Hydraulic Tubing

8/4/2019 Hydraulic Tubing

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48 Airframe Technology

A i r c r a f t M a i n t e n a n c e T e c h n o l o g y A P R I L 2 0 0 0

Lifelines to power and motion control

By Terry Karl and Mark Morrow

hen a fully loaded B747-400 final-

ly lands after 10 or more hours in

flight, some 600,000 lbs. of aircraft

and passengers travelling at

approximately 170 miles per hour

must come to a stop safely.While we all know that hydraulic pumps, valves, brakes,

and thrust reversers must work properly to accomplish this,

we often dont think about the ubiquitous metal tubes that

carry the hydraulic power through and to these subsystems.

These lifelines of the hydraulic system must also be in

optimum operating condition to meet these demands.

Most of todays attention in hydraulic system design

and maintenance is paid to the critical, large subsystems

and components, such as primary and secondary flight

control actuators, engine-driven pumps, auxiliary pumps,

electric motor pumps, and power transfer units. The emerg-

ing technologies surrounding the all electric aircraft and

EHAs (electrohydrostatic actuators) command much more

attention than simple hydraulic tubing. These technologies

and accessories drive both the complexity and costs asso-

ciated with aircraft hydraulic systems. As many mechanicsare aware, however, even with all these advancements and

the increased sophistication of hydraulic systems today,

fluid leakage remains as one of the most aggravating caus-

es for unscheduled maintenance actions. One of the lead-

ing causes of fluid leakage involves hard tubing and tube

fittings. This article focuses on hydraulic tubing and various

methods of maintenance and repair for these tubes.

Hydraulic power lifelinesHydraulic tubing and flexible hose styles are, indeed,

the lifelines of hydraulic power and motion control. These

distribution systems carry high-pressure (typically 3,000 psi)

hydraulic fluid power to all the various points in the aircraft

where work needs to be done. Although not nearly as

glamorous as the power and motion control accessories,

nevertheless, problems associated with tubing and fittings

cause some 30 to 60 percent of aircraft delays and cancel-

lations. A typical commercial transport can contain up to

several hundred yards of tubing and thousands of fittings

and fitting connections. Hydraulic tube sizes can rangefrom as small as a quarter inch up to one and a half inch-

es. Material types used range from aluminum (typically

6061T6) to corrosion-resistant steel (CRES alloys such as

300 series and more robust materials such as 21-6-9) to

exotic titanium material (such as 3Al-2.5V). Tubing wall

thickness for a given tube size can vary also, since the effort

to save weight in aircraft design is optimized. Therefore,

the mechanics dilemma in hydraulic tube repair can be

magnified quite quickly.

Tube failures

Many factors can contribute to tube failures. The mostcommonly cited factors are chafing (either against structure

or at clamping points), installation stress (created by runs

WProblems associated with tubing andfittings cause some 30 to 60 percent of aircraft

delays and cancellations.


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of tubing going from fixed point to

fixed point), and corrosion (due to

environmental conditions). While

thorough precautions are taken in the

selection of materials, design, and

installation by the airframe manufac-

turers, it is still possible that tubeleaks can occur. To minimize these

possibilities, maintenance of tubes

and fittings should always be accom-

plished in accord with the airframe

manufacturers recommended prac-

tices as detailed in the airframe main-

tenance manual.

Tubing maintenancebasics

Hydraulic system tubing mainte-

nance can be proactive in the sensethat routine inspections for evidence

of weeping and leakage can be done

visually or by wiping exposed sur-

faces with a clean, white rag to deter-

mine if fluid is present. Tube clamp-

ing points should be examined visu-

ally for evidence of looseness and

possible chafing against the tubes

they are clamping. Loose or damaged

clamps or clamps whose protective

elastomer padding is

worn or damagedshould be replaced.

Tubes should be

examined visually

for evidence of

scratches, nicks,

dents, or any other

malformations or

defects that could

cause failure points.

