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National Bureau of Standards OCT 3 I960

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^&cknlccil ^^ote 41

EXPENDABLE MODULES AS BASES FOR

DISPOSAL-AT- FAILURE MAINTENANCE

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
T

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March.

3, 1901, as amended by Congress in PubUc Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

of standard practices, codes, and specifications. The work includes basic and applied research,

development, engineering, instrumentation, testing, evaluation, calibration services, and various

consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the

Bureau's unique competence is required. The scope of activities is suggested by the listing of

divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series,

Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in NBS Circular 460, Publications of the

National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS

technical ^^ote

41

FEBRUARY 1960

EXPENDABLE MODULES AS BASES FOR

DISPOSAL-AT-FAILURE MAINTENANCE

R. O. Stone, P. Melssner, and K. M. Schwarz

The work described in this Report was sponsored by

the Department of the Navy, Bureau of Aeronautics, now integrated into the Bureau of Naval Weapons

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $2.25
FOREWORD

The work covered by this Note was performed by the

National Bureau of Standards on behalf of the Department of

the Navy, Bureau of Aeronautics, now integrated into the

Bureau of Naval Weapons . The program was under the

administration of the Electricity and Electronics Division

and the technical supervision of the Engineering Electronics

Section. A major objective of the program was the establishment of criteria for determining the conditions under which

airborne electronic assemblies may be expended at failure

rather than repaired. The criteria developed under this

objective are presented in this Note.

Gustave Shapiro, Chief

Engineering Electronics Section

Chester H. Page, Acting Chief

Electricity and Electronics Division

11
TABLE OF CONTENTS

Page

FOREWORD . ii

LIST OF FIGURES v ABSTRACT 1

1

.

INTRODUCTION 2

2

.

PRESENT MAINTENANCE CONDITIONS 2

3

.

THE EXPENDABLE MODULE 3

3.1 Advantages of Expendable Modules 6

3.2 Possible Disadvantages of Expendable Modules.. 9

4

.

SUBDIVISION OF EQUIPMENT 12

4.1 Determination of Optimum Module Size 13

4.1.1 Mean Life of Module vs. Size of Module. 14

4.1.2 Unit Cost of Module vs. Size of Module. 19

4.1.3 Solution of a Typical Problem 22

4.1.4 Conclusions as to Optimum Module Size.. 28

4.1.5 A General Solution for Determining the

Optimum Expendable Module Size 33

5. COST COMPARISON OF REPAIRABLE ASSEMBLY AND EXPENDABLE ASSEMBLY MAINTENANCE SYSTEMS 37

5.1 Repair of Defective Assemblies 38

5.2 Costs of the Repairable Assembly System 45

5.2.1 Failure and Other Calculations for

a 6-tube Assembly 48

5.2.2 Processing and Repair Costs 54

5.2.3 Evaluation of Cost Factors 56

5.2.4 Total Costs of Repairable Assembly

System 60

iii
TABLE OF CONTENTS (CONT'D)

Page

5.3 Total Costs of the Expendable Assembly

System 63

5.4 Conclusions as to Cost Difference 64

6. FACTORS OTHER THAN COST IN EXPENDABILITY DECISION.. 66

6 . 1 Improved Reliability , 67

6 .2 Reduction in Specifications , » 68

6.3 Improved Systems Maintenance 69

6.4 Freedom of Design 70

7 . CONCLUSIONS , ... ... 71

REFERENCES » 74

GLOSSARY 76

IV 1
LIST OF FIGURES

Figure Page

1 MEAN LIFE OF TYPICAL MODULE VS„ SIZE 18

2 MODULE UNIT COST VS. MODULE SIZE 21

3 COMPARISON OF VARIOUS FACTORS AS A

FUNCTION OF MODULE SIZE » 27

4 VARIOUS FACTORS VS. MODULE SIZE 29

5 MAINTENANCE SYSTEM BASED ON THE REPAIR

OF ASSEMBLIES AT THE DEPOT LEVEL 40

6 MAINTENANCE SYSTEM BASED ON EXPENDABLE

ASSEMBLIES « 41

EXPENDABLE MODULES AS BASES FOR DISPOSAL-AT-FAILURE MAINTENANCE

R. 0. Stone, P. Meissner , and K. M. Schwarz

The maintenance of future electronic equipment

will more than likely exceed the capabilities of

existing maintenance activities if present

maintenance procedures are continued. Design

trends such as miniaturization, printed circuitry,

encapsulation, and modular construction all point

toward disposal-at-failure maintenance. Disposal-atfailure modular design would help to solve some of

the future maintenance problems and would also be

compatible with future design trends.

It has been suspected that disposal-at-failure

maintenance would be too costly to be practical.

This report compares the costs to procure, support

and maintain an. equipment designed with expendable

modules to the costs to procure, support, and

maintain a similar equipment designed with repairable modules. It was found that the total costs in

either case were approximately equal. Since the

cost factor is not important in the decision between

expendable assembly or repairable assembly

maintenance, other factors, such as improved

reliability of equipment, smaller size, lighter weight,

and improved maintenance, all advantages that may

be gained through the use of expendable design,

should be carefully studied. These advantages may

be so important as to completely determine a

decision in favor of expendability

.

Modules of various electrical sizes (from 1 to

12 tubes) have been compared from the standpoint

of total over-all procurement cost, in order to

determine an optimum module size. On the basis of

calculations made, it has been concluded that the

optimum module which would lead to lowest over-all

procurement cost and at the same time be of aid in

the solution of logistic and maintenance problems

would be one containing from 4 to 8 tubes. This

is true for both disposal-at-failure and repairable

modules

.

- 1 -
- 2 -

1 . INTRODUCTION

The difficulties that have arisen in the maintenance of

complex electronic equipment used in modern aircraft are well

known and have been described in detail both in the press and

in official reports .-' '^ '^ '"*' ^'^ Disposal-at-failure

maintenance has frequently been suggested as a means of

surmounting some of the difficulties. This type of

maintenance has not been widely accepted, however, because

of the lack of guidelines for determining the limits within

which it may be practiced. Guidelines specifically applicable

f

to Bureau of Aeronautics airborne electronic equipment are

presented in this Note. It is believed that these guidelines

can be applied with some modification to aircraft electronic

equipment in general.

Such terms as piece part, module, assembly, FASRON, and

& R are frequently used herein. These and other terms are

defined in a Glossary at the end of this paper.

2. PRESENT MAINTENANCE CONDITIONS

To become familiar with BuAer maintenance procedures

and costs, NBS personnel visited approximately one-half of

the BuAer maintenance activities, as well as the Aviation

Now integrated into the Bureau of Naval Weapons.

See Glossary

.
- 3 -

Supply Office in Philadelphia and the Electronics Supply

Office in Great Lakes, Illinois. Also, discussions were held

with many equipment manufacturers and with staff members of

various universities and government laboratories to acquire

first-hand information on packaging and other electronic

design trends.

It was learned that the combat effectiveness of the

military weapons is reduced because of the present maintenance

and supply conditions. Many weapons are ineffective owing

to electronic failures that have not been corrected. Also,

large amounts of equipment are required for the maintenance

of the electronic portions of the weapons.

A solution to the maintenance problem may be the use of

standard plug-in modules that can be disposed of at failure

rather than repaired. This type of maintenance is discussed

in the following sections.

3. THE EXPENDABLE MODULE

A module is defined as a modularly dimensioned assembly.

The phrase "modularly dimensioned" means that the assemblies

are to be physically dimensioned in specified size increments

that will permit a group of modules to be combined with a

minimum of waste space. The modular dimensions must be

See Glossary.
- 4

-

selected so that modules can be grouped efficiently around

those portions of the equipment which, for electrical or

physical reasons, are not contained in modules. When a piece

of equipment has been modularly constructed, it is referred

to as "modulized . "^'^

An expendable module is one which need not be disassembled

for piece part* repair when it becomes defective; it is simply

disposed of following suitable accountability procedures.

Hence, this type of repair action has been designated

"disposal-at-failure" maintenance. Within the Bureau of

Aeronautics a disposable item would be coded "Consumable" (C)

as opposed to "Repairable" (R) . At the present time, most of

the electronic equipment is designed to be repaired through

the replacement of parts down through the smallest piece parts.

Current design trends indicate increasing use of modular

construction techniques for compact, intricate electronic

equipment. The subdivision of equipment into assemblies has

advantages from the standpoint of design and manufacture when

high part densities are required. From the standpoint of

maintenance, it is essential that the equipment be divided

into assemblies to permit accessibility to all parts of the

equipment. Although the assemblies need not conform to any

particular set of modular dimensions, there are important

advantages in the use of standardized modules.

See Glossary.

J
- 5 -

The problem of installing an equipment in different

aircraft has given rise to the integrated package concept.

This involves the mounting of several different pieces of

equipment on a common frame, eliminating separate cases for

each piece of equipment. A common power supply may also be

used . This concept could permit important savings in weight

and space, although it has only limited application at the

present time because of the wide variety of equipment and

case sizes in use. A piece of equipment composed of modules

could be integrated into a new aircraft by rearrangement of

the modules to fit the available space, or the modules of

several different' equipment types could be grouped together.

The same effect could, of course, be achieved by redesigning

all of the equipment to fit each new aircraft, but the procurement, logistic, and maintenance problems would become

intolerable

.

Because of the advantages that are possible through the

use of standardized modules, it seems reasonable to assume

that future equipment will make increasing use of modular

construction. Hence, the term "module" is used quite

frequently in these pages, although the advantages of

expendability could be realized with expendable assemblies

which were not necessarily modular in form.
6

-

3.1 Advantages of Expendable Modules.

(a) Improved Reliability of Electronic Equipment ; The

environmental conditions in which equipment must operate are

becoming more severe as new weapon systems are developed.

Improved reliability may be gained by use of expendable

modules that are embedded in plastic or sealed hermetically.

Data as to how much the reliability of a module may be

improved by embedment or hermetic sealing has not been

obtained. Most of the organizations contacted in regard

to this problem felt that considerable improvement in

reliability may be possible; however, they had made few

actual measurements. One manufacturer of electronic systems

containing encapsulated modules felt that encapsulation

had increased the mean life of the modules four times. If

considerable improvement in reliability can be generally

gained through embedment or hermetic sealing, designing

equipment with expendable modules so protected would be a

tremendous step toward more reliable electronic systems.

J

3^ See Glossary.
- 7

-

Use of expendable modules would eliminate many of the

factors which now contribute to the lowering of equipment

reliability. At present, printed circuits, resistors,

transistors, etc., may be easily damaged during repair. Replacement parts may be inferior to the original parts, resulting

in lower reliability. Maintenance repairs increase the wearout of electronic assemblies, particularly subminiature

assemblies.

