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Criticality | Introduction | Types | Qualitative Analysis | Quantitative Analysis

Criticality

Criticality is a measure of the frequency of occurrence of an effect.

– May be based on qualitative judgment or

– May be based on failure rate data (most common).

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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.

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Qualitative Approach:

–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 that results in a particular severity classification (e.g., results in an end effect with a severity of I, II, etc.).

  • Category I-Catastrophic: A failure that may cause death or weapon system loss (i.e., aircraft, tanks, missiles, ships, etc.)
  • A failure that may cause severe injury, major property damage, or major system damage that will result in mission loss.
  • Category III – Marginal: A failure that 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.
Read More:   Reliability | Reliability Analysis | Methods | Failure Rate | Mean Time Between Failures | MTBF | Failure Mode Effective Analysis | FMEA

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

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