Tubing should also

be free from exces-

sive accumulations of other fluids

such as cleaning fluids and other

foreign substances that might

eventually lead to corrosive dam-

age to the tube. Finally, tubing

should not be in direct contact

with airframe structure, othertubes and/or hose or other acces-

sories that might lead to chafing.

In some cases, common air-

craft cleaning solutions may also

have long-term damaging effects

on tubing life. If there are any

doubts as to whether any of the

cleaning solutions might con-

tribute to decreased tubing life,

consult with the airframe manu-

facturers technical product support

personnel.Also, while it may seem self-evi-

dent, always check the tube fitting, or

B nut connections to ensure they are

properly torqued. Consult the air-

frame technical manual for various

torque levels given different tube and

fitting materials.

When inspection does reveal

any of these problems, replacing the

entire tube assembly or sections

thereof may be required. Several

repair or replacement strategies maybe employed, depending on the cir-

cumstances and what is permissible

according to the airframe manufac-

turers maintenance manual.

Repair and/or replace-ment strategies

Following are repair or replace-

ment alternatives, depending upon

the aircrafts situation, the resources

available at the time,

the mechanics skilland training level, and

the urgency of bring-

ing the aircraft back

into flight-worthy con-

dition:

Minor repair of

the tube assembly If

the tube is scratched

or nicked not deeper

than 10 percent of the

tube wall thickness,

Airframe Technology 49


Table I

Tube Flattening Limits

Tube Operating Pressure

Material 1000 PSIG Maximum Greater Than 1000 PSIG

Aluminum 10 Percent 5 Percent

321 CRES 10 Percent 5 Percent

21-6-9 CRES 5 Percent 3 Percent

Carbon Steel 7.5 Percent 7.5 Percent

Titanium 5 Percent 3 Percent

Ovality, or flatness, in the tube bend area can be measured by using the following formula:

Ovality Percentage = [(Max OD Min OD)/Nominal OD] X 100

Table II

Tube Bend Radii Recommendations

Minimum Bend Radii (inches)

Tube O.D. 6061T6 300 Series 21-6-9 TI-3AL-2.5V

Dash No. (in.) Aluminum Stainless Steel Stainless Steel Titanium

-3 3/16 3/8 3/8 9/16 9/16

-4 1/4 _ 1/2 _ 3/4

-6 3/8 _ 3/4 1-1/8 1-1/8

-8 1/2 1 1 1-1/2 1-1/2

-10 5/8 1-1/4 1-1/4 1-7/8 1-7/8

-12 3/4 1-1/2 1-1/2 2-1/4 2-1/4

-16 1 2 2 3 3-20 1-1/4 2-1/2 2-1/2 3-3/4 3-3/4

-24 1-/12 3 3 4-1/2 4-1/2

Figure 1. Tube Repair Methods


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Airframe Technology

typically such damage may be bur-

nished away using hand tools.

However, if the scratch or nick is on

the outside radius of a bend, then the

tube should be repaired or replaced

as outlined below. Tubes with severe

die marks, splits, cracks, evidence oftorsional twist, or dents should also

be replaced.

Replace the entire tube assembly

Obtain a complete new tube

assembly from stock. Make sure the

new tube assembly can be installed

without pre-stressing the end fitting

connections. Too much pre-stress to

the end fitting connections may sig-

nificantly affect tube life. Use

Illustrated Parts List part numbers,

and follow the appropriate sectionsof the airframe manufacturers maint-

enance manual.

Install a tempo-

rary hose assembly

Many manufacturers

permit the use of a

temporary repair

that consists of ahose assembly fabri-

cated from field

assembly or

reusable hose fittings and bulk hose.