(b) Improved Systems Maintenance ; Use of expendable

modules should improve the present quality of electronic systems

maintenance. Since most technicians are in the service for a

comparatively short time, it is not possible or practical to

train thetii to be fully capable of piece -part* repair and at

the same time to be fully capable of maintaining the various

electronic systems, such as the navigation and communication

systems. Under disposal -at-failure maintenance, the training

required for piece-part repair could be considerably reduced,

and more time could be spent on the operation and maintenance

of electronic systems. Expendability will not solve all the

maintenance problems, since much effort is devoted to systems

check and test, adjustment and alignment, and the replacement

of parts that would not be contained in modules. The use of

See Glossary
- 8 -

expendable modules, however, would enable the maintenance

personnel to devote a much greater portion of their efforts

to these tasks.

(c) Reduction in Number of Spare Parts ; The number of

Spare parts would be drastically reduced by use of expendable

modules. A typical repairable four-tube module contains

approximately 40 different parts which must be supported with

spare parts. Expendable modules would need to be supported

with spare modules only. Reduction in the number of different

spare parts that must be supplied would greatly reduce the

tasks of the Aviation Supply Office (ASO) and the Electronic

Supply Office (ESO) . Such reduction would also benefit the

maintenance activities which often experience difficulties in

obtaining necessary spare parts.

^^^ Reduction in the Number of Specifications i The use

of standard expendable modules may reduce the number

of military specifications. At present, a specification is

required for each of the parts in an assembly. Use of

expendable modules would require only a single specification

for a complete module;.

(e) Better Storage Possibilities ; Expendable modules

will probably be embedded or hermetically sealed. Such

protected modules should withstand long storage periods with

little or n& added protection. They should require considerably

less storage volume than that required for repairable assemblies
- 9 -

and spare parts, many of which are stored in individual

protective containers much larger than the assemblies or

parts.

(f ) Usefulness under Emergency Conditions : In times of

emergency, disposal-at-fallure maintenance should be quite

successful, provided that it has been adopted and practiced

before an emergency arises. Piece -part repair of an assembly

probably could not be made at the front-line level, and

time would not permit such repair to be made at higher

maintenance levels. Consequently, defective assemblies would

often be expended, even if they were intended to be repaired.

As a result, the present maintenance program would surely

fail, since it and the equipment would not have been designed

for such practice of unintended expendability . For this reason,

disposal-at-failure maintenance may be highly successful, in

that it would realistically provide for wartime conditions.

3.2 Possible Disadvantages of Expendable Modules

„

(a) Decline of Technical Skills in the Military : Some

members of the military maintenance organization have suggested

that the use of expendable modules or assemblies would lead

to a decline in the skill of the technicians, since, the

technicians would be deprived of valuable repair experience.

It is true that a knowledge of parts and circuit details would

be of less importance than it now is, but the skills required
- 10 -

to locate faults in electronic systems would still be necessary,

The services of high-level technicians would be just as

important, although the emphasis in training would be shifted.

Much more time would be spent in learning electronic system

operation and fault location, and less time would be spent on

circuit theory and parts. Present maintenance activities

are generally so understaffed that a reduction in the number

of skilled personnel is unlikely. The main purpose of expendability is to enable the present maintenance organization to m

handle a greatly increasing workload in the future through

more efficient use of personnel. It is highly improbable that

adoption of the expendability concept would reduce the number

of skilled personnel or lower the level of technical skills.

(b) Increased Supply Burden: Most expendable modules will

be so designed that a piece-part repair will be difficult or

impossible; therefore, spare modules would always have to be

available for correcting equipment failures. These conditions

would put more stringent requirements on the supply system,

particularly at remote bases or on aircraft carriers. Nevertheless, since only complete modules would be supplied, rather than

the numerous different component parts now needed, the supply

system should be capable of providing sufficient spare modules.

The reliability of the standard expendable modules should be

ascertained prior to acceptance by the military. With this

knowledge of the reliability, the number of spare modules

I
- 11 -

necessary for satisfactory operation of an equipment type for

a known period of time could be calculated. Such calculations

have been considered and are discussed in sections 4.1.1 and

4.1.3.

(c) Reduction in Failure Data : Under disposal-at-failure

maintenance, no failure data on the parts in expendable

modules could be obtained from field usage, since piece-part

repairs would not be made . These data are important for

reliability study and improvement. They possibly could be

obtained by returning some of the failed modules to the

manufacturer for evaluation.

Excellent field failure data for the modules themselves

could no doubt be obtained. Small, inexpensive elapsed time

indicators could be included in the modules so that complete

failure data could be recorded. An accurate knowledge of the

failure rate of the modules would be necessary for predicting

and improving the reliability of electronic equipment.

(d) Excessive Cost ; The belief has been frequently

expressed that the cost of supplying spare expendable modules

would be excessive. Controverting this belief, this study

concludes that an equipment type built with expendable four

to eight tube modules would cost about the same to procure,

support, and maintain as would a similar equipment type built

with repairable four to eight tube modules. (See sections

4.1.2, 4.1.3 and 5)

.
- 12 -

4. SUBDIVISION OF EQUIPMENT

Under the present BuAer maintenance system which is based

on piece-part repair, effective subdivision of an electronic

set and its units plays an important part in facilitating

maintainability. Under a disposal-at-fallure maintenance

system, effective subdivision would become even more important.

The decisions made at the beginning of the design period

would affect not only the maintainability and the electrical

performance characteristics of the equipment, but also the

logistics and the total costs of procuring and supporting the

equipment.

The primary objective in subdividing a set into units is

to optimize maintainability by the front-line maintenance

activities. For achievement of this objectiv.e it is generally

agreed that a set should be divided into functional units.

Functional guidelines, however, are too broad and require much T

refinement. A functional unit could be a receiver, a

transmitter, a synchronizer, etc., (large expensive units); or |

a functional unit could be a video amplifier, an afc amplifier,

a horizontal sweep circuit, etc., (small inexpensive units).

A better guideline from the standpoint of maintainability

may be : Subdivide the set into units in such a manner as

to have a specified probability that an average

technician under specified conditions could locate

and exchange a faulty unit in a specified time.

See Glossary
- 13 -

Under this guideline, the primary subdivision of a set

depends on Its complexity and fault location possibilities.

Generally, equipment of greater complexity will require use

of larger primary units; equipment of lesser complexity will

permit use of smaller primary units. This guideline may need

to be modified when the systems and the weapon are being

Integrated .or when short life components must be used. The

physical (ilze 6"f a unit may often be determined by' the available

space and environmental conditions In the weapon.

Secondary subdivision, subdivision of the primary units

into assemblies or modules, may often be required not only to

facilitate maintenance by the front-line activities, but also

to ease the burden on the supply system and the storage

facilities and to reduce the total procurement costs. In the

present study, efforts have been made to determine the optimum

module size, in terms of number of tubes, that will permit

achievement of these ends.

4.1 Determination of Optimum Module Size.

The details involved in determining an optimum module

size (number of tubes) are best illustrated by means of a hypothetical example based on typical present-day conditions.

Such a hypothetical example is presented later in this report

(Section 4.1.3). In the hypothetical example, total procurement costs for typical modules of various electrical sizes
- 14 -

have been calculated. Before these calculations could be

made, however, two relationships had to be established: the

relationship between the reliability of a module and the

number of tubes in the module, and the relationship between

the unit cost* of a module and the number of tubes in the

module. The information needed for establishing these

relationships was obtained from a study of currently available

reliability prediction techniques and an analysis of equipment

now in use by the Bureau of Aeronautics.

4.1.1 Mean Life of Module vs. Size of Module.

Reliability prediction has received a great deal of

attention during the past few years. Means for predicting the

reliability of electronic equipment have been developed and

, . . . , . , 8,9,10,11,12 ^, „ T are being continuously improved. ' » ' ' Use of general

failure rates for the electronic parts permits prediction of

the mean life of a module within rather broad limits. For a

specific piece of equipment, more precise estimates of the mean

times to failure for the various assemblies may be made by use

of actual parts distribution and stress level information.

Further data may be derived from tests of the engineering and

production prototypes of the equipment. In the present study,

typical failure rates and parts distributions have been used to

obtain an over-all system average. Absolute accuracy, however,

is not important in determining the optimum module size (number

of tubes). Relative accuracy, the reliability of a module as

See Glossary.
- 15 -

a function of the number of tubes in the module, is the

important consideration.

The following procedure was used in predicting the

reliability of typical or average modules. This is the

13 procedure suggested by Aeronautical Radio, Inc. (ARINC) and

the Advisory Group on Reliability of Electronic Equipment

(AGREE)

.^

Step 1 . Define the equipment explicitly and uniquely in

terms of functions and boundary points . Step 2 . Specify the components within the system .

All of the components (assemblies) that form the

complete system should be considered when system

reliability is being predicted.

Step 3 . Select the parts which affect the system

unreliability . Parts having very little effect can be disregarded

Those having a dominant effect due to their large

number or high failure rate must be considered.

Step 4 . Determine a failure rate for each part or class

of parts used in each component of the system .

If the parts are not analyzed singly, the failure

rate of the part is the failure rate of its class.

Failure rate data may be obtained from references

8, 9, 10, and 11.

Step 5. Determine a preliminary figure for the failure

rate of each component within the system . Add the failure rates for all the parts in the

component as determined in Step 4.
- 16 -

Step 6 . Determine the correction factors to be used to

modify the preliminary figures for the failure

rate of each component . When stresses are applied to the whole component,

it is more practical tM apply a single correction

to the component rather than to correct the

failure rate of each individual part.

Step 7. Determine the failure rate for each component . Multiply the preliminary figure for the failure

rate of each component by its correction factor

as determined in Step 6.

Step 8. Determine a preliminary figure for the failure

rate of the system . Add the failure rates for all the components

within the system to obtain the preliminary

figure for the system failure rate.

Step 9. Determine the correction factors to be used to

modify the preliminary figure for the failure

rate of the system . As in Step 6, the equipment may be subject to

special stresses that have not been considered in

the computation of the failure rates of the parts

and components; these stresses must be considered

at this stage

.

