systems maintainability concepts and metrics

Stracener_EMIS 7305/5305_Spr08_03.19.08

Systems Maintainability Concepts and Metrics

Dr. Jerrell T. Stracener, SAE Fellow

Leadership in Engineering

EMIS 7305/5305Systems Reliability, Supportability and Availability Analysis

Systems Engineering ProgramDepartment of Engineering Management, Information and Systems


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Maintainability References

• Systems Engineering and AnalysisBenjamin S. Blanchard and Walter J. FabryckyPrentice Hall, 3rd edition 1998, ISBN 0-13-135047-1

• Maintainability Principles and PracticesBenjamin S. Blanchard and E. Edward LoweryMcGraw-Hill

• MIL-STD-470 Maintainability Programs for Systems and Equipment

• MIL-STD-471 Maintainability Verification / Demonstration / Evaluation

•MIL-HDBK-472 Maintainability Prediction


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Objective of maintainability

Design and develop systems/equipment which can be maintained

in the least time, at the least cost, and with a minimum expenditure of support resources, without adversely affecting the item performance or safety characteristics

•Maintainability greatly influences reliability and availability of a system or subsystem.

•Maintainability must be addressed early in the design stage to

prevent or reduce failure or down times of the system.


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Maintenance and Design

• The system’s design determines its requirements for maintenance

– Reliability (How often maintenance)

– Configuration (How much time for access)

– Built in Test (Fault Isolation Time)

– Subassembly life span (Inspection/forced replacement)

– Adjustment/alignment requirements (Inspection)

– Capacity/fill rate (Servicing)

– Corrosion susceptibility (Inspection/repair)


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Maintainability Definitions

• Maintainability is an inherent design characteristic of a system or product and it pertains to the ease, accuracy, safety, and economy in the performance of maintenance actions.

• Maintainability is the probability that a failed system will be restored to specified performance within a stated period of time when maintained under specified conditions.


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Maintainability Definitions continued

• Maintainability is a characteristic of an item, expressed by the probability that preventive maintenance (serviceability) or repair (repairability) of the item will be performed within a stated time interval by given procedures and resources (number and skill level of the personnel, spare parts, test facilities, etc.).

• Maintainability is the ability of an item to be retained in, or restored to, a specified condition when maintenance is performed by people having specified skill levels, using prescribed procedures and resources.


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• Maintainability analysis: The sequential development and review of data – concurrent with, or preceding design development – to aid in describing the optimum design characteristics of the equipment or system. The elements considered in the review are (1) quantitative requirements, (2) support resources, (3) cost, (4) operational objectives, and (5) safety. The results of the review are translated into criteria which are applied to the design process.

• Maintainability engineering: An organization that is associated with the functions of maintenance engineering, maintainability-design liaison, systems analysis, design services, support documentation, systems planning, safety engineering, and systems integration and test.



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Maintainability Metrics

• Times– MTTR : Mean Time to Repair– T5o% : Median Time to Repair– TMAX : Maximum Time to Repair

(usually 95th percentile– LDT : Logistics Delay Time– SDT : Supply Delay Time– MDT : Mean Down Time– DTM : Down Time for Maintenance– DTS : Down Time For Supply

• Events– MTBM : Mean Time Between Maintenance– MTTPM : Mean Time to Preventive Maintenance– MTBPM : Mean Time Between Preventive Maintenance

• Manpower– CS : Crew Size– MMH/FH : Man-hours per flight hour

• Diagnostics– FD : Fault Detection– FI : Fault Isolation– FA : False Alarms


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System Time Relationships

TotalTime

InactiveTime

ActiveTime

UpTime

Not Operating Time

AlertTime

InactiveTime

InactiveTime

DownTime

Maintenance Time

ModificationTime

DelayTime

Time to Restore Functions During

Mission

Corrective Maintenance

Time

Preventive Maintenance

Time

SupplyDelayTime

Administrative DelayTime


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Maintenance Categories

Maintenance

Preventive Maintenance(retain item functionality)

Corrective Maintenance(reestablish item functionality)

•Test of all relevant functions, •Inspect to detect hidden failures•Service to replace consumables•Activities to compensate for drift andto reduce wear out failures

•Overhaul to increase useful life•Time Change•Prognostics health management: monitor and repair before failure

•Failure detection•Failure isolation•Repair•Functional test


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Maintenance Definitions

• Maintenance:– All actions necessary for retaining an item in, or restoring it to, a

serviceable condition– Includes servicing, repair, modification, overhaul, inspection, and

condition verification

• Maintenance categories:1. Preventive Maintenance

Maintenance performed to retain an item in satisfactory operational condition by providing system inspection, detection and prevention of incipient failures, overhaul, lubrication, calibration, etc

Prescribe procedures to reduce the probability of failure or degradation

2. Corrective MaintenanceInitiated after fault detectionTo Regain state of system for performing required function


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Maintenance ProcessTop-Level Unscheduled Maintenance Process

