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Systems Engineering Program. Department of Engineering Management, Information and Systems. EMIS 7305/5305 Systems Reliability, Supportability and Availability Analysis. Systems Maintainability Concepts and Metrics. Dr. Jerrell T. Stracener, SAE Fellow. Leadership in Engineering.

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Systems maintainability concepts and metrics

Systems Engineering Program

Department of Engineering Management, Information and Systems

EMIS 7305/5305

Systems Reliability, Supportability and Availability Analysis

Systems Maintainability Concepts and Metrics

Dr. Jerrell T. Stracener, SAE Fellow

Leadership in Engineering


Systems maintainability concepts and metrics

  • 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


Objective of maintainability

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.


Maintenance and design

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)


Maintainability definitions

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.

5


Maintainability definitions continued

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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Systems maintainability concepts and metrics

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.

Maintainability Definitions continued

7


Maintainability metrics

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

System Time Relationships

TotalTime

InactiveTime

ActiveTime

UpTime

DownTime

Corrective MaintenanceTime

Maintenance Time

Not Operating Time

Preventive Maintenance Time

ModificationTime

AlertTime

SupplyDelayTime

DelayTime

InactiveTime

Administrative DelayTime

Time to Restore Functions During Mission

InactiveTime


Maintenance categories

Maintenance Categories

Maintenance

Preventive Maintenance(retain item functionality)

Corrective Maintenance(reestablish item functionality)

  • Failure detection

  • Failure isolation

  • Repair

  • Functional test

  • 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

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Maintenance definitions

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 Maintenance

Initiated after fault detection

To Regain state of system for performing required function

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Maintenance process

Maintenance Process

Top-Level Unscheduled Maintenance Process

Second Level Unscheduled Maintenance Process Definition

Corrective Maintenance Activity

On-Equipment Maintenance &Checkout Begins

On-Equipment Maintenance &Checkout Ends

Cannot Duplicate ProblemMaintenance Activity

Repair ComponentIn Place

Repaired ComponentTo Supply

RemoveComponent

AcquireSpare

ReplaceComponent

Component BenchCheck Serviceable

ComponentTo IntermediateLevel Maintenance

Component BenchCheck and Repair

Component Not Repairable ThisStation (NRTS)

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(ComponentTo Depot)

DEPOTREPAIR

(ComponentFrom Depot)


Systems maintainability concepts and metrics

Corrective Maintenance Cycle

Failure Occurs

Detection

Failure Confirmed

Preparation for

Maintenance

Active Maintenance Commences

Location and

Isolation

Faulty Item Identified

Disassembly

(Access)

Disassembly Complete

or

Repair of

Equipment

Removal of

Fault Item

Installation of

Spare/Repair Part

Re-assembly

Re-assembly Complete

Alignment

and Adjustment

Condition

Verification

13

Repair Completed


Corrective maintenance task definitions

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.

14


Systems maintainability concepts and metrics

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

15


Corrective maintenance detailed process

Corrective Maintenance Detailed Process

Fault Occurs

Time based on Probability of Failure

Latent

Time

Delay due to Probability of Detection

Fault Detection

Logistics Delay due to Resource Unavailability

Fault Isolation

Task Time based on Steps

Issue Spare

Supply Delay due to Spare Unavailability

Logistics Delay due to Resource Unavailability

Down

Time

Open Access

Time based on Steps

Logistics Delay due to Resource Unavailability

Repair

Time based on Steps

Remove & Replace

Logistics Delay due to Resource Unavailability

Close Access

Time based on Steps

Logistics Delay due to Resource Unavailability

Verification

Time based on Steps

Maintenance Time

Fault Corrected

Delay Time

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Fd fi fa

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 maintenance

AKA CND (can not duplicate)

% isolated faults that can not be confirmed in repair of item removed from system

AKA RTOK (retest okay)

17


Maintenance analysis

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

FTA

Identify CM

FMEA

LORA

MTA

Identify PM

RCMA

18


Maintenance task analysis mta

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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Systems maintainability concepts and metrics

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 example

MTA Example

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Maintainability metrics1

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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Maintainability metric definitions

Maintainability Metric Definitions

  • 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 definitions continued1

Maintainability Definitions continued

  • 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).


Maintainability definitions continued2

Maintainability Definitions continued

  • 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


Maintainability definitions continued3

Maintainability Definitions continued

  • 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.


Maintainability definitions continued4

Maintainability Definitions continued

  • 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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