Aircraft contain numerous systems that require tubing, bearings, valves, and more. One of the most important, but often overlooked, structural components are flexible hoses. Flexible hoses are utilized in aircraft fluid systems in particular to connect moving parts with stationary parts in areas that are subject to vibration or where increased flexibility is needed. More than that, flexible hoses can serve as a connector in metal tubing systems.
Hoses are usually made from synthetics which can meet strength, durability, and workability requirements for aircraft. The most common synthetic materials used in the manufacturing of flexible hoses include Buna-N, neoprene, butyl, ethylene propylene diene rubber (EPDM), and Teflon. Though Teflon is in a category of its own, the others are all synthetic rubbers.
Buna-N is a synthetic rubber compound with an increased resistance to petroleum products but cannot be used for phosphate ester base hydraulic fluids.
Neoprene is also a synthetic rubber compound that contains an acetylene base. It has ample resistance to petroleum, though it is not as good as Buna-N.
Butyl is a synthetic rubber compound made from raw petroleum materials. It is highly compatible with phosphate ester hydraulic fluid but cannot be utilized with petroleum products.
Flexible Hose Construction
Consisting of a seamless synthetic rubber inner tube encapsulated in layers of cotton braid and wire braid alongside an outer layer of rubber-impregnated cotton braid, this type of hose finds use in fuel, oil, coolant, and hydraulic systems. Furthermore, these hoses are normally classified by the quantity of pressure they are designed to handle under normal operating conditions: low, medium, and high.
Keep in mind that flexible hoses used in brake systems often contain a stainless steel wire braid installed over the hose to protect the hose from damage.
Flexible Hose Identification
To distinguish various hoses in any system, lay lines and identification markings consisting of lines, letters, and numbers are oriented on the hose. For instance, most hydraulic hoses are marked to identify their type, the quarter, and manufacturing date, as well as a 5-digit code identifying the manufacturer. These markings are in colored letters and numerals that indicate the natural lay of the hose and this configuration is repeated at intervals no longer than 9 inches along the length of the hose.
Flexible Hose Inspection
Flexible hoses and hose assemblies must be routinely checked for deterioration. Leakage, separation of covers and braids, cracks, hardening, lack of flexibility, or excessive “cold flow” are indications of hose deterioration. Cold flow is defined as the deep, permanent impressions in the hose as a result of the pressure imposed by hose clamps or supports. For a hose equipped with swaged end fittings, the entire assembly must be replaced if failure occurs. Replacing such hose assemblies requires that you obtain a new hose assembly with factory installed end fittings. For hoses with reusable end fittings, a replacement line can be designed with the appropriate tooling to comply with the assembly instructions of the manufacturer.
Flexible Hose Testing & Installation of Flexible Hose Assemblies
Prior to installation, all flexible hoses must be proof-tested after assembly with pressure applied to the inside of the hose assembly. In most cases, the proof-test medium is a liquid or a gas. When testing with a liquid, all trapped air is bled from the assembly before tightening the cap. Hose tests that use a gas are carried out underwater. For hoses that have been repaired or overhauled using existing hardware and new hose material, the hoses must be tested to a minimum 1.5x system pressure. Finally, a hydraulic hose burst test stand is utilized for testing flexible hose.
Flexible hoses must be installed in a way that does not impose a mechanical load on the hose. A slack or bend in the hose line of 5% to 8% of its total length should be provided when pressure is applied. It is important to note that a flexible hose contracts in length and expands in diameter when placed under pressure. When hose assemblies are subjected to vibration or flexing, slack must be provided between rigid fittings, and one should install the hose so that flexure does not occur at the end fittings. Additionally, the hose must remain straight for a minimum of two hose diameters from the end fittings and clamp locations that restrict or prevent hose flexure.
Twisting should be avoided at all costs as it can result in a rupture of the hose and loosening of the attaching nuts. Swivel connections at one or both ends have the ability to relieve twist stresses. A spiral around the hose is a good indication that twisting is occurring. To avoid sharp bends in your hose assembly, elbow fittings or elbow-type end fittings can be used. Sharp bends can reduce the bursting pressure of hose below its rated value. The last aspect to consider when installing flexible hoses consists of ensuring that the assembly is free of all other lines, equipment, and adjacent structures.
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