California State University at Northridge
MAINTAINABILITY DESIGN TIME TRADE-OFFS
A project submitted in partial
satisfaction of the requirements
for the degree of Masters of
Science in Engineering.
by
John Lawrence Hole
January 1982
The project of John Lawrence Hole is approved:
Date
R . Lanq'is
H. Vaish
Date
Cornmi ttee Chairman
Date
California State University at Northridge
January 1982
ii
TABLE OF CONTENTS
Page
List of Tables and Figures
iv
Abstract
v
Chapter I
INTRODUCTION . .
Chapter II
ATTRIBUTES .
10
Chapter III
EVALUATION
15
Chapter IV
PREDICTION .
27
Chapter v
CONCLUSIONS.
53
BIBLIOGRAPHY
1
54
iii
LIST OF TABLES AND FIGURES
Table No.
Title
2-I
Attribute Comparison .
12
2-II
Attribute Summary . . .
14
3-I
Data .
20
3-II
Vacuum Fill Panel.
21
3-III
Hydraulic Test Panel .
23
3-IV
Summary Table.
24
3-V
Time Allocation . .
26
Figure No.
Title
1-I
Price Versus Availability.
4
1-II
Cost Versus Availability •
5
3-I
Illustration of Relative Importance
of Maintainability Attributes.
17
4-I
Procedural Flow Block Diagram.
35
4-II
Related Constraint Matrix.
47
4-III
End Item/Corrective Maintenance
Action Matrix. . . .
. ....
48
End Item/Preventive Maintenance
Action Matrix. . . .
. ...
50
End Item/Operational Function
Matrix . . . . . . . .
. .
52
4-IV
4-V
iv
.
.
ABSTRACT
~~INTAINABILITY
DESIGN
TI~ill
TRADE-OFFS
by
John Lawrence Hole
Master of Science in Engineering
There is an ever increasing need for today's designers
to meet the demands of reducing the life cycle cost of new
systems and equipment.
With the current run-away support
cost it becomes imperative that new designs be affordable
over the life of the item.
The Department of Defense, in
particular, has stated that due to increasing support costs,
performance costs and support costs should be made more
equal.
As a result it may become necessary to sacrifice
some performance for reduced maintainability costs.
The
object is to increase the availability of the item by
reducing its mean time to repair and reduce the cost to
perform the maintenance action.
This report is directed mainly at aerospace ground
support systems but the material presented here can be
v
applied to a much broader class of hardware.
The method is
a generalized one being applied to the field of interest of
the author.
The first step in this method is to establish the
attributes which typify the system/equipment under
consideration.
The next step is to evaluate the attributes.
The evaluation process ranks the attributes and assigns a
value to their relative importance.
The last step in the
process is to make a prediction of the anticipated
maintainability of the design.
This step provides the
feedback that allows for evaluation of the current design.
The procedure can be repeated at each design iteration
to further optimize the maintainability design.
Time and
money invested in maintainability design will always mean
savings in life cycle cost.
Chapter I
INTRODUCTION
In today's world of more complex systems and equipment
together with higher costs of production and upkeep, it is
imperative that new attention be directed to the initial
design phase of these items in order to implement new cost
savings ideas.
With any system or equipment one of the main objectives
is to maximize the availability and minimize the cost.
Availability is the time an item is available for use.
More specifically, availability is the probability of being
in service during a scheduled operating period and performing
a prescribed mission.
It is the opposite of down time.
Availability, A, is defined ideally by
A=
MTBF
MTBF + MTTR
(1)
The MTBF is the mean time between failure, up time, and is
the inverse of the failure rate, which is assumed to be
constant.
Calculations of MTBF will not be considered in
this report.
The MTTR is the mean time to repair and is
considered the downtime or repair time.
The MTTR is briefly made up of the following main
elements:
time to realization, access time, diagnosis time,
spare part procurement time, replacement time, checkout
1
navid Smith, Maintainability Engineering, p. 8
1
2
time, system alignment time, logistic time, and administrative time.
The intent here is to reduce the MTTR, thus
increasing availability, by guiding the initial design so
as to reduce the above listed time elements.
By making a
more efficient design, the active time elements performed
by maintenance personnel can be reduced.
In the application of these ideas it is realized that
since everything is subject to failure at some point in
time, one must design for the greatest time between the
failures and design for the shortest time to repair the
failure.
Both aspects relate directly to the cost of the
designed item.
It costs additional to increase the MTBF
and it costs additional in terms of design time and hardware
to reduce the MTTR.
Typically the relationships for cost
and availability is similar to the one shown in Figure 1-I
for the production of an item.
That is, the cost of design
and manufacture (cost before delivery) increases with
availability, and the curve of costs of contractual repair,
poor publicity, etc.
availability.
(costs after delivery) decreases with
The sum of these curves yields a curve which,
being the total costs of the supplier, must relate to price.
The corresponding price relationship for the purchase of
the same item is similar to that shown in Figure 1-II.
This
shows how the established purchase price, the total deter7
mined from Figure 1-I, and the operating and maintenance
costs of the customer combine to yield the total cost of
ownership.
This is shown as a function of desired availability.
3
Again there is a value of availability corresponding to
minimum cost, but in this case it occurs at a higher
availability than for the supplier's minimum cost.
The conclusions reached from these two figures are
significant.
The customer wishes to purchase an item of
predetermined availability for a given price.
The
manufacturer, however, wishes to sell the item of equal
availability but at a higher price.
preceding figures.
This is shown in the
This problem does have a solution.
The final price or cost will be the result of a trade-off
of dollars for availability.
A similar type of trade-off
will be shown later for the design time (dollars) and
improved maintainability (availability).
The more frequent causes of unreliability are
equipment complexity, incorrect rating of components,
intrinsic inability to meet operating conditions, component
unreliability, manufacturing errors and poor maintainability.
Generally speaking, in practice it is found that about
one-half of all system failures are directly attributable
to the design.
In most design efforts the amount of time allotted
for_the design is limited by either a calendar time limit
or dollar limit.
the first step.
Realizing that these limits exists is
The next step is to decide how that
amount of time is to be apportioned.
To do this, one must
first look at the overall picture for the item to be
designed.
