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Aviation Maintenance Technician Handbook - General2

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 vibrationOccasionally, 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 tubingAll tubing is pressure tested prior to initial installation,.and is designed to withstand several times the normal.operating pressure to which it will be 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 tubingAluminum 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. 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 metalTitanium 3AL-2.5V.This type of 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 assemblyTitanium and titanium alloys are oxygen reactive. If a.freshly formed titanium surface is exposed in gaseous.oxygen, spontaneous combustion could occur at low.pressuresMaterial 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.7-1.Color of Band1100White3003Green2014Gray2024Red5052Purple6053Black6061Blue and Yellow7075Brown and YellowFigure 7-1. Painted color codes used to identify.aluminum alloy tubing.On large aluminum alloy tubing, the all