FMECA is just FMEA with Criticality Analysis. This presentation covers the Milstd 1629A approach to criticality analysis.

1. Criticality – Mil-Std-1629 Approach
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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2. Criticality Analysis
Qualitative analysis:
– Used when specific part or item failure rates are not
available.
Quantitative analysis:
– Used when sufficient failure rate data is available to
calculate criticality numbers.
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3. Qualitative Approach
Because failure rate data is not available, failure mode
ratios and failure mode probability are not used.
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.
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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4. 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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5. Mil-Std-1629 Severity Levels
Category I - Catastrophic: A failure which may cause death
or weapon system loss (i.e., aircraft, tank, missile, ship, etc...)
Category II - Critical: A failure which may cause severe
injury, major property damage, or major system damage which
will result in mission loss.
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.
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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6. Quantitative Approach
The quantitative approach uses the following
formula for Failure Mode Criticality:
Cm = βαλpt
Where Cm = Failure Mode Criticality
β = Conditional probability of occurrence of next
higher failure effect
α = Failure mode ratio
λp = Part failure rate
T = Duration of applicable mission phase
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7. 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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8. 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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9. Quantitative Approach
The quantitative approach uses the following
formula for Item Criticality within a particular
severity level:
Where Cr Item Criticality
n = The current failure mode of the item being
analyzed
j = The number of failure modes for the item
being analyzed.
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10. 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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11. Criticality Analysis Exercise
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
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 ___
Cm for R9 short resulting in performance degradation is ___
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12. Criticality Analysis Exercise
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13. Criticality Analysis Exercise
Item Criticality
Severity Levels
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14. Criticality Analysis - Answers
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
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
Cm for R9 short resulting in performance degradation is 0.012
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15. Criticality Analysis - Answers
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16. Criticality Analysis - Answers
Criticality Number x 10-6
R9(4)
R9(3)
R9(2)
Severity Levels
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