16. Lesson: Criticality Assessment
Documentation levels, controls, and redundancies can be determined by assigning a criticality to each subsystem, experiment, or component. A component determined to be vital for crew safety (Criticality 1) requires a complete set of documentation and controls. Systems with this criticality are extremely costly to develop and to control since the data package generally begins with the pedigree of the raw material and carefully documents and controls each step of the development, fabrication, and test process.
Components which are required for primary mission success (Category 2) can receive somewhat less rigid treatment although in actual practice the controls on Criticality 2 hardware are almost identical with Criticality 1 components. Items which affect secondary objectives should be placed in Category 3. Considerable relaxation of documentation and control is possible for components such as these. For example, qualification testing can be reduced and verification by analysis is acceptable. Verification by similarity (i.e., use in a comparable but not identical manner) can be utilized.
Peripheral items, such as cameras, small experiments, and crew equipment of a noncritical nature should be categorized as Criticality 4. These items require only enough documentation and controls to ensure they are safe and represent no hazard.
Background:
The Skylab Program made use of failure analyses to categorize systems and assign criticality numbers. Standard documentation and controls were determined for each category. The experience, however, fell somewhat short of expectation since it was difficult for a subsystem engineer to accept that his component "wasn't important." The tendency was to over categorize and, thus, over control the development. It required management pressure to ensure the proper level is assigned.
These lessons learned are from SKYLAB LESSONS LEARNED AS APPLICABLE TO A LARGE SPACE STATION, A dissertation submitted to the faculty of The School of Engineering and Architecture Of the Catholic University of America For the Degree Doctor of Engineering by William C. Schneider, Washington, D.C., 1976.
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Last Revised:
March 07, 2004
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