Other “FMEA” definitions

By Murray Wiseman

Extracted from Chapter 2 of Reliability-centered Knowledge

Derivatives of the original FMEA worksheets and process have added to and subtly altered the four knowledge elements

Function,
Failure,
Failure Mode, and
Failure Effect;

from their original meanings as they were defined by Nowlan and Heap and more recently in the SAE JA1011-1999 standard. Contending standards and older standards such as “FMECA” (Failure Modes, Effects, Criticality Analysis – MIL STD 1629A) and FMEA/AIAG (Automotive Industry Action Group 1995)[1] use several of the same words but ascribe to them different meanings. Hence alternate definitions of FMEA terminology have been and continue to be used extensively in many industries. Understandably, this has led to confusion and miscommunciation. Table 1 provides a comparison of alternate FMEA terms.

Table 1

Terminology	Non SAE-JA1011 definition	SAE-JA1011 definition
FMEA	A systematic tool for identifying: effects or consequences of a potential product or process failure, methods to eliminate or reduce the chance of a failure occurring	Different definition: A tool for determining the functions, functional failures, causes, and effects of a failure of an item in its operating context
Potential Failure	Incorrect material choice, inappropriate specifications, operator assembling part incorrectly, excess variation in process resulting in out-spec products. Example: Air Bag (excessive air bag inflator force, operator may not install air bag properly on assembly line such that it may not engage during impact	Different definition: An indicator that a failure mode has occurred and is in the process of degrading to a functional failure. At the time of detection, however, it has no dire consequences.
Basic and Secondary functions	Basic Function: ingress to and egress from vehicle, Secondary function: protect occupant from noise	Similar definition: Primary function: why item purchased / installed. Secondary function: All other functions (protective, environmental, appearance, control-containment-comfort, health and safety, efficiency, structure-superfluous). See page 140.
Failure Mode	Physical description of a failure. e.g. noise enters at door-to-roof interface	Different definition: The cause (at a practical causality depth) of a failure.
Failure Effects	Impact of failure on people, equipment. E.g. driver dissatisfaction.	Different definition: The typical worst case scenario of relevant events touched off by a failure mode occurring before, during, and after the failure.
Failure		The way in which a function is lost or compromised.
Failure Cause/Mechanism	Refers to the underlying (root) cause of a failure. E.g. insufficient door seal.	Somewhat different definition: In SAE JA-1011 there is only one active definition for all these terms. In other words, Failure Cause = Failure Mode = Failure Mechanism = Root Cause. It is the failure mode (or modes) retained in an analysis (e.g. using a cause and effect diagram if required.) for which there is a practical consequence mitigating activity.
Severity	A rating corrsponding to the seriousness of an effect of a "potential failure mode". (scale: 1-10)	None.
Occurrance	A rating corresponding to the rate at which a first level cause and its resultant failure mode will occur over the design life (scale 1-10)	None.
Detection	A rating corresponding to the likelihood that the detection methods or current controls will detect the potential failure mode (scale 1-10)	None.
Risk Priority Number (RPN)	Severity ´ Occurrence ´ Detection	None. Note that SAE JA1011 does not preclude the use of RPN. Neither does RPN detract from SAE JA1011, but merely adds another dimension to the analysis, if required.
Consequences	Unclear or varied.	None in FMEA but are adressed in the subsequent decision process of RCM. The consequences of failure are one of: Hidden, Safety/Environmental, Operational, or Non-Operational.

Conclusion

No definition is “right” or “wrong” per se. Rather they (alternate meanings) should be recognized as being different. Discussions among operators, engineers, and maintainers should should initially clarify which set of definitions is to be used.

Do you have any comments on this article? If so send them to murray@omdec.com.