Its fabrication, assembly, testing, evaluation,
4
(determines price)
Manufacturer 1 s
Cost
($)
Design
Man.ufacture
etc.
Cost
After
Delivery
Availability(%)
0
Figure 1-I.
100
Price Versus Availability
5
r
Total Cost of
Ownership
Owner 1 s
Cost
($)
Maintenance
0
.
1 -I I .
F 1gure
2
Availability (%)
Cost Versus Aval"1 a b"l"
1 1ty 2
David Smith, Maintainability Engineering, p.lO.
100
6
installation, operation, and its maintainability must be
considered.
Today the maintainability of an item is
costing more than any other aspect over the life of an
item.
For this reason the designer must weigh all of the
aspects of this design to obtain the most efficient design.
To do this, he must make a design time tradeoff.
A research of available literature provides no
information on techniques for assuring a good maintainability design or technique for improving on past similar
designs.
There is a definite need to establish a
quantitative approach in order to successfully improve
on succeeding designs.
The technique established here is
only one way in which the problem may be approached.
As
more historical data is gathered in the field of maintainability a better present day evaluation of current designs
can be made.
Emphasis should be placed on the accuracy of
accumulation of such data.
The initial attempts at
gathering this type of data have not been too successful.
Individual time evaluation are complicated.
The objective of this report is to provide a set of
the best all around attributes of a good maintainability
design and then provide a meaningful technique by which to
arrange and weigh the factors according to their importance
in a particular type of design.
The weighting or ranking
of attributes is derived in order to generate a priority
list.
Once a priority list is established a breakdown of
.
--------
7
the time apportionments can be assigned.
It is a matter
of design time tradeoffs so that the factors more critical
to reducing support costs are given the most time
consideration.
As directed by the Department of Defense, in current
studies of Life Cycle Costing (LCC), the performance costs
and support costs should be made more equal.
is the total cost of
acqui~ition
system over its full life.
Life cycle
and ownership of that
It includes the cost of
development, acquisition, operation support and disposal.
As demonstrated by the military, there is no longer
unlimited funds available to maintain old levels of status.
Today the design to cost philosophy restricts the design to
the money available for the life cycle.
In the past the design effort has mainly been toward
performance.
Now with the emphasis on maintainability, the
design engineer is in need of a design time tradeoff guide
that will assure him the appropriate proportion of time
being dedicated to the respective aspects of maintainability,
preventive and corrective.
This would be a general multi-
purpose type guide.
The maintenance concept must be developed at the
inception of a program prior to the start of actual
equipment design and remain sensitive to the users viewpoint.
The maintenance design actions should be one of
optimization not minimization.
It should be treated from a
practical rather than theoretical viewpoint.
8
Maintainability is the engineering tool with which the
seller is expected to hold the users support costs to
predictable and reasonable limits.
It is also the
analytical tool that integrates maintenance factors with
others to predict the ability of an assumed design to meet
a predetermined operational requirement.
In the end, the
reputation of the Contractor is based largely on the basis
of whether or not the user himself can maintain the new
system.
These concepts of maintainability and reliability are
fairly recent.
The first formal recognition of maintain-
ability was made by the United States Military in 1954,
which was the result of the evolution of a few basic
concepts from the 1890's.
The concepts were:
no equipment
has 100% reliability, all equipment will need maintenance,
approximately one-third of operating costs and personnel
are directed for maintenance, and the best way to design
for good maintainability is by a design team which can
handle the wide scope of factors that need to be considered
in a good design.
The first attempt to standardize was in 1966 with
attempts to standardize the approach, prediction, method
and procedures for design.
It became apparent that more
attention was being directed to the subject of maintenance
by the interest shown in engineering and management of
9
industry and government, by new school .programs on the
subject, by professional societies, and an increased
number of technical papers on the subject.
Maintainability is not a new concept but it does
continue to change in shape, form, and dimension.
not matured as
parameter.
a
It has
design discipline and as a contractual
Chapter II
ATTRIBUTES
In the initial design phases of a program a designer
is often left to choose his own direction.
Those with
more experience tend to choose their direction in a more
logical fashion.
What is often needed, but rarely used,
is a guideline or outline that will break down the task
into meaningful divisions that can be considered separately.
The task considered here is one of providing a good
maintainability design.
having such a list
o~
The main reasons therefore for
guideline is to direct a designer's
thought process so as to have complete coverage of all
aspects of a good maintainability design and to ensure that
no weak points are caused from lack of attention.
Examination of several references shows considerable
variation in a design checklist.
This is largely due to
different vantage points from which the author may be
writing.
What is needed here is a general guideline that
will provide the designer with a complete set of attributes
that he will feel confident will provide all the aspects
needed for a good maintainability design.
·In order to provide the best guideline, it was decided
to compile several of the established sources.
The sources
selected were the Proceedings of the NMSE System Performance
Effectiveness Conference, MIL-STD-470, Maintainability by
10
11
Blanchard and Lowery, and this author's evaluation based on
observations of systems and equipment used by the military
services and civil service.
Table 2-I shows the comparison
of the maintainability attributes considered important by
the various sources.
In compiling these sources a complete comprehensive
list can be generated.
The attributes in the complete list
were selected on the basis of which ones were mentioned in
the literature.
Table 2-II shows the compiling of attri-
butes along with a brief description of each.
The summary
list by no means includes all possible attributes.
It
does provide a broad list of major items, that if considered
diligently
will provide a good maintainability design.
The other attributes not included should not be completely
discarded but merely considered as subparts to the major
attributes.
12
Table 2-I.
Attribute
Attribute Comparison
3
NMSE
confer4
MILence
Proceed- STDings
470
Blanchard 5 Authors
and
EvaluLowery
ation
Accessibility - work
space & clearance
X
X
X
X
Test Points - detection
of malfunction
X
X
X
X
Controls
X
X
X
Labeling & Coding identification
X
Displays
X
Manuals & Checklists maintenance data
X
Test Equipment
X
Tools - accessories &
support equipment
X
X
X
X
X
X
X
X
Connectors - coded
X
X
Cases, Covers, Doors
X
Mounting & Fasteners
X
Handling - mobility &
storability
X
Safety
X
X
X
X
X
X
X
X
X
Interchangeability
X
HIL-STD Parts
X
Compatibility - facility
& equipment
X
Fail Safe Features
X
Tolerenced for Wear
X
Corrosion Control
X
Preparation to Begin
Maintenance
X
Localization of Malfunction
X
Case of Fault Correction
X
X
Adjustment & Calibration
X
X
X
X
X
13
Table 2-I.
