Visually

Inspect

lawn mower

flow - First level

5.0

Prepare for lawn mowing

SJart lawn mower ~* ^ rr^ '^•^•^^V^H

— fr

Move to mowing area

Mow lawji

Move to cleaning area

Clean lawn mower

Store lawn

' t

s ; L

V

L2

•

Operational flow - Second leve]

1.9

1.7

Operate

controls

1.6

Ref

Prepare for lawn

mowing

Turn on ignition

Position

lawn mawer

- Ref

Check gasoline level

And

V

Start

1.10

Lawn mower

Insert ignition key

Check oil level

Verify voltage

Maintenance flow - First level

1,5

Check air filter

f

2.0 - Ref -

Start

15.00

lawn

mower

G	Visually
	inspect
	mower for
	damage

16.0

Check

fuel

subsystem

T7.0

Check blade

far freedom

on motion

19.0

Restart

lawn

mower

3.0 r Ref -n

Move to

area

S.O

Maintenance flow - Second level

Check

electrical subsystem

G

30.0

31,0

18.1 '

Charge battery

18.2

Remove ancf replace plugs

IS.3

Check ignition switch

Transport fawn mower to repair facility

Repair

lawn mower

CO

Figure 5-5. Simplified lawn mowing capability (sample).

Sec. 5.2 Allocation of Requirements

135

decomposed and defined in functional terms. Operational and maintenance functions are evaluated with the objective of identifying specific design criteria. This process is illus­trated in Figure 5-7.

The functional analysis serves as a baseline for the definition of equipment re­quirements, software requirements, personnel requirements, maintenance and logistic support requirements, human factors requirements, and so on. In the past, these require­ments have been derived largely on an independent basis. The results have often been conflicting as different baselines have been followed. In other words, electrical design has evolved from one baseline, reliability requirements from another, maintainability requirements from another, logistic support analysis (LSA) requirements from a different source, and so on. This, in turn, has led to some costly results.

The purpose here is to develop one set of requirements and define one baseline from which all lower-level requirements evolve. Figure 5-8 specifies some of the areas that require a functional analysis as an input. It is an objective to ensure that all logistics-

rt

related requirements evolve from the same functional definition.

5-2. ALLOCATION OF REQUIREMENTS

The preceding discussion pertains to the first step in the process of translating system requirements into specific design criteria for various elements of the system. The func­tional analysis provides a description of major system functions and results in a prelim­inary synthesis of the overall system configuration.³ The next step involves the allocation of top-level system factors to various subsystems and lower-level elements of the sys­tem,⁴ In essence, a system can be broken down into different categories of components, as illustrated in Figure 5-9, each of which must support the overall performance and effectiveness requirements at the system level. The question is: What should be included in a "design-to" specification covering a given element of the system? Hopefully, the response is meaningful in terms of meeting some overall mission-oriented objective.⁵

Referring to System XYZ in Figure 5-3, the objective in this instance is to allocate system-level requirements to Units A, B, and C The allocation should include reliability factors, maintainability factors, personnel factors, economic factors, and the other fac­tors discussed in Chapter 2, as they are applicable to the design for supportability.

² Refer to B. S. Elanchard, and W* J. Fabrycky, Systems Engineering and Analysis, 2nd ed., Prentice-Hall, Inc., Englewood Cliffs, N.J., 1990, for additional coverage of functional analysis.

^Synthesis refers to the combining and structuring of pans and elements of an item in a manner that forms a functional entity. This may be accomplished as an iterative process in design,

⁴Allocation refers to the distribution, allotment, or apportionment of top-level requirements to lower indenture levels of the system.

⁵ If an item is to be procured from an outside supplier, design criteria are established to the level needed to effect the necessary design controls on the supplier and are included in the specification. On the other hand, the level to which criteria are established may be different if the item is to be developed within the major producer's organization.

