Chapter Objective: Upon completion of this chapter, you will have a working knowledge of aircraft hydraulic and pneumatic tubing and their associated hardware.

Tubing assemblies are used to transport liquids or gas (usually under pressure) between various components of the aircraft system. Tube assemblies are used in aircraft for fuel, oil, oxidizer, coolant, breathing oxygen, instruments, hydraulic, and vent lines. You must be familiar with the procedures for testing and fabricating tubing assemblies, and you must recognize the various tools and equipment and how to identify the different uses of tubing in naval aircraft. Tube assemblies are fabricated from rigid tubing and associated fittings. 

TUBING AND TUBE ASSEMBLIES 

Learning Objective: procedures used in the fabrication of hydraulic and pneumatic tubing assemblies.

The tubing used in the manufacture of rigid tubing assemblies is sized by outside diameter (OD) and wall thickness. Outside diameter sizes are in sixteenth-inch increments; the number of the tube indicates its size in sixteenths of an inch. Thus, No. 6 tubing is 6/16 or 3/8 inch; No. 8 tubing is 8/16 or 1/2 inch, etc. Wall thickness is specified in thousandths of an inch. The most common types of tubing are the corrosion-resistant steel tubing for high pressure and the aluminum alloy tubing for high pressure and general-purpose.

Corrosion-resistant steel tubing (CRES) is used in high-pressure hydraulic systems (3,000 psi and above) such as landing gear, wing flaps, and brakes. The tubing does not have to be annealed for flaring or forming. The flared section is strengthened by cold working and consequent strain hardening. Table 6-1 lists the most commonly used corrosion-resistant steel tubing in naval aircraft and includes some of the designations by which the corrosion steel tubing is known. Application notes are intended as guidelines. Corrosion-resistant steel tube assemblies fabricated with corrosion-resistant steel tubing MIL-T-6845 are authorized for repair or replacement for any line provided no attempt is made to weld or braze the tubing. MIL-T-6845 tubing is not to be substituted for British DTD-5016 annealed stainless steel tubing.

Aluminum alloy tubing is used for both high-pressure and general-purpose lines. Table 6-2 lists the most commonly used aluminum alloy tubing and its applications. Use of aluminum alloy tubing is limited in certain areas of airborne hydraulic systems by MIL-H-5440. Refer to the applicable drawing and the illustrated parts breakdown to determine the correct tubing for a particular system. Aluminum alloy tube assemblies fabricated with aluminum alloy tubing 6061-T6 are authorized for repair or replacement for any aluminum line. MIL-T-6845 Cres tubing (304-1/8H) is a suitable substitute for all aluminum alloy tubing when 6061-T6 is unavailable.

Corrosion-resistant steel 21-6-9 and titanium alloy 3AL-2.5V are presently being incorporated into new model aircraft. Repair and fabrication of assemblies using these materials may require special procedures. Refer to the applicable maintenance directives for specific details.