Attribute Comparison (Cont.)
Attribute
NMSE 3
confer4
ence
MILProceed- STDings
470
Verification of Correction
X
Logically Sequenced
Maintenance Tasks
X
Tedious Job Elements
X
Reduce Incorrect Assembly
X
Range of Personal Physio
Parameters
X
Requirement for Depot or
Factory Maintenance
X
Blanchard 5 Authors
and
EvaluLowery
ation
Environment
X
Reliability
X
Cables
X
Equipment Racks
X
Packaging
X
Parts/Components
X
3 Robert J. Suslowitz, Proceedings of the NHSE Performance
Effectiveness Conference, p 87
4 Department of Defense, MIL-STD-470, pp 6-8
5 Benjamin S. Blanchard, Maintainability, p 36
-
.
---- ----
-
14
Table 2-II.
Attribute
Attribute Summary
Description
Accessibility
Providing for adequate work space
and clearance
Test Points
Adequate number of points to detect
and isolate malfunction
Controls
Adequate number of controls to
operate as well as isolate and
correct malfunction
Labeling & Coding
Sufficient identification to provide
ease of operation as well as repair
in conjunction with maintenance data
Manuals & Checklists
Providing maintenance and operation
data sufficient for qualified
operator
Tools
Minimize use of non-standard tools
and accessories and provide all
non-standard tools
Connectors
Selection to be made to eliminate
any possible misconnection despite
labeling
Handling
Provide for mobility and storability
in a-variety of situations
Safety
Operator safety must not be
compromized
Adjustments &
Calibration
Provide for ease of making adjustments and extended calibration
periods
Chapter III
EVALUATION
The Attribute Summary established in Chapter II will
now provide the basis for evaluation of existing designs.
This evaluation process will then in turn provide a
ranking of importance of the attributes and also provide a
weighing of those attributes in order to determine a design
time apportionment.
The basis on which importance will be
made is the effect of the attributes on the Life Cycle
Cost (LCC) of the item under consideration.
It is necessary to mention here that the ranking and
weighing established here applies only to the type of
items under consideration and may or may not apply to other
systems or equipment.
However, what is presented here is
a technique that can be applied to other types of systems
or equipment.
The end result will always be a maintain-
ability design time tradeoff.
The ranking and weighing of attributes is established
to give an objective meaning to the apportionment of
design time.
A listing of common attributes in itself would
not provide much assistance in improving the maintainability
of new designs.
Improvement is the motivation behind this
evaluation process.
The ranking and weighting gives the
much needed direction to the designer to improve upon past
experience.
The objective is the accomplishment of a
15
16
satisfactory tradeoff of design time invest3ent for the
maximum gain in maintainability savings.
~£
a result of
the evaluation an established guideline will make the most
efficient use of the allocated design time.
To demonstrate the significance of apportioning or
trading off of maintainability design time, Figure 3-I
shows a possible relation between the maintainability
attributes as their importance varies with time.
That is
to say, the investment of additional design time will gain
the most benefit if put into the attribute having the
most importance.
labeled Controls.
For example, consider the curve
It can be seen that additional design
time is only beneficial up to a point.
After that point
additional time spent or invested would not add up to the
maintainability of the item being designed.
In fact i t is
possible to detract from the maintainability of the end
item.
In order to demonstrate the evaluation process
presented, two similar evaluations have been made.
The
systems used in the evaluation are .basically similar.
Both are used in the control and monitoring of fluids.
They both contain fluid control valves, tra..Tlsmission
lines, safety valves, and indicators.
The type of main-
tenance actions performed on each system would be similar.
Because of these basic similarities these systems were
chosen for evaluation.
Similarity is not required but helps
to illustrate the procedure.
17
MAINTENANCE
DIAGNOSTIC
TIME
CONTROLS
TEST
POINTS
MANUALS &
CHECKLISTS
----ACCESS I BI LITV
MAINTAINABILITY DESIGN TIME
Figure
3~r.
Illustration of Relative
Importance of Maintainability
Attributes6
6 Robert J. Suslowitz, Proceedings of the NMSE
Performance Effective Conference, p. 89.
18
Considering the examples for evaluation, several
assumptions and considerations have been specified.
The
design time listed for each attribute includes time spent
researching and selecting the component parts that were
purchased.
As part of the design time, selection of similar
parts were made to reduce the total number of spare parts
required.
There was no functional redundance designed into
the systems.
The cost of purchased parts for both units
were taken from recent purchase requisitions.
Failures of
the units are assumed to be random failures and not initial
break-in or wear out failures.
One factor to take note bf
is that there is no increase in the maintenance of the
system with age.
This can be seen from the fact that items
that do tend to wear out are either rebuilt or recalibrated
on a preventative maintenance basis and most of the remaining
items are of no consideration such as transmission lines and
safety devices.
The life cycle cost for the 10 year period considered
will be determined by taking the value for the estimated
savings in maintenance hours per year and multiplying by
the maintenance cost per hour.
This value is then adjusted
for the time value of money for a 10 year period to give
the total maintainability savings.
The cost of implementing
this savings is the design cost plus the additional parts
cost.
The difference between the gross savings and the
implementation costs results in the net savings or the
L.C.C. savings.
19
The evaluation of the design cost was made from design
time historical data of Hughes Aircraft Co.
Hardware costs
were based on parts count from design documentation times
current hardware purchase prices.
Computation of the
average design time per component was determined by dividing the total design hours for a particular assembly by the
total number of parts contained in that assembly.
shows the cumulative data.
Table 3-I
For example, Table 3-II shows
for two calibration parts times the average from Table 3-I
of 3.5 hours per component yields a total of 7.0 hours.
Two calibration parts at a cost of five dollars each yields
$10 for parts cost.
Estimates on yearly savings for each maintainability
implementation were based on observed evaluations of maintenance personnel in action.
