Failure mode, effects, and criticality analysis

Methodology

Slight differences are found between the various FMECA standards. By RAC CRTA–FMECA, the FMECA analysis procedure typically consists of the following logical steps:

• Define the system
• Define ground rules and assumptions in order to help drive the design
• Construct system block diagrams
• Identify failure modes (piece-part level or functional)
• Analyze failure effects/causes
• Feed results back into design process
• Classify the failure effects by severity
• Perform criticality calculations
• Rank failure mode criticality
• Determine critical items
• Feed results back into design process
• Identify the means of failure detection, isolation and compensation
• Perform maintainability analysis
• Document the analysis, summarize uncorrectable design areas, identify special controls necessary to reduce failure risk
• Make recommendations
• Follow up on corrective action implementation/effectiveness

FMECA may be performed at the functional or piece-part level. Functional FMECA considers the effects of failure at the functional block level, such as a power supply or an amplifier. Piece-part FMECA considers the effects of individual component failures, such as resistors, transistors, microcircuits, or valves. A piece-part FMECA requires far more effort, but provides the benefit of better estimates of probabilities of occurrence. However, Functional FMEAs can be performed much earlier, may help to better structure the complete risk assessment and provide other type of insight in mitigation options. The analyses are complementary.

The criticality analysis may be quantitative or qualitative, depending on the availability of supporting part failure data.

System definition

In this step, the major system to be analyzed is defined and partitioned into an indented hierarchy such as systems, subsystems or equipment, units or subassemblies, and piece-parts. Functional descriptions are created for the systems and allocated to the subsystems, covering all operational modes and mission phases.

Ground rules and assumptions

Before detailed analysis takes place, ground rules and assumptions are usually defined and agreed to. This might include, for example:

• Standardized mission profile with specific fixed duration mission phases
• Sources for failure rate and failure mode data
• Fault detection coverage that system built-in test will realize
• Whether the analysis will be functional or piece-part
• Criteria to be considered (mission abort, safety, maintenance, etc.)
• System for uniquely identifying parts or functions
• Severity category definitions

Block diagrams

Next, the systems and subsystems are depicted in functional block diagrams. Reliability block diagrams or fault trees are usually constructed at the same time. These diagrams are used to trace information flow at different levels of system hierarchy, identify critical paths and interfaces, and identify the higher level effects of lower level failures.

Failure mode identification

For each piece-part or each function covered by the analysis, a complete list of failure modes is developed. For functional FMECA, typical failure modes include:

• Untimely operation
• Failure to operate when required
• Loss of output
• Intermittent output
• Erroneous output (given the current condition)
• Invalid output (for any condition)

For piece-part FMECA, failure mode data may be obtained from databases such as RAC FMD–91 or RAC FMD–97. These databases provide not only the failure modes, but also the failure mode ratios. For example:

Each function or piece-part is then listed in matrix form with one row for each failure mode. Because FMECA usually involves very large data sets, a unique identifier must be assigned to each item (function or piece-part), and to each failure mode of each item.

Failure effects analysis

Failure effects are determined and entered for each row of the FMECA matrix, considering the criteria identified in the ground rules. Effects are separately described for the local, next higher, and end (system) levels. System level effects may include:

• System failure
• Degraded operation
• System status failure
• No immediate effect

The failure effect categories used at various hierarchical levels are tailored by the analyst using engineering judgment.

Severity classification

Severity classification is assigned for each failure mode of each unique item and entered on the FMECA matrix, based upon system level consequences. A small set of classifications, usually having 3 to 10 severity levels, is used. For example, When prepared using MIL–STD–1629A, failure or mishap severity classification normally follows MIL–STD–882.

Current FMECA severity categories for U.S. Federal Aviation Administration (FAA), NASA and European Space Agency space applications are derived from MIL–STD–882.