Sec. 5.2 Allocation of Requirements

137

T

OPERATIONAL FUNCTIONAL FLOW DIAGRAM

MAINTENANCE FUNCTIONAL FLOW DIAGRAM

I

RELIABILITY MODELS AND BLOCK DIAGRAMS

FAILURE MODE, EFFECT, AND CRITICALITY ANALYSIS (FMECA)

FAULT TREE ANALYSIS (FTA)

SYSTEM SAFETY AND HAZARD ANALYSIS

RELIABILITY-CENTERED MAINTENANCE (RCM)

MAINTAINABILITY ANALYSIS

MAINTENANCE TASK ANALYSIS (MTA)

LOGIC TROUBLESHOOTING FLOWS

OPERATIONAL SEQUENCE DIAGRAMS (OSDs)

OPERATOR TASK ANALYSIS (OTA)

LOGISTIC SUPPORT ANALYSIS (LSA)

OTHER SYSTEM DESIGN APPLICATIONS

Figure 5-8. Functional requirements flow.

i

				System
1		i			i i
Software		Facility		Equipment		Personnel Data
		I			i
		Unit A		UnitB		UnitC
				i
					i i
				Assembly I		Assembly 2 Assembly 3

Figure 5-9. Hierarchy of systems components.

Tap. 5

Sec. 5.2 Allocation of Requirements

13S

r

•e has

stems, a reli-il flow t>le ap-Don of b block

L

ionic in ical are

lanning

Ј mode

•j

pansion f, levels 1 on are

--idual re-

the reli-0.95 for

Weeks 1, h in turn

ntified in

rates (X) ctor for a :tion, one in Figure : assembly pated fre-ogistic re-

le maturity experience rilar in na-weighting rsses.

he confines of

ve cov-

Level I

Level II

Level III

Level IV

Level V

Figure 5-10. Reliability block diagram approach (N levels). (NAVAIR 00-65-502/ NAVORD OD 41146, Reliability Engineering Handbook, U.S. Navy, Washington, D.C., revised March 1968.)

When accomplishing reliability allocation, the following steps are considered ap­propriate:

1. Evaluate the system functional flow diagram(s) and identify areas where design is known and failure-rate information is available or can be readily assessed. Assign the appropriate factors and determine their contribution to the top-level system reliability requirement. The difference constitutes the portion of the reliability re­quirement which can be allocated to the other areas.

Sec. 5.2 Allocation of Requirements

141

System XYZ

MTBF = 450

A = 0.002222

^^0.9978

Unit ,4

Unit

Unite

Complexity Factor

= 11 %

MTBF = 4050 X = 0-000246

Complexity Factor

= 84%

MTBF-536 A = 0.001866

Complexity Factor

= 5%

MTBF = 9050 X = 0-000110

Assembly 1

Assembly 2

Assembly 3

Complexity Factor

= 6% A-0,000116

Complexity Factor

= 83% A = 0.001 550

Complexity Factor

- 11% A = 0.000200

r

L

1

Input

					C	*to_
						*
a		b
					d

Parallel-Series Combination, Reliability Block Diagram

_r J

Figure 5-11. System XYZ functional breakdown

altered or traded off as long as the combined unit-level requirements support the system objective. In other words, the failure rate of Unit B may be higher and the failure rate of Unit A may be lower than indicated without affecting the requirement of 0.002222, and so on! The techniques of trading off different parameters to meet an overall require­ment are discussed further in Chapter 6.

The reliability factors established for the various items identified in Figure 5-11

Sec. 5.2 Allocation of Requirements

143

1

» 5

or

Met

450(1 - 0.9989) 0.9989

= 0.5

Thus, the system's Met requirement is 0.5 hour, and this requirement must be allocated to Units A, B, C, and the assemblies within each unit. The allocation process is facili­tated through the use of a format similar to that illustrated in Table 5-1.

