Transcript Criticality Analysis

Criticality – Mil-Std-1629 Approach
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CRITICALITY is a measure of the frequency of
occurrence of an effect.
– May be based on qualitative judgement or
– May be based on failure rate data (most common)
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Slide 1
Criticality Analysis
Criticality Analysis
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Qualitative analysis:
– Used when specific part or item failure rates are not
available.
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Quantitative analysis:
– Used when sufficient failure rate data is available to
calculate criticality numbers.
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Slide 2
Criticality Analysis
Qualitative Approach
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Because failure rate data is not available, failure mode
ratios and failure mode probability are not used.
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The probability of occurrence of each failure is grouped
into discrete levels that establish the qualitative failure
probability level for each entry based on the judgment of
the analyst.
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The failure mode probability levels of occurrence are:
– Level A - Frequent
– Level B - Reasonably Probable
– Level C - Occasional
– Level D - Remote
– Level E - Extremely Unlikely
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Slide 3
Criticality Analysis
Quantitative Approach
Failure Mode Criticality (CM) is the
portion of the criticality number for an
item, due to one of its failure modes,
which results in a particular severity
classification (e.g. results in an end
effect with severity I, II, etc...).
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Slide 4
Criticality Analysis
Mil-Std-1629 Severity Levels
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Category I - Catastrophic: A failure which may cause death
or weapon system loss (i.e., aircraft, tank, missile, ship, etc...)
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Category II - Critical: A failure which may cause severe injury,
major property damage, or major system damage which will
result in mission loss.
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Category III - Marginal: A failure which may cause minor
injury, minor property damage, or minor system damage which
will result in delay or loss of availability or mission degradation.
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Category IV - Minor: A failure not serious enough to cause
injury, property damage or system damage, but which will
result in unscheduled maintenance or repair.
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Slide 5
Criticality Analysis
Quantitative Approach
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The quantitative approach uses the following
formula for Failure Mode Criticality:
Cm = βαλpt
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Where Cm = Failure Mode Criticality
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β = Conditional probability of occurrence of next
higher failure effect
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α = Failure mode ratio
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λp = Part failure rate
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T = Duration of applicable mission phase
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Slide 6
Criticality Analysis
Criticality Analysis Example
A resistor R6 with a failure rate of .01 failures per million hours is located on the Missile
Interface Board of the XYZ Missile Launch System. If the resistor fails, it fails open 70
% of the time and short 30 % of the time. If it fails open, the system will be unable to
launch a missile 30 % of the time, the missile explodes in the tube 20 % of the time, and
there is no effect 50 % of the time. If it fails short, the performance of the missile is
degraded 50 % of the time and the missile inadvertently launches 50 % of the time.
Mission time is 1 hour.
λp = 0.01 in every case
α = 0.7 for open
β = 0.3 for unable to fire
β = 0.2 for missile explodes
β = 0.5 for no effect
α = 0.3 for short
β = 0.5 for missile performance degradation
β = 0.5 for inadvertent launch
Cm for R6 open resulting in being unable to fire is (.3)(.7)(.01)(1)=0.0021
Cm for R6 open resulting in a missile explosion is (.2)(.7)(.01)(1)=0.0014
Cm for R6 open resulting in no effect is (.5)(.7)(.01)(1)=0.0035
Cm for R6 short resulting in performance degradation is (.5)(.3)(.01)(1)=0.0015
Cm for R6 short resulting in inadvertent launch is (.5)(.3)(.01)(1)=0.0015
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Slide 7
Criticality Analysis
Quantitative Approach
Item Criticality (Cr) is the criticality
number associated with the item under
analysis. For a mission phase, Cr is the
sum of the item’s failure mode
criticality numbers, Cm, which result in
the same severity classification.
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Slide 8
Criticality Analysis
Quantitative Approach
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The quantitative approach uses the following
formula for Item Criticality within a particular
severity level:
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Where Cr Item Criticality
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n = The current failure mode of the item being
analyzed
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j = The number of failure modes for the item
being analyzed.
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Slide 9
Criticality Analysis
Criticality Analysis Exercise
Criticality Analysis:
Determine failure mode criticality values
and item criticality values for the R9
resistor, and create an item criticality
matrix.
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Slide 10
Criticality Analysis
Criticality Analysis Exercise
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A resistor R9 with a failure rate of .04 failures per million hours
is located on the Power Supply Board of the XYZ Missile Launch
System. If the resistor fails, it fails open 70 % of the time and
short 30 % of the time. If it fails open, the system will be unable
to launch a missile 30 % of the time and there is no effect 70 %
of the time. If it fails short, the performance of the missile is
degraded 100 % of the time. Mission time is 1 hour.
λp = __ in every case
α = __ for open
β = __ for unable to fire
β = __ for no effect
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α = __ for short
β = __ for missile performance degradation
Cm for R9 open resulting in being unable to fire is ___
Cm for R9 open resulting in no effect is ___
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Cm for R9 short resulting in performance degradation is ___
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Slide 11
Criticality Analysis
Criticality Analysis Exercise
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Slide 12
Criticality Analysis
Item Criticality
Criticality Analysis Exercise
Severity Levels
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Slide 13
Criticality Analysis
Criticality Analysis - Answers
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A resistor R9 with a failure rate of .04 failures per million hours
is located on the Power Supply Board of the XYZ Missile Launch
System. If the resistor fails, it fails open 70 % of the time and
short 30 % of the time. If it fails open, the system will be unable
to launch a missile 30 % of the time and there is no effect 70 %
of the time. If it fails short, the performance of the missile is
degraded 100 % of the time. Mission time is 1 hour.
λp = 0.04 in every case
α = 0.70 for open
β = 0.30 for unable to fire
β = 0.70 for no effect
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α = 0.30 for short
β = 1.00 for missile performance degradation
Cm for R9 open resulting in being unable to fire is 0.0084
Cm for R9 open resulting in no effect is 0.0196
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Cm for R9 short resulting in performance degradation is 0.012
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Slide 14
Criticality Analysis
Criticality Analysis - Answers
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Slide 15
Criticality Analysis
Item Criticality
Criticality Analysis - Answers
R9(4)
R9(3)
R9(2)
Severity Levels
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Slide 16
Criticality Analysis