Bending reduces strength due to a reduction in wall thickness. The extrados (outside radius) of the bend stretches and become thinner, while the intrados (inside radius) compresses and gets thicker. The tighter the bend, the greater reduction in wall thickness.

5050-0 temper aluminum tubing (.035 wall thickness) is used on a large number of light aircraft for wheel brake and gear retract hydraulic systems, and fuel systems.  In the "0" condition, 5052 is soft and workable. The ends are easily flared and 5052 has higher fatigue strength than most aluminum alloys.

Be careful when replacing tubing that you are using the same type of tubing that you are replacing. 5050-0 is for low pressure systems only. In some applications such as light aircraft fuel injector lines, a stronger stainless steel tubing is used. Some hydraulic systems also use tubing other than 5052-0. 

The use of aluminum alloy tubing is limited in  certain areas of airborne hydraulic systems by  MIL-H-5440. Consult the applicable drawing  or illustrated parts breakdown to determine the correct tubing for a particular system. Tubing that conforms to Federal Specifications WW-T700/ 1 and WW-T-700/6 will not be used in  hydraulic systems. 



Aluminum Alloy Tubing. Aluminum alloy tubing  that conforms to Federal Specification WW-T-700/1 is  used for general-purpose lines and conduits of low or  negligible fluid pressure, such as instrument lines and  electrical and ventilating conduits.
Aluminum alloy tubing  that conforms to Federal Specifications WW-T-700/4 and  WW-T-700/6 is the most widely used for general-purpose lines of low and medium pressure. 

It is easily flared and soft enough to be formed with hand tools. Handle it with care to prevent scratches, dents, and nicks. Aluminum alloy tubing is used with either of two connection types: a flared joint for mechanical connectors or the beaded end for clamps and flexible hoses. In hydraulic systems, 5052-O aluminum alloy tubing that conforms to Federal Specification WW-T-700/4 is used for reduced pressure (1,500 psi maximum) and return lines. For high-pressure lines (3,000 psi), MIL-T-7081 aluminum alloy tubing (6061) is in considerable use. 

Stainless Steel Tubing. Corrosion-resistant steel (CRES) tubing, MIL-T-8504 and MIL-T-6845, is used in  high-pressure hydraulic systems (3,000 psi) such as landing gear, wing flaps, and brakes. It is preferred that external brake lines be made of stainless steel components (i.e., tubing, sleeves, nuts) to prevent damage caused by flying gravel and stones and ground-handling accidents. In other systems where stainless steel tubing is used (regardless of the tube size), it is preferred that only stainless steel nuts be used. This will minimize sleeve and nut cracking, which may be induced by overtorque of "B" nuts and vibration. CRES tubing does not have to be annealed for flaring or forming. In fact, the flared section is somewhat strengthened by the cold working and consequent strain hardening. The high tensile strength of stainless steel tubing permits the use of a thinner wall than does aluminum alloy tubing. Therefore, the weight is about the same as thicker-walled aluminum alloy tubing.

Tubing is sized by the Outside Diameters

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, the 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.

Typical wall thickness for 5052-0 Aluminum temper fluid tubing is .035 inch.