IEGR 459: Intro to Logistics Management and
Supply Chain
Logistics Measures and Considerations
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Maintainability
System Effectiveness
Supply Chain Factors
Transportation, Packaging and Handling Factors
Sept. 19, 2011
Fall 2011
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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
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Times
– MTTR
– T5o%
– TMAX
– LDT
– SDT
– MDT
– DTM
– DTS
Events
– MTBM
– MTTPM
– MTBPM
Manpower
– CS
– MMH/FH
Diagnostics
– FD
– FI
– FA
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Mean Time to Repair
Median Time to Repair
Maximum Time to Repair
(usually 95th percentile
Logistics Delay Time
Supply Delay Time
Mean Down Time
Down Time for Maintenance
Down Time For Supply
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Mean Time Between Maintenance
Mean Time to Preventive Maintenance
Mean Time Between Preventive Maintenance
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Crew Size
Man-hours per flight hour
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Fault Detection
Fault Isolation
False Alarms
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Maintenance Categories
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 Categories
• Mct – Mean Corrective-maintenance time
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Arithmetic average maintenance corrective cycle times
Equivalent to MTTR.
Probability functions for repair time
• Normal distribution
• Exponential distribution
• Log-normal Distribution
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Repair-time distributions.
Corrective maintenance cycle.
Maintainability Measure
Example:
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
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Maintainability Measure
Example:
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Normal distribution of repair times.
Example: Convert maintenance times of 40 and 50 mins into standard (Z)
Lognormal distribution of repair times.
Mean Preventive Maintenance time (Mpt)
Mpt – Mean or average elapsed time for performing preventative
or scheduled maintenance of an item.
Mpt = ∑(fpti)(Mpti)
∑fpti
fpti = Frequency of the individual (ith) preventative
maintenance actions per system hour
Mpti = elapsed time required for the ith preventative
maintenace action
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Mean Preventive Maintenance time (Mpt)
Mpt – Mean or average elapsed time for performing preventative
or scheduled maintenance of an item.
Mpt = ∑(fpti)(Mpti)
∑fpti
fpti = Frequency of the individual (ith) preventative
maintenance actions per system hour
Mpti = elapsed time required for the ith preventative
maintenace action
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Median active corrective maintenance time
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)
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Median active preventive maintenance time
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.
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Mean active maintenance time (M).
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.
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Maximum active corrective maintenance time (Mmax)
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
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More Definitions
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
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Other Maintenance Measures
Maintenance Labor hours (MLH)
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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
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Cost per maintenance action ($/MA)
$/OH
$/month
$/mission
Ratio of maintenance cost to total life-cycle cost
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Composite view of uptime/downtime factors.
The relationship between maintenance downtime and logistic and factors.
The elements of effectiveness.
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
Up-Time
A=
Up-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.
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Systems Measures of Effectiveness
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System Effectiveness
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)
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Availability
= (loading time – downtime)/loading time
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Performance rate
= {(output)(Actual time)/(loading time – downtime)} x (idea cycle time)/(actual cycle
time)
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Quality rate (Q)
= {input – (Quality defects + Startup defects + rework)}/input
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Cost Effectiveness
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Cost Effectiveness
• With respect to availability
Cost effectiveness = Availability/ Life-cycle cost
• With respect to Performance
Cost effectiveness = Performance/ Life-cycle cost
• With respect to Logistics effectiveness
Cost effectiveness = Logistics effectiveness/ Life-cycle cost
• Cost effectiveness with respect to overall equipment effectiveness
• Cost effectiveness with respect to facility space
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