Second Level Unscheduled Maintenance Process Definition

On-Equipment Maintenance &Checkout Begins

On-Equipment Maintenance &Checkout Ends

Corrective Maintenance Activity

Repair ComponentIn Place

Cannot Duplicate ProblemMaintenance Activity

RemoveComponent

AcquireSpare

ReplaceComponent

Repaired ComponentTo Supply

Component BenchCheck and Repair

Component BenchCheck Serviceable

Component Not Repairable ThisStation (NRTS)

ComponentTo IntermediateLevel Maintenance

(ComponentTo Depot)

(ComponentFrom Depot)

DEPOTREPAIR


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Detection

Preparation forMaintenance

Location andIsolation

Disassembly(Access)

Removal ofFault Item

Re-assembly

Alignmentand Adjustment

ConditionVerification

or

Repair ofEquipment

Installation ofSpare/Repair Part

Failure Occurs

Failure Confirmed

Active Maintenance Commences

Faulty Item Identified

Disassembly Complete

Re-assembly Complete

Repair Completed

Corrective Maintenance Cycle


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Corrective Maintenance Task Definitions

• Maintenance, corrective: That maintenance performed to restore an item to satisfactory condition after a malfunction has caused degradation of the item below the specified performance. The major tasks associated with corrective maintenance are:

Preparation – gathering tools; obtaining, setting up, and calibration maintenance aids; warming up equipment; etc.

Localization – determining the location of a failure to the extent possible without using accessory support equipment.


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• Disassembly – equipment disassembly to the extent necessary to gain access to the item that is to be replaced.

• Interchange – removing the defective item and installing the replacement.

• Reassembly – closing and reassembly of the equipment after the replacement has been made.

• Alignment – performing any alignment, minimum tests, and/or adjustment made necessary by the repair action.

• Verification checkout – performing the minimum checks or tests required to verify that the equipment has been restored to satisfactory performance.

Corrective Maintenance Task Definition Continued


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Fault Detection

Fault Isolation

Open Access

Remove & Replace

Close Access

Verification

Repair

Time based on Probability of Failure

Delay due to Probability of Detection

Task Time based on Steps

Time based on Steps

Corrective Maintenance Detailed Process

Issue Spare

Fault Occurs

Fault Corrected

Logistics Delay due to Resource Unavailability

Supply Delay due to Spare Unavailability


Time based on Steps




Time based on Steps

Time based on Steps

DownTime

LatentTime

Maintenance Time

Delay Time


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FD/FI/FA

• Fault detection (FD)– % of faults detected before noticeable impairment of system

performance occurs– Time from fault occurrence to detection

• Fault Isolation (FI)– % of detected faults isolated to single item– % of detected faults isolated to group of items

• False Alarm (FA)– % of detected faults that can not be confirmed in system

maintenanceAKA CND (can not duplicate)

– % isolated faults that can not be confirmed in repair of item removed from systemAKA RTOK (retest okay)


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Maintenance Analysis

• FMEA : Failure Mode Effects Analysis• RCMA: Reliability Centered Maintenance Analysis• CM : Corrective Maintenance• PM : Preventive Maintenance• LORA : Level of Repair Analysis• MTA : Maintenance Task Analysis• FTA : Fault Tree Analysis

FMEA

RCMA

Identify CM

Identify PMLORA MTA

FTA


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Maintenance Task Analysis (MTA)

• will detail the resources required to implement effective corrective and preventative maintenance tasks for a system and/ or equipment

• a detailed analysis performed for each of the corrective and preventative maintenance tasks (earlier identified in the LSA process)

• Defines support resources that will be required to conduct each of the maintenance tasks.

• Determines task intervals and task elapsed times.

• Level of Repair Analysis (LORA), establishes at which level (or line) of maintenance that a task will be performed.

.

.

.


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• support resources determined and detailed for each corrective and preventative maintenance task – Support Equipment; Standard and special; – Tools; Standard and special; – Personnel; – Facilities; – Storage and packaging; – Transportation consideration;

• provides critical input to support and address the needs other ILS elements, such as training and technical publications development. Technical publications utilize data such as the detailed task description, which also provide input for the development of maintenance training courses and support material.

