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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). Criticality Analysis. Qualitative analysis:

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Criticality mil std 1629 approach
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
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.


Qualitative approach
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


Quantitative approach
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...).


Mil std 1629 severity levels
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.


Quantitative approach1
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


Criticality analysis example
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


Quantitative approach2
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.


Quantitative approach3
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.


Criticality analysis exercise
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 exercise1
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 ___



Criticality analysis exercise3
Criticality Analysis Exercise

Item Criticality

Severity Levels


Criticality analysis answers
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



Criticality analysis answers2
Criticality Analysis - Answers

R9(4)

R9(3)

Criticality Number x 10-6

R9(2)

Severity Levels


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