1 Logistics Systems Engineering Availability NTU SY-521-N SMU SYS 7340 Dr. Jerrell T. Stracener, SAE Fellow.

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Logistics Systems EngineeringAvailability

NTUSY-521-N

SMUSYS 7340

Dr. Jerrell T. Stracener, SAE Fellow

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Performance Measures (DoD Sys Eff)

• Availability:– A measure of the degree to which an item is

in an operable state at the start of a mission when the mission is called for at a random time. Expressed as inherent, achieved, or operational.

• Readiness:– The probability that military forces, units,

weapons systems, equipment and personnel will be capable of undertaking the mission

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Performance Measures (DoD Sys Eff)

– and function for which they are designed or organized, at any random point in time.

• Sustainability:– The capability of military forces, units,

equipment and personnel to maintain a specified level of mission activities for specified times.

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Commercial System Measures

• Aircraft– Top ten delay/cancellation systems

(delays>15 min and cancellations)Delay rates (rate/100 revenue departures)Total interruptions (delays > 15 min &

cancels)Daily utilization ratesdispatch reliability (%)Maintenance required per cycle

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Commercial System Measures

• Vehicles– Average miles between failure

• Ships– Average steaming days between main turbine

failures

• Power Plants– Unit availability (average operating hours per

year)

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MIL-STD-721 Definitions

• Availability– A measure of the degree to which an item is

in an operable state at the start of a mission when the mission is called for at an unknown (random) time.

• Reliability– The probability that an item can perform its

intended function for a specified interval under stated conditions.

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MIL-STD-721 Definitions

• Mean Time Between Failures (MTBF)– A basic measure of reliability for repairable

items. The mean number of life units during which all part of the item perform within their specified limits, during a particular measurement interval under stated conditions.

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Availability, MTBF, Reliability

Failed StateOperational

State

1MTBF

1MTTR

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Availability Definition

• Easy to understand• Difficult to compute• Uptime and downtime are difficult to

define• Steady state value

DowntimeUptime

UptimeA

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Types of Availability

• Inherent Availability (Ai):– The designed in availability. Ai calculations

ignore all logistics contributions to downtime.

MTTRMTBF

MTBFA i

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Types of Availability

• Operational Availability (Ao):– The expected in-service availability. Ao

calculations include the impact of logistics on availability. Exactly which logistics elements are included must be defined in advance.

MLDTMTTRMTBF

MTBFAo

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Major Factor Influencing Availability

• System Reliability Design Characteristics– MTBF: a reliability function which assumes

that operation occurs after early failure (infant mortality) and prior to war-out, i.e., a constant failure rate exists.

– Mean Time Between Maintenance Action: a reliability function which accounts for all causes of maintenance activity, whether a failure occurred or not.

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Major Factor Influencing Availability

• System Maintainability Design Characteristics– MTTR: a maintenance function, includes

corrective maintenance time (CMT) and preventive maintenance time (PMT)

• Support System Design Characteristics– Mean Logistics Down Time: a maintenance

related logistics function which involves spares provisioning and logistics delay time (LDT) administrative delay time (ADT)

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System Availability Calculation

• Produces reasonable results if and only if MTBF >>MTTR

• Causes availability to be understated by ignoring the case where multiple subsystem failures overlap in time

n_subsys2_subsys1_subsyssys AAAA

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Effect of Availability Model Assumptions

• Identical redundant elements– Simplifies calculations– Most common method of applying

redundancy

• Hot standby– Can be optimistic, ignores startup time,

software load time, etc– Often justified, commonly used method of

applying redundancy

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Effect of Availability Model Assumptions

• Independence of elements– Produce idealized, optimistic results– Sometimes not justified, particularly with

software base systems

• Perfect switching– Produce optimistic results– Difficult to justify for most systems

• Unrestricted repair– Minor impact if n is small and MTBF>>MTTR– Allows simpler calculation formulas to be used

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Effect of Availability Model Assumptions

• MTBF>>MTTR– Allows the use of simple formulas, not custom

model solutions– Usually the case for modern electronic

systems

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Choosing an Availability Analysis Method

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Advantages of Simulation

• Permits controlled experimentation with:– consideration of many factors– manipulation of many individual units– ability to consider alternative polices– little or no disturbance of the actual system

• Effective training tool• Provides operational insight• May dispel operational myths

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Advantages of Simulation

• May make middle management more effective

• May be the only way to solve problem

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Disadvantages of Simulation

• Costly (very costly?)• Uses scarce and expensive resources• Requires fast, high capacity computers

(use of PC’s?)• Takes a long time to develop• May hide critical assumptions• May require expensive field studies• Very much dependent on availability of

data and is validity

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When simulation models make sense

• When mathematical models do not exist, or analytical methods of solving them have not yet been developed

• When analytical methods are available, but mathematical solution methods are too complex to use

• When analytical solutions exist and are possible, but are beyond the mathematical capabilities of available personnel

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When simulation models make sense

• When it is desired to observed a simulated history of the process over a period of time in addition to estimating relevant parameters

• When it may be the only possibility because of difficulty in conducting experiments and observing phenomena in their actual environment

• When time compression may be required for systems over long time frames

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Availability Model Development Approach

• Define model elements and specifications– Operational Activity Element Specifications– System State Conditions and Attribute Specs.– Operational Activity Demand Generation– System Component Level of Detail

Determination– Support System Resource Definition and

Specifications

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Availability Model Development Approach

• Define Model Structure (Processing, Inputs and Outputs)– Model Processing Definitions– System Failure Processing– System Unscheduled Maintenance Processing– Model Inputs– Model Outputs

• Implement Model Structure on the Comp.– Model Activities

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Availability Model Development Approach

• Implement Model Structure on the Comp. (continue)– Model Output Measure Calculations

Implementation

• Perform Full Model Test & Evaluation Using Sample Data– Validate the model’s ability to produce

expected results

• Install Model at User Site and Perform Checkout, Train Users

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Major Availability Analysis Process Elements

• Reliability Analysis Techniques (typical)– Failure Modes and Effects Analysis– Failure Modes and Effects Criticality Analysis– Reliability Block Diagrams– Failure Rate Estimation

• Maintainability Analysis Techniques (typical)– Maintenance Task Time Analysis– Engineering scale models

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Major Availability Analysis Process Elements

• Supportability Elements (typical)– Logistics Support Analysis– Spares Provisioning Levels

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Power Plant Exp. Avail. Relationship

• Availability

• Plant Availability

where Ai = availability of major equipment or system or A = 1 - Ui

where Ui = unavailability = 1 - Ai

DowntimeUptime

UptimeA

n_subsys2_subsys1_subsyssys AAAA

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Power Plant Exp. Avail. Relationship

• Typical Fossil Unit Availability Experience (adjusted availability %)– Boiler 88.5%– Turbine 94.2%– Condenser 98.5%– Generator 95.6%– Other 97.6%

• Average fossil fueled unit availability %4.74)024.044.015.058.115(.1A

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Availability Analysis: A value-added process

• Availability analysis provides the “glue” which ties system RMS performance evaluation together:– Considers operational environments and

stresses– Identifies dominant failure modes– Incorporates repair and replace times

estimates– Evaluates overall support system

performance

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Availability Analysis: A value-added process

• It provides a rational structure for evaluating system design and development decisions based on system level performance measures.