February 2017 – Why Do a FMEA/FMECA if We Don’t Have To?

Is it Really Worth the Effort?
You bet. Different types of components, or parts of a process, can fail in a number of ways, called failure modes, and each failure mode has an associated failure rate. Failure Mode Effects Analysis (FMEA) and Failure Mode Effects Analysis and Criticality Analysis (FMECA) can find potential design weaknesses you can address to make your product or process safer and more reliable. These analyses can also assess likelihood of each failure mode causing system-level consequences. Both FMEAs and FMECAs are often performed on designs or procedures. The idea is to predict whether things, software, or processes meet reliability or safety requirements in the intended operating environments.

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What is a FMEA
A FMEA is a step-by-step approach to determine failures or process errors. Some examples of when a FMEA on a product, process, or service should be performed:
  • During the design process, to see whether a product or process can be expected to meet reliability and safety goals
  • When considering modification or redesign of an existing product or service, to determine whether changes might degrade (or perhaps improve) reliability or safety
  • When there are recurring failure reports from the field
A FMEA worksheet is typically an Excel spreadsheet that includes the following columns, with one row for each failure mode – that is, for each way that each component or process step could fail:
  • An unique ID assignment for reference
  • Identification of the unit containing the potential failed component
  • A reference designation to identify exactly which component is under consideration
  • The failure mode (for a product, perhaps open circuit, gain too low, fractured, or excessively worn; for a process, perhaps too late, wrong mixture, or failure to perform)
  • The local effect (for a product, typically the effected circuit on a circuit board)
  • The next higher (or intermediate) effect (for a product, typically a failed circuit board or LRU)
  • The end (or system-level) effect
  • The failure probability (likelihood of the failure during a specified period of operation, usually called the exposure time)
  • Remarks (sometimes an explanation is useful)
FMEA chart 2
The above image corresponds to a component or piece-part FMEA which takes on a related, but different approach. A component FMEA or FMECA can provide useful design information and help answer questions about a design’s reliability, safety, and cost. 

Some FMEA worksheets also include additional columns for each failure mode:
  • Identification of the failed component’s function
  • The cause of the failure (often just guesses but sometimes very meaningful )
  • The operational mode or mission phase of the system for each particular failure mode (the effects of a certain failure in standby mode may be quite different when in operational mode or maintenance mode)
  • Method of failure detection, if any (background health checks, human alerts, others)
  • Compensation provisions, if any (redundant components or systems, other design features)
FMEA chart 1

The image above is an example of a functional FMEA. A functional FMEA is one common type of FMEA where a system’s functions are failed rather than individual components.

FMECA
FMECA is a variation of FMEA that adds a column for criticality analysis, which determines the seriousness of system-level consequences of each component failure mode. FMECA therefore identifies potential failures that require the greatest attention – the focus is whether such failures at their stated likelihoods are acceptable, and if not then how the design team can mitigate or eliminate them.

Table I of MIL-STD-882, “System Safety,” provides widely-accepted definitions of severity categories (from Catastrophic to Negligible). Table II provides subjective definitions of failure probability levels (from Frequent to Improbable). Table III provides overall risk assessment using data from Tables I and II, and defines four levels of risk as High, Serious, Medium, and Low. Risk provides an objective way to represent each potential failure’s likelihood and consequences as a single term, and therefore is a generally-accepted basis for determining a system’s acceptability.



Published Standards and Guidelines
Different industries use various FMEA and FMECA standards and guidelines that specify formatting and other requirements. Some of the main published standards include MIL-STD-1629, “Procedures For Performing a Failure Mode, Effects, and Criticality Analysis,” and SAE ARP 4761, “Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment.” (The Government has officially cancelled MIL-STD-1629 but it remains a widely-used reference in the reliability community.)

MIL-HDBK-217, “Reliability Prediction of Electronic Equipment,” provides failure rates for, well, electronic components. Other sources provide similar information for various mechanical components. 

RAC FMD-2016, “Failure Modes/Mechanism Distributions,” provides useful probability data for determining failure rates of a component’s failure modes. For example, you may find a certain electrical connector’s failure rate, but that doesn’t tell you how the failure rate is split between open path failures and shorted path failures. FMD-2016 provides that kind of information (in this example, it’s roughly 84% open and 16% shorted).

Unfortunately, the analyses often must consider components or failure modes that aren’t in any published references. In those cases, you can sometimes find listings for failures that are arguably close to the ones you need to address, and then apply some engineering judgment to determine how to adjust the published failure rates for your particular failure modes and their environments.

Learn More – Why Omnicon?
The Omnicon Group is a long-time recognized leader in conducting FMEA and FMECA. We work with you to tailor any type of required reliability analysis to your needs and provide results that are most useful to you. Reliability has been our number one priority since our start in 1984. 

Omnicon is proud of its many awards and achievements in reliability, maintainability, and testability engineering. We have more engineers named Reliability Engineer of the Year than any other company in the world. Many companies rely on Omnicon’s engineering to improve the quality of their hardware, software and system designs. Our worldwide engineering and design expertise is unparalleled. We are ISO-9001-2008 and AS9100C certified. Our engineering processes are highly structured and are tailored to meet military, commercial, and FAA guidelines. Above all, Omnicon maintains an outstanding track record of on-time performance and ongoing customer satisfaction.

Tags: FMEA, FMECA, Guidelines, reliability, safety

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