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Guide through chapter 7

Aircraft fluid lines are usually made of what?

Metal tubing(rigid fluid lines) or flexible hose

This is used in stationary applications and where long relative runs are possible, they are widely used in aircraft for fuel, oil, coolant, oxygen, instrument, and hydraulic lines.

Metal Tubing (rigid fluid lines)

This is generally used with moving parts or where the hose is subject to considerable vibration

Flexible Hose

If a tube bursts or cracks what is it generally the result of?

Excessive Vibration
Improper Installation
Damage Caused by collision with an object

True or False: During replacement tubing should be of the same size and material as the original tubing


True or False: All tubing is pressure tested prior to initial installation, and is designed to withstand several times the normal operating pressure to which it will be subject


What type of tubing material is there for Rigid Fluid lines?

Aluminum Alloy Tubing
Titanium 3AL - 2.5V

In the early days of aviation what type of tubing was used extensively in aviation fluid applications?

Copper Tubing

In modern aircraft these types of alloys have generally replaced copper tubing

Aluminum Ally
Titanium 3AL - 2.5V

Tubing made from what is used for general purpose lines of low or negligible fluid pressures, such as instrument lines and ventilating conduits.

1100 H14 (1/2 Hard)
3003 H14 (1/2 Hard)

Tubing made from these materials are used in general purpose systems of low or medium pressures, such as hydraulic and pneumatic 1000 to 1500 PSI systems, and fuel and oils

2024-T3 (Solution Heat treated & then cold worked
5052-O (Annealed recrystallized)
6061-T6 (solution heat treated and then artificially aged

This is used extensively in high pressure hydraulic systems (3000 PSI or more) for operation of landing gear, flaps, brakes, and in fire zones.

Annealed CRES 304
CRES 321
CRES 304 1/8 Hard
CRES = Corrosion Resistant Steel

Because of its high tensile strength, it permits the use of tubing with thinner walls; consequently , the final installation weight is not much greater than that of the thicker wall aluminum alloy tubing

Annealed CRES 304
CRES 321
CRES 304 1/8 Hard

These materials are used where there is risk of FOD i.e; the landing gear and wheel wells.

Steel Lines

Although identification markings for steel tubing differ, each usually includes what?

Manufacturer's name or trademark
SAE (Society of Automotive Engineers) #
Physical condition of the metal

This type of tubing and fitting is used extensively in transport category and high performance aircraft hydraulic systems for pressures above 1500 PSI.

Titanium 3LA-2.5V

What type of fittings are used with titanium tubing?

Cryofit Fittings
Swaged Fittings

Why should you not use titanium tubing and fittings in any oxygen system assembly?

Titanium and titanium alloys are oxygen reactive, if fresh formed titanium surface is exposed in gaseous oxygen spontaneous combustion could occur at LOW pressures

Before making repairs to any aircraft tubing, it is important to what?

Make accurate identification of tubing materials

To positively identify the material used in the original installation what should you do?

Compare code markings of the replacement tubing with the original markings on the tubing being replaced

On large aluminum alloy tubing, where is the alloy designation? On small aluminum tubing??

- Stamped on the surface
- Stamped on the surface but more often it is showed by a color code not more than 4 inches width, painted at the two ends and approximately midway between the ends of some tubing

Name the color bands for these aluminum alloy #s:

Blue & Yellow
Brown & Yellow

When the band consists of 2 colors, what width is used for each color?

1/2 the width

If the code markings are hard or impossible to read, it may be necessary to do what?

Test samples of the material for hardness by hardness testing

How are metal tubings sized and in what fractional size is it measured?

Measured by OUTSIDE diameter & in 16-ths of an inch

What is important to know when installing tubing?

Outside Diameter
Wall Thickness

In addition to other classifications of means of identification tubing is manufactured in various types of?

Wall Thickness

Wall thickness is usually printed on the tubing and is measured in what?

Thousands of an inch

How would you determine that inside diameter of a metal tubing?

Subtract (2x)twice the wall thickness (w.t.) from the outside diameter (o.d.)

Tube forming consists of 4 processes, what are they?


True or False: Scratches, abrasions, or minor corrosion on the outside of fluid lines may be considered negligible and can be smoothed out with a burnishing tool or aluminum wool.