The observations were made on
similar mechanical equipment that did not have the maintainability attributes discussed.
The individual operations
performed by maintenance personnel were evaluated and
multiplied by the approximate number of times that operation
was to be performed per year.
To illustrate, consider the
attribute of adjustments and calibrations on the Vacuum
Fill Panel (Table 3-II).
The value of sixteen hours was
derived from the following:
from
~ts
two hours per gage to remove
assembly and transport to the centralized calibra-
tion lab, times two gages in the assembly, times the calibration period of four times a year.
16 hours per year.
This product yields
20
Table 3-I
Data
Vacuum Fill Panel
(VFP)
Hydraulic Test Panel
(HTP)
Number of parts
178
236
Design hours
620 hrs.
Design hours/
component
3.5
1240 hrs.
5.3
21
Table 3-II
Vacuum Fill Panel
Attribute
Accessibility
Equipment
Implementation
Design
Costs
. Hrs Parts
(3 ea) Hinged
3.5
door set
Front panel loca- 3.5
tion and hardware
Lower panel access 3.5
holes
Est. Save
in M
hrs/year
r;cc saving
(present value)
$30
9
$2045
25
8
1811
15
3
625
Test Points
N/A
Controls
Quarter turn
hand valves (8)
3.5
96
4
784
Labeling &
Coding
Panel engraving
(35) & ref des
marking (25)
3.5
60
15
3450
3
12
635
2772
Manuals &
Checklists
N/A
Tools
No special tools
required
Connectors
N/A
Handling
Handle set
Caster Set
3.5
3.5
5
20
Relief valve
3.5
35
7.0
7.0
52.5
10
40
60
Safety
Adjustments
& Calibration
2 Calibration
parts
2 Hand valves
15 fittings
16
2253
22
To establish the life cycle cost the average value
used for the cost of design time was determined to be currently $22. per hour.
The average value used for the cost
of maintenance actions was determined to be $15. per hour.
The life cycle cost was determined by taking the difference
between the annual maintenance time savings times the cost
per hour, adjustment for the time value of money, and the
total design cost of time and hardware.
Assuming a stan-
dard industry interest rate of 10%, the factor for the time
value of money based on a uniform series compound amount is
15.94.
For example, Table 3-II shows a savings of $2253.00
for adjustments and calibration.
follows:
This was determined as
(16 hours/year x $15/hour x 15.94) (66.5 hours x $22/hour + $110)
=
$2253.00
Tables 3-II and 3-III show the evaluation of the
Vacuum Fill Panel and Hydraulic Test Panel respectively.
The total savings of each equipment implementation was
computed.
Each attribute subtotal was then divided by the
grand total savings of the assembly to determine its percentage of the overall savings.
piled in Table 3-II.
This data has been com-
This percentage now becomes the
weighting factor for the attributes.
now be ranked as to their importance.
The attributes can
The attributes hav-
ing no weighting or data are those that did not apply to
the particular type of design under consideration here.
should be remembered that the original list arrived at
It
Table 3-III
Hydraulic Test Panel
Equipment
Implementation
Attribute
Accessibility
Hinged door
Front panel
mounting
Design
Costs
Hrs Parts
Est. Save
in M
hrs/year
LCC Saving
(present value)
5.3
5.3
$30
25
3
8
$ 571
1771
Test Points
N/A
Controls
Quarter turn hand
valves {12)
5.3
180
6
1138
Panel engraving
(28) & ref des
marking (37)
5.3
60
15
3410
12
2733
48
6551
Labeling
Coding
&
Manuals &
Checklists
N/A
Tools
No special tools
required
Connectors
N/A
Handling
Caster Set
5.3
20
Safety
Relief Valves (3) 15.9
Safety marking
5.3
105
10
Adjustment &
Calibration
6 Calib parts
6 Hand valves
28 Fittings
31.8
31.8
148.4
30
120
112
24
Table 3-IV
Summary Table
VACUUM FILL PANEL
Attribute
HYDRAULIC TEST PANEL
Rahkihg %
Safety
At·tribute
Ranking %
Safety
Accessibility
31
Adjust & Calib.
41
Labeling & Coding
24
Labeling & Coding
21
Handling
24
Handling
17
Adjust & Calib.
16
Accessibility
14
Controls
5
Controls
Tools
Tools
Test Points
Test Points
Manuals & Checklists
Manuals & Checklists
Connectors
Connectors
7
25
in Chapter 2 is a general list and all factors may not apply
to one particular design.
The attribute of Safety has been
listed first because of its obvious importance, but a
weighing factor cannot really be measured, especially
considering the possibility of loss of human life.
The final step is the establishment of Table 3-V.
This is accomplished by first taking the average of each
attribute in Table 3-IV.
The attributes are then ranked
in decreasing order of their importance established by their
percentage value.
Table 3-V provides the design time apportionment for
the new design.
The designer needs now to allocate to the
respective attributes the percentage indicated of his
available design time for maintainability.
This will also
provide the confidence that the final design will indeed
have an improved efficiency of maintainability based on a
good design time trade-off.
26
Table 3-V
Time Allocation
Attribute
Desi·gn Time Allocation, %
Safety
Adjust & Calib
28
Labeling & Coding
23
Accessibility
22
Handling
21
Controls
6
Tools
Test Points
Manuals & Checklists
Connectors
Chapter IV
Prediction
The prediction of the expected number of hours that a
system or equipment will be inoperative while it is undergoing maintenance is of vital importance to the user
because of the adverse effect that downtime has on mission
success.
Once the operational requirements of a system are
designed, it is important that a technique be utilized to
predict its maintainability in quantitative terms as early
as possible during the design phase.
This prediction should
be updated as the design is finalized to assure compliance
with specified requirements.
A significant advantage of using a maintainability
prediction procedure is that it identifies for the designer,
areas of poor maintainability which justifies product
improvement, modification, or a change of design.
Another
useful feature of maintainability prediction is that it
permits the user to make an early assessment of whether the
predicted downtime, the quality and quantity of personnel,
tools and test equipment are adequate and consistent with
the needs of system operational requirement.
