Rigid Fluid Lines Their Materials and Sizing


Fluid lines are important for a variety of aircraft operations and applications including those related to fuel, oil, oxygen, engine coolant, instrument, and hydraulic lines. The aircraft depends on many of these systems to sustain vital operations, and thus maintaining the fluid lines that enable them to perform is also critical. Rigid fluid lines are a special type of fluid line that is typically used in stationary applications where a straight tubing run is possible.

When maintaining or replacing rigid tubing, it is important to ensure that the same size and tubing material of the original assembly is used as improper installation can lead to failure. The most common rigid tubing materials include copper, aluminum alloy, steel, and titanium. When aviation was in its infancy, copper was the main tubing material for most applications. Since then, copper has been mostly replaced with other materials such as aluminum and steel. The aluminum alloy tube generally serves fuel and oil lines, as well as hydraulic and pneumatic systems that are 1,000 to 1,500 psi. Steel tubing is corrosion resistant, and also efficient for high pressure hydraulic systems that exceed 3,000 psi. Because of this, steel tubing is often used for landing gear, brakes, flaps, and more. Lastly, titanium tubing boasts a higher strength to weight ratio than steel tubing, thus it is mainly used for high performance and transport aircraft for pressure systems exceeding 1,500 psi.

When replacing an aircraft fluid line or rigid tubing, one should always identify which material is present in the assembly. While discerning between differing metals may be somewhat simple, it may be harder to identify between different grades of a certain metal. To accurately confirm which tubing material has been used for the original aircraft fluid line, one should refer to code markings of the old and new material. This coding may be stamped onto the surface of a larger tube, while smaller tubes will often have colored bands that denote what grade of material they are composed of. If no confirmation can easily be made, one can always conduct hardness testing on a sample of the tube.

Beyond the material of replacement rigid tubing, size is also important. The standard sizing of metal tubing is done by measuring the outside diameter in sixteenths of an inch, such as that number 5 tubing is 5/16” and number 7 tubing is 7/16”. Wall thickness may also be a concern when choosing between replacement tubing, and different tubes may vary. In general, both the size of the aircraft rigid tubing and wall thickness are printed on the tube itself. If the tubing material needs to be cut to fit a certain assembly, it is critical that the process produces a square end that is devoid of any burrs. To achieve this, tools such as tube or chipless cutters are recommended. Cutting with an excess 10% of tube length is also useful, as this can provide for bending as needed without the risk of becoming too small.

When it comes time to begin sourcing the aircraft fluid line parts and components that you need for your next project or operation, ASAP Aviation Unlimited has you covered with everything you are searching for. ASAP Aviation Unlimited is owned and operated by ASAP Semiconductor, and we can help you find the aircraft and marine parts that you are searching for, new or obsolete. As a premier supplier of parts for the aerospace, civil aviation, and defense industries, we're always available and ready to help you find all the parts and equipment you need, 24/7x365. ASAP Semiconductor is an FAA AC 00-56B accredited and ISO 9001:2015 certified enterprise. For a quick and competitive quote, email us at sales@asapaviationunlimited.com or call us at +1-919-348-4040.


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June 1, 2020

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