If a fluid line assembly is to be replaced, what can often be salvaged?

The fittings

When cutting tubing it is important to produce a cut that is what? What can tubing be cut with?

Square End
Free of burrs
Tube Cutter (w/any soft metal copper, aluminum or aluminum alloy), Hacksaw

A new piece of tubing should be cut approximately WHAT % longer than the tube to be replaced to provide for minor variations in bending.

10 percent longer

This tool is capable of removing both the inside and outside burrs by just turning the tool end for end

Deburring Tool

Why must extreme care in the deburring operation by taken?

Wall thickness of the end of tubing may be reduced or fractured, slight damage of this type can lead to fractured flares or defective flares which will not seal properly

When using a fine tooth hacksaw, preferably one have how many teeth per inch?

32 teeth per inch

What is an easy way to hold small diameter tubing, when cutting it?

Place the tube in a combination flaring tool
Clamp the tool in a vise

Making the cut about one half inch from the flaring tool, keeps what from happening?

Sawing vibrations to a minimum
Prevents damage to the tubing

The objective of this procedure is to obtain a smooth bend without flattening the tube.

Tube Bending

True or False: Tubing under 1/4" in diameter usually can be bent without the use of a bending tool


True or False: A small amount of flattening in bends is acceptable, but the small diameter of the flattened portion must not be less that 50% of the original outside diameter

False; Must not be less than 75%

Tubing with these types of bends should not be installed, what are they?


With this type of equipment, proper bends can be made on large diameter tubing and on tubing made from hard material.

Tube bending machines i.e. CNC tube bending machine

The ordinary production tube bender will accommodate tubing ranging from what size outside diameters.

1/4" to 1 1/2" o.d.

True or False: A filler of metallic composition or of dry sand may e used to facilitate bending

True (alternative bending method)

True or False: When using a filler method, the cut of the tube should be slightly longer than required

True, the extra length is for inserting a plug (may be wooden), tube ends can be closed by flattening or soldering metal disk in them

What two kinds of flares are generally used in aircraft tubing?

Single Flare
Double Flare

Why is it important that the flare tubing be properly shaped?

- Flares are frequently subjected to extremely high pressures
- If not shaped properly, the connection all leak or fail

A flare made to small produces?
A flare made to large produces?

- To small produces a weak joint which mate leak or pull apart
- To large interferes with the proper engagement of the screw thread on the fitting and will cause leakage

This is the result of the tubing not being cut squarely

Crooked Flare

True or False: If a flare is not made properly, flaws CAN be corrected by applying additional torque when tightening the fitting

False, flaws cannot be corrected

The flare and tubing must be free of what?

Cracks, dents, nicks, scratches, or any other defects

A flaring tool used for aircraft tubing has male and female dies ground to produce a flare of what degree?

35 to 37 degrees

True or False: Under no circumstances is it permissible to use an automotive-type flaring tool which produces a flare of 45 degrees


A single-flare hand flaring tool consists of?

Flaring block or grip die
A yoke
Flaring pin

What two types of flaring tools are used to make flares on tubing?

Impact type
Rolling type

On what material should rolling-type flaring tools only be used on? And which should it not be used on?

Soft copper, aluminum and brass tubing
Do not use with corrosion resistant steel or titanium

What should always be read since there are several different types of rolling type flaring tools that use slightly different procedures?

Tool manufacturer's instructions

A double flare is used on what material tubing and with what diameters? Why is this necessary?

- Aluminum alloy tubing 3/8" outside diameter and UNDER
- Necessary to prevent cutting off the flare and failure of the tube assembly under operating pressures

Why is a double flare better?

Smoother and more concentric therefore seals better
More resistant to the shearing effect of torque

To prevent thinning out of a flare wall, what should you not do?

Over tighten

Although the use of flare less tube fittings eliminates all tube flaring, another operation is necessary prior to installation of a new flare less tube assembly, what is this operation?


Final tightening depends upon the tubing:
For aluminum tubing?
For steel tubing?