When determining the maintainability of a system it is
important to remember tnat maintainability is a characteristic of design and installation which is expressed as the
probability that an item will conform to specified
27
28
conditions within a given period of time when maintenance is
performed in accordance with prescribed procedures and
resources.
One of the most important aspects of making
predictions is to have recorded maintainability data and
experience which has been obtained from comparable systems
and components under similar conditions of use and
operation.
A transferability is assumed that allows data
accumulated from one system to be used to predict the
maintainability of the comparable system under study.
Usually during the early design phase of the life cycle,
commonality can only be inferred on a broad basis.
However,
as the design-becomes refined commonality is extendable if
a high positive correlation is established relating to
equipment functions, maintenance task times, and levels
of maintenance.
The two parameters necessary for any prediction are
the failure rates of components at the assembly level of
interest and the repair time required at the maintenance
level involved.
There are many sources of failure rates
of parts as a function of use and environment.
Repair
times are determined from prior experience, simulation or
past data from similar applications.
Maintainability
predition is a tool-for design enhancement because i t
provides for the early recognition and elimination of areas
of ineffective design.
Effective prediction leads to
reduction of life cycle costs.
29
The following procedure is presented in the interest of
completeness for a study in maintainability.
The procedure
is presented in a general parametric format.
It is
applicable to all systems and equipment where failure rates
and repair times are known.
The example used in the previous
section will not be presented here.
The example dealt with
a system consisting totally of mechanical components.
As
is the case for a large number of mechanical components, no
historical failure rates have been established.
This procedure is based on the use of historical
experience, subjective evaluation, expert judgement and
selective measurement for predicting the downtime of a
system/equipment.
The procedure uses existing data to the
extent available.
It provides an orderly process by which
the prediction can be made and integrates preventive and
corrective maintenance.
Task times to perform various
maintenance actions are estimated and then combined to
predict overall system/equipment maintainability.
This procedure recognizes that throughout a mission, a
system/equipment performs various operational functions and
that the maintenance time depends upon the specific
operational function which is in process.
As a general
definition applicable to all systems either mechanical or
-
electro-mechanical, an operational function is defined as
that particular function which the system is performing
at the specific interval of time during which the
30
maintainability analysis is being conducted.
In other
words, the procedure requires the development of a mission/
maintenance profile which specifies the various operational
functions of the system and the scheduled preventive maintenance actions required for each operational function.
Another significant assumption is that the estimate of
task times can be made best by a maintenance analyst working closely with the design engineer, or by the design
engineer himself.
Therefore, it is assumed that the main-
tenance task times so estimated are practical, realistic
and applicable for performing a maintainability prediction.
Time analysis can be performed as soon as sufficient
system/equipment definition exists.
This level of defini-
tion for initial estimations is usually available fairly
early in the definition phase.
The application of the
time estimating procedures will permit the updating of the
equipment design and can, therefore, take place throughout
the design and development phases.
The intrinsic maintainability of the system/equipment
is predicted under the assumption of optimum utilization of
specified support equipment and personnel.
The intrinsic
maintainability is given by the following parameters.
Mean Corrective Downtime - MCDT
Mean Preventive Downtime - MPDT
Total Mean Downtime - TMDT
The following information concerning the operational
and maintenance environment of the system and subsystems is
31
required to make the initial maintenance task time analysis.
Subsequent schematics, assembly drawings, etc., will be used
for updating this time estimation as the system design
continues.
a.
System Block Diagram
b.
Functional Flow Diagrams
C.
Subsystem Block Diagrams
d.
Subsystem Flow Diagrams
e.
End Item List
f.
End Item Failure Rates
g.
Maintenance Concept
h.
Maintainability Goais
i.
Operational Resources (facilities,
personnel, support equipment, etc.)
j.
A detailed definition of the task being
performed
k.
Location at which the task is being performed
1.
Environmental Constraints
This procedure utilizes expert judgment and existing
data sources on maintenance task time, but the procedure
does not
rely only on existing data.
The appl.icability of
the data is decided by the analyst and is supplemented by
his expert judgment in estimating maintenance task time when
such information is not available.
The analytical foundation of the task analysis procedure
integrates the development of task performance time for
32
preventive and corrective maintenance actions.
A maintenance
action is defined as the exclusive maintenance task which
occurs at a specific location and within a specific set of
conceptual and physical constraints.
This maintenance
action permits the logical development of elapsed times and
subsystem equipment levels.
The mean corrective maintenance time for the system/
equipment will vary for each individual scheduled preventive
maintenance action applicable to a specific operational
function.
The task analysis procedure permits the evalu-
ation of these times from the end item up to the system
level.
The products of the procedure are:
a.
The elapsed time to perform preventive
maintenance action, assuming that no
detectable malfunctions exist in the system.
b.
The elapsed time to correct malfunctioning
end items detected during each preventive
maintenance action of an operational function.
c.
The distribution of corrective maintenance
times for detectable malfunctioning end items
for each preventive maintenance action of an
operational function.
d.
The mean corrective downtime (MCDT) for
detectable malfunctioning end items for each
preventive maintenance action of an operational
function.
33
e.
The distribution of corrective maintenance
task times for the system and subsystems.
f.
The preventive downtime (PDT) for the system
and subsystems for a specified calendar time.
g.
The total mean corrective downtime (MCDT)
for the system and subsystems for a specified
calendar time.
h.
The total mean downtime for integrated
preventive and corrective maintenance for
the system and subsystems for a specified
calendar time.
These maintenance downtimes relate only to the inherent
maintainability of the equipment, since administrative and
other delays are not normally definable during the design of
the equipment.
The estimated elapsed time required to perform
maintenance on a system will vary as function of the conceptional and physical constraints within which the estimation
was made.
These constraints consist of the availability of
physical resources (i.e., personnel, spares and consumables,
support equipment, and facilities) and applicable maintenance
and operational concepts (i.e., testing concept, level of
repair, mission descriptions, etc.).
The applicability of
specific constraints must be documented if a given time
estimate is to be meaningful.
Only signal elapsed times
are estimated for each maintenance task.
This number should
approximate the mean time required to perform the task under
34
actual conditions.
The correlation between estimated and
verified task times justifies the use of single values
for the purposes of this procedure.
A series of mission/maintenance profiles will be
established based on the system operational requirements.
