aging aircraft wiring

8/9/2019 Aging Aircraft Wiring

1/89

NAVAL POSTGRADUATE

SCHOOL

Monterey,

California

THESIS

AGING

AIRCRAFT

WIRING:

A

PROACTIVE

MANAGEMENT

METHODOLOGY

by

Vasileios

Tambouratzis

June

2001

Thesis Advisor:

Associate Advisor:

Donald

R .

Eaton

Raymond

E.

Franck

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Master's

Thesis

4 TITLE A N D

SUBTITLE

ging

Aircraft Wiring:

A

Proactive

Management

Methodology

6.

AUTHOR(S)

Tambouratzis, Vasileios

5.

UNDING NUMBERS

7.

ERFORMING

ORGANIZATION

NAME(S)

AND

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Naval

Postgraduate

School

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CA

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11 . UPPLEMENTARY

NOTES

Th e views

expressed

in

this thesis

are

those

of the author

and

do

not

reflect

the

official policy or position of the

Department

of

Defense or

the

U.S.

Government.

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13 .

ABSTRACT

maximum 20 0 words)

During the last years, military budgets have been dramatically reduced an d the services have been unable to

acquire

sufficient

ne w

systems.

Military aviation is

on e

of

the

areas

that

have

been severely

impacted.

Th e

result

is

that

the

current

fleet

faces

significant

aging

aircraft

problems.

Aircraft wiring is one

of

the areas that have severly affected by the aging process.

Recent

accidents involving

aging

wiring

problems

and

reduced

operational

readiness

du e

to aging

wiring

have made clear

that

aging aircraft

wiring presents a difficult an d complicated problem fo r the military aviation. However,

current maintenance practices

fall

short in

successfully

inspecting an d

maintaining

wiring.

Th e purpose

of this

thesis

is

to

provide

a

proactive management plan

to

deal

with

aging

wiring. Th e objective

is

to

come

up with a systematic process

in

order

to identify an d prevent serious failures

caused

by electrical faults

of

wiring systems.

This

process

will

be

based

on

the

principle

of

Reliability

Centered Maintenance (RCM).

14 .

UBJECT

TERMS

Aging

Aircraft, Aging

Aircraft Wiring,

Reliability

Centered

Maintenance

15 . UMBER

OF

PAGES

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CODE

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ABSTRACT

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ABSTRACT

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AGING

AIRCRAFT WIRING:

A

PROACTIVE MANAGEMENT

METHODOLOGY

Vasileios Tambouratzis

Captain,

Hellenic

A ir

Force

B.S., Hellenic

Air

Force Academy, Technical Department,

1993

Submitted

in

partial

fulfillment

ofthe

requirements

for the

degree

of

MASTER

OF SCIENCE IN MANAGEMENT

from

the

NAVAL POSTGRADUATE SCHOOL

June

2001

Author:

Approved

by :

Donald R.

Eaten, Thesis Advisor

J. Euske,

Dean, Graduate

of

Business an d Public

Policy

ixx


5/89

V


6/89

ABSTRACT

During the as t

years, military

budgets have been

dramatically

reduced nd th e

services

have been unable to

acquire sufficient

new

systems.

Military aviation is

on e

of

the

reas

hat have been everely

mpacted.

he

esult

s ha t

he

urrent fleet faces

significant

aging

aircraft

problems.

Aircraft wiring

s

ne

f the reas

hat have everely ffected

by

he

ging

process. ecent

ccidents nvolving

ging wiring

roblems nd

educed

perational

readiness

ue

o

ging

wiring

have

made

lear

ha t

ging

ircraft

wiring

presents

difficult nd omplicated roblem or he ilitary viation.

owever,

urrent

maintenance

practices fall

short

in

successfully inspecting

an d maintaining

wiring.

The

purpose

of

this

thesis is to

provide

a

proactive management

plan

to deal

with

aging wiring.

The

objective

is to

come

up

with a

systematic process

in order to

identify

and

prevent

serious

ailures

aused

by

electrical

faults of

wiring

ystems. This

process

will

be

based

on

th e principle of

Reliability

Centered

Maintenance

(RCM).

v


7/89

V


8/89

TABLE OF

CONTENTS

I. NTRODUCTION

A. B A C K G R O U N D

B.

P U R P O S E

C. RESEARCH

QUESTIONS

D.

SCOPE

E.

EXPECTED BENEFITS

OF

THIS THESIS

F.

ORGANIZATION

II. HE

AGING A I R C R A FT PROBLEM

A. INTRODUCTION

B.

MILITARY

FLEET

C.

COMMERCIAL FLEET 4

D.JOINT INITIATIVES 5

III. AGING AIRCRAFT

WIRING

9

A.

INTRODUCTION

9

B. AIRCRAFT WIRING 9

1

General

9

2. nsulation 1

3. Circuit

Breakers

3

C. CAUSES OF AGING WIRING 3

1.

General

3

2.

Environmental Factors 5

3.

Wiring Design 6

4.

Wiring

Installation 9

D.

AGING WIRING

EFFECTS 9

1 General 9

2.

Short

Circuit

9

3.

Arc Tracking 0

4.

Results

1

E. C U R R E N T

MAINTENANCE

PRACTICES 3

1 General

3

2. O

Level Wiring Maintenance 4

3.

Visual

Inspections 7

4. Summary

9

IV .

ELIABILITY CENTERED MAINTENANCE OF AGING WIRING 1

A.

INTRODUCTION

1

B.

RELIABILITY CENTERED MAINTENANCE 1

1 Background 1

2.

