Rigid Tubing Color Coding and Inspection

Used in stationary applications where long, relatively straight runs are possible, rigid tubing is a recognizable technique for the funneling of a liquid. Hydraulic systems use rigid tubing to carry pressurized liquid from the reservoir through various filters and valves. In the combustion system, hot exhaust gas is expelled out of an aircraft through a rigid tubing, known as the exhaust valve.

One key example of rigid tubing on an aircraft, however, are the aircraft fluid lines. Because of the properties of jet engine fuel, the material of the rigid tubing should be carefully considered. Aluminum alloys 2024-T3, 5052-O, and 6061-T6 are a popular choice of material for rigid tubing for fuel lines. In comparison, CRES 304 steel is used in for tubing in the high-pressure hydraulic systems.

Rigid tubing is sized by outside diameter, which is measured fractionally in sixteenths of an inch. To ease construction, the diameter of the tube should be printed onto the rigid tubing. Although steel is heavier than aluminum, the overall application weight is more or less the same. Along with diameter, rigid tubing can be categorized by wall thickness. The steel tubing used in a high-pressure hydraulic system has a thinner diameter because the steel itself is stronger. If aluminum was used in a hydraulic system, the thickness would have to be significantly thicker. Along with the diameter, the material type should be marked on the rigid tube.

To help identification, color codes are painted onto the tubing. Now, an aircraft is not constructed a series of vibrantly painted pipes. Instead, the color codes are discrete, 4-inch-wide patches on either end, or the midway point of the pipe. Aluminum alloy number 5052 is purple. Aircraft fluid lines are further color-coded to notate the type of system and its contents. A rigid tubing marked with grey tape and triangles means is carrying fluid related to the deicing system.

During inspection, checks should be made to ensure that there are no dents or scratches in the tubing. Manufacturers release damage limits for the specific type of tubing. Aluminum tubing has a higher damage limit than the thinned-walled steel. It is possible to remove a dent in the tubing that is not deeper than 10 percent of the wall. A bullet can be drawn through the tubing, pushing out the dent. A dent that is significant in size or has a crack in it however should be replaced. The color-coding system should help ensure the correct tubing is used in replacement.

The proper torque values should be noted before attempting to replace a rigid tube fitting. Tubing made out of soft aluminum alloy can use a flare fitting consisting of a sleeve and a nut. The sleeve helps to protect the tubing allowing it to withstand additional pressure. A flaring tool is used to produce the correct flare degree. Overtightening or failing to sufficiently tighten the fitting can lead to system leakage or the line to give way under pressure.

Despite their seemingly straightforward application, rigid tube fitting requires care and attention when it comes to installation and maintenance. The color code systems should be utilized from assembly and steps should be taken to ensure system regularity.

At ASAP Aviation Procurement, owned and operated by ASAP Semiconductor, we can help you find all the rigid tubing 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, 24/7-365. For a quick and competitive quote, email us at sales@asap-aviationprocurement.com or call us at 1-702-919-1616.


January 21, 2019