These profiles shall specify the schedules of operational
functions and preventive maintenance actions for a given
calendar time.
The mean corrective downtime and preven-
tive downtime for the system are calculated in sequence by
function, mission/maintenance profile, and complete system.
A procedural flow block diagram of the procedure is shown
in Figure 4-I.
An explanation of each block in the diagram
follows:
The end items (1) of the system are identified and
categorized under the appropriate headings as:
system,
subsystem, assembly, etc., down to the smallest piece of
equipment on which a specific maintenance action will be
accomplished.
item.
The failure rate is identified for each end
The preventive maintenance actions of an operational
function (2) to be performed on the categorized end items
are defined (e.g., check out, servicing, adjustment, etc.).
The physical and conceptual constraints previously
described must be defined and documented for each function.
The corrective maintenance actions (3) to be performed on
appropriate categorized end items are defined.
These
actions will include, but are not necessarily limited tothe
( 2)
DEFINITION OF:
I
PREVENTIVE
MAINTENANCE
OPERATIONAL
FUNCTION
END(1)ITEM
DEFINITION
~
( 5)
•
I PREVENTIVE
·r
MAINTENANCE TASK
TIME ANALYSIS
~ALFUNCTION
(4)
•
r
(3)
DEFINITION OF:
CORRECTIVE
MAINTENANCE
ACTIONS
PREVENTIVE MAINTENANCE
ACTION TASK TIMES
(END ITEM DISTRIBUTION
AND TOTAL TIME)
( 7)
TOTAL MAINTENANCE
TASK TIME ANALYSIS
ETECTION
ANALYSIS
I
TOTAL PREVENTIVE
• DOWNTIME
TOTAL MEAN
H
~CORRECTIVE DOWNTIME
(6)
CORRECTIVE
MAINTENANCE TASK
TIME ANALYSIS
I
•TOTAL MEAN DOWNTIME
MAINTENANCE TASK
TIMES PER PREVENTIVE AND
OPERATIONAL FUNCTION
(END ITEM DISTRIBUTION AND MCDT)
"---------~CORRECTIVE
Figure 4-I.
Procedural Flow Block Diagram
w
\.Jl
p •
36
maintenance actions of test, remove,
replace, adjust,
repair, etc., specified by the applicable constraints.
The
physical and conceptual constraints previously described
defined a set of qorrective action which can be undertaken.
The detectable end item malfunctions for each preventive
maintenance action of an operational function are defined,
(4).
Those end items which can be detected asmalfunctioned,
but which cannot be corrected (within the constraints of the
location at which the maintenance is occurring), aregrouped.
No troubleshooting will be conducted within these groups of
end items since, by definition, no corrective action can
be undertaken.
A task analysis is conducted for each pre-
ventive maintenance action,
(5).
A distribution of end
item task times and a total time for each operation function is generated.
The total task time for the operational
function is compared to the allocated time to determine if
the maintainability design of the equipment is adequate.
If not, the distribution of end item task times permits
identification of critical design points.
A task
analysis is conducted for corrective maintenance (6)
associated with each of the preventive maintenance of an
operational function.
This analysis is conducted by
deriving the troubleshooting, repair, and verification time
for each end item previously defined as a detectable and
repairable malfunction.
These times are described in terms
of a distribution of end item corrective maintenance times
37
versus frequency of occurrence and by a mean corrective
downtime (MCDT) for the specified operational function.
As
before, the MCDT and distributions are used to identify
critical design points.
The preventive and corrective
maintenance times and associated MCDT's are integrated (7)
over the previously specified calendar time (e.g., 6 months)
to derive the total preventive downtime, total mean corrective downtime, and the total me-an .downtime, where all times
are related to the inherent maintainability characteristics
of the system and exclude administrative and other delays.
A parametric description of the previously described
procedure is provided in the following paragraphs and is
referenced to the numbered blocks in Figure 4-I.
1.
such that
Each end item of the system is described by Ii'
r 1 , r 2 • • In-l' In are inclusive of all end items
within the system.
The failure rate of each end item is
given by i\.i' where i\.i is assumed to be a constant over the
specified calendar time, and where i\.. is the failure rate
1
of end item Ii' etc.
2.
The preventive maintenance actions of an opera-
tional function associated with the system are given by
P. such that P , P • • • • P
, and P are inclusive of
2
1
J
~1
m
all preventive maintenance actions and where each action is
defined by type (e.g., inspection, servicing, etc.) and by
physical and conceptual constraints (e.g., personnel,
spares and consumables, support equipment, facilities,
testing concepts, etc.).
A new function must be defined
38
if the type o·r ·constraints are modified.
A subset of I. is associated with each function P .•
1
J
The operational functions of the system are given by Or
such that o 1 , o 2 • • . • or-l' Or are inclusive of all
operational functions, where each function is defined by
the end items of the system being used.
An operational
function must be defined for each different subset of I,
utilized during the system.
••• I
By definition, all end items
n- 1 , I n will appear in at least one subset
of I., associated with the preventive maintenance action
1
of an operational function.
3.
The corrective maintenance actions associated with
the system are given by c 1 , c 2 • • • cg-l' and cg are
inclusive of all corrective maintenance actions where each
action is a maintenance action taken to correct a detected
malfunction indication (e.g., test, remove, replace, adjust,
repair, verify, etc.) within a specific subset of constraints.
The actions are assigned to each system end
items so that the times involved are exclusive to the individual end items,
(e.g., the removal of an end item, assum-
ing that all access to that item has been accomplished).
4.
The concept on which the analytical procedures are
based prescribes that only those system end items (I.) which
1
can cause identifiable malfunction indications during the
preventive maintenance, Pm' action or operational, Or'
functions will contribute to the mean corrective downtime
for that function.
The probability that corrective
39
maintenance will occur is a function of the
~A.,
l
of the
subset of Ii associated with the specific function.
The
end items r 1 , r 2 . . . In are assumed to have only one
failure mode for purposes of this discussion.
However, in
some cases it may be necessary to specify the various
possible failure modes to provide adequate downtime estimation accuracies.
The determination of the necessary
level of definition required to achieve specific accuracies
of the task time estimation is beyond the scope of this
paper.
5.
