1. Design for Maintainability

2. Maintainability Definition
Maintainability, as a characteristic of design, can be defined on the basis of a combination of the following factors:
Maintenance times
Maintenance frequency
Maintenance cost
The above three factors are dependant on the fact that the system is operated and maintained in accordance with prescribed procedures and resources.
Maintainability

3. Maintainability Measures of Maintainability From a systems perspective
Corrective maintenance
Preventive maintenance
From a software perspective
Adaptive maintenance
Perfective maintenance
Maintainability

4. Maintenance Elapsed -Time Factors
Corrective and preventive active maintenance times
Administrative and logistic delay times
Total maintenance downtime
Maintainability

5. Mean corrective maintenance time(Mct)
Mean corrective time is a composite value representing the arithmetic average of the individual maintenance cycle times(Mcti)
Mean time to repair(MTTR)
Basic measure of maintainability
Sum of corrective maintenance times divided by the total number of failures within an item
Average time it takes to fully repair a failed system
Includes fault isolation,replacements of failed item(s) and check out
Its also called the mean corrective maintenance time,Mct
Maintainability

6. Corrective Maintenance Cycle
Detection
Failure
Occurs
Failure Confirmed
(Notification)
Preparation for
Maintenance
Reassembly
completed
Alignment
And
Adjustment
Active Maintenance
Commences
Localization
And
Isolation
Condition
Verification
(Checkout)
Repair
Completed
Faulty Item
Identified
Disassembly
(Access) OR
Disassembly
Completed
Removal of
Faulty Item
Installation of
Spare/Repair
Part
Repair of
Equipment
Corrective Maintenance Cycle
Maintainability

7. Histogram of maintenance times
Frequency of Occurrence 14 12 12 10 10 9 8 7 6 5 4 4 3 2
29.5
39.5
49.5
59.5
69.5
79.5
89.5
99.5
Time(min)
Histogram of maintenance times
Maintainability

8. Arithmetic Mean
Maintainability

9. Mean Preventive Maintenance Time ( )
Preventive Maintenance refers to the actions required to retain a system at a specified level of performance
Includes active maintenance time
Does not include logistic delay and administrative delay time
Maintainability

10. Logistics Delay Time (LDT)
Refers to maintenance downtime, which is expended as a result of waiting for spare part to become available
Waiting for availability of an item of test equipment in order to perform maintenance, waiting for transportation and so on
LDT does not include active maintenance time
It does constitute a major element of total maintenance downtime (MDT)
Maintainability

11. ADT & MDT Administrative Delay Time (ADT) Maintenance Down Time (MDT)
That portion of downtime when maintenance time is delayed for administrative reasons.
Examples:
Personnel Assignment Issues
Labor Strike
Maintenance Down Time (MDT)
Total elapsed time required to repair or restore a system to full operational status or to retain a system in that condition
MDT includes:
Mean active maintenance time
Logistics delay time (LDT)
Administrative delay time (ADT)
Maintainability

12. Uptime/Downtime Factors
Maintainability

13. Maintenance Labor Hours
Elapsed time vs. labor hours
Elapsed time reduction via more personnel Can be expensive
Maintainability is primarily concerned with ease and economy of maintainability
Maintainability

14. Maintenance Labor Hours
Important factors in the labor-time element
Maintenance labor-hours per system operating hour (MLH/OH)
Maintenance labor-hours per cycle of system operation (MLH/cycle)
Maintenance labor-hours per month (MLH/month)
Maintenance labor-hours per maintenance action (MLH/MA)
Note: Any of these factors can be specified in terms of mean values.
Maintainability

15. Maintenance Frequency Factors
The reliability factors MTBF and λ are the basis for determining the frequency of corrective maintenance.
Maintainability deals with the design characteristics necessary to minimize corrective and preventive maintenance
In this area reliability and maintainability factors should be compatible and mutually supportive.
Maintainability