T

c d

Table 5-1. system xvz allocation

n fa

Sr

B

I-

L

I,

i-

cr

to

lo

I	2	3	4	5	6	7
			Contribution	Percent	Estimated	Contribution of
	Quantity of		of Tola!	Contribution	Corrective	Total Corrective
	Items per	Failure Rate	Failures	cr = cyiEC,	Maint. Time	Maint. Time
Item	System (Q)	(\) X 7000 hr	Cf = (C)(X)	x /OO	Met, f/ir)	C, = (Cf)(Mct)
1. Unit /I	\	0.246	0.246	11%	0.9	0.221
2. UnitB	1	1.866	1.866	84%	0.4	0.746
3. UnitC	I	0.110	O.I 10	5%	1.0	0.110
Total			E Cf = 2.222	100%		Ј C, = 1 .077
— EC 1.077
Met Fc		j- "irct-m YV^ — fl If		55 hour (requirement: 0.5 hour)
		>r k>vsiem ai^ — — -~ u.h-? ^i^ ^^ ^^ T ^Tl

Referring to Table 5-1, each item type and the quantity (Q) of items per system are indicated. Allocated reliability factors are specified in column 3, and the degree to which the failure rate of each unit contributes to the overall failure rate (represented by C}) is entered in column 4. The average corrective maintenance time for each unit is estimated and entered in column 6.⁹ These times are ultimately based on the inherent characteristics of equipment design, which are not known at this point in the system life cycle. Thus, corrective maintenance times are initially derived using a complexity factor which is indicated by the failure rate. As a goal, the item that contributes the highest percentage to the anticipated total failures (Unit B in this instance) should require a low Met, and those with low contributions may require a highe^Mct. On certain occasions, however, the design costs associated with obtaining a low Met for a complex item may lead to a modified approach which is feasible as long as the end result (Met at the system level) falls within the quantitative requirement/⁰

3)

⁹It should be noted that the Met, values are not allocated using the weighting factors as was the case for reliability, cost, and MMH/OH. Rather, the Met/ values are estimated and then a calculation is made to see if the system Met meets the specification requirement.

'°Note that, in any event, the maintainability parameters are dependent upon the reliability parameters. Also, it will frequently occur that reliability allocations are incompatible with maintainability allocations (or vice versa). Hence, a close feedback relationship between these activities is mandatory.

Sec. 5.2 Allocation of Requirements

145

Following the completion of quantitative allocations for each indenture level of equipment, all values are included in the functional breakdown illustrated in Figure 5-12. The illustration provides an overview of major system design requirements.

C. Allocation of Logistics Factors

In addition to reliability and maintainability parameters and their impact on design, one must also consider other factors that are critical to successful system operation. These factors, some of which were introduced in Sections 2.3 through 2.7, deal with supply support, test and support equipment, personnel and maintenance organization, facilities, and transportation.

As mentioned earlier, all elements of the system must be addressed to include the various activities depicted in Figure 2-37. Thus, it may be necessary to establish some additional design criteria covering the various elements of logistic support. A few ex­amples are noted below.

1. Test equipment utilization in the intermediate maintenance shop shall be at least 80%, and test equipment reliability shall be at least 90%.

System XYZ

At MTBF

= 0.9989 = 450 = 0.5 MMH/OH =0.2 Skill Level = GS-5

Cost = $500,000

Unit ,4

Unitfi

UnitC

MTBF = 4050

M_ct - 0.9

MMH/OH =0.02 Skill Level - GS-7

Cost = $170,000

= 536

MTBF

MMH/OH =0.17 Skill Level = GS-7

Cost = $250,000

MTBF = 9050 M_cl = 1.0

MMH/OH =0.01 Skill Level = GS-7

Cost = $80,000

Assembly 1

Assembly 2

Assembly 3

M

= 0.000116 = 0.5

ct

MMH/OH =0.015

Skill Level = GS-9

X =0.001550

Met = 0-4

MMH/OH =0.13

Skill Level = GS-9

M

_cl

= 0.000200 = 0.3

MMH/OH = 0.025 Skill Level = GS-9

Figure 5-12. System XYZ functional requirements.

i- 5 Sec. 5.3 Design Criteria 147

t

be The criteria thus established have a direct impact on system/equipment design, which,

in turn, affects logistic support.