Step 10. Determine the failure rate of the system . Multiply the preliminary figure for the system

failure rate by the correction factors determined

in Step 9.
- 17 -

Step 11. Determine the predicted reliability function

for the systerir

The reliability function for the system is

given by

_ ^

V _ -t X (system failure rate)

Step 12 . Determine the predicted mean number of hours

between system malfunctions . The predicted mean number of hours between

malfunctions, C » is given by:

€ =

1

system failure rate

The above procedure, together with parts distribution

information, has been vised to predict the mean time between

failures of typical modules of various sizes. (See Figure 1.)

The information on parts distribution (the numoer and types of

parts per vacuum tube) was obtained from our study of present

airborne equipment and data from the Vitro Laboratories. The

example given below shows how the reliability prediction was

carried out. The example is based on a typical four-tube

module or assembly. The parts listed are those which will

affect the reliability of the module. The class failure

rates and the correction factors were determined from an

13 ARINC report, to which the reader is directed for detailed

information.
- 18 -

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ol-H
- 19 -

No. of

Parts Type

Class Hourly

Failure Rate Failure Rate

4 Vacuum Tubes

(90% rated,

100% filament)

90 X 10-6 360 X 10-6

11 Capaicitors

(80% rated)

2 X

10-6 22 X 10-6

16 Resistors

(80% rated)

6 X 10-6 96 X 10-6

3 Coils 3 X 10-6 9 X 10-6

1 Printed Circuit 6 X

10-6 6 X 10-6

1 Connector 4 X 10-6 4 X 10-6

36 497 X 10-6

Component tolerance : ±10%

Maximum temperature of tubes: 150°C.

Correction factor due to temperature: .85

Correction factor due to no malfunction detection: 1.2

Failure rate of module = .85 X 1.2 X 497 X 10-6 ^ 500 X 10-6

Mean time between failures Ri 500 X 10-6

2000 hours

4.1.2 Unit Cost of Module vs. Size of Module.

For establishing a relationship between the cost of a

module and its size (number of tubes), the unit costs for

representative assemblies currently used in BuAer equipment were

obtained. After most of the BuAer electronic equipments of

recent design were analyzed, 300 different assemblies used

in 18 different equipment types were selectecj as representing
- 20 -

an approximation to modularly construced assemblies. Only

those assemblies that contained 10 or less tubes and were

easily removable from their equipments were selected for use

in this study.

The unit costs for the 300 assemblies were obtained from

spare parts lists filed in BuAer equipment contracts as

contract amendments, and also from the Aviation Supply Office

in Philadelphia. Because design and tooling costs are included

in first contracts, the latest subsequent contracts that

excluded these costs were used whenever possible.

The best correlation of cost to a physical property was

the cost per tube. Figure 2 is a curve showing the unit cost

of assemblies versus the number of tubes in the assemblies.

The vertical bars show the range of costs of assemblies versus

the number of tubes. The circles indicate the average cost

of the assemblies versus the number of tubes. The curve shown

is the linear least-squares fit of the average costs. The

correlation between the cost and the number of tubes is

rather loose, with a wide dispersion in the costs for an

assembly of any given size. This is to be expected, since a

large number of variables have not been considered. Electrically

and physically the assemblies examined differed widely. Some

assemblies contained tuning elements, precision potentiometers,

relays, etc.; some were encapsulated, some were plug-in, etc.

The average costs of some of the assemblies may not be

representative because only a few manufacturers are
- 21 -

320

300

280

260

240

220

200

80

Ico

O '^ ^ 3l60h

140-

120-

100

80

60

40

20

COST = 29.5 + 16.8 V,

3 4 5 6 7

NUMBER OFTUBES.V^

MODULE SIZE

8 9 10

FIG. 2. MODULE UNIT COST VS. MODULE SIZE
- 22 -

represented; for instance, about 80% of the one-tube assemblies

considered were made by one manufacturer. There is not

sufficient standardization and modular design in present

operational equipment to make possible the construction of an

accurate curve for cost prediction; however, with the rapid

movement toward standard modular design, it should be possible

in the near future to construct curves for more accurately

predicting unit costs.

As shown by Figure 2 , it costs approximately $40 to procure

a one-tube assembly, $24 more for a two-tube assembly, and

approximately $17 more for each additional tube. In the

absence of the actual cost or information to the contrary, this

curve can be used to approximately predict the unit cost of an

assembly.

4.1.3 Solution of a Typical Problem.

The data used and the conditions stated in the following

problem are based as nearly as possible on present-day

requirements. The solution may be considered as a solution

to an actual present-day problem.

Assume that an electronic set containing 240 tubes is

required as part of a jet aircraft. The set is to be designed

for disposal-at-failure maintenance. The following operational

conditions are known prior to the design of the set:

See Glossary
- 23 -

1. Total of 300 sets to be installed and operated

in jet aircraft

.

2. Program life* for the sets to be 5 years, with

an average of 400 hours of operation per year, total of 2000

hours operation.

3. Attrition rate of aircraft is 1.5% per month.

4. For desired front-line ease of maintenance, the

sets are to.be divided into ten 24-tube units, each of which

is assumed to be different.

5. The sets will be distributed to 20 geographical

locations

.

6. One bench set-up and one spare set will be

required at each location for front-line maintenance; one set

will be required at each of two Overhaul and Repair Depots.

This gives a total of 342 sets to initially outfit and support

7. All spares are to be procured for life-of-type

.

8. Spares can be shifted to necessary location at

any time.

9. There must be a 95% probability that sufficient

spares are procured to meet requirements over the program life,

Since the attrition rate of the operating planes is 1.5%

per month, the average number of sets operating in planes over

the program life will be 200. Including fixed installations,

the average number of sets operating will be 242. In the

solution of the following problem, figures for the attrition

*See Glossary.
- 24 -

rate are not required if the figure for the average number of

sets operating is used. It can be shown that use of this

figure is equivalent to explicitly taking account of the

attrition rate.

Problem: How should the units be subdivided for

achievement of optimum military value?

(a) Consider each unit to contain one 24-tube module.

A complete set will contain ten of these 24-tube modules,

each module being of a different type.

Assume that the modules have equal failure rates and unit

costs

Mean time between failures (MTBF) ~ 340 hours.

(From Fig. 1.)

Unit cost of modules ~ $433. (From Fig. 2.)

For a 95% probability that sufficient spares are procured

for the equipment, there must be a probability of —i/ -95 that

sufficient spares are available for each of the 10 different

module types.

10 I P(s,nrt) = -^ .95 = .9949

The number of spares necessary for each module can be

determined from the following equation:

^( ^.

^ -nrt (nrt)"i

P(s,nrt) = T e --

lifeo TM'

P(s,nrt) = probability of sufficient spares

s = number of spares m=0,l,2,3, s n = average number of sets supported

r = failure rate of module = 1

MTBF

t = operating time = program life in hours
- 25 -

Substituting appropriate values for the above quantities,

this equation becomes:

-242 X 2000

.9949= i e ''' ^42^^00^-

m=0 _ 340

m:

s = 1459 spares for each of the ten different

module types.*

Knowing the number of modules in each set and the number

of spares for each module, we can calculate the total procurement cost for the original and spare modules

Total cost for spare modules = 10xl459x$433 ^ $6 ,300 ,000

.

Total cost for the initial quantity of modules

for the 342 sets = 342xl0x$433

$1,500,000

Total procurement cost of 24-tube modules^ $7,800,000

(b) Repeating these calculations for the case in

which each unit is subdivided into two 12-tube modules,

we get

MTBF for a 12-tube module % 680 hours. (From Fig. 1.)

Unit cost of a 12-tube module ^ $231. (From Fig. 2.)

Number of spares requii*ed for each module type - 787.

Total cost for spare modules=10x2x787x$231 ^ $3,600,000

Total cost for the initial quantity of modules

for the 342 sets = 342xl0x2x$231^ $1,600,000

Total procurement cost of 12-tube modules^ $5,200,000

This equation is solved by using the normal distribution

approximation for P(s,nrt).
- 26 -

(c) These calculations have been carried out for a

range of module sizes so that the size which leads to the

lowest over-all procurement cost may be determined . The

resulting values are tabulated in Figure 3.

Note that in Figure 3 two sets of values are given for

the one-tube modules. The larger modules will generally be

unique within a set because of the large number of tubes

which they contain. It has been observed, however, that at

the one- or two-tube level considerable repetition may occur.

This is an advantage from the logistic standpoint in that

fewer different module types are required. The advantage of

this repetition was tested by carrying out two different

calculations for the one-tube modules. In the first calculation it was assumed that each type was used only once in the

equipment; in the second calculation it was assumed that each

type was used twice, on the average. Thus, only one-half as

many types would be required. These calculations are indicated

below

Each unit is divided into 24 one-tube modules.

Case (a): Each module is considered as unique.

Case (b) : Each module is considered as used twice.

MTBF for a one-tube module '^ 7500 hours. (From Fig. 1.)

Unit cost for a one-tube module % $40 . (From Fig. 2.)
- 27 - i d d (A o 0) o e o u

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- 28 -

(a) (b)

Each module type used Each module type used

only once per equip- twice per equipment,

ment

.

Number of spares

for each module

type

93 167

Total cost for

spare modules: 24 x 10 x 93 x $40 = 12 x 10 x 167 x $40 = $894,000 $701,600

Total cost for the

initial quantity

of modules for the

342 sets: 342 x 24 x 10 x $40 = 342 x 24 x 10 x $40 = $3,280,000 $3,280,000

Total procurement cost for

modules: $4,200,000 $3,982,000

4.1.4 Conclusions as to Optimum Module Size.

Contrary to popular belief, it would probably be poor

practice to attain an expendable modular desig,n by simply subdividing the equipment into modules that cost less to replace

than to repair. Curves of the several calculations are shown

in Figure 4. These curves display some interesting information

As shown in the figure, it would cost just as much to procure

the sets and spares composed of one-tube modules as it would

to procure the sets and spares composed of eight-tube modules.

For the electronic set under consideration, it would cost no

more to expend modules containing eight tubes than to expend

modules containing one tube. The lowest-cost design is that
- 29 -

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-sousing modules containing between three and five tubes. The

comparatively flat total-cost curve for modules containing

between one and eight tubes is perhaps a fortunate condition;

when the optimum module size is being determined, considerable

latitude exists with respect to costs. There are, however,

the following important factors to be considered.

(a) Ease of maintenance . From the maintenance viewpoint,

larger modules are advantageous. It is considerably less

difficult' to locate a defective six or eight tube module than

to locate a one or two tube module. Since ease of maintenance

is the primary goal in the subdivision of equipment, it should

be given the most weight.