R C M Principles 3

3. R C M Benefits

6

4. R C M

Categories 7

a.

Run-to-Failure

8

b. Preventive Maintenance 9

c. Predictive Maintenance 0

d.

Proactive

Maintenance

0

C.

R C M

APPLIED

IN

AGING AIRCRAFT WIRING

1

1 Scope

of

the Analysis 1

2.

R C M Process 2

l


9/89

a.

eneral

2

b.

Functional Failure Analysis 4

c.

ignificant

Item

Selection 5

d. R C M Decision Analysis 7

e.

Age Exploration 2

3.

Results

4

D.

TECHNICAL

S O L U T I O N S

ZZZZZZZZZZZZZZZZZZZZZ

66

1 General 6

2.

Smart

Wiring

6

3.

Non-Destructive Wiring Inspection

Methods

7

4.

Arc Fault Circuit Interrupters

9

V. O N C L U S I O N S A ND RECOMMENDATIONS 1

A.

INTRODUCTION

B.

C O N C L U S I O N S

ZZZZZZZZZZZZ.T2

1

Aircraft Wiring Ages and

Degrades

Over Time

2

2.

Aging

Wiring

Severly

Impacts

Aircraft Safety

2

3. Current Maintenance Practices

do

no t

Adequately Address Wiring

2

C.

RECOMMENDATIONS

3

1

R C M

Analysis

Should

be

Followed

fo r

Every

Type

of

Wiring

3

2.

An Accurate Wire Discrepancy

Data

Collection

System

Needs

to

be

Established 4

3. Technical

Solutions

C an

Assist

in

a

Proactive

Management Plan..

4

LIST OF

REFERENCES

5

INITIAL

DISTRIBUTION

LIST Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z

79

ll


10/89

ACKNOWLEDGMENT

The

uthor would

ike

o cknowledge

hose

ndividuals

who

provided

heir

support

throughout

the

information

gathering

phase

of

this

thesis.

would

also

ike

to

thank my

wife, Maria, for

her

patience

and

support

during

the

thesis

process.

X


11/89

I.

NTRODUCTION

A. BACKGROUND

During

th e as t

years, military

budgets have

been

dramatically reduced

and the

services

have been unable to

acquire sufficient

new

systems.

Military

aviation is

one of

the

areas that

have been

severely

impacted. The only way

to

meet

the mission

demands in

a

constrained

funding

environment

is

to

extend

the

service

life

of

selected

aircraft.

There

re

many

ld

ircraft

20

o

5+

years)

hat

re

he

backbone

f

the

operational force, some

of which

will

be

retired

an d

replaced

with

new

aircraft. However,

fo r he most part, eplacements

re number of years

way.

or many ircraft,

no

replacements are planned,

and many are

expected

to

remain

in service another 25 years.

For xample,

t will be t east nother

0-15

ears

t

best, before here will be

significant

number ofreplacements

fo r

the F-16 A/C.

The aging of aircraft

ha s resulted in

extremely

challenging

problems dealing with

the

ong-term

ffects

of

structural

ging

nd

repair, but

what

is

the ffect

of

aging

on

other

ystems?

Until

ecently,

he

ging

of electrical ystems, nd

wiring

pecifically,

received little

attention.

This is

changing

dramatically, in

part

due

to a

number

of

serious

accidents nvolving iring roblems.

ecent

ccidents n

both

he ommercial nd

military

viation

have

made

lear

that

th e

effects

of

age

on

aircraft

wiring

need

to

be

examined

in

th e

same

way

as

structures.


12/89

Wiring s he

vital

lectrical nd optical network hat arries data, ignals nd

power

to

an d

from

th e

various

systems.

Wiring

goes

into

every

nook

an d

cranny.

In

fact,

wiring

is

embedded

into the

aircraft

the way

nerves

are

embedded

into flesh.

Like

ll

materials,

wire ge s nd degrades over ime. Vibration, moisture nd

temperature

can

adversely affect wiring

haracteristics. Shorts, arcing an d open circuits

are

the

results

of wire

insulation

degradation which

can be

a

serious

flight

safety

concern.

Wiring problems have often aused ires or ircraft ystems malfunctions eading o

aircraft

loss.

Wiring-related problems

are a

leading

cause

of

unscheduled maintenance ours

for

ircraft.

ignificant

portion

of

aircraft

maintenance

man-hours s

xpended

n

troubleshooting wiring

to

ffect

repairs

of avionics

nd

weapon

ystems. However th e

maintenance philosophy is fly to fix . Aircraft wiring is not repaired unless it actually

causes system failure

or

is

a

safety

hazard. Moreover today's

typical nspections

are

visual

an d

they

do

not

ge t

to

th e

heart

of

aircraft

wiring

problems.

Obvious

failures

such

as

severed

wires are detected

but

individual visual inspections

do

not

reveal the slow

but

continuous

rosion

of

wiring

insulation

that

results

from

thousands of

bumps nd

jolts

over th e

aircraft

lifetime.

B.

PURPOSE

The

purpose

of this

thesis

is

to

provide

a

proactive

management plan to

deal with

aging

wiring.

The

objective

is to come up with a systematic process in order to identify


13/89

and

prevent

serious

failures

caused

by

electrical

faults

of

wiring ystems.

This

process

will

be

based on

the

principle of

Reliability Centered

Maintenance (RCM). Research

will

include an evaluation

of

the current

maintenance

philosophy

for

aging

wiring, analysis of

wiring

rom

R C M

perspective, nd

will

uggest

proactive

management

plan

or

dealing with aging

aircraft

wiring

systems.

