Criticality MilStd1629 Approach n CRITICALITY is a measure

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) http: //www. fmea-fmeca-com Slide 1 Criticality Analysis

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. http: //www. fmea-fmeca-com Slide 2 Criticality Analysis

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 http: //www. fmea-fmeca-com 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. . . ). http: //www. fmea-fmeca-com Slide 4 Criticality Analysis

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. http: //www. fmea-fmeca-com Slide 5 Criticality Analysis

Quantitative Approach n n The quantitative approach uses the following formula for Failure Mode Criticality: Cm = βαλpt n Where Cm = Failure Mode Criticality n β = Conditional probability of occurrence of next higher failure effect n α = Failure mode ratio n λp = Part failure rate n T = Duration of applicable mission phase http: //www. fmea-fmeca-com Slide 6 Criticality Analysis

Criticality Analysis Example A resistor R 6 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 R 6 open resulting in being unable to fire is (. 3)(. 7)(. 01)(1)=0. 0021 Cm for R 6 open resulting in a missile explosion is (. 2)(. 7)(. 01)(1)=0. 0014 Cm for R 6 open resulting in no effect is (. 5)(. 7)(. 01)(1)=0. 0035 Cm for R 6 short resulting in performance degradation is (. 5)(. 3)(. 01)(1)=0. 0015 Cm for R 6 short resulting in inadvertent launch is (. 5)(. 3)(. 01)(1)=0. 0015 http: //www. fmea-fmeca-com 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. http: //www. fmea-fmeca-com Slide 8 Criticality Analysis

Quantitative Approach n The quantitative approach uses the following formula for Item Criticality within a particular severity level: n Where Cr 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. http: //www. fmea-fmeca-com Slide 9 Criticality Analysis

Criticality Analysis Exercise Criticality Analysis: Determine failure mode criticality values and item criticality values for the R 9 resistor, and create an item criticality matrix. http: //www. fmea-fmeca-com Slide 10 Criticality Analysis

Criticality Analysis Exercise n A resistor R 9 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 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 R 9 open resulting in being unable to fire is ___ Cm for R 9 open resulting in no effect is ___ Cm for R 9 short resulting in performance degradation is ___ n n n n http: //www. fmea-fmeca-com Slide 11 Criticality Analysis

Criticality Analysis Exercise http: //www. fmea-fmeca-com Slide 12 Criticality Analysis

Item Criticality Analysis Exercise Severity Levels http: //www. fmea-fmeca-com Slide 13 Criticality Analysis

Criticality Analysis - Answers n A resistor R 9 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 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 R 9 open resulting in being unable to fire is 0. 0084 Cm for R 9 open resulting in no effect is 0. 0196 Cm for R 9 short resulting in performance degradation is 0. 012 n n n n http: //www. fmea-fmeca-com Slide 14 Criticality Analysis

Criticality Analysis - Answers http: //www. fmea-fmeca-com Slide 15 Criticality Analysis

Item Criticality Analysis - Answers R 9(4) R 9(3) R 9(2) Severity Levels http: //www. fmea-fmeca-com Slide 16 Criticality Analysis