Criticality – Mil-Std-1629 Approach

1. 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)

2. 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.

3. 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

4. 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...).

5. 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.

6. 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

7. 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

8. 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.

9. 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.

10. Criticality Analysis Exercise Criticality Analysis: Determine failure mode criticality values and item criticality values for the R9 resistor, and create an item criticality matrix.

11. 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 ___

12. Criticality Analysis Exercise

13. Criticality Analysis Exercise Item Criticality Severity Levels

14. 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

15. Criticality Analysis - Answers

16. Criticality Analysis - Answers R9(4) R9(3) Item Criticality R9(2) Severity Levels