C.

RESEARCH

QUESTIONS

The

questions

that

this

thesis

is

posing

and

trying to answer

are:

•

ow we define aging

aircraft?

What

are

the

associated

problems with

aging

aircraft?

•

hat

is th e

current

status

in U S A F ,

U SN

and

commercial

airlines

with

respect

to

aging?

•

hat

are

the

functions

and

th e

characteristics

of

th e

aircraft

wiring

systems?

•

hat are

the causes

of aging wiring?

•

ha t

are

th e

consequences of aging, in

wiring

systems?

• hat is

th e

current

maintenance

practice? Does it

adequately

address

wiring

problems?


14/89

•

ha t

is

Reliability

Centered

Maintenance

(RC M )

an d

what are

the potential

applications

to

the

problem

of aging

wiring

?

•

ha t

are

the

technical

solutions

that

can facilitate

a

management

plan

fo r

aging

wiring

based

on

R C M ?

D. COPE

The

cope

will

nclude:

n nalysis

of the

ging

wiring

problem

nd how t

affects

readiness

an d

aircraft

safety,

n

evaluation

of

the

current

maintenance

practice

fo r

aging

wiring,

n nalysis of

th e

Reliability

Centered Maintenance

R C M )

philosophy,

an d

a feasibility

tudy

of implementing

a proactive an d RCM-based, management plan

fo r aging

wiring.

The

thesis will conclude with a

recommendation fo r applying this plan

to

aging

aircraft

fleets.

E.

XPECTED

BENEFITS

OF

THIS

THESIS

This

tudy

ill rovide

he

ecessary

nformation

equired

o

mplement

proactive

nd

eliability-Centered-Maintenance ased

anagement

lan

or

ging

wiring.

xpected

esults

nclude

ncreased perational

eadiness,

eduction n

maintenance

costs an d

increased aircraft

safety.


15/89

F.

ORGANIZATION

Chapter I

provides n ntroduction

to

he

general ging

ircraft problem.

This

chapter

includes

a

description

of

the

aging

aircraft

issue,

an

overview

of

th e

current

status

in

the

military

an d the commercial sector

along

with

th e

initiatives that

these parties have

taken to deal with this problem.

Chapter

II

provides description

of

th e ircraft

wiring,

nalyzes

how

ging

affects it

and

what

are th e

results

of aging

wiring,

an d

provides

an overview of the current

maintenace

practice

concerning

aging

wiring.

Chapter

IV

describes

he

Reliability Centered Maintenance

oncept

and

how

it

ca n be applied in

aging

wiring,

devises a

proactive wiring

maintenance

plan and

provides

technical

solutions

that

are

based

on

R C M .

Finally,

hapter

rovides

he

onclusions

nd

ecommendations

f

his

research.


16/89

THIS

PAGE

IS

INTENTIONALLY

LE F T

BL AN K


17/89

II . HE AGING AIRCRAFT

PROBLEM

A.

INTRODU TION

Both

military an d

the commercial

air

fleets, face an impending crisis. Aircraft

are

getting older, nd s hey ontinue o ge,

problems

esulting rom ging

ircraft

will

become increasingly more

urgent.

The military and the airlines continue to

fly planes

as

they

age,

and many

of

these

aircraft have

already

exceeded their economic design

goal

(generally onsidered

to be

the period

of service,

fter

which

a

substantial ncrease n

maintenance osts

s xpected o

ak e place

n order

o

ssure

ontinued

operational

safety). Experience

proves

that

high-cycle

planes, ven

those

that

are

well

onstructed

an d kept n good

epair,

re

vulnerable

o many problems uc h s tructural atigue,

corrosion,

ystem

egradation, s hey ge . ge-related ncidents ay ecome

commonplace

an d

result

in

loss

of

aircraft,

loss

of

mission

and,

most

importantly,

loss

of

human

lives.

But,

how someone

can

decide

if

an aircraft

is old?

What

are

the

criteria

in

such a

decision?

No ingle

riterion identifies

ircraft

as

old .

The

age of a

plane

actually

depends

on

many

factors. Measuring

chronological

age is

on e means

of

establishing

the

age of

an

aircraft.

Considering

th e

number

of flight

cycles

a

plane has

accumulated

is

equally

mportant n

etermining

he

ea r

n lane. omplete

light

ycle

s

composed f

on e ake-off,

ressurization,

epressurization

nd

anding,

ince

hese


18/89

activities

ypically

place he

most

tress

on

an

ircraft. Consequently, o

obtain true

picture of th e age of

an

aircraft, both

th e number of

years

an d the number of

cycles

that

a plane

has

lown re

elevant

actors.

s

ircraft

ge

nd

ycles

ccumulate, ging

problems

will

inevitably occur. Hence,

th e

need

fo r

inspection

an d maintenance

increases

as aircraft

grow

older.

[Ref.

]

B.

MILITARY

FLEET

Any discussion of th e

wisdom

of

retaining instead of

replacing capital equipment,

such

as

aircraft, is usually

based

on

economic considerations. For

example, if th e costs of

maintaining he

quipment xceed

he

apital, nterest, nd amortization harges

n

replacement quipment,

he

ecision o purchase he eplacement

s

traightforward.

Often

the

replacement

equipment

offers

an improved productivity

as

well.

[Ref.

2]

In

he ase f

ilitary

ircraft, perational

eadiness

nd

afety-of-flight

considerations

also

enter

into

the

decision

to

repair

or

replace.