16. Some Definitions MTBF = Mean time between failure
λ= Rate of breakdown = 1/MTBF
Maintainability

17. Maintenance Frequency Factors
(Continued)
MTBM:Mean time between maintenance is the mean average time between all maintenance actions 1
MTBM=
where: 1 1 +
MTBMs
MTBMμ
MTBMμ: Unscheduled (corrective maintenance)
MTBMs: Scheduled (preventive maintenance)
MTBMμ should be equivalent to the MTBF if all failures are considered
MTBM is crucial to determine the achieved and operational availability.
Maintainability

18. Maintenance Frequency Factors
(Continued)
Example :
Consider a communication equipment with an MTBF (where all failures are considered) of 20 years and has a schedule of maintenance every 6 months, what is the MTBM ?
If all failures were measured to determine the MTBF is safe to assume that MTBF=MTBMμ
With a MTBMs= 0.5 years 1 1
MTBM= = =
0.55 years 1 1 1 1 + +
MTBMs
MTBMμ
0.5 years
20 years
Maintainability

19. Maintenance Frequency Factors
(Continued)
MTBR:Mean time between replacement a factor of MTBM refers to all item replacement.
In corrective and schedule maintenance a replacement of a spare part is not always necessary.
A low MTBR will mean a higher frequency of replacement which in turn will required higher levels of maintenance support.
A maintainability objective in system design is to
maximize the MTBR
Maintainability

20. Maintenance Cost Factors
Important factor in the total life-cycle cost
Make design decisions in the early stages of System Development
Life-cycle must be considered when defining the system requirements
Maintainability

21. Maintenance Cost Factors
Cost-related indices appropriate in system design
Cost per maintenance action ($/month)
Maintenance cost per system operating hour ($/OH)
Maintenance cost per month ($/month)
Maintenance cost per mission or mission segment ($/mission)
Ratio of maintenance cost to total life-cycle cost
Maintainability

22. Related Maintenance Factors
Various logistics factors based from the System
Supply Responsiveness
Test and Support Effectiveness
Maintenance facility availability and use
Transportation modes
Maintenance organizational effectiveness
Maintainability

23. Related Maintenance Factors
Cost-effectiveness
Total system cost
(Live cycle)
System effectiveness
Availability
Dependability
Capability
Uptime
(Operating/standby)
Downtime
(Non-use)
Active maintenance time
Logistic delay time
Administrative delay time
Logistic support resource consumption
Maintainability

24. Maintainability Requirements in the System Life-Cycle
Conceptual Design
========================
Feasibility study
Operational requirements
Maintenance Concept
Functional requirements
Technical performance measures
Advance system planning
Maintenance concept,
quantitative and qualitative
maintainability requirements
for system (MTBM, MTBR, Mct,
Mpt, MLH/OH, cost/MA, etc.),
maintainability planning.
Preliminary System Design
System functional analysis
Preliminary synthesis and allocation of design criteria
System optimization
System synthesis and definition
Allocation of maintainability requirements, maintainability analysis and trade-offs, maintenance engineering analysis, design support, maintainability predictions, formal design review and approval.
Detail Design & Development
System / product detail design
System prototype development
System prototype test and evaluation
System modification
Maintainability analysis and trade-offs, maintenance engineering analysis, design support, maintainability predictions, maintainability demonstration, formal design review and approval.
Production / Construction
==============================
Fabrication, assembly, and test of system and its components
System construction
Maintainability test and evaluation; maintainability data collection, analysis, and corrective action
System Utilization &
Life-Cycle Support
================================
Consumer use of system and its components
Life-cycle system support
Maintainability data collection, analysis, and evaluation; system modification (as required)
System
Retirement
and
Phaseout
Feedback Loop
Maintainability

25. Maintainability in the System Life Cycle
Maintainability is considered early in the process
During the conceptual design phase.
Requirements developed from feasibility study
Qualitative Requirements
Quantitative Requirements
Maintainability testing is included as part of the overall system test and evaluation activity
After the physical models are developed and evaluation process is established.
Maintainability

26. Maintainability in the System Life Cycle
System Requirements
Definition of system performance factors, the mission profile, and system use requirements.
Definition of the operational life cycle.
Definition of the basic system maintenance.
Definition of the environment in which system is expected to operate and be maintained.
Maintainability

