked, in most instances, with 1" tape or decals. On lines 4" in diameter (or larger), lines in oily environment, hot lines, and on some cold lines, steel tags may be used in place of tape or decals. Paint is used on lines in engine compartments where there is the possibility of tapes, decals, or tags being drawn into the engine induction system.
In addition to the above-mentioned markings, certain lines may be further identified regarding specific function within a system (e.g., drain, vent, pressure, or return). Lines conveying fuel may be marked FLAM; lines containing toxic materials are marked TOXIC in place of FLAM. Lines containing physically dangerous materials, such as oxygen, nitrogen, or FreonTM, may be marked PHDAN.
Aircraft and engine manufacturers are responsible for the original installation of identification markers, but the aviation mechanic is responsible for the replacement when it becomes necessary. Tapes and decals are generally placed on both ends of a line and at least once in each compartment through which lines run. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessories within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.
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 and 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.
Hose Identification
Lay lines and identification markings consisting of lines, letters, and numbers are printed on the hose. 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 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 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.
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 and Replacement of Flexible Hose
To make a hose assembly, select the proper size hose and end fitting. 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.
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.
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.
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-tightplus-turns method. Because of the variations in hose clamp design and hose structure, the values given in Figure 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.