Fortunately,

inspection

an d

maintenance procedures

have

been

developed to reduce the likelihood

of

failures during

the design service life. However, several political changes, including the en d of

the Cold

War,

have caused th e military to

change

their approach to

force management. Since the

budget o

develop

new ircraft ystems has been educed, he only way o meet he

mission

demands

is

to extend

the

service

life

of some aircraft.

Th e

U.S . A ir Force has many old ircraft hat orm he backbone of

the otal

operational force structure.

Th e

oldest are the more than

500

jet tanker aircraft, the

KC -


19/89

135, that were first introduced

into

service more than

40

years ago. The B-52H

bomber

an d

th e

C-130

irlifter

became operational

35

to

40

years

ago.

The

F-15

air superiority

fighter,

the A-10

close ai r support aircraft an d

th e

E-3

(AWACS),

20 to 25

years

ago.

The

F-16 multirole fighter

an d

th e KC-10 jet

tanker

are

15 to

20

years

old.

For

the

most

part,

replacements

fo r

these

aircrafts

are

a

number

of

years

away and

th e

program

schedules

continue to

be

constrained

by

and

subject to th e vagaries

of

annual

funding cycles.

For

example, t

best,

t

will

be

5

o 20

years t

east, before

here

will

be

ignificant

number

of

replacements

fo r the

F-16.

The

remainder

of

the

aircraft mentioned

above

have

no planned

replacements

an d are expected to remain in service an additional

25

years or

more.

[

Ref.

2] . Table

1

shows the current age

status

of

th e

U S A F

fleet.


20/89

MC

Type

Avg

Age (yra)

Total A/C

A C Type

Avg

Age {yrs)

Total i

A/OA-iO

15.8

22 0

F-16 .1

80 2

B-1

|

10.3 77

EF-111

29.2

33

B -2

i

3. 3

20

F-117

6.4

57

B-52

i

35.8

85

G- 3

6 6

3

C-5

i 15.8

81

G-4

12

14

C- 9

26.5 23

G-7

12

9

KC-10

i

12.7

59

G-9

10.6

4

C-12

18 34

G-10 2.6

C-17

2.7

34

G-11 2.2

2

C-18

|

11.4

6

H -1 26.5

70

C-20

I

9.9

13

H-53

24.9

4 6

C-21

12.7 76

H-60 8. 4

59

C-23

j

12.9

3

RQ-1 0.9

2

C-25

j

6. 9

2

t-1 2 9 j

17 9

C-27

I

5.4

7

T-3 2. 6

11 0

C-130

l 25.1 30 6

T-37

34.2

41 9

C-135

35.7 30 0

T-38

30.2

47 1

C-137

I

21.3

6

T-39

36.6

3

C-141

31

141

T-41

27.5 •

3

E- 3

17.8 32

T-43 23.5

1 1

E-4

I

23.3

4

U-2

13.6

28

E- 8

1-2

2

V-18 13.5

3

F-4

F.15

i

27.9

3

i

11.9

61 8

i

Töt^

18.8

4,481

Table 1. USAF

Active

Fleet

May

1998

From [Ref. 3]

The

Navy

aces

imilar

problem.

he

Navy

urrently

perates

ver

2,100

aircraft

that are

over

fifteen

years old,

965

of

which

are

more than

25

years old.

Figure

displays

th e

aging trend for the

current

fleet.

10


21/89

Naval

Aviation-Average Age

31

26

A/C Age

21

11

1973 1981 1989

997

2005 2013

Fiscal Year

-Average

Age:

17.2

yrs

Figure 1 Aircraft Average Age Trend From [Ref.

4]

Some naval

aircraft are over

30

years old, uch

as

th e

CH-53D

an d the

CH-46.

Some others have replacements on the

way, such as th e F/A-18 E/F Super

Hornet

for the

F-14 Tomcat an d

older

F/A-18 C/D Hornets; the Boeing 737

fo r

the C-9;

th e

V-22 for

th e

H-53

nd H-46; and

the

CH-60

for

the

H-46.[Ref.5]. However, several platforms

do

not

have

replacements

currently

planned,

an d

even the

ones

with replacements

coming,

will

continue

to

operate

for

several

more years

before

new

systems

come

into service. Like

th e

Air Force

ase,

xtending

naval

ircraft

ervice

ife

by ontrolling

ging

mpacts

s

critical

fo r future

mission

accomplishment.

The

number

of

military

operations

during

he

as t

years,

ha s

been

very

high.

Although

he operational

nvironment

s

very

demanding,

he number

of

aircraft

has

shrunk

with

th e

emaining

orce

ging

apidly.

Aging ircraft

nd

ts

mplication

s

1 1


22/89

relatively

new

topic an d

wasn't

considered

by the aircraft

designers

because they

thought

that

aircraft nd elated systems would be eplaced before

ge

became n ssue. Even

aircraft hat re

elatively young

re

eing

tressed o heir imits. specially

Navy

aircrafts,

that are flown under adverse

conditions

(salt water,

catapult

launches

an d

hard

landings), xperience ncreased

ging

problems. For

xample,

he F-18 ommunity s

expecting

to spend

$878

million

over th e

next

12

years

to

conduct

a

service

life

extension

program (SLEP)

fo r

35 5 F/A-18 C /D aircraft.

As result, military aviation readiness s alling. Figure

2

depicts

th e daunting

trends fo r naval

viation,

while

Figure 3 hows th e increase

in

maintenance

man-hours

per flight hour.