Such hose assemblies are typically

limited for use until the aircraft reach-

es the next possible maintenance

facility that is capable of performing a

permanent repair. Care should be

taken to ensure the hose assembly

meets the pressure requirements of

the system it is being used with andthat there are no fluid compatibility

problems. In the case of hydraulic

systems using either phosphate ester

(typically known as Skydrol or Hy-

Jet) or MIL-H-5606 petroleum-based

products, Teflon-lined, wired-rein-

forced hose is preferred. Hose assem-

blies should be clamped in the loca-

tions where the hard tube was

clamped, and care should be taken

that all other installation practices for

hose are followed.Major repair of the tube assembly

Many, if not all, airframe manufac-

turers permit repair of tube assem-

blies either on the aircraft or in the

maintenance shop. Damaged sec-

tions of the assembly may be cut out

and replaced in the form of a splice

(see Figure 1.). Damaged end fittings

may be cut off and replaced, but care

should be taken that the repaired

assembly fits the installation and that

no pre-stress is imparted to therepaired tube due to misalignment or

differences in length caused by the

repair. After the defective tube assem-

bly is removed from the aircraft, there

are five basic steps to follow:

1. Cut the defective area out of theassembly

2. Deburr the cut ends3. Form the replacement tube sec-

tion4. Install the new tube section

and/or fittings5. Inspect the new tube assembly

Always ensure that the repair

tube material is of the correct materi-

al type, size, and wall thickness in

accordance with the manufacturers

instructions.

Generic tube repairStep by step description of a

basic tube repair:

Tube cutting Cut the tube

using a chipless tube cutter, high-

grade hacksaw, or other production

method that ensures a square-cut

end, with a minimum amount of

burrs. The cutter should be moved

slowly and uniformly to ensure that

tube deformation does not occur.Tube deburring After cutting

the tube, carefully remove any burrs

from both the outside diameter and

inside diameter of the tube. Use of a

deburring tool helps to prevent the

inclusion of metallic chips inside the

tube which would contaminate the

hydraulic system. Cut and deburred

tube ends should be protected from

further damage or the collection of

dust or dirt if they are to be left unat-

tended for any period of time.Tube bending In most cases it

will be necessary to form the

replacement section of tubing to fit

the aircraft. Obtaining a smooth

bend with an absolute minimum of

tube flattening is needed to ensure

high integrity and life of the

replacement section as well as

avoiding any unwanted fluid flow

restriction. Tubes may be formed

either by hand or by powered tube

benders, but care must be taken,depending upon the nature of the

tube material, as well as the wall


Figure 5(d) -

Swag the Fitting

Figure 5(a) - Mark the Tube

Figure 5(b) - Position the Fitting

Figure 5(c) - Position the Tool


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Airframe Technology 51thickness. Excessive flattening, kink-

ing, wrinkling, or other deformation

of the tube must be avoided. Table I

on pg.52, shows the acceptable lim-

its of tube flattening.

The maximum OD and the

minimum OD are the largest andsmallest cross-sectional diameters

measured within the area of the

bend. The ovality in the bend area

should not exceed the values in

Table I. Recommended minimum

bend radii are as shown in Table II,

pg.52.

Depending on the capability of

the apparatus used to bend the

tube, it may be necessary to use

larger bend radii.

Installing tube fittings Afterthe replacement section has been

formed and is ready for installation,

attachment of the permanent tube

fittings is required. While each of

the permanent tube fitting styles has

some unique requirements, there

are generic steps which are similar:

1. Mark the parent tube and the

replacement section to indi-

cate where the repair fitting

is to be positioned

2. Position the tube fitting rela-tive to the tubing

3. Position the installation tool-

ing

4. Install the tube fitting

5. Inspect the installed joint

Figures 5(a) through 5(e) depict

this sequence of events for the axi-

ally swaged, Rynglok Tube Fitting

System.

Inspection of the repaired tube

assembly If possible, the repairedtube assembly should be proof test-

ed using appropriate equipment, in

accordance with the airframe manu-

facturers maintenance manual

instructions, prior to being installed

on the aircraft. Alternatively, the

repaired tube assembly may be

installed on the aircraft and tested as

the hydraulic system is tested before

deeming the aircraft flight-worthy.

