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Logistics Measures and Considerations

IEGR 459: Intro to Logistics Management and Supply Chain

Sept. 19, 2011Fall 2011

• Maintainability• System Effectiveness• Supply Chain Factors• Transportation, Packaging and Handling Factors

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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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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 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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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 Categories

• Mct – Mean Corrective-maintenance time• Arithmetic average maintenance corrective cycle times • Equivalent to MTTR.

• Probability functions for repair time• Normal distribution• Exponential distribution• Log-normal Distribution

Repair-time distributions.

Corrective maintenance cycle.

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Maintainability Measure

Corrective Maintenace Time (minutes)

40 58 43 45 63 83 75 66 93 92

71 52 55 64 37 62 72 97 76 75

75 64 48 39 69 71 46 59 68 64

67 41 54 30 53 48 83 33 50 63

86 74 51 72 87 37 57 59 65 63

Figures of Histogram and Frequency Polygon of maintenance actions

Example:

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Maintainability Measure

Example:

Normal distribution of repair times.

Example: Convert maintenance times of 40 and 50 mins into standard (Z)

Lognormal distribution of repair times.

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Mpt = ∑(fpti)(Mpti) ∑fpti

Mean Preventive Maintenance time (Mpt)

Mpt – Mean or average elapsed time for performing preventativeor scheduled maintenance of an item.

fpti = Frequency of the individual (ith) preventativemaintenance actions per system hour

Mpti = elapsed time required for the ith preventative maintenace action

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Mpt = ∑(fpti)(Mpti) ∑fpti

Mean Preventive Maintenance time (Mpt)

Mpt – Mean or average elapsed time for performing preventativeor scheduled maintenance of an item.

fpti = Frequency of the individual (ith) preventativemaintenance actions per system hour

Mpti = elapsed time required for the ith preventative maintenace action

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The median maintenance time is that value that divides all the downtime values so that 50% are equal to or less than the median and 50% are equal to or greater than the median. The median will usually give the best average location of the data sample. The median for a normal distribution is the same as the mean; the median in a log-normal distribution is the same as the geometric mean (MTTRg)

Median active corrective maintenance time

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The median preventative maintenance time is that value that divides all the downtime values so that 50% are equal to or less than the median and 50% are equal to or greater than the median.

Median active preventive maintenance time

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Mean active maintenance time (M). M is the mean or average elapsed time required to perform scheduled (preventive) and unscheduled (corrective) maintenance. It excludes logistics delay time and administrative delay time.

Mean active maintenance time (M).

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Mmax can be defined as that value of maintenance downtime below which a specified percentage of all maintenance actions can be expected to be completed. Mmax is related primarily to the log-normal distribution, and the 90th or 95th percentile point is generally taken as the specified value

Maximum active corrective maintenance time (Mmax)

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Logistics delay time LDT - -Logistics delay time is the maintenance downtime that is expended as a result of waiting for a spare part to become available, waiting for necessary test equipment to perform maintenance. waiting for transportation, waiting to use a facility required for maintenance, and so on. LDT does not include active maintenance time but does constitute a major element of total maintenance downtime (MDT).

Administrative delay time (ADT). Administrative delay time refers to that portion of downtime during which maintenance is delayed for reasons of an administrative nature: personnel assignment priority, labor strike, organizational constraint, and so on. ADT does not include active maintenance time but often constitutes a significant element of total maintenance downtime (MDT).

Maintenance downtime (MDT). Maintenance downtime constitutes the total elapsed time required (when the system is not operational) to repair and restore a system to full operating status and/or to retain a system in that condition. MDT includes mean active maintenance time (M) , logistics delay time (LDT), and administrative delay time (ADT). The mean or average value is calculated from the elapsed times for each function

More Definitions

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Other Maintenance Measures

Maintenance Labor hours (MLH)• MLH per System Operating hours (OH)• MLH per cycle of system operation• MLH per month• MLH per maintenance action

Maintenance Frequency• Mean time between maintenance (MTBM)• Mean time between replacement (MTBR)

Maintenance Cost factors• Cost per maintenance action ($/MA)• $/OH• $/month• $/mission• Ratio of maintenance cost to total life-cycle cost

Composite view of uptime/downtime factors.

The relationship between maintenance downtime and logistic and factors.

The elements of effectiveness.

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Availability

Availability – The probability or the percentage that a system will be ready or available when required for use and be able to complete its overall mission in a satisfactory manner

A =A = Up-TimeUp-TimeUp-Time + Down-TimeUp-Time + Down-Time

• inherent Availability – probability that a system or equipment, when used under stated conditions in an ideal support environment (i.e., readily available tools, spare: maintenance personnel, etc.), will operate satisfactorily at any point in time as required. It excludes preventive or scheduled maintenance actions, logistics delay time and administrative delay time

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• Achieved availability - the probability that a system or equipment, when used under stated conditions in an ideal support environment (i.e., readily available tools, spares, personnel, etc.), will operate satisfactorily at any point in time. Achieved availability includes preventative maintenance and excludes logistics delay time and administrative delay time

• Operational availability - the probability that a system or equipment, when used under stated conditions in an actual operational environment,

will operate satisfactorily when called upon.

Systems Measures of Effectiveness

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System Effectiveness

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System effectiveness - Function of performance (P), Availability (A) and dependability (D)

SE = (P) (A) (D)

Overall equipment effectiveness - Function of performance rate (P), Availability (A)and quality rate (Q)

SE = (P) (A) (Q)• Availability

= (loading time – downtime)/loading time

• Performance rate = {(output)(Actual time)/(loading time – downtime)} x (idea cycle time)/(actual cycle time)

• Quality rate (Q) = {input – (Quality defects + Startup defects + rework)}/input

Cost Effectiveness

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Cost Effectiveness

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• With respect to availabilityCost effectiveness = Availability/ Life-cycle cost

• With respect to PerformanceCost effectiveness = Performance/ Life-cycle cost

• With respect to Logistics effectivenessCost effectiveness = Logistics effectiveness/ Life-cycle cost

• Cost effectiveness with respect to overall equipment effectiveness

• Cost effectiveness with respect to facility space