The task times for preventive maintenance actions
are given by:
PDTm
=
m
~
i=l
T.
l
m
m = The total preventive maintenance performance
time for action P
m
where: PDT
T.
lm
= The time to perform the maintenance task
on end item Ii as required by action Pm.
A distribution of the individual task times within each
action can be developed to identify critical design points
as previously described.
40
6.
The end items defined in 4 as being detectable
during a specific preventive maintenance action or an
operational function serve as a starting point in the conduct of the corrective maintenance task analysis.
The
fault isolation concept for the system under action,
an Or function, is defined.
Pro
of
The troubleshooting, repair,
and verification time for repairable end items, or the
troubleshooting for non-repairable end item groups are
derived based on the defined fault isolation concept.
for action P :
m
Item 1:
Item 2:
Item n:
Item l.th
T2
Tn
m
=
(L.:T
52
( L.: T
=
m
T.
lm
=
+ T
m
s
( L.: T
)
nm
sl
m
c2
+ Tc
+ T
m
v2
m
+ Tv
nm
nm
+ Tc.
l
m
+ Tv.
l
m
where:
T.
= The total time required to correct
lm
malfunctioning end item I.l during
action P
m
of an operational function
Thus,
41
T
=
s.
1
m
The troubleshooting test times required to
isolate end item I. during action P
m
The time required to remove, replace, adjust,
1
T
=
c.
1
m
or otherwise repair malfunctioning end item I.
1
during action P
T
=
v.
1
m
m
The time required to verify that the system is
good, given that I. is replaced, repaired,
1
adjusted, etc., during action P
m
For function 0 :
r
Item 1:
Item 2:
Item n:
Item ith
Tl
T2
T
=
(Z::T
=
(Z::T
r
r
nr
T.
1r
=
=
(Z::T
(Z::T
sl
s2
sn
s.
1
) + T
+ T
cl
vl
r
r
r
+ T
c2
r
r
+ T
r
v2
r
+ Tc
+ Tv
n
nr
r
+ Tc.
1r
r
+ Tv.
1r
where:
T.
1r
=
The total time required to correct malfunctioning end item I. during function 0
1
=
r
The fault isolation test times required to
isolate end item Ii during function Or
=
The time required to remove, replace, adjust,
calibrate, or otherwise correct the malfunctioning end item I. during function 0
1
=
r
The time required to verify that the system is
good, given that I. is replaced, repaired,
1
adjusted, etc., during function Or.
42
In addition, the time to isolate the non-repairable end
item groups during action P
m
is given by:
where:
T.
Jm
=
The total time required to isolate the jth
group during action Pm of an operational
function.
The troubleshooting time required to isolate
the jth group during action P .
m
The time required to isolate the non-repairable end item
groups during function 0
r
is given by:
where:
T.
Jr
=
The total time required to isolate the jth
group during the function or
The troubleshooting time required to isolate
the group during function 0
r
The mean-corrective downtime of the system or identification of the requirement to shift it to another maintenance
or operational function during action P
~A.i
MCDT
m
=
m
T.l
+
m
~A.i
m
m
is given by:
43
where:
=
MCDT
m
The mean-corrective-downtime for the system
during action Pm of an operational function
=
The failure rate of detectable malfunctioning end item I. during action P
1
m
The failure rate of the ith end item in the
jth non-repairable group which can be
isolated during action Pro.
The mean-corrective-downtime of the system or identification of the requirement to shift to another maintenance or
operational function during function or is given by:
~A.i
MCDT
r
=
r
T.
1r +
~A.i
r
+
~A.i.
Jr
~T
s.
Jr
~A.·1.
Jr
where:
MCDT
r
=
The mean-corrective-downtime for the system
during function 0
=
r
The failure rate of the ith end item in the
jth non-repairable group which can be
isolated during function or
44
7.
A total maintenance time analysis is conducted to
define the total time required to perform preventive maintenance, and the total mean-corrective-downtime, for maintenance of the system.
The total time for preventive
maintenance is given by:
where:
= Total preventive-downtime during the
specified calendar time.
m
=
Frequency of occurrence of the mth
preventive maintenance action during the
specified calendar time
The mean-corrective-downtime for the system is derived from
the mission/maintenance profiles.
The mean-corrective-
downtime for the system is given by the weighted (normalized
failure rates) of the MCDT for each action Pm of an Or
operational function.
Therefore,
l r + A·l
gr
~(A.
=
MCDTs
~(A.
lr
MCDT
+
A.
l
r +
+
gr
~(A.
lm
+ A.l
gm
+ A.
~(A.
l
) MCDT
m
l·
Jm
m
)
where:
MCDT
s
= The mean-corrective-downtime for the system
for the given mission/maintenance profile
45
Applying the equation to a hypothetical mission/
maintenance profile results in:
MCDT
s
=
+
+
+
+
[I:<
L:<
A..
l
L:<
L:<
L:<
+
+
l
l
A..
l
A.·l
A..l
A.i
l
A..l
+
m2
+
+
A..l
rl
+
gr
1
+
ml
+
gm
1
MCDT
m.
l
1
MCDT
gr
r2
2
+ MCDT
gm
A..l
rs
rl
1
A..
+
A_.
l
gm
r2
A_.
l
MCDT
)
gr
A..
ml
A.i
L:[<
A..l
+
A_.
\L[(
+
+
rl
2
gr
MCDTr ]/
3
3
+
A.·l
r2
+
A..
l
m2
gr
2
A.i m )
2
Ai
)] \
gm
2
r3
A..l
>]
gr
3
46
The total mean-corrective-downtime of the system for
the mission/maintenance profile is given by:
=
MCDTt
f
(MCDT .)
s
where:
MCDTt
=
The total mean-corrective-downtime of the
system for the mission/maintenance profile
f
=
The number of detectable failures occurring
during the calendar time
The total mean-downtime of the system with a specified
mission/maintenance profile is given by:
T
p
=
~a
PDTm +
m
MCDTt
where:
T
p
=
The total mean-downtime of the system with
a specified mission/maintenance profile for
the calendar time
=
The frequency of occurrence of the action
P
during the calendar period
m
The use of a mix of mission/maintenance profiles for
the system gives a total mean-downtime of:
ka
=
T
p
ka
p
p
47
where:
Tt
=
The total mean-downtime of the system for a
given mix of mission/maintenance profiles
ap
=
The frequency with which the pth mission
maintenance profile will occur during the
calendar time
The development of the system maintenance times is
initiated by establishing and grouping the physical and
conceptual constraints existing within the maintenance
environment.