(b) Modification . Often, after the sets and spares have

been procured, the sets must be modified. Since modification

of expendable modules may be impossible or difficult, replacement modules may be necessary; therefore, smaller modules

would be more satisfactory. The weight to be given the modifi-;

cation factor is dependent upon the technical knowledge and

design experience that can be used in developing and building

the equipment. When an advanced equipment requiring an

extension of the technical art is being considered, the modification factor should be given considerable weight. When an

equipment evidencing mature design is being considered, this

factor may be given less weight. 4
- 31 -

There appears to be a growing interest in deferred

14 procurement of aircraft spares, particularly when highcost items are involved. Under a deferred procurement policy,

the services would procure only enough spares for a year or so

of operation. The manufacturer would be authorized to hold a

small buffer stock sufficient to cover lead time. After

sufficient usage data had been accumulated and the design

had become stabilized, a major procurement would be placed

covering the remainder of the program. The present policy

is based upon life-of-type* buying, in which efforts are

made to contract for all spares at the beginning of a program.

In the case of expendable modules, deferred procurement

would be advantageous, since it would prevent the scrapping

of large quantities of spare modules due to modifications.

(c) Logistics . From the standpoint of logistics,

several conflicting factors are evident. The types of spares

(the number of line items) increase rapidly as the size of

the module is reduced. (See Fig. 4). This would suggest

larger modules to minimize the number of line items controlled

by supply. However, as the size of the modules increases, the

storage volume of spares required goes up rapidly. (See Fig, 4)

This would suggest smaller modules to minimize the storage and

liandling problems. In the supply system, both of these factors

are important, but in front-line operation the volume of spares

^i See Glossary.
- 32 -

merits more serious consideration. Since improved front-line

operation is the primary goal, the volume of spares should be

given much more weight than the number of line items.

Included in Figure 3 are the results of repetition when

one-tube modules are used . It was assumed that each of

the modules was used twice. This reduces the types of

spares or number of line items from 240 to 120. Due to

repetition, the cost and volume of the spares was reduced by

approximately 15%. The total procurement cost was reduced

by very little, approximately 3%.

Designs using one-tube modules tend to be more attractive

due to the benefits of repetition. However, extensive use

of repetition (many identical modules throughout military

equipments) would be necessary before significant advantages

could be gained. Generally, a design using one-tube modules

is more difficult to develop and has lower reliability, poorer

maintainability, and larger size and wei,ght than one using

larger modules. These disadvantages probably outweigh the

benefits that reasonably may be expected from the universal

use of standard one-tube modules.

Although the solution and discussion of this typical

problem does not define a particular module size, some general

conclusions are possible. Small modules, one to three tubes,

are unfavorable; they offer no economic advantages and have

several important disadvantages. Large modules, those

I
- 33 - containing more than eight tubes, are unfavorable due to the

rapid increase in total costs. The optimum expendable module

size probably is in the range of four to eight tubes.

4.1.5 A General Solution for Determining the Optimum

Expendable Module Size

.

In the typical problem presented in section 4.1.3, one

of the conditions was a 95% probabil ity that sufficient

spares were procured for the life of the program. In actual

practice the number of spares procured is not determined in

this manner. This condition was used in the problem to assure

equal chances of having sufficient spares when comparing modules

of various sizes.

At present the number of spares procured is governed by

typical usage rates. Generally, the usage rates are

determined by calculating the average number of spares

necessary as shown by field usage reports The usage rate of

an expendable module should be approximately equal to its

failure rate. By use of the failure rate to determine the

average number of spare modules necessary, it is possible to

obtain a general relationship between the size of the modules

contained in an expendable modular equipment and the total

program procurement cost of the equipment . This relationship

is given by the following equation

.
- 34 -

Ct = [t ^eNa ^ ^ N, Cf + C^V,m

No. of No. of

spare modules

modules initially

operating

Cost of

module

m

Ce ^- 5^ C^

No. of Supply

modules costs

to be for

supplied modules

C = total cost of initial modules, spare modules, and

* maintaining their supply.

T = program life in operating hours (2000 hrs)

.

T. = MTBF* for typical one-tube stage, in operating hours

(8000 hrs).

V = number of tubes in set

.

Ng^ = average number of sets operating over program life (242)

i

.

V = size of module (number of tubes)

.

m

N. = number of sets operating initially (342)

.

C- = fixed cost per module ($29.5).

C„ = cost per tube of module ($16.8).

Cg = cost to establish a line item in supply system ($75).

C = cost per year to keep an item in supply system ($28)

.

Ty = operating hours per year (400)

.

The numbers in parenthesis represent typical values.

The above equation may be solved for the value of V^

which results in the minimum program cost:

V (for minimum program cost) = NiCf +

^ C„ + Ce

TpNaCv

See Glossary
- 35 -

Substituting the typical values given in parenthesis, we

find that the value of V^ for minimum program cost is slightly

more than 3 tubes per module.

The module size for minimum program cost is independent

of the number of tubes in the set. The module size varies as

the square root of the reliability level, since T. is directly

proportional to the reliability art. Future trends in the

electronics field should lead to designs containing larger

modules: component part and circuit reliability are continuing

to improve, program lives are tending to be shorter, and

attrition rates of operating equipment are becoming higher

(smaller N in the equation)

.

Th6 above equation does not include all of the costs that

may in some manner be related to the module size. The cost

to locate and exchange a defective module is probably a

function of the size of the module. It is known that more

time is required to locate a small module than to locate a

large one; however, the relationship of the time to locate and

exchange a defective module versus its size has not been

determined. This problem has been considered in connection

15 with the maintenance of Army missile equipment. If this

relationship were determined, it would be possible to convert

this time function into costs in the above equation. Since

the technician's time is probably more valuable than the costs
- 36 -

incurred by the military to support him, a military value

rather than costs should be established for a technician's

time. In this value, the reduced down-time of aircraft due

to more effective use of maintenance personnel should be

considered. '

The costs of the instrumentation for maintenance may be

a function of the module size. Some maintenance personnel

feel that a module checker to check each module individually

would be necessary. The procurement and maintenance costs of

a module checker would probably be a function of the module

size used in the equipment. Where bench set-ups and standard

test Equipment are used, the instrumentation cost is almost

independent of the module size. Since the type of maintenance instrumentation that may be necessary is not known, this

factor has not been included in the equation.

The dashed curve in Fig. 4 shows the calculated total

procurement cost for the initial modules and spares versus

module size for the problem in section 4.1.3, when the

general equation is used. This curve should show small

differences since the probability of having sufficient spares

is quite different for the two cases. However, for a large

number of equipments, either of the two solutions for the

optimum module size should be satisfactory.
- 37 -

5. COST COMPARISON OF REPAIRABLE ASSEMBLY AND

EXPENDABLE ASSEMBLY MAINTENANCE SYSTEMS,

In this section, a cost comparison is made between the

present maintenance system based on the use of repairable

assemblies and a system based on the use of optimum expendable

assemblies. Preferably, the total cost of both alternatives

over the program life should be used to permit the inclusion

of fixed costs, indirect costs, and other factors such as

16 reliability and availability. However, acquisition of the

information needed for such a comparison was beyond the scope

of the present study. The problem is considerably simplified

by concentrating on those areas in which the costs of the two

systems are known to differ. This is the general approach

that has been used in the comparison.

In the repair of equipment built with either repairable

or expendable assemblies, the steps involved in locating a defective assembly should be essentially the same. Hence,

the following discussion is concerned with the handling and

disposition of a defective assembly after it has been located.

Sec Gl ossa ry .
- 38 -

5.1 Repair of Defective Assemblies.

Before cost differences between the repairable assembly and

expendable assembly systems can be analyzed, a familiarity with

the present system for repairing electronic equipment is

necessary, particularly with regard to the handling of assemblies. Within the Bureau of Aeronautics there are three

levels at which defective assemblies may be repaired. These

levels are Squadron, FASRON*, and the Overhaul and Repair

Depot.* The squadron is primarily concerned with the operation

of aircraft and equipment and has very limited facilities for

the repair of electronic equipment. For every few squadrons

there is a FASRON (Fleet Aviation Service Squadron) which has

quite complete repair facilities. The FASRON is generally

located sufficiently close to the squadrons to permit use of

the FASRON facilities by squadron personnel. Some squadrons,

particularly those that operate in remote areas, are designated as self-supporting, and have their own facilities

similar to the FASRON. Carriers have electronic shops with

facilities similar to the FASRON. The & R Depots are

completely equipped for all types of maintenance and repair

work. There are relatively few of them, however, and material

sent to them for repair often must be packed and shipped over

long distances. For efficient operation of the depots with

reasonably stable workloads, a certain backlog of work is

*See Glossary
- 39 -

desirable; this backlog requires extra storage and handling

facilities

.

It has been determined that a considerable quantity of

equipment is being sent to the depots for piece-part repairs.

Indications are that this quantity will increase due to the

complexity of new equipment and the intricacies of modern

construction techniques. Hence, the main emphasis in this

study has been placed on determining the costs incurred in the

repair of electronic material at the depot level. These costs

include handling of the material at the originating activity,

shipment to the & R Depot, screening, storage, and repair.

Figure 5 illustrates these various steps „ After the item has

been repaired it is returned to Supply in RFI (ready for issue)

condition, and is treated essentially as a new item.

Consequently, the costs from this point on are the same whether

the item is a repaired assembly or a new expendable assembly.

Figure 6 illustrates the steps involved in locating and

replacing a defective expendable assembly.

For the repairable case , in determining total costs over

the program life of an equipment type, various costs must be

considered in addition to the direct cost of making repairs.

If the equipment is to be repaired only at the depot level,

at least three & R depots in different geographical areas

are customarily designated as overhaul points. If the equipment

See Glossary

.
Squadron Level

(Flight Line)

40

Defective

Unit

Remove Unit from Plane

Intermediate Level

(FASRON or Equivalent)

fl n

Isolate Defective Assembly —

Exchange through

Local Supply

Depot Level

Skilled Personnel

Bench Set-ups

Test Equipment

Repair Par

Repair Manua

)ment \

•ts

I luals I

Repairable

Noi"mal 1

1

Ur§

Storage

1

1

Re]Dair

Salvage or Disposal

[on ^pnnn

& R Depot

Return to Supply

Ready-f or- Issue

FIG. 5. MAINTENANCE SYSTEM BASED ON THE REPAIR OF

ASSEMBLIES AT THE DEPOT LEVEL
- 41 -

Squadron Level

(Flight Line)

Remove Unit from Plane

Intermediate Level

(FASRON or Equivalent)

Defective

Unit

Isolate Defective Assembly

Exchanfie through

Local Supply

Return of Selected Units

for Failure Analysis

Disposal

FIG. 6. MAINTENANCE SYSTEM BASED ON EXPENDABLE ASSEMBLIES
- 42 -

is to be used in large numbers in a variety of aircraft,

then all nine & R Depots may be designated. This means

that bench set-ups and test equipment must be provided at

each depot for all of the units and assemblies. In addition,

a complete operating set with all of its accessories is provided

for substitution checking of repaired units and for training

of the repair personnel.