12


23/89

Trends-Readiness

Percent

Mission

Capable

1995

996

997

99 8

Figure

2.

Readiness Trends

From

[Ref. 4]

Trends-Maintenance

95

6

7

8

9

Figure

3.

Maintenance

Man-Hour

Trend

From

[Ref. 4]

1 3


24/89

C.

COMMERCIAL

FLEET

Since

deregulation

of the

irline ndustry, he

apid

growth of U.S .

ir

arrier

passenger

traffic

ha s

been

accompanied

by

high

demand

an d

increased

delivery

times

fo r

ne w

aircraft.

The

on g ead time or

the

acquisition

of

new aircraft has hus orced the

airlines

to operate

some

of their

aircraft

beyond

originally

expected

engineering

life.

Various

factors

force airlines to operate

irplanes

beyond their economic

design

goals. New aircraft production cannot

keep

pace

with

industry

growth

an d

probably will

no t

be

able

to

match

th e

demand

in

the

near

future.

This

lag

in

production

has

resulted,

an d

will continue to

result, in

the

extended us e

of

numerous aircraft beyond their intended

life

spans.

Due

to

backlogs in orders fo r new

aircraft,

delivery

may

be

delayed

for several

years after th e order

is

placed. Thus, to meet consumer demand, airlines continue to fly

aircraft that they

expected to

retire. Furthermore,

new planes

are

being used not to

replace

old

ircraft,

bu t

o

upplement

he xisting leets, hus xpanding

he

leet o match

passenger demand. Low fuel prices also make it

economical

to continue

to use th e

older,

less

fuel-efficient

planes rather

than retire

them.

The

verage

ge

of th e

U.S .

ommercial ai r

carrier

fleet

has

increased

from

4.6

years

in 1970 to

8

years in

1999.

The U.S.

commercial

fleet breakdown is presented in

Figure 4.

By early 1999, 41 percent of

the fleet

was

at least 20 years old an d nearly 80 0

more

aircraft were

apidly

pproaching hat

ge .

n

the

past, 20-year-old

aircraft were

most often replaced by newer aircraft

fo r

airline service. However, this

is

no

longer true

and the number

of

20-year-old aircraft

is

xpected

to

ncrease.

Although

hronological

14


25/89

age

alone is

not

a

direct

measure

of potential

aging

problems,

it

can

alert

operators

to

problems

when

age

correlates with

high

numbers

of flight hours and

flight cycles.

6,50

3,50 -,

24,90

24,10%

jB

20+

years

15-20

years

□ 10-15 years

05-10

years

under

5

years

Figure 4.

U.S. Commercial Fleet Age Breakdown From [Ref. 6]

D.

JOINT INITIATIVES

As previously

shown,

both

the

military

and

the commercial

aviation,

experience

urgent

aging

problems. Table

2

gives

a

clear

picture

of the

extensiveness

of

the

problem.

1 5


26/89

Number

GE

Of

Planes

0+Years

0+Years

Major U.S . Airlines

3696

International

Airlines

3646

U.S . Cargo carriers

82

International

cargo

95

U.S . Air

Force

4421

90 %

83

97%

96 %

71

41%

36 %

81 %

84

42

Table

2.

Ages

of

Aircraft Serving

in

Composite

Fleets,

as

of

1999

From

[Ref.

7]

The designers of

th e

aircraft

in

service

today,

would

have

never

dreamed

these

planes would

be

operational fo r so many

years.

ndeed, not much

thought

was given to

the

ging ssue,

ecause he

ircrafts

ere

o

av e een

etired

on g efore

ging

problems

became

significant.

In

order

to ffectively

deal with aircraft

aging,

th e military

an d the

commercial

sector

have joined forces.

The

Navy, Air

Force, Federal

Aviation

Administration (FAA),

NAS A nd private

erospace

ndustry re jointly ttempting o

nsert

echnology

nd

improve aintenance/support ctions o

ddress

he ging

ircraft

ssue. arious

organizations

nd

joint programs uc h

s

he White House

Commission on

Aviation

Safety

an d Security

(WHCSS), th e

A ir Transport

Association's

Aging

System

Task Force

(ASTF), he

FA A

Aging

Aircraft Task Force, the

Air

Force

Aging

Aircraft

Office,

th e

16


27/89

NAVAIR ging

ircraft

ntegrated Product Team IPT), he nter-agency/inter-service

aging

aircraft

planning

(JACG),

have

been

developed. Furthemore,

NAS A, FAA, Navy

an d Air Force have jointly held

five

conferences to ddress th e aging aircraft

problem.

These initiatives demonstrate

the

seriousness of the aging problems an d th e coordinated

attempts

of

government

an d industry.

17


28/89

THIS

PAGE

IS

INTENTIONALLY

LEFT

BLANK

18


29/89

III.

GING

AIRCRAFT WIRING

A.

NTRODUCTION

In

dealing

with the

problems

of

extending

th e

life

of aging

aircraft,

most

emphasis

seems o be placed on tructural ssues. ndeed, he ging of

aircraft

has esulted n

extremely difficult

problems

dealing

with

the

ong-term

effects

of

structural

ging

nd

repair,

but what is th e

effect of

aging

on other

systems?

Until recently, wiring ha s

often

been

forgotten

or

treated

as

an

afterthought.

The

aging

of

electrical

systems,

and

wiring

specifically, received little attention.

This

is

in the

process

of

changing

dramatically,

in

part

du e

to

a

number

of

serious

accidents

involving

wiring

problems.

