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Criticality – Mil-Std-1629 ApproachPowerPoint Presentation

Criticality – Mil-Std-1629 Approach

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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:
- 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

- 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

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

- 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 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

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 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 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:

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

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 - 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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