The training of technicians is an item of major expense.

It is anticipated that the use of expendability will permit

much more effective use of present maintenance personnel. The

time and effort required to correct a malfunction should be

greatly reduced, since it would not be necessary, in the

expendable case, to perform piece-part repairs of defective

assemblies. While the capability of making piece-part repairs

exists at all of the maintenance levels which have been

discussed, the tendency is for such repairs to be relegated to

the depot level due to the pressure of more urgent duties on

the front-line organizations. Consequently, for purposes of the

present study, it has been assumed that the piece -part repair

of assemblies is performed at the & R depots, where the

repair work is done by civilians. It has been assumed that the

location and replacement of a defective assembly is essentially

the same, as far as the front-line organization is concerned,

whether the assembly is repairable or expendable. Hence, the

value of the military technician's time is not used in
- 43 -

determining the relative costs of the repairable and expendable cases. Civilian personnel at the & R depots are

presumed to have received technical training prior to employment, and this is reflected in the higher hourly wage rate.

On-the-job training is considered as part of the overhead

cost of the depot

.

In order to carry out repairs on assemblies, the

designated & R Depots must have complete stocks of all "bits

and pieces." These parts must be procured, distributed,

and stored, and must be requistioned as needed. Since there

is always a degree of uncertainty as to what the usage rates

will be for various parts, it is unavoidable that excess stocks

of some parts will be procured, while other parts may have to

be reordered. Both of these situations increase the cost of

maintaining stocks of spare parts. For assurance that new

parts will be interchangeable with those in the original equipment, various procurement documents and lengthy specifications

are required for each part

.

Detailed instruction manuals with pictorial and schematic

diagrams, servicing instructions, and lengthy parts lists

are required to aid in the piece-part repair of assemblies.

It is felt that the use of expendable assemblies would permit

many of the circuits to be represented merely as block diagrams,

reducing the need for schematics, and greatly simplifying the

See Glossary
- 44 -

instruction manuals. If expendable assemblies were modified

during production, or from one manufacturer to another, it

would not be necessary to send out detailed change notices, so

long as the performance of the assemblies remained essentially

the same

.

In the operation of the repairable case, a certain

quantity of spare units is necessary due to the time

required to perform a repair. In addition, some of the units

may be damaged beyond repair, or may be lost during shipping

and handling; such losses are reflected in the system recovery

rate, which is expressed as the percentage of defective items

that are restored to ready-for-issue status. Items which do

not get repaired are in effect expended and must be replaced

by spares. If the repair time (including handling, shipping

and storage, as well as actual repair) is long, then there is

a period at the beginning of a new program during which no

repaired items are available, and sufficient spares must be

obtained to tide the program over this period. Eventually,

a "steady-state" condition is reached; however, if the repair

time is a significant portion of the total program life, then

this initial quantity of spares is quite important. The lower

the system recovery rate, the more spares are required; if the

recovery rate were zero this would correspond to the

expendable case. With a low recovery rate fewer repairs are

made, and the fixed costs of the repair system must be prorated

'See Glossary
- 45 -

over fewer items, leading to an increased repair cost per

item.

5.2 Costs of the Repairable Assembly System.

The gathering of actual cost data has been a difficult

problem. In some areas, fairly detailed data is available,

while in others only rough estimates can be made. This is

partly because of the variety of conditions which are encountered: shipping distances and methods vary; electronic items

vary in size, weight, and complexity; procedures differ somewhat at various depots; storage time varies with system

requirements; and the actual repair costs vary widely. The

number of any one assembly type that is repaired may vary from

a few per year to several hundred. Assemblies can fail in an

endless number of ways. Repair time can vary from minutes to

hours. Parts requirements can vary from simple, common items

to special parts that may take weeks to obtain. Hence, any

statement of repair costs must be a fairly broad average. The

general problem of obtaining data on repair costs has been

17 described in a BuSandA study, which established formulas for

use in determining the economic disposition (by repair or by

disposal) of new and used aviation material.

The calculation of over-all costs for the repairable case

is best illustrated by means of a hypothetical example.
- 46 -

Efforts have been made in this example to select typical

values, and to use the best available cost data. The

conditions selected are those that were used in the determination of the optimum module size (Section 4.1.3). These are

listed below:

1. A set contains 240 tubes, grouped into ten units

of 24 tubes each. Each unit is further divided into four

6-tube assemblies. Each assembly is assumed to be different.

2. Three hundred of these sets will be installed in

aircraft.

3. This equipment is to be in use for 5 years at an

average of 400 hours per year, for a total of 2000 hours of

operation.

4. Attrition rate for the aircraft is 1.5% per month.

5. The sets will be distributed to 20 geographical

locations.

6. One bench set-up and one spare set are required at

each location for front-line maintenance; one set is required

at each of two & R Depots. This gives a total of 342 sets

which must be outfitted and supported initially.

The use of 6-tube assemblies has been chosen for this example

as representing the optimum size repairable assembly for the

given conditions. This can be shown by repeating the

calculations for assemblies of different sizes. I
- 47 -

7. All spares are procured for life-of-type

.

8. Spares may be redistributed as needed at any time.

For calculation of the cost of maintaining the 342 sets

which have been postulated, the number of sets operating at

any time and the rate at which failures occur must be determined

Initially there are 300 sets installed in aircraft, but this

number decreases at the rate of 1.5% per month throughout

the program. This can be expressed:

n.Ctj' = Ni e"^^

nj^(t) is the number of sets in aircraft at time t.

Nj^ is the initial quantity of sets in aircraft,

300 in this case.

a is the value of the exponent corresponding to the

specified rate of attrition, which for this case

is 0.0151. (When t = 1 month, e"^ = 98.5% = .985,

a = .0151)

t is time in months.

The quantity of bench set-ups needed is assumed to remain

constant throughout the program. For simplicity, their

operating time is assumed to be equal to the operating time of

the sets in the aircraft.

N^ is the number of bench sets.

The total number of sets operating at any time, t, is

thus

n. (t) + N^^ = N. e"^^ + N^^

This is read "n^ as a function of time."
- 48 -

5.2.1 Failure and Other Calculations for a 6-tube Asseml^ly.

It is assumed that the failure rate for the assemblies is

constant with respect to time, which appears to be characteristic of electronic equipment. The failures are randomly

distributed, which leads to rather involved probabilistic

calculations. However, for large populations these random

fluctuations average out, and calculations based on average

rates afford a reasonable approximation. The number of any one

type of assembly that fails in a given period of time ie

equal to the number of equipment -hours divided by the mean life

of the assembly

f(t) = (Equipment -hours) * 9^

where f(t) is the number of assemblies that have failed and

^a is the mean life of the assembly. For the 6-tube

assemblies under consideration, a mean life of 1360 hours

has been calculated.

The equipment -hours can be found by taking the area under

a curve of number-of -equipments versus time. This area is

given by

(Equipment -hours) = *^ (ni(t) + N^,) dt X hg|

where hq) is the number of operating hours per month. In the

present case, h^ = 400 -f 12 = 33.3 hours per month. I
- 49 -

Evaluating the integral

J^ [°i(t) + Nbj dt = y pi e-at + nJ

xl Ni e~^* dt +y Nu dt

o

F-F'T L-a Jo

= Ni \^ L + % t

^ [l - e-at]+ Nb t

(Equipment hours) at time t = f-^p- (1 - e

^'^) + N^j 1 1 X h^

f (t) = 1-^ (1 - e-at) ^ ^^ A ^"^ "^

As assemblies fail they are sent to the O Ss R Depot for

repair. A period of time is required for completion of the

repair process, which includes shipping, handling, storage,

and actual repair. This time is designated T^.

.

If f(t) expresses the failures as a function of time, then

f(t ~ Tp) expresses the repaired assemblies as a function of

time. Since this expression does not apply for times less

than Tr (no repairs are completed within a period T^ of the

start of the program) the expression must be slightly modified.
- 50 -

This is done by multiplying the expression by the function

H(t - Ty), which is defined as zero for values of time less

than Tj. and as unity for time greater than Tj.

.

k(t) = f(t - T^) H(t - Tr) H(t - Tr) = -(V l?Z I I l^

/o for t <

I

1 for t >

where k(t) expresses the number of repaired assemblies as a function of time.

One further modification is needed, since only a portion

of the defective assemblies are actually repaired. A certain

percentage are screened out at the & R Depot as unsalvageable, or disappear along the way. The proportion of the

defective assemblies that are eventually restored to readyfor-issue condition is designated rg.

rg = system recovery rate.

Hence, the complete expression for the repaired assemblies,

as a function of time, is given by:

k(t) = rg f(t - Tj.) H(t - Tr) g^

The number of spares needed at time t is equal to the Jk

number of failures that have occurred up to that time, minus

the number of assemblies that have been repaired
- 51 -

s = number of spares.

s(t) = f(t) - k(t)

f (t) =1-^ (1 - e-^^) + Nb t ^TM

^a

k(t) = rg f(t - Tj.) H(t - Ty)

= ^s 1-^ (l - e-^^* - |4i|x-e-(t-Tr)j,,^(, '^^'K Nb(t - T,)|-i' H(t-Tr)

_,^J|

^m Factoring out —^— and subtracting k(t) from f (t)

s(t) = -^-^1-^ (1 -e-''^) +Nh t

1 - -at

]

-^ |l - e-a(t - TrL Nb(t-Tr) H(t-Tr)V

With these expressions it is possible to determine

(a) The number of failures that will occur.

(b) The number of defective assemblies that will be

sent back for repair (assumed to equal the

number of failures).

(c) The number of repairs that will be completed.

(d) The number of spares that will be consumed.

For the expendable case the number of spares consumed

will be equal to the number of failures. For the repairable

case, the four quantities listed above must be determined. For
- 52 -

the hypothetical model under consideration the values selected

are as follows

N^ (initial quantity of sets in aircraft) = 300

a (attrition rate) = ,0151

N]3 (number of bench sets) - 42

hm (number of equipment operating hours per month) = 33.3

9^ (m^an life of as&embly) =: 1360

rg (system recovery rate of assembly) = . 50

Tp (time required for repair process) = 10 months

A program life of five years, or 60 months will be

considered

t = 60 months

(a) The number of failures = f(t)

h. = r^|-sr (1 . e-at) ^ Ni, tl -^

= [^ {l - e-(-OlSl) «0)}. (42) (60)1

^

= (".350) (fy)

= 352 failures for each assembly type.