For aluminum alloy up to and including 1/2" outside diameter, tighten nut from 1 to 1 1/6 turns
For steel and aluminum alloy tubing over 1/2" outside diameter, tighten from 1 1/6 to 1 1/2 turns

After presetting the sleeve, disconnect the tubing from the fitting and check the follow points; The tube should extend how far beyond to the sleeve pilot? Otherwise what can happen?

3/32" to 1/8"
Blowout may occur

The sleeve pilot should contact the tube or have a maximum clearance of how far for aluminum alloy and steel tubing

0.005" for aluminum alloy
0.015 for steel

True or False: A slight collapse of the tube at the sleeve cut is permissible


Tubing may be beaded with what?

Hand beading tool
Machine beading rolls
Grip dies

How is the method of beading determined?

Depends on the diameter and wall thickness of the tube and the material from which it was made

Hand beading tool is used with tubing have what outside diameter?

1/4" to 1" o.d.

Fluid lines in aircraft are often identified by what type of markers? What do they identify

Color codes, words, and geometric
These markers identify each line's function, content and primary hazard

True or False: Paint is used on lines in engine compartments, where there is a possibility of tapes, decals, or tags being drawn into the engine induction system


True or false: On lines 4" in diameter or larger, lines in oily environments, hot lines, and on some cold lines, steel tags may be used in place of tape or decals


Lines conveying rule may be marked how?


Lines containing toxic material are marked how?


Lines containing physically dangerous materials, such as oxygen, nitrogen, or Freon may be marked how?


Who is responsible for original installation of identification markers, and who is responsible for their replacement when it becomes necessary

Aircraft and engine manufacturers
Aviation mechanics

Depending on the type and use, fittings will have what two types of threads?

Pipe threads
Machine threads

These threads are similar to those used in ordinary plumbing and are tapered both internal and external

Pipe threads

How are external and internal threads referred to as?

External threads - male threads
Internal threads - female threads

When two fitting are joined, male and female, the thread taper forms what?

A seal

What should be used when joining pipe threads to prevent seizing and high pressure leakage.

Some form of pipe thread lubricant approved for the particular fluid application

Where should lubricants not be used?

Oxygen lines, the oxygen will react with petroleum products and can ignite

True or False: Use care when applying thread lubricant so that the lubricant will not enter and contaminate the system


These type of threads have no sealing capability and are similar to those used on common nuts and bolts, they also have no taper and will not form a fluid tight seal.

Machine threads

This type of fitting is used only to draw connections together or for attachment through bulkheads.

Fitting with machine threads

What types of seals is used to make the connection fluid tight, when a fitting with a machine thread is used.

Flared tube connection
A Crush washer
Synthetic seal

The size of fittings with machine threads is given in dash numbers, which equals what?

The nominal outside diameter in 16-ths of an inch

Fittings attach one piece of tubing to another or system of units...what four types are there?

Bead and clamp
Flared fittings
Flare-less fittings
Permanent fittings

What is usually the deciding actor in the selecting of a connector

The amount of pressure that the system carries and the material

When a fluid line passes through a bulkhead, and it is desired to secure the line to a bulkhead, what type of fitting should be used?

Bulkhead fitting

The type of fitting is used only in low-or medium-pressure systems, such as vacuum and coolant systems - requires a bead, section of hose, and hose clamps

Bead and clamp fitting

These types of fittings may be used as connectors in all systems regardless of pressure

Flared, Flare-less, or permanent type fittings

A flared tube fitting consists of what?

A sleeve and nut

True or false: Tubing used with flared fittings must NOT be flared before installation


For what reason does the sleeve in a flared tubing support the tube?

So that vibration does not concentrate at the edge of the flare, and distributes the shearing action over a wider area for added strength

Fitting combinations composed of different alloys should be avoided for what reason?

To prevent dissimilar metal corrosion

How are standard AN fittings identified?

AN steel fittings are colored black
AN aluminum fittings are colored blue
Aluminum bronze fittings are cadmium played and natural in appearance

These flare less fittings are designed primarily for high pressure (3000 psi) hydraulic systems that may be subjected to severe vibration or fluctuating pressure

MS flare less fittings

MS flare less fittings eliminate what? yet still provides what?

Eliminates tube flaring
Provides a safe & strong dependable tube connection

MS flare less fittings consist of 3 parts what are they?


What purpose does the reverse angle prevent and provide?

