Contents
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).
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 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.
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