(b) The number of defective assemblies of any one type

that will be sent back for repair is assumed to equal the

number of failures

Number of defective assemblies sent back for repair = 352
~ 53 -

(c) Because of the recovery rate of 50%, one-half

of the defective assemblies will eventually be repaired:

The number of repaired assemblies = 352 x 0.5

= 176 repairs.

(d) It might at first appear that if 352 failures

occurred and 176 repairs were made , the number of spares

needed would simply be 352 - 176 = 176 spares. However, some

of the repaired assemblies do not become available until

after the 60-thohth period. Hence, the number df spares needed

must be palculated using the equation for s(t):

The number of spares needed = s(t)

s(t) = 0a

I Q

-r< N

-at ) + Nv

•]

_i 1 _ e'a(t - Tr) + Nb(t - Tr)|H.(t - T^) \

The first term in square brackets is equal to 14,350 from

part (a), t is 60 months and Tj. is 10 months. Hence,

t > Tj., and H(t - T^) =1.

33.3 s(t) = 1360

/l4, 350-0. sr^{l-e-(-015^)<6«-l«>)+ (42X60-10)

= lli§|^'3^^-^-^ [l2,600]}

s(t) = 197 spares.
- 54 -

5.2.2 Processing and Repair Costs.

Having determined the number of defective assemblies

that will be sent back for repair, and the number that will

actually be repaired, we can calculate the costs incurred

in processing and repairing these assemblies. Since the

recovery rate of the assemblies under consideration is

50%, one-half of the defective assemblies will be repaired

and the other half will be considered expended. It will

be assumed that all of the defective assemblies reach the

& R Supply activity where they are screened to determine

whether they are in repairable condition. Hence, all process

costs through the screening process are incurred by all

assemblies. However, only half of the assemblies incur the

actual cost of repair.

The steps involved in the processing and repair of

defective assemblies are listed below. Certain intermediate

steps have been lumped together for convenience. Under these

steps there are various types of costs, which in general can

be considered as either paperwork costs or materials handling

costs. The symbol F will be used for paperwork and K for

materials handling. Subscripts indicate the step with which

the cost is associated.
- 55 -

(1) Processing at the originating activity:

Fq = paperwork

Kq = materials handling

Pq = packing and marking

(2) Shipping to & R Depot:

T = transportation cost, based on average distance,

average weight, and normal mode of transportation.

(3) Receiving at the St R supply activity:

Yj. = paperwork

Kj. = materials handling

(4) Screening:

S = cost to screen an item.

(5) Storage :

Fh = paperwork

Kh = materials handling

H = holding costs

(6) Transfer to O & R Shop:

F^ = paperwork

Kt = materials handling

(7) Repair:

L = labor, including overhead

B = material

(8) Return to Supply:

Fg = paperwork

Kg = materials handling
- 56 -

The processing costs under steps 1 through 4 (screening)

are Incurred by all of the defective assemblies, while the

remaining steps are Incurred only by the proportion actually

repaired. This proportion Is equal to the recovery rate.

The total cost (Cr) of processing the defective assemblies

may be represented thus:

Cr = (Number of defective assemblies) x (recovery rate) x

(cost of steps 1 through 8) + (number of defective

assemblies) x (one minus the recovery rate) x (cost

of steps 1 through 4).

Using the previous notation this may be written:

Cj. = f (t) X rg X IFq + Kq + Pq + T + Fj. + Kj. + S + Fii +

Kjj + H + Ft+Kt+L + B + Fs+Kjl+f(t)x

(1 - Tg ) x JF^j + Kq + Pq + T + Fj. + Kj. + sl .

5.2.3 Evaluation of Cost Factors

Values have been assigned to the various cost factors.

Based on the 6-tube assemblies which are being considered,

and the best available information which has been obtained,

the following estimates have been made

,
- 57 -

The cost of processing an assembly from the originating

activity through screening at the & R Depot is $7.33. The

cost of sending an assembly through the complete repair

process is $7.33 + $37.81 = $45.14.

F (paperwork) $ .07

Kq (materials handling) .95

Pq (packing & marking) 2.16

T (transportation) 2.00

Fj. (paperwork) .73

Kj. (materials handling) .07

S (screening) 1.35

Costs through screening $7 .33

Fh (paperwork) 1 .69

Kh (materials handling) .03

H (holding cost 1 .30

Ft (paperwork) 1 .31

Kt (materials handling) .07

L (labor) 20 .40

B (material) 12 .00

Fs (paperwork) .96

Ks (materials handling) .05

Costs to complete repair $37.81
- 58 -

It must be remembered that the above figures are obtained

on the basis of the hypothetical 6-tube assembly, and that

many factors are averaged in determining the various cost

values. In considering a specific equipment or a proposed

design, the data could probably be refined to fit actual

conditions more closely.

The actual cost of repair parts has been included in

the repair cost as item B; however, there are additional

costs to consider in connection with the supply of these

parts. These are the paperwork costs associated with

purchasing and keeping track of the parts over the life of the

program. It is somewhat simpler to consider these as over-all

costs, rather than to prorate them over the individual parts,

since it is not known how many of each part will be used, and

these costs are relatively independent of the number of parts

used

.

Each new part that is stocked by the supply system must

be procured, given a stock number, cataloged, inventoried

periodically, and reordered as necessary. Thus, there are

paperwork costs to be considered, in addition to the cost of

the parts themselves. Some of these costs are incurred at the

Aviation Supply Office (ASO) and some are incurred at the

using activities (the two & R Depots in the present case).

It has been estimated that the average cost at ASO to procure

a new part (generally referred to as a line item), including
- 59 -

preparation of bids, purchase orders, etCo, is $21. The cost

of establishing a line item, including stock numbering,

cataloging, etc., averages about $50. The cost of maintaining

records, analyzing inventories, etc., averages about $23 for

each year that the item is stocked.

In the case of the assemblies themselves, each type of

assembly would be considered a line item whether the assembly

were repairable or expendable. Hence, the cost to keep records

on the assemblies would be the same for both systems. However,

in the case of repairable assemblies all of the numerous

different repair parts must be stocked, in addition to spare

assemblies. Not all of the parts are actually new to the

supply system, since many of the parts would have been used

in previous equipments. In the present example, a set

containing 240 tubes has been postulated, and should contain

about 2400 parts, assuming average circuitry. Assuming that

10% of the parts are new, there will be 240 new parts added to

the supply system for repair purposes. Presumably, the cost

of the parts already in the supply system has been charged

against previous equipment. The cost incurred at ASO to

purchase and keep track of these 240 new items over the fiveyear program life can be calculated as follows:

CpjL = 240 new parts x ($21 to procure each part + $50 to

establish each part) + 240 x $28 per year x 5 years.

= $50,640

where C -, is the supply cost incurred at ASO for the 240 new
- 60 -

parts (exclusive of the cost of the parts themselves).

In the present example, it has been assumed that the

repairs will be performed at each of two & R Depots;

hence, the cost of stocking the parts at these depots must

be considered. It has been estimated that a cost of about

$12 is incurred for each line item that is received at an

activity. Since the & R Depots try to schedule their work

3 to 6 months in advance, they probably would reorder the

necessary parts twice a year. The number of different parts

ordered each time is rather a matter of conjecture, but

assuming this figure to be 10%, then the number of different

parts ordered each time is 10% of 2400 or 240. If orders

are placed twice each year for five years, there are ten

orders altogether, making a total of 10 x 240 or 2400 items

that are received. The cost of receiving these items is

2400 X $12 or $28,800. Since this cost is incurred at each

of two & R Depots, this figure must be doubled:

Cp2 = $28,800 X 2

= $57,600

5.2.4 Total Cost's of Repairable Assembly System.

It is now possible to arrive at the complete cost for

the repairable case, with regard to those areas in which the

costs differ from the expendable case. This complete cost is

made up of the following items
- 61 -

(1) Cost of the initial quantity of assemblies.

(2) Cost of the calculated number of spare assemblies.

(3) Cost of performing the calculated number of

repairs, including the processing costs on those

assemblies that are screened out as unsalvageable.

(4) Cost of stocking repair parts in the supply system.

(5) Cost of jigs, fixtures, and test equipment at the

designated & R Depots.

(6) Cost of maintenance manuals.

For the present example, these costs are evaluated

as follows:

(1) Cost of assemblies for 342 initial sets:

240 tubes per set

6 tubes per assembly

240 -r 6 = 40 assemblies per set

342 X 40 = 13,680 assemblies for initial equipment

13,680 X $130 per assembly ^ $1,780,000

(2) Cost of spare assemblies;

197 X 40 = 7,880 spare assemblies

7,880 X $130 per assembly ^ $1,020,000

(3) Cost of processing and repairing defective assemblies:

The number of failures has been calculated to be

352 for each of the 40 types of assemblies.

352 X 40 = 14,080 assemblies which will be sent back

for repair.
- 62 -

Cj. = 14,080 X 0.5 X $45.14 + 14,080 x (1-0.5) i $7.33

= 14,080 X 0„5 X ($45.14 + $7,33)

= 7,040 X $52.47

% $370,000

(4) The cost of stocking repair parts in the supply system:

This has been calculated earlier:

Cp]L *• <^p2 = $50,640 + $57,600

% $110,000

(5) Cost of jigs, fixtures, prorated share of test

equipment for the repair of assemblies:

$1,000 per assembly at each of two & R Depots:

40 X $1000 X 2 = $80,000

(6) Cost of maintenance manuals:

$16,000

The costs of the repairable system are recapitulated

below:

(1) Cost of assemblies in original equipment $1,780,000

(2) Cost of spare assemblies 1,020,000

(3) Cost of processing and repairing

defective assemblies 370,000

(4) Cost of stocking spare parts 110,000

(5) Cost of depot repair set-ups 80,000

(6) Cost of maintenance manuals 16,000

$3,376,000
- 63 -

5.3 Total Costs of the Expendable Assembly System.

The cost of the repairable case may be readily compared

with the expendable case, for the hypothetical example, since

the number of failures has been determined, and each failure

will require a replacement assembly. The number of spare

assemblies is increased over the repairable case since no

repairs are performed on assemblies. The cost of maintenance

manuals has been reduced, though this cost cannot be eliminated

entirely, since instructions are still needed for locating

defective assemblies, making adjustments, and for maintaining

those portions of the equipment that are not contained in

expendable assemblies. The costs of the expendable case are

evaluated as follows:

(1) Cost of assemblies for 342 initial sets:

13,680 assemblies at $130 per assembly ;^$1 ,780,000,

as before

.

