Downloaded from https://www.everyspec.com FM 1-509 CHAPTER 2 PNEUDRAULIC LINES RIGID TUBING Characteristics. Rigid tubing assemblies are made of aluminum alloy, stainless steel, or copper. Some air systems aboard aircraft have copper tubing. Two aluminum alloys are commonly used: 5052 and 6061. Alloy 5052 may be used for lines carrying pressures up to 1,500 psi; alloy 6061 may be used for pressures up to 3,000 psi. As a general rule, exposed lines and lines subject to abrasion, intense heat, or extremely high pressure are made of stainless steel. The tubing used to produce rigid tubing assemblies is sized by its outside diameter (OD) and wall thickness.
Outside diameter sizes are in sixteenth-of-an-inch increments, the number of the tube indicating its size in sixteenths of an inch. For example, number 6 tubing is 6/16 or 3/8 inch, number 8 tubing is 8/16 or 1/2 inch, and so forth. Wall thickness is specified in thousandths of an inch. Most aircraft maintenance manuals contain a table which lists the original material and acceptable substitutes and gives the wall thickness for each. Damage. Nicks and scratches on tubing can be reduced by using care. Most damage of this kind occurs because of careless handling of tools during aircraft maintenance.
Any dent less than 20 percent of the tubing diameter is not objectionable unless it is on the heel of a short-bend radius. A nick no deeper than 15 percent of wall thickness of aluminum, aluminum alloy, copper, or steel tubing may be reworked by burnishing with hand tools; any nicks in excess of 15 percent of wall thickness should be rejected. Nicked tubing is not only weaker due to internal pressure, but it must be burnished out to reduce a notch effect. If nicks are not burnished out, stress concentration under vibration will cause the tubing to become weak and break at nicked points. Burnishing removes notches by removing the surrounding material.
Burnishing is not allowed in the heel of bends where material has already been stretched thin during forming. Tubing which is nicked in a bend should be replaced if it is carrying over 100 psi pressure. For tubing carrying pressures of 100 psi or less, a nick no deeper than 20 percent of wall thickness of aluminum, aluminum alloy, copper, or steel tubing may be reworked by burnishing with hand tools. When tubing is used for fluids at atmospheric pressure or less, the bursting strength of the tubing is unimportant; however, it must be leak-tight at all times. Dents can be removed from tubing by drawing a bullet through the tubing.
The diameter of the bullet must be equal to or slightly less than the inside diameter of the tubing. All tubing is pressure-tested before installation and is designed to withstand several times the operating pressure which it will be subjected 2-1