Recent accidents

in

both

ommercial

an d

military aviation

have

made

clear

that

the

effects

of

age

on

aircraft

wiring

need to

be

examined in

the

same

way

as is

done with

structures.

B.

IRCRAFT

WIRING

1.

eneral

One way to realize what

wiring

performs in

an

aircraft is to compare it

with

veins.

Think

of the

human

body. How

important

is

blood

to

the body?

H ow

is

blood

distributed

to

all the

living

organs

in

order

fo r

a human

being

to function,

cope

with

the

environment

and

urvive?

Th e

blood

is distributed

to

the

living

organs

by

veins. Let's

compare

th e

19


30/89

veins

to

wire

fo r an

aircraft.

An

aircraft needs

electrical energy

to function

like the human

body

needs

blood. This

electrical energy

is

distributed

by

wiring.

Bundles

of wire

carry

the

electrical

energy like veins carry th e blood. There are

arteries, big an d small carrying blood, like

power

wire busses carry power. Power

busses

are like major arteries.

ndividual wire bundles for specific controls, instruments, lights

an d

electronic

items are like

small

arteries.

They

all

perform

vital functions in

the

control

of

th e aircraft during

all

kinds

of

environmental

conditions. f

a vein

carrying

blood,

s

damaged

or cut,

then

then

there

is a limited period

of

time fo r

a

person to react before

he

or

sh e

experiences

weakness,

loss

of

functionality

for organs

an d

possibly

eventual

death.

If

an y

of

th e wire

bundles

in

an

aircraft, experiences a

fire,

th e plane

has a limited

time

to

respond o

damage

o he

wiring

harness.

The

ircraft xperiences

weakness,

oss of

functionality of

controls

and

vital

instruments before

losing

altitude, speed

an d results in

sudden

death

(crash).

Wiring

is,

thus,

the

vital

electrical

network

that

carries

the

data,

signals

an d

power

to

an d

from

systems.

Wiring

goes into every nook an d cranny. As previously shown, it is

embedded

nto

he

ircraft

he way

veins

and nerves

re mbedded

nto

lesh.

his

provides he opportunity

o monitor

nd

nterrogate he

health

tatus

of

systems nd

framework

components.

Electrical wire

onsists

of

a

onductor

hat

s ncased

n protective

ayer of

insulation. Wire

is routed throughout an aircraft in a series

of bundles with clamps

and

connectors.

Safe

routine

practices

include

measures to prevent

wires from

wear,

abrasion,

20


31/89

contamination

an d

contact

with

other

components;

to

gently

bend

and

turn

wires

during

installation to

prevent

cracking

of

th e

insulation

an d

to

physically

eparate wires

ro m

systems

whose

signals may

interfere with

on e

another.

[Ref.

8]

The

bulk

of

aircraft wiring

failures

re

attributed

to

broken wire nd nsulation

damage. Table

3 shows the kinds

of failure

seen

on

a typical

A ir Force

fighter aircraft.

Broken

Wire

46%

Insulation

Chafing.

Damage

30%

Outer Layer

Chafing

14%

Failure

in

Connector

10%

Table

3.

Wire Failure Data fo r a Typical Fighter From

[Ref.

9]

2. nsulation

Wiring nsulation s he

irst

ine of

defense. t

provides

protective barrier

between

a

conductive

wire

an d other

conductive

objects,

such

as the

airframe

or

a

nearby

conductor. Insulation

can

be

made very

thick

if

necessary.

But

aircraft wiring

needs

to

be

thin

to

conserve

weight,

pliant

to

bend without

cracking,

abrasion

resistant,

an d

have

high

dielectric (insulating)

strength.

Historically scientists

have

had

difficulty

in

designing

an d

manufacturing

insulation

that

simultaneously

meets

all

requirements.

Soft,

flexible

wiring

tends

to

erode

more

easily than a hard

surface.

If

it

is too

soft,

the

conductor will

push

through du e o

tress

t

bends. ventually, he nsulation

becomes

racked or worn

2 1


32/89

through. A ingle

rc

rom worn

or

racked pot an ause rc racking where he

insulation

burns

along

a

length

of

wiring exposing more

of the

conductor.

Eventually

th e

problem

eleases nough urrent

o ause

he

breakers o

hrow,

but

no t

before

many

wires

have

been

affected

and

toxic

fumes

are

spread by

convecting

air

currents.

[

Ref.

10 ]

Most military

an d

commercial aircraft produced over th e last twenty years us e a

wire insulation

construction

based

on

either military

specification

MIL-W-81381

or

MIL-

W-22759.

The

insulation materials used are principally aromatic polymide

(also

known

as

Kapton)

or

cross-linked

ethylene

tetrafluoroethylene (EFTE).

The

problem

of smoke

and

fires

is

particularly

acute

in

old wiring insulated

with

aromatic

polymide

nsulation

Kapton) which

ppeared o meet

equirements

of

light

weight nd

high

dielectric onstant. But Kapton ails he

test of time.

This

particular

insulation

s

omposed

f

ubstance

with

oosely

onded

enzine

molecules

hat

eventually

urn nto arbide rystals.

When moist or wet, arbide rystals eact with

moisture

to

form

a

flammable

ga s

[Ref.

10].

The

Navy,

which

commonly

operated

in

the

harshest

of

environments,

was on e of

the irst users o

notice

he problems ssociated

with

Kapton nsulation nd banned

Kapton's se.

etween

996

nd

998,

he

oD ingle rocess nitiative, bliged

McDonnell

ouglas o

tandardize

ll

military

ircraft

roduction

n

omposite

insulation. Composite

wire

saved

weight,

reduced

part number complexity

an d

improved

safety.

