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# Criticality Mil-Std-1629 Approach - PowerPoint PPT Presentation

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). Criticality Analysis. Qualitative analysis:

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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)

• 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.

• 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

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...).

• 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.

• 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

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

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.

• 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.

Criticality Analysis:

Determine failure mode criticality values and item criticality values for the R9 resistor, and create an item criticality matrix.

• 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 ___

Item Criticality

Severity Levels

• 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

R9(4)

R9(3)

Criticality Number x 10-6

R9(2)

Severity Levels