FMEA (failure modes and effects analysis) is a bottom-up method that systematically works out the ways a product or process can fail and what each failure leads to. FTA (fault tree analysis) works the other way round: it puts a single undesired event at the top and breaks it down, through logic gates, into the causes that could produce it. The international reference for both comes from the International Electrotechnical Commission (IEC): IEC 60812 for FMEA and IEC 61025 for FTA.
According to the IEC, the current third edition of IEC 60812 was published on 10 August 2018 and the second edition of IEC 61025 on 13 December 2006. The 2018 edition added alternative ways of calculating the risk priority number (RPN) and a method based on a criticality matrix.
In short:
- FMEA asks "how can this part or step fail, and what happens if it does?"; FTA asks "through which combinations of causes can this bad event occur?" (NASA Fault Tree Handbook).
- FMEA prioritises using severity, occurrence and detection ratings; since 2019 the automotive industry uses Action Priority (AP) instead of the RPN (VDA QMC).
- The two methods do not replace each other; an FMEA can be used to check that a fault tree is complete.
What is FMEA and what is it for?
The IEC 60812 abstract states that the purpose of FMEA is to establish how items or processes might fail. The method identifies failure modes together with their effects on the item or process, and allows those modes to be prioritised to support decisions about treatment. The standard applies to hardware, software, processes including human action, and their interfaces. It also states a clear limit: IEC 60812 is a generic standard and gives no specific guidance for safety applications.
According to the American Society for Quality (ASQ), FMEA was developed by the US military in the 1940s. When it is performed during design it is called a design FMEA (DFMEA); when used for manufacturing and process control, a process FMEA (PFMEA). ASQ stresses that FMEA has the greatest leverage early in development, when changes are still cheap.
Where a criticality ranking based at least on the severity of consequences is added, IEC 60812 calls the analysis FMECA (failure modes, effects and criticality analysis).
How is fault tree analysis (FTA) carried out?
NASA's 2002 Fault Tree Handbook describes FTA as a deductive method: it starts from an undesired event, such as the failure of a main engine, and determines its causes through a systematic, backward-stepping process. IEC 61025 likewise covers identifying appropriate assumptions, events and failure modes, and sets out identification rules and symbols. In practice, the steps can be sequenced as follows:
- Define the top event: Write the undesired event clearly and measurably. A vague definition makes the tree branch unnecessarily.
- Develop it with gates: According to the handbook, an OR gate shows that the output event occurs if one or more of the inputs occur; an AND gate shows that it occurs only if all inputs occur.
- Reach the basic events: The lowest events that are not developed further (a component failure, a human error) are the basic events.
- Find the minimal cut sets: A cut set is a combination of basic events that can cause the top event; a minimal cut set is the smallest such combination. A single-element minimal cut set is a weak point that alone can cause the top event.
- Quantify if needed: If basic event probabilities are known, the tree can be used to calculate the probability of the top event and the importance of each cause.
What is the RPN, and what did Action Priority (AP) change?
In a classic FMEA, each failure mode receives ratings for severity, occurrence (likelihood) and detection. A paper presented in 2016 by NASA's Goddard Space Flight Center defines the RPN as the product of the likelihood, severity and detection/prevention rankings. The same paper warns of a practical trap: if the severity scale is numbered in reverse (worst case equals 1), the RPN gives erroneous results and the order of improvement actions is inverted. ASQ notes that severity is usually rated on a scale from 1 to 10.
In the automotive industry, the joint FMEA handbook of the US-based AIAG and Germany's VDA introduced a new structure in 2019. According to the VDA QMC white paper on the handbook, the method was divided into seven steps: planning and preparation, structure analysis, function analysis, failure analysis, risk analysis, optimisation and documentation of results. In the risk analysis step, the severity, occurrence and detection tables were revised and Action Priority (AP) replaced the RPN. In practical terms, different rating combinations that give the same product are no longer treated as equal; the decision is driven by the action priority read from the tables.
Which method applies depends on customer and sector requirements. FMECA and criticality matrices are more common in aerospace and defence projects, the AIAG-VDA approach in the automotive supply chain.
How are FMEA and FTA used together?
The NASA handbook explains the relationship with a clear warning: combining several FMEAs to build a fault tree cannot be done and is bad practice. The reason is that FMEA treats causes one by one without showing the relationships among them, whereas a fault tree models those relationships as necessary and sufficient conditions. On the other hand, an FMEA is a useful tool for checking the completeness of a fault tree and for deciding whether, and in what detail, a tree is needed.
A common division of labour in practice:
- FMEA: screens all components and process steps and answers "what are we missing?" from a broad perspective.
- FTA: examines a few safety-critical top events in depth and exposes common-cause and redundancy weaknesses.
- Shared output: actions from both analyses are tracked in a single list, with owners and dates.
Conclusion: which analysis, and when?
When a broad screening is needed, FMEA comes first; when the causes of a single critical event need to be understood in depth, FTA does. A practical start is to agree in writing which standard and rating tables the team will use, to fix the direction of the severity scale, and to begin the analysis early in design. The analysis creates value when it is treated as a living record updated as the design and process change, not as a document filed away after one pass.
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