[Ref. 1]

22


33/89

3.

ircuit Breakers

The

rimary

evice

or protecting n

ircraft

ro m

he

azards

f

lectrical

malfunctions

is

the

circuit

breaker.

Its

role

is

to

protect

wire

from

damage

du e

to

current

overloads.

Circuit breakers

are capable

of

responding

to

the

thermal effects of the

current

carried by he

wire nd

re

lexible nough

o

work with

wide

variety

of

loads

n

multiple platforms under diverse

environments.

Aerospace

circuit

breakers

are

based

on

the

principle

of

sensing heat.

he y use hermal lements

esigned o protect wiring

insulation

systems

based

upon

historical

insulation

aging-versus-temperature

data.

They

are

designed o protect he wiring ircuits by opening utomatically prior o damage

occurring through

excessive

heating

under

overload

conditions

[Ref. 12].

C. CAUSES OF AGING WIRING

1.

eneral

Wire

ystems

ink

electrical, lectro-mechanical nd lectronic ystems. Wiring

has

emerged

as

vital

in the control

an d safety of these ystems, due to their increasing

complexity. However,

ll

lectrical wire ystems re ubject

to ging: he

progressive

deterioration

of

physical

properties

and performance of wire systems with

use

and with

the

passage

of time.

23


34/89

Wire degradation

is

cumulative over

time. For instance, in

Figure 5 th e

correlation

between flight

hours

and

the occurrence of wire

degradation,

is

clearly

revealed.

As an

aircraft

ages, the

number

of

wire defects

increases.

3Ü-4QK 0-50K

G-60K

0-70K

FHqht hoursfaircraft

Bare

Wire

70K+

-

•

• >5Q% nsulation Gone

o

O

Figure

5

Age-Related

Wire

Failure

From

[Ref.

9 ]

The

causes of the aging

wiring

can

be

summarized

as

follows:

Environmental

factors

Wiring Design

•

Wiring

Installation

24


35/89

2.

Environmental

Factors

Environmental amage s efined s egradation ue

o xposure

o

he

atmosphere,

vibration,

heat,

water

intrusion,

corrosion

an d

other

such

effects.

Vibration is

on e

of the factors affecting

wire

aging. Vibrations

that

occur

naturally

in

flight

cause

wires

to

rub

against aircraft parts,

an d

against

themselves. This protracted

rubbing

auses

he protective nsulation o wear

hin

nd ventually

xpose

he

ore.

Vibration is also, not constant

throughout

the

frame

of th e aircraft. It

varies

greatly an d as

such

t

is

ffecting

he

wiring

unning

hrough

those

reas

differently

s

well.

Wheel

wells,

ngine ompartments, reas

near

he ir-conditioning

packs

ll have

different

vibration

cycles

and yet the

current approach to wiring does

not take those differences

into account.[Ref.

13]

Moisture

is

another contributor to

aging

wiring.

Most insulation

material

is

very

complex

long

chain

polymer

an d moisture

accelerates

changes to

this complex

polymer

which

decreases

the

insulation

qualities

over

a

short

period oftime.

[Ref.

13]

Temperature

is

another player. Besides th e internal

overheating,

there

is also th e

external heat

coming from just about

an y

device

on

aircraft.

A great

amount

of

energy

is

used

in

aircraft,

and

energy

is

heat.

Wiring

connectors

can

also

be

affected

by environmental factors.

Th e

connectors

are

usually

good

for

about

500

open/close

cycles.

Over

th e

course

of

twenty

years,

it

is

quite

ikely

hat

om e

high ailure units

will

ause

his imit

o be

xceeded.

As

he

connectors

re opened

to

llow

access,

moisture

often

enters.

On losure

the

moisture

25


36/89

reacts with he metals

f

the onnector. luminum/nickel plated

onnectors

orrode

easily. But, ven tainless teel housings

an

orrode

over

time. Corroded onnectors

crack and break to xpose wiring o th e

elements nd the

breakage

results n loose or

intermittent

connections.

[

Ref.

10]

Finally, he evere aunch, ecovery

an d

salt

water

environment

in which Navy

aircraft

operate

compounds

the problem

further.

3.

Wiring Design

Some of

th e more

common

answers, that

on e

ca n ge t when asking people

dealing

with

airplanes

fo r

their

opinion

on

wiring,

are:

• ire is wire. It

is

never

different.

•

iring

is

a

necessary

evil.

•

ire just

connects

the

pieces

that

really

do

something,

like

radios

an d

computers.

•

nybody

ca n

design

wiring.

•

iring

costs

too much.

•

iring

adds

too much

weight.

•

iring

can

ruin

the

electro-magnetic

interference

(EMI)

test

results.

26


37/89

•

ire

consumes

all the

maintenance

hours.

•

he

only thing

ever

new about

wiring is

a

new

way that it

can

fail.

All he bove

omments

how

that wiring,

t

the basic

evel,

s not n overly

complicated engineering discipline.

It

is

not

difficult

to understand continuity,

electrical

isolation, nd hat

most

onnectors dhere o

righty-tighty, efty-loosey. ust

bout

everyone can

design

a

wiring

harness

but

will it work

long, work

well,

be

cost

effective,

be

light, no t

corrode,

be

maintainable

?

This

is

where

wiring

design

as

a

specialty

matters,

and

where

wiring

design

ca n

have

a

big

impact

on

the

aging

aircraft

situation.