Prevents inward collapse of the tubing when tightened and provides a partial sealing force to be exerted against the periphery of the body counterbore

The counterbore should within the body of a MS flare less fitting is what for steel connectors and aluminum fittings

15 degrees for steel connectors
45 degrees for aluminum fittings

A popular system for connecting and repairing hydraulic lines on transport category aircraft is the use of what?

Permaswage fittings

Create a permanent connection that is virtually maintenance free, used to join hydraulic lines in areas where routine disconnections are not required and are often used with titanium and corrosion resistant steel tubing

Swaged fittings

Swaged fittings are installed with what?

Portable hydraulic powered tooling

What should always be used to install swaged fittings?

Manufacturers instructions

This is a tube fitting that is mechanically attached to the tube by axial swaging, the movement of the ring along the fitting body results in deformation of the tube with a leak tight joint

Permalite fitting

Many transport category aircraft use what type of fittings to join hydraulic lines in areas where routine disconnections are not required

Cryofit Fittings

Cryogenic sleeves are made of what type of alloy

Shape memory alloy, Tinel

Describe the process that happens to install a cryofit fitting

- Sleeve is manufactured 3% smaller
- Frozen in liquid nitrogen & expands 5%
- Fitting removed and inserted onto tube
- During warming period fitting contacts forming a permanent seal

Cryofit fittings can only be removed by what means?

Cutting the tube at the sleeve

What should be done before installing a line assembly in an aircraft?

- Inspect line carefully
- Remove dents and scratches, be sure all nuts and sleeves are snugly mated and securely fitting by proper flaring of the tubing
- Line assembly should be clean and free of all foreign material

Why should you never apply compound to the faces of the fitting or the flare?

It will destroy the metal-to-metal contact between the fitting and flare, a contact which is necessary to produce the seal.

The use of what prevents over tightening or under tightening?

Torque wrenches and the prescribed torque values

True or false: It is permissible to tighten the nut an additional 1/6 turn (total 1/3), should a connection leak.


Name 3 steps in the installation of a flare less tube

1) Tighten the nut by hand until increase in resistance
2) Use a wrench and turn the nut 1/6 turn *use on the connector to prevent it from turning while tightening the nut
3) After the tube assembly is installed, the system should be pressure tested

In regards to installation of rigid tubing, how far should you never tighten the nut beyond

1/3 turn, this is the maximum the fitting may be tightened without the possibility of permanently damaging the sleeve unit

True or false: Minor dents and scratches in rigid tubing may NOT be repaired


Scratches and nicks not deeper than WHAT % in the wall thickness in ALUMINUM alloy tubing, and which are not in the heel of a bend, may be repaired by what means?

Not deeper than 10%
By burnishing with hand tools

Since the damage limits for hard, thin walled corrosion-resistant steel and titanium tubing are considerably less than for aluminum tubing, what should you consult for damage limits

The aircraft maintenance manual

What needs to happen to lines with severe die marks, seams, or splits in the tube

Need to be replaced

This is a ball bearing or slug normally made of steel or some other hard metal, in the case of soft aluminum tubing, a hard wood slug or dowel may even be used.

A bullet

True or false: Any crack or deformity in a flare is unacceptable and is cause for rejection


True or false: A dent of less than 45% of the tube diameter is not objectionable, unless it is in the heel of a bend

False, less than 20%

When repairing a low-pressure line using a flexible fluid connection assembly, you should position the hose clamps carefully to prevent what?

Overhand of the clamp bands or chafing of the tightening screws on adjacent parts

When replacing rigid tubing what must you ensure?

- The new line is the same as that of the line being replaced
- Never select a path that does not require bends in tubing

Why are bends necessary?

To permit the tubing to expand or contract under temperature changes and to absorb vibrations

This type of fluid line is used in aircraft systems to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed?

Flexible Hose Line, can also be used as connector in metal tubing system

What is NEVER used in construction of flexible fluid lines?

Pure rubber

What is used in place of pure rubber on flexible hose lines?

Synthetic materials
Ethylene propylene diene rubber

This is a synthetic rubber compound which has excellent resistance to petroleum products


What should you not use Buna-N for

Phosphate ester base hydraulic fluid (skydrol)

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