27. Component Selection and Application
Select standardized components and materials.
Select items with built-in self-test features and diagnostics.
Select items that do not require highly specialized skills.
Maintainability

28. Component Selection and Application…
Ensure appropriate accessibility to subsystems for maintenance.
Incorporate modularized functional-package approach.
Avoid selection of short-life components.
Incorporate proper identification of components.
Maintainability

29. Maintainability Analysis Method
Maintainability Analysis: Iterative process of system synthesis, analysis and evaluation
Tools/Methods Used for Maintainability analysis
1, Reliability-Maintainability Trade-off Evaluation
2, Reliability-Centered Maintenance
3, Repair versus Discard Analysis
4, Maintenance Prediction
5, Maintenance Task Analysis (MTA)
Maintainability

30. Maintainability Analysis Method
Reliability- maintainability trade off Evaluation:
Reliability = The probability that a system will perform in a satisfactory manner for a given period when used under specified operating conditions.
E.g.; Equipment A MTBF = 100 hrs
Equipment B , MTBF = 125 hrs
>> Reliability of equipment B is higher.
Maintainability = It is the ability of a product to be maintained
E.g.; Equipment A Mct = 2 hrs, Equipment B Mct = 3 hrs
>> maintainability of the equipment A is higher
Maintainability

31. Maintainability Analysis Method
Example:
Requirement: To replace an existing equipment item with a new item for the purpose of improving operational effectiveness.
Alternatives: 3 different design configurations are available
Evaluation: Reliability-Maintainability-Cost>>decision
Result: Configuration A satisfies reliability maintainability with least cost.
Maintainability

32. Maintainability Analysis Method
Reliability- Centered Maintenance(RCM)
Cost-effective preventive maintenance program
Best initiated during the early process
RCM analysis leads to specification for preventive maintenance or recommendation for redesign
Repair Versus Discard Analysis:
It is economically feasible to repair certain items and/or to discard certain when failure occur.
Maintainability

33. Maintainability Prediction
Maintainability Predictions are done periodically at different stages in the design process.
Predictions of the MTBM, Mct, Mpt, MLH/OH, and so on, are made and compared against the requirements identified earlier in the Maintainability Allocation Process.
Areas of noncompliance are evaluated for possible design improvement.
Maintainability

34. Prediction of Mct
The Prediction covers subsystems, units, assemblies, subassemblies and parts.
Maintainability Characteristics such as localization, isolation, accessibility, repair, and checkout are evaluated.
Times applicable to each part are combined to provide factors for the next-higher level (subassemblies, assemblies, etc.).
Maintainability

35. Design Review and Evaluation
The characteristics of the system and its elements are evaluated in terms of the initially specified maintainability requirements for the system.
A check list may be developed to facilitate the review process. Ex:
Have servicing and lubrication requirements been held to a minimum (if not eliminated)?
Have adjustment, alignment, and calibration requirements been minimized (if not eliminated)?
Have all system maintainability requirements been met?
Maintainability

36. Maintainability Demonstration
It is conducted as part of the system test and evaluation effort
It is usually accomplished during the latter part of detail design.
It should be conducted in an environment that simulates, as closely as practical, the operational and maintenance environment planned for the item.
Maintainability

37. Checklist for review cont
Standardization; Minimize spares?
Modular packaging achieved?
Are modules interchangeable?
Diagnostic testing provisions?
Quick disassembly possible?
Accessibility to parts requiring replacement?
Is alignment, adjustment and calibration in field minimized?
Are the parts labeled?
Servicing and lubrication minimized?
Handling of heavy items designed?
Maintainability

38. Design Review & Evaluation
Done at various stages of design and development
Create a formal checklist for the reviews
Maintainability requirements adequately defined and specified?
Compatible with other requirements ? Realistic ?
Compatible with maintenance concept?
Maintainability

39. Maintainability Demonstration
It is usually accomplished during the latter part of detail design.
It should be conducted in an environment that simulates, as closely as practical, the operational and maintenance environment planned for the item
Maintainability