Criticality Analysis Slide 1 Criticality – Mil-Std-1629 Approach n 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)

Criticality Analysis Slide 2 Criticality Analysis n Qualitative analysis: –Used when specific part or item failure rates are not available. n Quantitative analysis: –Used when sufficient failure rate data is available to calculate criticality numbers.

Criticality Analysis Slide 3 Qualitative Approach n Because failure rate data is not available, failure mode ratios and failure mode probability are not used. n 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. n 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

Criticality Analysis Slide 4 Quantitative Approach Failure Mode Criticality (C M ) 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...).

Criticality Analysis Slide 5 Mil-Std-1629 Severity Levels n Category I - Catastrophic: A failure which may cause death or weapon system loss (i.e., aircraft, tank, missile, ship, etc...) n Category II - Critical: A failure which may cause severe injury, major property damage, or major system damage which will result in mission loss. n 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. n 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.

Criticality Analysis Slide 6 Quantitative Approach n The quantitative approach uses the following formula for Failure Mode Criticality: n C m = βαλ p t n Where C m = Failure Mode Criticality n β = Conditional probability of occurrence of next higher failure effect n α = Failure mode ratio n λ p = Part failure rate n T = Duration of applicable mission phase

Criticality Analysis Slide 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. λ p = 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 C m for R6 open resulting in being unable to fire is (.3)(.7)(.01)(1)= C m for R6 open resulting in a missile explosion is (.2)(.7)(.01)(1)= C m for R6 open resulting in no effect is (.5)(.7)(.01)(1)= C m for R6 short resulting in performance degradation is (.5)(.3)(.01)(1)= C m for R6 short resulting in inadvertent launch is (.5)(.3)(.01)(1)=0.0015

Criticality Analysis Slide 8 Quantitative Approach Item Criticality (C r ) is the criticality number associated with the item under analysis. For a mission phase, C r is the sum of the item’s failure mode criticality numbers, C m, which result in the same severity classification.

Criticality Analysis Slide 9 Quantitative Approach n The quantitative approach uses the following formula for Item Criticality within a particular severity level: n Where C r Item Criticality n n = The current failure mode of the item being analyzed n j = The number of failure modes for the item being analyzed.

Criticality Analysis Slide 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.

Criticality Analysis Slide 11 Criticality Analysis Exercise n 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. n λ p = __ in every case n α = __ for open n β = __ for unable to fire n β = __ for no effect n α = __ for short n β = __ for missile performance degradation n C m for R9 open resulting in being unable to fire is ___ n C m for R9 open resulting in no effect is ___ n C m for R9 short resulting in performance degradation is ___

Criticality Analysis Slide 12 Criticality Analysis Exercise

Criticality Analysis Slide 13 Criticality Analysis Exercise Item Criticality Severity Levels

Criticality Analysis Slide 14 Criticality Analysis - Answers n 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. n λ p = 0.04 in every case n α = 0.70 for open n β = 0.30 for unable to fire n β = 0.70 for no effect n α = 0.30 for short n β = 1.00 for missile performance degradation n C m for R9 open resulting in being unable to fire is n C m for R9 open resulting in no effect is n C m for R9 short resulting in performance degradation is 0.012

Criticality Analysis Slide 15 Criticality Analysis - Answers

Criticality Analysis Slide 16 Criticality Analysis - Answers R9(2) R9(4) R9(3) Item Criticality Severity Levels