Ground Rules and Assumptions

Additional Resources > Windchill Risk and Reliability Practitioner’s Guide > Failure Mode and Effects Analysis > FMEA Planning > Ground Rules and Assumptions

While the FMEA objective is to identify all potential failure modes within a design, the emphasis must be on the early identification of catastrophic and critical failure possibilities so that they can be eliminated or minimised quickly through early design correction. Consequently, the FMEA can begin at the higher system levels as soon as preliminary design information is available and then be extended to the lower system levels as more information becomes available.

To ensure that all team members share a common understanding of the level to which the analysis is to be performed and the time line by which it must be completed, an analysis approach (functional, interface or detailed) for each item must be identified and a schedule established. A well-designed FMEA plan will also include guidelines and assumptions for each of the topics in Table 6-2.

Table 6-2. Guidelines and Assumptions for a FMEA Plan
Topic	Topic Intent
Worksheet Format	Indicates which FMEA worksheet is to be used for each approach, based on those defined in the corporate FMEA standard.
Indenture Level	Indicates the lowest indenture level at which failure modes must be documented. For example, MIL-STD-1629 indicates that the lowest level for Task 101 is based on three criteria: • Lowest level specified for logistics support analysis • Lowest indenture level at which items are assigned a severity classification of catastrophic or critical • Specified or intended maintenance and repair level of items assigned a severity classification of marginal or minor
Coding System	Indicates how the system functions and equipment are to be consistently labelled on the FMEA worksheets so that they can be used to track failure modes. This coding system must be consistent with the numbering used on the functional and reliability block diagrams and must demonstrate the relationship of each failure mode with the system.
Failure Definition	Provides general statements of what constitutes a failure for each item, in terms of performance parameters and allowable limits for each specified output; also notes acceptable degradation limits.
Rating Scales	Indicates the assessment ratings to be used for severity, occurrence (if criticality analysis is to be performed) and detection. Standard classifications exist for use in the military, aerospace and automotive industries. The number of classifications and their descriptions can, however, be tailored to the industry and systems being analysed. If more than one set of rankings is to be used, a cross-reference mapping must be provided so that all scales can be merged.
Coordination of Effort	Indicates how FMEA results are to be used by other departments to support reliability, maintainability, safety, and survivability and vulnerability programmes.

Although every effort to identify and record all ground rules and assumptions must be made prior to beginning the analysis, both ground rules and assumptions may need to be added or changed as design requirements are modified. Communication of such changes to all involved, however, is critical to the success of the FMEA.