System Definition
System definition requires a review of all available design information. The information for defining the system is likely to be found in the following technical specifications and development plans:
• Customer specifications.
• Engineering specifications.
• Quality specifications.
• Reliability specifications (necessary for criticality analysis).
• Engineering drawings.
• Computer-aided design (CAD) data.
• Predecessor history, including:
◦ Trade-off studies.
◦ Stress analysis results.
◦ Test results.
In addition to stating system objectives, the above resources specify design and test requirements for operation, reliability and maintainability, and give acceptable performance limits under specified operational and environmental conditions. These documents also generally define what constitutes a failure and describe what contributes to the various types of system failure.
The existing technical specifications and development plans are used to write functional narratives for each mission, mission phase and operational mode. These functional narratives, which reference the existing technical specifications and development plans as data sources, identify:
• Primary and secondary mission objectives.
• Mission functions and operational modes using a top-down approach.
• Alternative operational modes if more than one method for performing a function exists.
• All multiple functions using different equipment or groups of equipment.
• Functional outputs for each system level.
• Conditions that constitute system and part failure.
• Profiles of anticipated environmental conditions for each mission and mission phase.
• Amount of time an item spends operating in each operational mode during different mission phases or when only its function is required.
Once functional narratives are written, they serve as detailed system definitions for the FMEA plan and are used in the summary of the final FMEA report.