TUBING FABRICATION AND MAINTENANCE
Chapter Objective: Upon completion of this chapter, you will have a working
knowledge of aircraft hydraulic and pneumatic tubing and their associated
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: Recognize the various
materials, tools, equipment, and testing
procedures used in the fabrication of
hydraulic and pneumatic tubing assemblies.
TYPES OF TUBING
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
Corrosion-Resistant Steel Tubbing
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
Aluminum Alloy 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.
alloy tube assemblies fabricated with aluminum alloy
tubing 6061-T6 are authorized for repair or
replacement for any aluminum line.
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.
Fittings for tube connections are made of
aluminum alloy, titanium steel, corrosion-resistant
steel, brass, and bronze. Fittings are made in many
configurations and styles. The usual classifications
are flared-tube fittings, flareless-tube fittings, brazed,
welded, and swaged fittings (figs. 6-1 through 6-4).
Refer to table 6-3, for identification of fittings.