Functional FMEA

The Failure Mode and Effects Analysis (FMEA) is a systematic, bottom-up method of identifying the failure modes of a system, item, function and determining the effects on the higher level. It may be performed at any level within the system (e.g., piece-part, function, blackbox, etc,). Software can also be analyzed qualitatively using a functional FMEA approach. Typically, a FMEA is used to address failure effects resulting from single failures [1]... 

A Level 3 FMECA is conducted to consider how the system failure may affect the platform (Level 4) and its operation application (Level 5). This type of FMEA is often referred to as a functional FMEA and may be very similar to an FHA (see Chapter 2). 

Before we go further into the FMEA process, it is useful to review what a Functional Block Diagram (FBD) is, as it forms an essential part of the Functional FMEA process. 

Note The Criticality is assigned to the severity at Level 3, and this requires detailed knowledge of the Level 3 architecture (e.g. redundancy) and Level 4 application (e.g. is the aircraft cleared for low-level IFR). A piece-part FMEA is useful for systems that rely on redundancy (since a functional FMEA may not reveal single component failures affecting more than one redundant element) and is particularly useful for assessing electronic components, mechanical elements and assemblies (refer ARP4176 para G.3.2). 

And finally, is this a functional or piece-part FMEA Functional FMEAs are typically performed to support the safety analysis effort, whereas piece-part" FMEAs are performed as necessary to provide further refinement of the failure rate (ARP4761 para G.3.2). 

Table 5.3 Example format for a system level 2 functional FMEA...

A functional FMEA may be conducted at any system level (e.g. the FH As in Section 3.3 can also be entitled Functional FMEAs) and is concerned with the function of the system under consideration and how that function might fad. The basic failure categories will be identical to those described in Section 3.2.2. 

Note A piece-part FMEA is often only effectively conducted by the design authority of the part being considered. For the purposes of supporting a 2X.1309 System Safety Assessment, the piece-part FMEA is thus seldom applied above System Level 3 and is only conducted(ARP4761 para G.3.2.2) when necessary (eg, when the more conservative results of a functional FMEA will not meet the ETA probability of failure budget). 

If it is a functional FMEA (conducted at Level 3 or 4), then drop down to a lower level (e.g. piece-part) and exclude components with no effect on the event under consideration... 

As this case study is required to support a CS25.1309 safety assessment, it is suggested that a Functional FMEA at the system integration level (i.e. Level 3 in Fig. 1.1) would be more appropriate than a piece-part FMEA at the component level. ° This Functional FMEA will have the objective of searching for single failure conditions in the Barometric Altitude Display System which might cause a catastrophic failure at the aircraft level (i.e. Level 4 in Fig. 1.1). 

Table 5.10 Functional FMEA for the Barometric Altitude Display System ... 

The FMEA is used primarily for doing subsystem and system hazard analyses, but a functional FMEA can also be useful in performing preliminary hazard analyses and operating hazard analyses. It helps to identify critical items in terms of safety and reliability. 

The functional FMEA requires less detail and can be performed with the upper branches of an analytical tree. The requirement is to identify subsystems and their functions, not details of subsystem configuration or design. 

The scope is easily defined if an analytical tree is used as a feeder document (Chapter 10). The indenture levels can be identified by specifying the number of tiers of the tree to be included in the analysis (limits of resolution). A functional FMEA includes upper and/or middle tiers only a hardware FMEA includes the entire tree. 

The functional FMEA tends to use deductive logic, that is, to ask what the cause may be and to focus on the functional failure modes and their causes. It can be used early in the program and is usually done at the subsystem or assembly level. The hardware FMEA tends to be more inductive, asking what ir and when questions, in that common component failure modes are listed and then the focus of the analysis is on the effects of each failure mode. It needs fixed design data and goes to the component level. 

From the analytical tree (and/or other input documents), prepare a systematic breakdown of the system to the specified level of detail (generally subsystem or assembly level for a functional FMEA and component level for a hardware FMEA). 

Column 1—Component Description. Enter the component description (narrative description and part number, if applicable). Note that this particular form was designed for a hardware FMEA. If a functional FMEA is being performed with this form, the component may be a subsystem, assembly, or subassembly. For some projects, breaking this column into two or three separate columns may be desirable. For example, if specific part numbers are assigned to all or most components and/or if separate item numbers are desired, a separate column (usually added as the first column with the component description becoming the second column) may be added just for item or part numbers. If the function of the component is not apparent or cannot... 

Column 3—Effects on Other Components. List the impact of each failure listed in column 2 on other components. As in column 1, components may also include subassemblies, assemblies, or subsystems, especially if a functional FMEA is being performed. List those components that will physically be damaged and those whose functions may be adversely affected, which are generally those located adjacent to or near the failed component physically and/or operationally. They are generally in the same or the next higher indenture level or tier. 

