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

•
Systems Engineering and Analysis
Benjamin S. Blanchard and Walter J. Fabrycky
Prentice Hall, 3rd edition 1998, ISBN 0-13-135047-1
• Maintainability Principles and Practices
Benjamin S. Blanchard and E. Edward Lowery
McGraw-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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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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• 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 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 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, maintainabilitydesign liaison, systems analysis, design services, support documentation, systems planning, safety engineering, and systems integration and test.
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•
Times
– MTTR
– T
5o%
– T
MAX
– LDT
– SDT
– MDT
– DTM
– DTS
:
:
:
:
:
•
Events
– MTBM
:
– MTTPM
:
– MTBPM
:
•
Manpower
– CS
:
– MMH/FH
:
•
Diagnostics
– FD
– FI
– FA
:
:
:
:
:
:
Mean Time to Repair
Median Time to Repair
Maximum Time to Repair
(usually 95 th percentile
Logistics Delay Time
Supply Delay Time
Mean Down Time
Down Time for Maintenance
Down Time For Supply
Mean Time Between Maintenance
Mean Time to Preventive Maintenance
Mean Time Between Preventive Maintenance
Crew Size
Man-hours per flight hour
Fault Detection
Fault Isolation
False Alarms
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Total
Time
Inactive
Time
Active
Time
Up
Time
Not Operating
Time
Alert
Time
Inactive
Time
Inactive
Time
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Time to Restore
Functions During
Mission
Down
Time
Maintenance
Time
Modification
Time
Delay
Time
Corrective
Maintenance
Time
Preventive
Maintenance
Time
Supply
Delay
Time
Administrative
Delay
Time
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Maintenance
Preventive Maintenance
(retain item functionality)
•Test of all relevant functions,
•Inspect to detect hidden failures
•Service to replace consumables
•Activities to compensate for drift and to reduce wear out failures
•Overhaul to increase useful life
•Time Change
•Prognostics health management: monitor and repair before failure
Corrective Maintenance
(reestablish item functionality)
•Failure detection
•Failure isolation
•Repair
•Functional test
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• 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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Top-Level Unscheduled
Maintenance Process
On-Equipment
Maintenance &
Checkout Begins
Second Level Unscheduled
Maintenance Process Definition
Corrective Maintenance Activity
Cannot Duplicate Problem
Maintenance Activity
Repair Component
In Place
Remove
Component
Acquire
Spare
Replace
Component
On-Equipment
Maintenance &
Checkout Ends
Repaired
Component
To Supply
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Component
To Intermediate
Level Maintenance
Component Bench
Check Serviceable
Component Bench
Check and Repair
Component Not Repairable This
Station (NRTS)
(Component
To Depot)
DEPOT
REPAIR
(Component
From Depot)
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Corrective Maintenance Cycle
Detection
Preparation for
Maintenance
Location and
Isolation
Disassembly
(Access) or
Removal of
Fault Item
Repair of
Equipment
Installation of
Spare/Repair Part
Re-assembly
Alignment and Adjustment
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Failure Occurs
Failure Confirmed
Active Maintenance Commences
Faulty Item Identified
Disassembly Complete
Condition
Verification
Re-assembly Complete
Repair Completed 13

• 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.
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Latent
Time
Corrective Maintenance Detailed Process
Fault Occurs
Time based on Probability of Failure
Delay due to Probability of Detection
Fault Detection
Down
Time
Fault Isolation
Issue Spare
Open Access
Repair
Remove & Replace
Logistics Delay due to Resource Unavailability
Task Time based on Steps
Supply Delay due to Spare Unavailability
Logistics Delay due to Resource Unavailability
Time based on Steps
Logistics Delay due to Resource Unavailability
Time based on Steps
Close Access
Verification
Logistics Delay due to Resource Unavailability
Time based on Steps
Logistics Delay due to Resource Unavailability
Time based on Steps
Maintenance Time
Delay Time
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Fault Corrected
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• 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)
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• FMEA
:
• RCMA :
• CM :
• PM
• LORA
• MTA
:
:
:
• FTA :
Failure Mode Effects Analysis
Reliability Centered Maintenance Analysis
Corrective Maintenance
Preventive Maintenance
Level of Repair Analysis
Maintenance Task Analysis
Fault Tree Analysis
FTA
FMEA
RCMA
Identify CM
Identify PM
LORA MTA
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.
.
.
• 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.
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Landing Gear Installation
Task:
Part Number:
Landing Gear Installation
Nomenclature: Landing Gear, Complete
Item Function: The LG provides the shock absorbing capability during landing and structural support the aircraft during ground operations.
LCN
Task Class: Scheduled
No. DETAIL MAINTENANCE TASK ANALYSIS
X TOTAL TASK
X.1 Attach LG to Trolley
X.2 Install Inboard Pin
X.3 Install Outboard Pin
X.4 Install drag brace assembly at aircraft interface
X.5 Disconnect trolley
X.6 Install retract actuator and bleed if required
X.7 Connect hydraulic dressings
X.8 Connect electrical harness
X.9 Connect hydraulic power unit
X.10 Perform functional check (retract and extend)
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
Repair Parts and Consumables: Hydraulic fluid, nitrogen, lubricant
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.
Test Equipment: None
PHS&T Requirements: None
Training Requirements: Train maintenance personnel on proper inspection, servicing and handling procedures.
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Scheduled Interval:
Task Type:
LRC Type:
Special Tools?
Installation
LRU
No
Support Equipment?
Support Items ID with ICC
Yes
Maintenance Concept: Organizational
Elapsed
Time in
Minutes
8
0
5
Skill Levels: Class 3
72
3
10
10
5
3
20
8
Crew
Size
1.5
2
1
1
2
2
1
2
2
0
3
Man
Minutes
16
0
15
109
6
10
10
10
6
20
16
If yes, list below
If yes, list below
Remarks
Support Equipment: Ground lock pins, Aircraft jacks, Trolley, Nitrogen Servicing Cart,
Hydraulic Servicing Cart, grease gun, drip pan/fluid receptacle
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.
Facilities Requirements: None
Hazardous Materials List: Hydraulic Fluid
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•
Times
– MTTR
– T
5o%
– T
MAX
– LDT
– SDT
– MDT
– DTM
– DTS
:
:
:
:
:
•
Events
– MTBM
:
– MTTPM
:
– MTBPM
:
•
Manpower
– CS
:
– MMH/FH
:
•
Diagnostics
– FD
– FI
– FA
:
:
:
:
:
:
Mean Time to Repair
Median Time to Repair
Maximum Time to Repair
(usually 95 th percentile
Logistics Delay Time
Supply Delay Time
Mean Down Time
Down Time for Maintenance
Down Time For Supply
Mean Time Between Maintenance
Mean Time to Preventive Maintenance
Mean Time Between Preventive Maintenance
Crew Size
Man-hours per flight hour
Fault Detection
Fault Isolation
False Alarms
Stracener_EMIS 7305/5305_Spr08_03.19.08
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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.
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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:
• M ct
– Mean active corrective-maintenance time
(arithmetic mean). Equal to MTTR.
• M pt
– Mean active preventive-maintenance time
(arithmetic mean).
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• M – mean maintenance time (function of M ct and M pt
).
• MTTR – Mean time to repair. Equal to M ct
.
• 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.
• M ct
– Mean active corrective maintenance time. Equal to ERT
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• ERT – Equipment active repair time. Equal to M ct
• M max
– Median active preventive-maintenance time.
confidence level).
.
• M pt
• MTTR g
– Geometric mean time to repair.
– maximum active corrective maintenance time (95%
• 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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