[Ref.

14 ]

In 1978,

a

standard

carrier-based F-14

Tomcat

fighter

had approximately

90,000

feet of

wire

in its wiring

system.

A

Boeing 74 7 had

approximately 500,000

feet of wire.

According to tudies onducted

by

NAVAIR,

his

would

translate to

oughly

786

nd

4,366

ounds

espectively,

ot ncluding onnectors nd

upporting

ardware.

his

imposed

high

pressure

in

wiring

design.

The

pressures

on

both

military

an d

commercial

operators

to

reduce weight

fo r advances

in

performance

an d

range

have

made

the

wiring

system

an easy

target

for weight reduction

initiatives

[Ref.

15].

These weight

reductions did

not come

free.

They

had

their

impacts in

both

wiring

design

nd

wiring maintenance.

Wiring was

relegated

to whatever

space

was

ef t over

when

hydraulic

lines,

ontrol

rods

and cables,

vionics

boxes and

other

equipment

was

installed.

In on e

case

th e

generator feeder wires

were

installed

riding

against

the

structure

and

hydraulic

lines

for

a

good

distance.

These conditions

required

significant

added chafe

protection.

[Ref.

15 ]

27


38/89

Some ther

xamples

f weight

eduction nitiatives ncluded eduction r

elimination

of

the equired lack n

wiring

nd eduction of insulation hickness nd

conductor

size.

The

result

was a number

ofproblems due

to lack

of slack

provisions, wire

and

onductor breakage

nd

breakage of

previously

undamaged

wire

while

rying

o

locate

a fault in

another wire

(maintenance

handling

difficulties).

Another problem with wiring design

is

that

every

aircraft misses

some

beneficial

new

innovation

during ts development process

because t

is too ate to ge t

it

in

the

design.

For

example, ircraft

that

were

designed

35 years

ag o re

penalized

by

th e

fact

that wire

insulation

used was

very

thick

(.015

in

plus)

compared

to

wires

available

just

a

short time

later.

With

minimum

gauge

practices

in

place at

the time

(typically

22

gauge)

these

factors

combined to

make

for a heavy wiring

system. The

aircraft till

in

service

from

this

time

carry

this

added

weight

around

th e

world

every

day. In

an

aircraft

like a

B-

52 ,

it

is

quite

possible

that

this

weight

penalty

could

amount

to

thousands

of

pounds

if

any

significant amount

of

th e

original wiring is

still installed.

[Ref.

14 ]

Age f

design

as n ve n

arger

ffect

n

onnectors. ontact

auge

ize

minimums av e

remendous mpact n

onnector

ount.

Many lder

ircraft

nd

avionics

ere

esigned

hen

onnectors

ere

vailable

with only

6

r

20 auge

contacts.

Pin density

of a 20 gauge onnector

is

half

of

a

22

gauge type. Going to 6

gauges

halves

it

again.

Aircraft disconnects ca n be greatly reduced by a redesign

Ref.

14].

28


39/89

4. iring

Installation

Installation

of

wiring

also

has

large

ffect

on

wire aging. Most of th e

current

insulation

types

are

unable

to

withstand

tight

radious

bends,

ye t

in

today's

aircraft,

there

are thousands

of

examples

of

this type bending.

The

clamping an d bundling

devices

also

ad d

to stress an d

strain

on

the

insulation.

Another problem with wiring installation

is

that

ince wiring

is

not treated

as

a

system,

it

is

therefore relegated to whatever space

is

left. The

result

is

poor

location

of

terminals,

connectors

an d

junction

points

an d

th e

relative

size

of

th e

maintenance

tools.

D. AGING

WIRING

EFFECTS

1.

eneral

As

previously dicussed, ircraft wiring

an

be

compromised

by

everal actors.

Wiring

design

and

installation,

and

environmental

factors

can

all

contribute

to

premature

aging wiring. Aging wiring

an

everely mpact

the aircraft afety. Two re

th e

main

effects

of

aging wiring: short circuit and

arc-tracking.

2. hort Circuit

When he rotective ayer f

nsulation

n wire s ompromised nd

he

conductor is

exposed,

th e potential

exists

for

a

hazardous

electrical

system malfunction

caused

by

a short

ircuit A

hort

circuit

occurs when

lectricity takes

n unintended

path. For

example,

condensation

and

other

conductive

materials

that

are sometimes

found

29


40/89

on

wire bundles an bridge he gap between wire onductor nd djacent metallic

structure.

When electrical current follows the unintended

path

to

the

-metallic

structure, a

short

circuit

that

could interrupt

th e function

of an

electrical

system

occurs. Short circuits

can transfer power

to adjacent

wires

or draw

an

excessive

current

from the power source,

overheating

wires

an d

creating

fire

hazards.

[Ref.

8]

3. Arc Tracking

Electrical rcing

s

type of short ircuit n which

high

current

crosses gap,

emitting

parks.

The parks nclude molten material

ro m

he

wire onductor

s

t s

vaporized

by

he

high

nergy discharge,

producing

xtreme ocalized

heat.

The

rcing

could

ignite

flammable

products

in

the

area

an d

could potentially

initiate

an

explosion

[Ref.

8].

Arc

tracking occurs when the insulation material chars.

Th e charred insulation

is

conductive,

can

sustain

an d

propagate

an

arc

along

the

length

of

a

wire,

an d

may

flash-

over o onsume djacent ire nsulation r ther

ombustible

aterial. With

he

exception

of

ntermittent

perating

nomalies,

either h

aging aircraft wiring

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