Failure detection methods

For each component and failure mode, the ability of the system to detect and report the failure in question is analyzed. One of the following will be entered on each row of the FMECA matrix:

• Normal: the system correctly indicates a safe condition to the crew
• Abnormal: the system correctly indicates a malfunction requiring crew action
• Incorrect: the system erroneously indicates a safe condition in the event of malfunction, or alerts the crew to a malfunction that does not exist (false alarm)

Criticality ranking

Failure mode criticality assessment may be qualitative or quantitative. For qualitative assessment, a mishap probability code or number is assigned and entered on the matrix. For example, MIL–STD–882 uses five probability levels:

The failure mode may then be charted on a criticality matrix using severity code as one axis and probability level code as the other. For quantitative assessment, modal criticality number ${\displaystyle C_m}$ is calculated for each failure mode of each item, and item criticality number ${\displaystyle C_r}$ is calculated for each item. The criticality numbers are computed using the following values:

• Basic failure rate ${\displaystyle {\lambda }_p}$
• Failure mode ratio ${\displaystyle \alpha }$
• Conditional probability ${\displaystyle \beta }$
• Mission phase duration ${\displaystyle t}$

The criticality numbers are computed as ${\displaystyle C_m={\lambda }_p\alpha \beta t}$ and ${\displaystyle C_r=\sum _{n=1}^N(C_m{)}_n}$. The basic failure rate ${\displaystyle {\lambda }_p}$ is usually fed into the FMECA from a failure rate prediction based on MIL–HDBK–217, PRISM, RIAC 217Plus, or a similar model. The failure mode ratio may be taken from a database source such as RAC FMD–97. For functional level FMECA, engineering judgment may be required to assign failure mode ratio. The conditional probability number ${\displaystyle \beta }$ represents the conditional probability that the failure effect will result in the identified severity classification, given that the failure mode occurs. It represents the analyst's best judgment as to the likelihood that the loss will occur. For graphical analysis, a criticality matrix may be charted using either ${\displaystyle C_m}$ or ${\displaystyle C_r}$ on one axis and severity code on the other.

Critical item/failure mode list

Once the criticality assessment is completed for each failure mode of each item, the FMECA matrix may be sorted by severity and qualitative probability level or quantitative criticality number. This enables the analysis to identify critical items and critical failure modes for which design mitigation is desired.

Recommendations

After performing FMECA, recommendations are made to design to reduce the consequences of critical failures. This may include selecting components with higher reliability, reducing the stress level at which a critical item operates, or adding redundancy or monitoring to the system.

Maintainability analysis

FMECA usually feeds into both Maintainability Analysis and Logistics Support Analysis, which both require data from the FMECA. FMECA is the most popular tool for failure and criticality analysis of systems for performance enhancement. In the present era of Industry 4.0, the industries are implementing a predictive maintenance strategy for their mechanical systems. The FMECA is widely used for the failure mode identification and prioritization of mechanical systems and their subsystems for predictive maintenance.

FMECA report

A FMECA report consists of system description, ground rules and assumptions, conclusions and recommendations, corrective actions to be tracked, and the attached FMECA matrix which may be in spreadsheet, worksheet, or database form.

Risk priority calculation

RAC CRTA–FMECA and MIL–HDBK–338 both identify Risk Priority Number (RPN) calculation as an alternate method to criticality analysis. The RPN is a result of a multiplication of detectability (D) x severity (S) x occurrence (O). With each on a scale from 1 to 10, the highest RPN is 10x10x10 = 1000. This means that this failure is not detectable by inspection, very severe and the occurrence is almost sure. If the occurrence is very sparse, this would be 1 and the RPN would decrease to 100. So, criticality analysis enables to focus on the highest risks.

Advantages and disadvantages

The Strengths of FMECA include its comprehensiveness, the systematic establishment of relationships between failure causes and effects, and its ability to point out individual failure modes for corrective action in design.

Weaknesses include the extensive labor required, the large number of trivial cases considered, and inability to deal with multiple-failure scenarios or unplanned cross-system effects such as sneak circuits.

According to an FAA research report for commercial space transportation,

Failure Modes, effects, and Criticality Analysis is an excellent hazard analysis and risk assessment tool, but it suffers from other limitations. This alternative does not consider combined failures or typically include software and human interaction considerations. It also usually provides an optimistic estimate of reliability. Therefore, FMECA should be used in conjunction with other analytical tools when developing reliability estimates.