Maintenance Task Analysis (MTA)


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MTA ExampleLanding Gear Installation

Task: Landing Gear Installation Scheduled Interval: Part Number: Task Type: Installation Nomenclature: Landing Gear, Complete LRC Type: LRU

Special Tools? No If yes, list below Item Function: The LG provides the shock absorbing capability during landing and structural support the aircraft during ground operations. Support Equipment? Yes If yes, list below

LCN Support Items ID with ICC Task Class: Scheduled Maintenance Concept: Organizational

No. DETAIL MAINTENANCE TASK ANALYSIS Elapsed Time in Minutes

Crew Size

Man Minutes

Remarks

X TOTAL TASK 72 1.5 109 X.1 Attach LG to Trolley 3 2 6 X.2 Install Inboard Pin 10 1 10 X.3 Install Outboard Pin 10 1 10 X.4 Install drag brace assembly at aircraft interface 5 2 10 X.5 Disconnect trolley 3 2 6 X.6 Install retract actuator and bleed if required 20 1 20 X.7 Connect hydraulic dressings 8 2 16 X.8 Connect electrical harness 8 2 16 X.9 Connect hydraulic power unit 0 0 0

X.10 Perform functional check (retract and extend) 5 3 15 X.11 Shim drag brace if necessary as part of functional check X.12 Install ground lock pins X.13 Remove aircraft from jacks

Task complete Tool Requirements: Torque wrench, common wrenches, sockets

Skill Levels: Class 3

Repair Parts and Consumables: Hydraulic fluid, nitrogen, lubricant

Support Equipment: Ground lock pins, Aircraft jacks, Trolley, Nitrogen Servicing Cart, Hydraulic Servicing Cart, grease gun, drip pan/fluid receptacle

Special Handling Requirements: Ensure LG is secure in the aircraft prior to disconnecting from trolley. Failure to do so may injure personnel and/or equipment.

Safety Considerations: Hydraulic fluid and lubricants may cause skin irritation. Avoid contact with skin

and clothing. Wash thoroughly after handling. Use caution when handling landing gear structural components, connecting

ground support equipment trolleys aircraft jacks, etc. Failure to do so may result in bodily injury.

Ensure all personnel on the ground are clear from the aircraft during landing gear extension and retraction. Failure to do so may result in bodily injury.

Test Equipment: None

Facilities Requirements: None

PHS&T Requirements: None

Hazardous Materials List: Hydraulic Fluid

Training Requirements: Train maintenance personnel on proper inspection, servicing and handling procedures.


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Maintainability Metrics

• Times– MTTR : Mean Time to Repair– T5o% : Median Time to Repair– TMAX : Maximum Time to Repair

(usually 95th percentile– LDT : Logistics Delay Time– SDT : Supply Delay Time– MDT : Mean Down Time– DTM : Down Time for Maintenance– DTS : Down Time For Supply

• Events– MTBM : Mean Time Between Maintenance– MTTPM : Mean Time to Preventive Maintenance– MTBPM : Mean Time Between Preventive Maintenance

• Manpower– CS : Crew Size– MMH/FH : Man-hours per flight hour

• Diagnostics– FD : Fault Detection– FI : Fault Isolation– FA : False Alarms


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• Mean Time Between Maintenance (MTBM): included both preventive and corrective maintenance requirements. Relative to corrective maintenance, MTBM considers primary (random) failures, secondary (dependent) failures, quality and manufacturing defects, operator- and maintenance-induced failures, and others.

• Mean Time Between Replacements (MTBR): Mean time between equipment item replacements for preventive- and corrective-maintenance purposes. This factor forms the basis for spare/repair-parts determination. It should be noted that a maintenance action (represented by the MTBM factor) does not always result in an item replacement.

Maintainability Metric Definitions


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• Maintenance downtime: That portion of downtime which can be attributed to preventive- and corrective-maintenance functions. Maintenance downtime may be expressed in a measure of central tendency (arithmetic mean, geometric mean, median, and mode). It may also be expressed in terms of a maximum value relative to a percentile point of distribution of downtime. Symbols of maintenance downtime are:

• Mct – Mean active corrective-maintenance time (arithmetic mean). Equal to MTTR.

• Mpt – Mean active preventive-maintenance time (arithmetic mean).



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• M – mean maintenance time (function of Mct and Mpt).

• MTTR – Mean time to repair. Equal to Mct.

• MDT – Maintenance downtime (total time during which an equipment item is not in condition to perform its intended function). MDT includes logistics time and waiting or administrative time.

• Mct – Mean active corrective maintenance time. Equal to ERT



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• ERT – Equipment active repair time. Equal to Mct.

• Mpt – Median active preventive-maintenance time.

• MTTRg – Geometric mean time to repair.

• Mmax – maximum active corrective maintenance time (95% confidence level).

• Logistics or Supply Time – That portion of nonactive maintenance time during which maintenance is delayed solely because a needed item is not immediately available.

• Wait or Administrative Time – That portion of nonactive maintenance time that is not included in logistics or supply time.



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• Maintenance indices: Primary expressions of quantitative measurements of equipment-maintenance characteristics.

MMH/OH – Maintenance man-hours per equipment or item operating hour.

MMM/OH – Maintenance man-minutes per equipment or item operating hour.

Cost/OH – Maintenance cost per equipment or item operating hour.

Others are MTBM, MDT, and achieved availability.

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