The allowable corrective maintenance actions
(e.g., remove/replace, repair, test--troubleshoot--,
adjust, etc.) are specified for each end item for each set
of constraints.
Step 1 - The constraints applicable to each preventive
maintenance action of an operational function, and to the
corrective maintenance action are related through the use
of the matrix shown in Figure 4-II.
CORRECTIVE :MAINTENANCE ACTIONS
cl
pl
rilril
:>Utf.l
HZZ
8.::1:!0
ZZH
rilrilE-l
:>E-lU
rilZ.::t!
O:::H
Pl~
c
g
X
X
X
p2
p
c2
X
X
Figure 4-II.
Related Constraint Matrix
m
X
48
An "X" at a row/column junction of the matrix
indicates that the applicable constraints to the actions
Pm of an 0 r will permit the accomplishment of the corrective maintenance actions (C ) •
g
Step 2 - An end item corrective maintenance action
matrix, as shown in Figure 4-III, is used as an aid in the
conduct of the task time analysis procedure.
The correc-
tive maintenance actions assigned to each end item are
described to match the established physical and conceptual
constraints.
A time value at the matrix junction of an end item row
and an action column indicates that this end item is acted
upon or utilized during that action.
For example, in
Figure 4-III end item (I.) is acted upon during corrective
l
maintenance action (C ).
1
END ITEM
CORRECTIVE MAINTENANCE ACTIONS
I.
A.
cl
Il
Al
Tll
I2
A2
0
l
l
I3
A.3
T3
c2
c3
0
Tl
T2
2
0
1
T2
T3
c
3
g
Tl
I
I
3
I
3
I
n
An
T
Figure 4-·III.
---
T
n
g
End Item/Corrective Maintenance
Action Matrix
g
49
This matrix will serve to establish the corrective
maintenance actions which can be undertaken on the system
end items within the specified physical and conceptual
constraints associated with the preventive maintenance
action of an operation function.
Step 3 - An end item/preventive maintenance action
matrix, as shown in Figure 4-IV, is used to calculate the
individual action performance time (PDT) and the related
mean-corrective-downtime (MCDT) •
50
END ITEM
I.
A..
1
PREVENTIVE MAINTENANCE ACTIONS
p
p1
pl
p2
Prev.
Corr.
Prev. Corr.
p2
1
I1
A.l
T1
I2
A.2
T2
I3
A.3
0
0
m
Prev.
p
m
Corr.
0
1
T2
1
0
1
0
T3
2
(I3)
l
TP.
l(::_l)
I
0
A.n
n
PDT
TR
1
ij
PDT
m
=
~T.
1
TR
1
~
MCDT
1
PDT 2
~
m
TR
~
MCDT
2
PDT
m
~
m
T.
PCDT
0
2
1
=
l'A.·1
m
Figure 4-IV.
+
m
+
End Item/Preventive
Maintenance Action Matrix
n
~
MCDT
m
51
Each preventive maintenance action column is divided
into two parts:
the first for the times required to per-
form the specified action on the affected end items of the
system; and the second part for the times required to
troubleshoot, repair, and verify detectable malfunctioning
end items.
The end item/corrective maintenance action matrix is
used to establish those end items which can be corrected
if malfunctioning within constraints specified for the
individual preventive maintenance actions exist.
The mean-
corrective-downtime for each preventive maintenance action
is calculated by establishing the troubleshooting paths to
each detectable end item malfunction, considering the
specified troubleshooting, logic and constraints, and the
interrelationships of the various detectable malfunctioning
end items.
The time
is placed at the end item (I }/
n
action (P ) matrix junction of Figure 4-IV. This time is
m
the summation of the troubleshooting, repair, and verification times for repairable end items.
The time
troubleshoot non-repairable end item groups is submitted as
required, as shown in Column P
2
Corr. of Figure 4-IV.
The
non-repairable group in this example consists of end items
r 4 through rn-1.
Ste~
4 - An end item/operational function matrix as
shown in Figure 4-V is used to calculate the MCDT
operational function.
r
for each
52
END ITEM
OPERATIONAL FUNCTIONS
I.
l
A·l
01
Il
Al
Tl
I2
A2
T2
I3
A3
0
I
An
0
n
r
0
0
T2
1
T
1
T3
r
0
0
3
T
n3
n2
MCDT
r
r
Tl
0
T
nr
MCDT
2
l
l:Ai
r
r
MCDT
3
+
T.
Figure 4-V.
The MCDT r
0
2
0
2:Ai
=
03
1
MCDT
MCDT
02
+
l:A'l .
Jr
r
I: Ts
l
r
l:A.
l.
Jr
End Item/Operational
Function Matrix
for each operational function is calculated
in a manner identical to that used for the MCDT
m
for
preventive maintenance actions.
Step 5 - The total preventive maintenance time, the
total mean-corrective-downtime, and the total mean downtime
for maintenance are calculated for each mission/maintenance
profile.
Chapter V
CONCLUSIONS
The designer now has available a means by which he is
able to improve the maintainability aspect of a new design.
By taking the general list of attributes and applying to it
the foregoing evaluation based on previous similar designs,
the designer is able to improve the new design.
This is
accomplished by correctly apportioning the design time
allotted.
The trading off of the design time has as its
result a greater savings in the life cycle cost.
With continuing improvement in design an expected
result will be continual cost savings in maintainability.
In this day of spiriting costs, a company's ability to stay
competitive may well lie in the skill of its designers to
supply the most efficient product at the least cost.
The limited data presented here is sufficient to show
the trends and patterns of such an evaluation.
Additional
data would further confirm the conclusion of improved
design through proper design time apportionment for further
studies, additional data on the detailed breakdown of maintainability repair times must be gathered.
This should be
performed for various types of equipment and various types
of repair functions in order to establish a good data base.
Improved efficiency in design must begin with an evaluation
of the past.
53
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