There are basically two types of failure mode and effect analyses. They are distinguished more by the target of the analysis than the actual analysis itself. In fact, the steps required in the performance of each are very similar only the items being analyzed differ. Perhaps the fundamental difference between the two is in their approach. The first type, often referred to as the functional FMEA, utilizes the deductive reasoning approach (i.e., it begins by assuming a failure and focuses... 

The functional FMEA targets any subsystems that may exist within an entire system. The functional FMEA will evaluate each subsystem and attempt to identify the effect of any failures in these subsystems. The analyst not only looks for the possible effects of subsystem failures on the system as a whole but also examines the effect of such failures on other subsystems within the system. Although functional FMEAs are not as common as the hardware FMEA, their basic utility should not be dismissed. When a complex system (such as a nuclear reactor, an airliner, an overhead bridge crane, or a new robotic milling machine) consists of numerous secondary subsystems, each with its own set of supporting subsystems, the functional FMEA should be performed to ensure proper system safety evaluation at every level. 

The second and more common hardware FMEA examines actual system assemblies, subassemblies, individual components, and other related system hardware. This analysis should also be performed at the earliest possible phase in the product or system life cycle. Just as subsystems can fail with potentially disastrous effects, so can the individual hardware and components that make up those subsystems. As with the functional FMEA, the hardware FMEA evaluates the reliability of the system design. It attempts to identify single-point failures, as well as all other potential failures, within a system that could possibly result in failure of that system. Because the FMEA can accurately identify critical failure items within a system, it can also be useful in the development of the preliminary hazard analysis and the operating and support hazard analysis (Stephenson 1991). It should be noted that FMEA use in the development of the O SHA might be somewhat limited, depending on the system, because the FMEA does not typically consider the ergonomic element. Other possible disadvantages of the FMEA include its purposefiil omission of multiple-failure analysis within a system, as well as its failure to evaluate any operational interface. Also, in order to properly quantify the results, a FMEA requires consideration and evaluation of any known component failure rates and/or other similar data. These data often prove difficult to locate, obtain, and verify (Stephenson 1991). 

The Functional Failure Modes and Effects Analysis (Functional FMEA) or Functional Failure Analysis (as it is usually called in the USA) is typically performed to support safety analysis efforts early in the lifecycle and intended for iterative application. It tries to find all hypothetical failure modes to the defined functions of the considered system and assesses the operational effect thereof. That way functional failure modes that may be eliminated/mitigated by functional level design changes can be found, the confidence in the overall design concept is strengthened and areas requiring risk reduction can be identified. 

The Functional Hazard Assessment (FHA) asks the question How safe does the system need to be considering the required functionality and the specific environmental context of the system. A typically used technique in that phase is the Functional Failure Modes and Effects Analysis (Functional FMEA) to find all theoretically possible failure modes which then can be traced to hazards. 

The basic objective of FMEA/FMECA is early identification of catastrophic, critical and other potential failures so that these can be eliminated or mitigated through design and manufacturing process at the earliest possible time. There are three kinds of FMEA/FMECA, namely, functional FMEA/FMECA, design FMEA/FMECA (DFMEA/DFMECA), and process FMEA/FMECA (PFMEA/PFMECA) (see Clause... 

The aim of functional FMEA/FMECA is to improve the design of the system. Similarly, DFMEA/DFMECA always looks to see that there is improvement in the design of subsystems/components. The objective of PFMEA/PFMECA is to improve the manufacturing process. Thus it transpires that the major objective of FMEA/ FMECA shall include ... 

There are three types of FMEA/FMECA functional FMEA/FMECA, DFMEA/ DFMECA, and PFMEA/PFMECA. Apart from these there are two other types of FMEA service FMEA and SWFMEA. In service FMEA the focus is on service issues. SWFMEA (discussed separately in later clauses) focuses on software issues. Functional FMEA/FMECA is also known as concept FMEA/FMECA or system FMEA/ FMECA. Some literature shows two types of FMEA/FMECA in the sense that one is functional and other is hardware FMEA/FMECA (where both PFMEA/PFMECA and DFMEA/DFMECA are considered under hardware FMEA/FMECA). These different types are a result of changes in analysis pattern and assessment, but the basic concepts/approaches are the same. 

Table IV/2.1.2-1 Documentation for Hardware and Functional FMEA/FMECA (Typical Indicative Only) ...

Failure is the loss of the ability of an item to provide its required function. FMEA is a logical process for identification of failure modes of the elements of a system with focus on causes of failures and the failure effects. FMECA is an extension of FMEA, where quantitative estimations of the likelihood and the severity of each failure mode... 

The functional FMEA is used to evaluate failures in one or many subsystems that function within a larger system, while the hardware FMEA examines failures in the assemblies, subassemblies, and components within those subsystems. The FMEA, therefore, has great versatility in the system safety process. The analysis can either be specialized, without regard for other subsystems which are not within the scope of the analysis, or it can be generalized to encompass total subsystem or system effects of a given failure condition. However, because the FMEA does not consider the human factors element or multiple failure analyses within a system, other types of system safety analysis tools and techniques should also be utilized. 

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