Care should be taken to perform all

normal hydraulic system tests.Permanent tube fittings:A bewil-

dering array Over the years, quite

a few permanent

tube fitting styles

have been devel-

oped, both for pro-

duction of the air-

crafts hydraulic

tube system, aswell as for repair of

the aircraft once in

s e r v i c e .

Development of

this wide variety of

choices is due in

large part to the complex variety of

installations on the aircraft. Many

mechanics have often encountered

situations where, among other

things, they have wondered exactly

how the installation was designedand installed on the aircraft and

whether any thought was given to

the poor mechanic who one day

might have to maintain these tubes.

As a result, each of the types of per-

manent tube fitting styles offers

attractive attributes, depending

upon a variety of circumstances.

Correspondingly, each tube fit-

ting system also entails some less

than desirable attributes. Each

mechanic and system maintenanceengineer must examine the primary

criteria of their aircraft needs, fleet

needs, logistics, mechanic skill and

training levels, and other pertinent

factors, when deciding what tube fit-

ting styles best meet their require-

ments. Having said that, it is worth

mentioning the most prominent per-

manent tube fitting styles available

today:

Weld-style fittings:Widely used

to produce aircraft hydraulic tubeassemblies, this method of attach-

ment may also be used for repair,

but requires the proper weld equip-

ment and inspection facilities and is

more difficult to accomplish on

board the aircraft. Mechanic skill

levels are relatively high. Welded

tube connections, accomplished

correctly, create joints of equal or

greater strength than the parent

tube, and are light in weight.

Bite-style fittings: This fittingstyle relies on sleeves that literally

bite into the parent tube to effect the

connection. They are relatively sim-

ple to accomplish, require a lesser

skill level on the part of the mechan-

ic, but also require a larger envelopein which to turn wrenches that

install the fitting.

External swage-style fittings:

Although used industry-wide, exter-

nal swage-type fittings require a

considerable amount of equipment

to accomplish repairs and a relative-

ly skilled workforce to install them.

The prevalence of the system pro-

vides for logistic advantages. In

some cases, the production tubes

were manufactured with this system,minimizing the envelope restrictions

for access to accomplish repairs.

This fitting style also uses an elas-

tomeric seal on the interior of the fit-

ting as a secondary seal, if required.

Shaped memory fittings:

Advancements in metallurgical sci-

ence allowed for the development

of a special fitting style that relies on

the memory of the metal. These

fittings are stored in cryogenic

dewars of liquid nitrogen andremoved when needed to be

installed on the hydraulic tubes.

Special equipment pre-chills the

tube ends to properly accept these

lightweight fittings; however, since

these fittings accomplish their

method of attachment to the tubing

by warming up in the ambient envi-

ronment, they are somewhat time

sensitive in their installation.

Axially swaged-type fittings:This

fitting style axially swages a perma-nently attached ring around a fitting

body, which permanently deforms


Figure 5(e) - Inspect the Installation


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both the fitting and the tube to effect a

metal-to-metal seal without the use of

elastomers. The fitting material is compat-

ible with all types of tubing and wall thick-

nesses. Mechanic training and skill level

are relatively low. Installation equipment

investment required is more than bite-typefittings and shaped memory fittings, but

less than external swage and weld-style

fittings.

The road aheadCare and maintenance of hydraulic

system tubing are as important as any

other aspect of system maintenance.

With the ever increasing complexity of

todays aircraft, proper attention to

hydraulic tubes on both a preventative as

well as a repair and replacement schemecan avert potential problems before they

impact the aircrafts flight operations.

Older and newer technologies and

approaches are available to meet each

individual aircrafts needs. Newer tech-

nologies attempt to address all elements

of cost associated with the repair and

maintenance of hydraulic tubing. Each

mechanic is encouraged to familiarize

themselves with alternative tube repair

technologies in an effort to ensure opti-

mized repair capability for the aircraft onwhich they work. AMT


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Terry Karl is the Engineering Manager,and Mark Morrow is the Distributor

Business Manager for Eaton AeroquipEngineered Systems Division.

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Hydraulic Tubing