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FMECA - The Process
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This section describes 4 important stages of FMECA development.
Contents
Boundaries
Define the System
Hierarchy
The FMECA Worksheet
Criticality Matrices
Boundaries
Boundaries / Scope are important factors which need to be considered and defined before any analysis takes place. An analysis boundary may for
instance, limit the analysis to ‘failures pertaining to the system under consideration’. Alternatively, the boundary could be set at a point within
the system. A boundary which is set outside the scope of the system can create significant difficulties during the development of the FMECA and
caution should be exercised before agreeing such a requirement.
Define the System
Before an analysis can take place, the system needs to be well defined. It is important to ensure that all components, sub-assemblies, modules,
LRUs etc. are represented within the system and at the correct level in the system hierarchy. There are a number of different tools and techniques
to assist this process, which include the following:
- Equipment Breakdown Structure (EBS) - This is normally used to describe the hierarchical structure of the system (see Hierarchy).
- Reliability Block Diagrams (RBD) - An RBD identifies the critical functional paths for a given function and clearly identifies any areas of redundancy.
Visit the Reliability Advice site to download a tutorial on RBDs [www.reliability-advice.co.uk/downloads.htm].
- Functional Block Diagrams (FBD) - This information is normally a primary requirement for performing a functional FMECA.
> Hierarchy
Most systems will involve some form of hierarchical structure in order to divide the top level system into a number of assemblies and sub-assemblies.
These levels of hierarchy can be described both graphically and by a numbering system often described as a Logistic Support Analysis (LSA) Control
Number (LCN). Part of a system (e.g. an Aircraft) could be broken down as follows:
1: System – top level equipment under consideration – e.g. Aircraft
1A: Assembly – e.g. Undercarriage
1A01: Module – e.g. Wheel
1A01A: Component – e.g. Tyre
The FMECA Worksheet
A FMECA will normally be presented in some form of spreadsheet format. There are a number of sources for guidance and standards regarding
formatting the FMECA. One of the most common of these is MIL-STD-1629A.
Criticality Matrices
A Criticality Matrix provides a graphical means for illustrating the distribution of failure criticality against severity, and assists in assigning
design mitigation and priorities for any failure detection methods.
An example of a Criticality Matrix is shown below:
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