(2) Cost of spare assemblies:

The number of failures has been calculated to be 352

for each of the 40 types of assemblies:

352 X 40 = 14,080 failures

14,080 assemblies x $130 per assembly ;^$1 , 830, 000

(3) Cost of maintenance manuals:

It has been assumed that the cost of the maintenance

manuals will be reduced by a factor of 4

$16,000 -5- 4 = $4,000
- 64 -

The costs of the expendable case are recapitulated below:

(1) Cost of assemblies in original equipment $1,780,000

(2) Cost of spare assemblies 1,830,000

(3) Cost of maintenance manuals 4,000

$3,614,000

5.4 Conclusions as to Cost Difference.

Thus, the cost of the expendable case is $3,614,000 as

compared to $3,376,000 for the repairable case, in terms of

the factors that have been considered. It must be remembered

that these are not the complete costs that are incurred in

the support of the equipment, since many factors which were

assumed to balance out have not been considered.

The small difference in costs between the expendable

case and the repairable case is probably insignificant. The

inaccuracies of the data and the assumptions necessary to

calculate the costs preclude precise comparison. Also, there

are undoubtedly significant costs for both cases that have been

omitted. It is believed, however, that if more complete repair

cost data were available, the difference between the expendable

and repairable cases would be diminished, because most of the

omissions relate to items that would add to the cost of the

repairable case. It should be emphasized that the costs being

considered in this study are the costs concerning only that
- 65 -

portion of an equipment that can be modulized. The costs of

procuring and maintaining those portions of an equipment that

cannot be modularly designed are at least twice the costs

of procuring and maintaining the modular portion. Hence,

small differences in the costs of the modular portion would

be insignificant when the total costs of procuring and

maintaining an equipment were under consideration.

It is concluded that the costs of the expendable and

repairable cases are so nearly equal that the cost factor

may be eliminated as a bar to the use of expendable

assemblies or modules within the optimum size of 4 to 8

tubes

.

A commonly discussed ratio of maintenance costs to

procurement cost is ten to one. During this report no

attempt was made to either substantiate or disprove this

ratio. The maintenance costs considered in this report are

probably a small portion of the total maintenance cost

for the following reasons. First, the initial steps

in the repair, namely, isolation and exchange of a defective

assembly, may be much more expensive than the actual repair

of the assembly. These steps are performed by military

technicians. The training costs and the support costs

(housing, meals, supervision, transportation, special

services, recruitment, extra personnel, etc.) for these
- 66 -

technicians add greatly to the direct labor cost. Expendability would not directly aid in reducing these costs.

Second, the repair of assemblies at a depot may be much

cheaper than in the field. Third, much of the total cost

may be incurred in maintaining those portions of the equipment

which are not contained in assemblies. Many maintenance

actions do not involve repairs. Some simply involve switch

settings and front-panel adjustments. Others involve routine

checks and inspection of the equipment.

6. FACTORS OTHER THAN COST IN EXPENDABILITY DECISION

The following factors which cannot be converted or

simply related to costs should be seriously considered in

the decision between expendability and repairability . These

factors all favor expendability, and with the cost factor

probably eliminated as a deterrent to the practice of

expendability, they may be so important as to completely

determine a decision in favor of expendability.
- 67 -

6.1 Improved Reliability

It may be possible to achieve higher reliability with

disposal-at-failure assemblies or modules than with repairable assemblies or modules. Hermetic sealing or encapsulation may be employed to improve the reliability of expendable

modules. Quantitative data as to how much the reliability

may be improved has not been obtained; however, several of

the manufacturers of hermetically sealed or potted assemblies

felt that the failure rates had been improved considerably,

from 30 to 400 percent

.

It is thought that performing repairs on a module lowers

its inherent reliability. Replacing the parts with lower

quality parts, damaging parts when soldering, handling, etc.,

can contribute to higher failure rates. Quantitative data as

to how much the reliability may be lowered has not been

obtained

.

It is generally agreed that modules have nearly constant

failure rates over the program life. Where this is the case

it makes no difference in the reliability of an equipment

whether a failed module is replaced with a new one or a used

one. When wear-out is a factor, modules may have a normal

failure rate, and the reliability will probably be improved by

replacing a failed module with a new one rather than a

repaired one.

The three factors discussed above indicate that an

improvement in reliability may be possible by use of expendable
rather than repairable modules.

- 68 -

Since a comparatively small

n

improvement in reliability offers considerable savings in

the maintenance and operational costs of equipment, improved

reliability may well be the most important factor in the

decision between expendable or repairable design.

The savings in maintenance costs resulting from improved

reliability can be evaluated in a straightforward manner by

following the procedure used in the previous illustrative

examples. However, the improved operational effectiveness,

which is a much more important aspect , must be evaluated

from a military standpoint. This aspect has been considered

18 in studies at Redstone Arsenal, and at International

19 Business Machines, Inc.

6.2 Reduction in Specifications

The use of expendable modules could lead to a reduction

in the quantity of military specifications. Specifications

stating the reliability , environmental, physical, and

electrical requirements of the module could possibly replace

the numerous specifications needed for spare parts. Although

such reductions in specifications are possible, there is

some question as to what the savings may be to the military.

If the military should reduce or eliminate the specifications

of parts, the module manufacturer would have to prepare

these specifications in order to insure his meeting the

module specifications.
- 69 -

It has been observed that many manufacturers prepare

their own specifications for internal company use, above

and beyond the existing military specifications. There are

two main reasons for this. First, because the military

specifications frequently are not sufficiently up-to-date

to cover worthwhile new developments, the manufacturer must

prepare his own specifications if he wishes to use the latest

parts and materials. Second, in order to secure the highest

quality in his equipment, the manufacturer frequently

tightens the existing military specifications to restrict his

suppliers to those who produce the best available parts and

materials and who are known to be reliable, consistent sources

Under such circumstances, some savings might be realized by

the military in the reduction of specifications.

6.3 Improved Systems Maintenance

The average military technician is not in the service

long enough to become thoroughly familiar with the over-all

operation of complex electronic systems, as well as the

detailed functioning of circuits and parts. Yet, for rapid

flight-line maintenance of aircraft, a knowledge of systems

is most important and should be the primary objective in

the training of flight-line technicians. The use of

expendable modules would free the technicians from considerations of individual parts and circuits, and should permit a
- 70 -

major step in the direction of improved systems maintonance.

The difficulty of obtaining both detailed and over-all system

maintenance has been described as follows:

"While recognizing the practical necessity for

performing the detailed maintenance functions at the

using level, it should be noted that the system also

suffered because of the tendency of the operational

personnel to assume a general familiarity with the

equipment without fully developing the detailed

knowledge necessary to perform the job. In other words,

a fascination with the technical aspects of the overall system interfered with the primary maintenance

objective, that of keeping the equipment operational . "•'^

Piece parts repairs, if they are to be made, should not

interfere with the primary mission of the front-line

technicians, which is to maximize the operational readiness

of aircraft . When repairs do not have to be made at the

front line, there should be sufficient spare equipment to

permit delaying of the repair work until a slack period,

allowing time for the ordering and receipt of parts. If

defective equipment is to be sent to a depot for repair,

sufficient spare equipment is required to cover the period

that it is out of service.

6.4 Freedom of Design

When an equipment is designed for piece -part repair, all

the parts subject to failure must be accessible and removable

These requirements are not compatible with other design

specifications such as small volume, light weight, resistance

to humidity, shock and vibration, etc. Replacement of piece
- 71 -

parts would not be necessary, of course, if an equipment were

built with disposal-at-failure assemblies or modules; the

designer would have almost complete freedom in the layout and

selection of parts. In airborne equipment, lightweight, small

volume, and environmental resistance are extremely important

factors. Most designers of equipment agree that an expendable

assembly or module may be smaller, of lighter weight, and

more resistant to environment than a repairable assembly or

module. Such improvements through the use of expendable

modular design may be decisive factors in the choice between

expendable or repairable modular design.

7. CONCLUSIONS

As mentioned in the text, the data used in the

calculations were typical data. Such data cannot be

considered as representing facts about any particular equipment; use of averages, estimates, and approximations were

necessary to arrive at these typical values. While the

approach followed in this study does not preclude certain

conclusions, such conclusions should be applied with care in

particular conditions. For instance, the optimum module

size for equipments having short lifetimes (perhaps missile

equipment) may be much larger than is indicated in (3) below;

the optimum module size for equipments having long lifetimes

(perhaps ground-based or shipboard equipment) may be smaller
- 72 -

than is indicated in (3) below.

(1) Performance of piece-part repair of electronic

equipment is beginning to exceed and will continue to exceed

the capabilities of most of the front-line maintenance

activities.

(2) Disposal-at-failure design would help to solve

some of the future electronic maintenance problems, and would

also be compatible with future design trends.

(3) The use of assemblies or modules (either expendable

or repairable) containing from 4 to 8 tubes would lead to

lowest over-all procurement cost of an aircraft equipment

type 'for its program life and at the same time aid in the

solution of logistic and maintenance problems.

(4) An aircraft equipment type built of expendable

4 to 8 tube assemblies or modules would cost about the same to

procure, support, and maintain as would a similar equipment

type built with repairable 4 to 8 tube assemblies or modules.

(5) The costs of the expendable and repairable cases

are so nearly equal that the claim of excessive cost for the

expendable case need no longer prohibit the use of expendable

assemblies or modules within the optimum size of 4 to 8 tubes.

(6) The following potential advantages of expendability

should be carefully studied; investigation may prove them to

be preponderant in a decision for expendability.
- 73 -

Improved reliability of electronic equipment

Improved systems maintenance

Smaller size of equipment

Lighter weight of equipment

(7) Although the guidelines and conclusions presented

in this Note pertain specifically to Bureau of Aeronautics

airborne electronic equipment, it is believed that they can

be applied with some modification to aircraft electronic

equipment in general

.

Now integrated into the Bureau of Naval Weapons.
- 74 -

REFERENCES

1. R. L. Kirkwood and L. W. Sepmeyer , "The Cost of Unreliability of Air Force Airborne Electronic Equipment as

Represented by the Cost of Maintenance," (Confidential)

Report No. RM-1002, The Rand Corporation, ISTanta Monica,

California, December, 1952.