In regard to the development of design criteria, a few examples are provided for illustrative purposes:

1. Through evaluating the combination of requirements covering reliability and cost, it is often possible to determine whether an item should be designed for repair at

is failure or for discard at failure. If the reliability of an item is high enough (e.g., one

anticipated failure in 50,000 hours of system operation) and the unit cost is low enough (e.g., $1,000), it may not be economically feasible to establish a repair capability with the associated logistic support to enable repair of that item when failure occurs. Thus, the item is discarded-at-failure and there is no need to incorporate provisions for acces­sibility, test points, modular packaging, and so on, within that item. This, of course, has a significant impact on spare parts, test equipment, personnel training, and mainte­nance data requirements. Although many decisions of this type will be based on ftulher analysis (refer to the discussion on trade-offs in Chapter 6), it is often possible to estab­lish general criteria which specify that items with a reliability exceeding a given value and a unit cost less than a specific amount will be designed for discard-at-failure. Ex­perience data from similar systems already in the operational inventory are used in es­tablishing such criteria.

2. Referring to Unit B of System XYZ in Figure 5-12, the allocated Met of 0.4 hour means that in the event of malfunction, the maintenance technician must be able to complete the corrective maintenance cycle (refer to Figure 2-10) in 24 minutes. Based on experience data for like equipment, about 60% of the total corrective maintenance time (on the average) involves malfunction localization and isolation. Assuming that this percentage is valid for System XYZ,, an estimate of allowable localization and isolation time for Unit B would be 14 minutes. Complying with the 14-minute goal would neces­ sitate the availability of a few readily accessible test points or readout devices to allow positive fault isolation to any one of the three assemblies of Unit B. The exact quantity and placement of test points depends on the degree of functional packaging, and a pre- liminaiy indication of such can be determined through an analysis of maintenance func­ tional-flow diagrams. The type of test and support equipment required will be based on the results.

Of the remaining 10 minutes in the maintenance cycle, the technician must accom­plish the disassembly, repair, and checkout functions (or removal and replacement func­tions) . This infers that each of the three assemblies of Unit B must be directly accessible and must not require the removal of another assembly to gain access. In addition, each assembly should be modular with plug-in and/or quick-release features, and should be interchangeable with like spares so as to minimize alignment and adjustment require­ments after item installation. These and other similar considerations are necessary to meet the Met objective.

3. The allocated skill level and maintenance maahoor requirements (refer to Fig­ure 5-12) indicate the personnel resources that wffl be available for the accomplishment of future system maintenance. The type md vnayh jiiij of tasks which an individual

Questions and Problems

149

10* Select a system of your choice and assign top-level requirements. Accomplish a reliability allocation to the second indenture levcL Accomplish a maintainability allocation to the same level. Allocate supportability factors as appropriate. Refer to Problem 2 in Chapter 3 and Problem 7 in Chapter 4.

11. What is meant by ^iLdesign criteria¹'? How are criteria developed? How are criteria applied to the design process?

12. From the allocations in Problem 10, develop design criteria for the system.

13. Referring to Figure 5-12, System XYZ has the following requirements: MTBF = 650, Met = 0,6, MMH/OH = 0.7, and unit cost = $100,000, Allocate these requirements to Units A, 6, C, and to Assemblies 1,2, and 3 of Unit B.

14. In Figure 5-13, allocate the quantitative factors to the unit level as indicated.

System ABC

MTBF =100hr = 1 hr = 2hr MMH/OH - 0.4 Cost =$100,000

Unit A

I'r.it B

UnitC"

-0.00125

MMH OH = Cost

X = 0.00275

Me: = ?

Mpi = 0

MMH OH = ?

Cost = ?

X Me,

MMH/OH Cost

0.00315

?

7

9

•

9

UnitO

UnitЈ

X Me,

= 0.00115

~ *?

A

-0.00170

MMH/OH = ? Cost = ?

MMH/OH = Cost = ?

Figure 5-13,

15. Refer to Section D of Appendix D, Complete Problem Exercise 1 using the Reliability Pre diction Program (RPP) Model.