2. Robert Lusser, "The Notorious Unreliability of Complex

Equipment," Redstone Arsenal, Huntsville, Alabama,

September, 1956.

3. G. B. Devey, "Reliability in Electronic Equipment,"

Proceedings of the IRE, April, 1950, pp. 344-355.

4. K. Henny, et al . , Reliability Factors for Ground

Electronic Equipment , McGraw-Hill Book Co., Inc., New York,

New York, 1956.

5. Stanford Research Institute, Stanford, California,

"Maintenance Minimization in Large Electronic Systems,"

January, 1952.

6. Collins Radio Co., Cedar Rapids, Iowa, "General Module

Standards," May 1, 1955.

7. Bureau of Aeronautics Military Specification, MIL-E-19600(AER)

,

September 1, 1956.

8- Advisory Group on Reliability of Electronic Equipment (AGREE)

,

Office of the Assistant Secretary of Defense (Research and

Engineering) , "Reliability of Military Electronic Equips

ment," Report, June 4, 1957.

9. Vitro Laboratories, Silver Spring, Maryland, "Techniques

for Reliability Measurement and Prediction Based on Field

Failure Data," Technical Report #80, October, 1955.

.0. R. R. Carhart, "A Survey of the Current Status of the

Electronic Reliability Problem," Report No. RM-1131, The

Rand Corporation, Santa Monica, California, August, 1953.

.1. Aeronautical Radio, Inc., Washington, D. C. , "Electronic

Reliability in Military Applications," General Report #2,

July, 1957.
- 75

12. Proceedings of the Conference on the Reliability of

Military Electronic Equipment, sponsored by the United

States Air Force and the Radio Corporation of America,

Philadelphia, Pa., August 16-17, 1955.

13. Aeronautical Radio, Inc., Washington, D. C. , "Investigation of Electronic Equipment Reliability," Air Force

Reliability Assurance Program, Progress Report No. 1,

February, 1956.

14. James W. Petersen, "Savings from Procurement Deferral

with Interim Contractor Support ; The Case of High Value

Airframe Spares," Report No. RM-2085, The Rand Corporation,

Santa Monica, California, January 10, 1958.

15. C. I. Cummings, "Some Army Problems in Missile Maintenance,'

Proceedings of the EIA Conference on Maintainability of

Electronic Equipment, December, 1957 and May, 1958.

16. Allen M. Dame, "Determining Total Equipment Cost," Naval

Aviation Electronic Service Unit (NAESU) , Naval Research

Station, Washington 25, D. C. , December 12, 1957.

17. S. J. Salerno, "A Proposal for Optimizing Certain Decisions

in the Aviation Supply System," U. S. Naval Supply

Research and Development Facility, Bayonne, New Jersey,

February, 1956.

NOTE: The project under which this report was written has

been transferred to Bureau of Supply and Accounts, Advanced

Supply Systems Research and Development Division, Logistics

Research Branch (Mr. J. R. Simpson), Arlington Annex,

Washington 25, D. C. , where work is being continued.

18. E. Pieruschka, "Optimum Allocation of Funds for Reliability

Programs of Guided Missiles," Redstone Arsenal, Huntsville,

Alabama, January, 1955.

19. R. E. Kuehn, "The Throw-Away Philosophy: Implications

and Problems," Supplement to the Proceedings of the EIA

Conference on Maintainability of Electronic Equipment,

December, 1957 and May, 1958.
- 76 -

GLOSSARY

AGREE - Advisory Group on Reliability of Electronic Equipment,

Office of the Assistant Secretary of Defense (Research

and Engineering)

ARINC - Aeronautical Radio, Inc.

Assembly - (See Equipment) A combination of subassemblies and

parts joined together to form an element of a unit, and

which performs a specific function necessary to the

operation of the unit as a whole. Example: amplifier,

oscillator, dyhamotor, etc.

Attrition rate - The rate at which equipment or aircraft are

retired from service, either because of fair wear and

tear, or as the result of damage beyond repair.

Embedment - The use of a casting compound, usually a plastic

resin, which permeates and encloses a device, and which

hardens to form a solid, durable structure. Embedment ^ includes both encapsulation and potting. In encapsulation

the embedded device is removed from a mold , while in

potting, the mold, which is usually a thin metal housing,

becomes a permanent part of the structure.

Encapsulation - (See embedment)

Equipment - Aircraft electronic equipment consists of those

onic

I

installed electronic articles which make up the electronic

configuration of an aircraft. Equipment is generally

subdivided as follows: system, set, unit, assembly, subassembly, part. Explanations of these terms appear elsewhere in the Glossary.

Failure rate - The rate at which failures occur in electronic

equipment, generally expressed as a function of equipment

operating hours. Mathematically, the probability of

failure in a small but finite interval of time following

a period of failure-free operation.

FASRON - Fleet Aviation Service Squadron. The FASRON provides

Facilities for use by the personnel of the supported

operating squadrons in the performance of intermediatelevel maintenance. FASRON personnel maintain the FASRON

test equipment and bench set-ups, and provide instructions

and assistance.
- 77 -

Field Engineer - A civilian employee with an engineering

degree or equivalent experience, employed by a private

company and assigned under contract to a field

maintenance activity for the purpose of assisting and

instructing in the maintenance of specific items of

electronic equipment

.

Life-of-type -r The period of years over which operation of a

given equipment type within an authorized operating

allowance is intended or can reasonably be expected.

Line item - A term used to describe entries (parts, devices,

materials) as they appear on requisitions, bills of

lading, and other supply documents. Each entry is

considered a line item regardless of the quantity of the

item.

Maintenance - Maintenance is the function of retaining

material in, or restoring it to, a serviceable condition

Its phases include servicing, repair, modification,

modernization, overhaul, rebuild, test, reclamation,

inspection and condition determination, and the initial

provisioning of support items.

Maintenance Activity - Any organization within the Bureau of

Aeronautics assigned the mission, task, or functional

responsibility of performing aircraft upkeep or rework.

Mean time between failures - Abbreviated MTBF. The average

number of hours of operation between failures.

Mathematically, the reciprocal of failure rate.

Modular - 1) Dimensioned in accordance with a set of

"' pFescribed size increments.

2) Composed of modules.

Module - A modularly dimensioned assembly.

Modulj.zed - Designed and constructed with modules.

MTBF - Mean time between failures. (Defined above).

& R - Overhaul and repair. (See & R Depot)
- 78 -

& R Depot - Overhaul and Repair Depot . The & R Depot

is the highest level of maintenance within the Bureau

of Aeronautics and contains complete factory-type

facilities and equipment. There are several & R Depots

strategically located within the Continental United States.

These are fixed installations, and are staffed by

civilian personnel under a military commander. The & R

Depots carry out scheduled overhaul of aircraft and

perform repairs which are beyond the capabilities of

the lower levels of maintenance.

Overhaul - Overhaul of equipment involves disassembly to as

thorough an extent as is required to permit complete

inspection, test, cleaning, adjustment, alignment, and

service. Parts and portions of the equipment which are

found to be unsatisfactory are restored or replaced, so

that after reassembly and test, the equipment will comply

with applicable specifications.

Packaging - The style of construction of electronic equipment , i.e., whether unitized, modulized, or conventional

chassis.

Packing - The wrapping and crating of equipment for storage

and shipment

.

Part - The smallest complete entity used in the construction

of equipment. Applies to all of the "bits and pieces"

of which equipment is composed, including the electrical,

electronic, and mechanical items and supporting hardware. Typical electronic parts are resistors, capacitors,

vacuum tubes, transformers, etc. Parts are not normally

susceptible to disassembly for servicing.

Piece part - Refers to the smaller electronic parts, which In

general are soldered in, and are frequently Inaccessible

Program life - The period of time over which operation of a

type of aircraft or equipment within an authorized

operating allowance Is intended or can reasonably be

expected.

Ready-for -issue - Abbreviated RFI . Equipment or material which

is in condition to be issued to a using organization,

having been tested and inspected for compliance with all

applicable specifications.

II
- 79 -

Recovery rate - The ratio of the number of defective items

eventually restored to RFI condition to the number of

defective items turned in for repair.

TCfeliability - The probability that the equipment will give

satisfactory performance for a given period of time when

used in the manner and for the purpose intended

.

RFI - Ready-for-issue. (Defined above).

Set - (See equipment), A unit or units and necessary

assemblies, subassemblies and parts connected or

associated together to perform an operational function

o

Examples: radio receiving set, radar set, sonar set.

System - (See equipment)

T) In equipment: A combination of two or more sets,

generally physically separated when in operation, and

such other assemblies, subassemblies, and parts

necessary to perform an operational function or functions

Examples: navigation system, communication system.

?) Conceptually: A concept or philosophy; a method

of achieving an objective. Example: A system (method)

of maintenance based on the use of throw-away assemblies.

3) Organizationally: The organization which gives

embodiment to a prescribed concept. Example: supply

system, maintenance system.

Trouble shooting - The process of locating and determining the

corrective action required to rectify discrepancies or

malfunctions in equipment.

Unit - (See equipment). A separate package consisting of an

assembly or any combination of parts, subassemblies, and

assemblies mounted together, normally capable of

independent operation in a variety of situations.

Examples: receiver-transmitter unit, power supply unit.

Note: The unit most nearly corresponds to the notion of

a "black box", having a sufficiently distinct function to

permit rapid location in trouble shooting, yet small

enough to be removed with reasonable effort.

Unit Cost - This term is used in procurement to refer to the

purchase price of one of an item, such as a resistor or

an assembly. The "item" might be an entire radar set

consisting of many separate pieces, in which case the

unit cost is the price of one complete radar set.

Usage rate - The rate at which spare parts and other materials

are used up and must be resupplied.

USCOMM —DC

TJ.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller. Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front

cover. WASHINGTON, B.C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Photographic Technology. Length. Engineering

Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Molecular Kinetics. Free

Radicals Research. Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory. Radioactivity. X-rays. High

Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic

Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Poly- mer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion. Metal

Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Air Conditioning, Heatmg, and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat Transfer. Concreting Materials.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital Circuitry.

Digital Systems. Analog Systems. Application Engineering.

• Office of Basic Instrumentation. • Office of Weights and Measures.

BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction. Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular Propagation

Services. Sim-Earth Relationships. VHF Research. Radio Warning Services. Airglow and Aurora. Radio

Astronomy and Arctic Propagation.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Research. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Propagation Obstacles Engineering. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Impedance Standards. Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards.

Radio Communication and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ultra High Frequency and SuperHigh Frequency Research. Modulation Research. Antenna Research. Navigation Systems. Systems Analysis. Field Operations.
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