Critical item list

Instructions for preparation of failure modes and effects analyses and critical items list Methodology for conduct of NSTS hazard analyses... 

Common techniques for hazard analysis are the failure modes and effects analysis (FMEA) and fault tree analysis (FTA). Many of the other techniques listed in Chapter 17 are also used. TTie FMEA is considered a reliability tool and used, in most NASA and NASA contractor organizations, by a separate reliability division or branch. The FMEA is used to generate another popular NASA tool, the critical items list (CIL). 

Critical items list (CIL) A listing comprised of all critical items identified as a result of performing the FMEA (NSTS 22254). 

Step 5 is concerned with developing a list of crihcal items. The list is prepared to facilitate the communication of important analysis results, generally to the management personnel, and it contains information on areas such as item identification, concise statement of failure mode, criticality classification, and degree of loss effect. Finally, Step 6 is concerned with documenting the analysis. This step is equivalent in importance to all the previous steps (i.e., Step 1 to Step 5) because poor documentation can result in ineffectiveness of the FMEA process. The FMEA report includes items such as system description, ground rules, system definition, failure modes and their effects, and critical items list. 

Critical items List The purpose of the FMEA is to identify and evaluate failure modes and the possible system effects of those failures. Since the potential for undesirable effects must be eliminated or controlled, the FMEA also provides recommended actions that must be taken to accomplish this goal. As part of this analysis process, the FMEA identifies any and all items within the system that, if a failure were to occur, would have a critical effect on the operation of that system. Therefore, to facilitate evaluation and analysis of these system effects, a critical items list is developed. The list provides detailed descriptive information on each item. It will explain its overall function within the system, as well as the function of any components that may make up that item. The failure mode determined as critical is then listed along with the potential effect(s) of such a failure. If an item on the critical items list is to be accepted as is, then acceptance rationale must be provided. Such rationale may include an explanation of any existing or planned design limitations that will prevent the failure during actual system operations, or the provision of excessive factors of safety that will render such fail-ure(s) extremely improbable. Another area for evaluating acceptance is the history, or lack thereof, and any known failures of systems similar in nature and operation. 

Finally, the most important element of the entire report, the FMEA, provides recommendations for management acceptance or rejection of the risk associated with any failure of any item on the critical items list. 

After completing a FMEA, the procedure involves development of a critical item list (CIL). This list contains... 

Of the 4,222 that are termed Criticality l/lR, 3,233 have waivers. Waivers are granted whenever a Critical Item List component cannot be redesigned or replaced. More than 36 percent of these waivers have not been reviewed in 10 years. 

FMEA is used to assist analysts to perform hazard analyses and it is regarded as a supplement rather than a replacement for hazard analyses. Safety analysts can use FMEA to verify that all safety critical hardware has been addressed in the hazard analyses. The FMEA for hardware systems is an important technique for evaluating the design and documenting the review process. All credible failure modes and their resultant effects at the component and system levels are identified and documented. Items that meet defined criteria are identified as critical items and are placed on the Critical Item List (CEL). Each entry of the CIL is then evaluated to see if design changes can be implemented so that the item can be deleted from the CIL. Items that cannot be deleted from the CIL must be accepted by the programme/project, based on the rationale for acceptance of the risk. The analysis follows a well-deflned sequence of steps that encompass (1) failure mode, (2) failure effects, (3) causes, (4) detectability, (S) corrective or preventive actions, and (6) rationale for acceptance. 

As noted in the Department of Energy Action Plan Lessons Learned fk>m the Columbia Space Shuttle Accident and Davis-Besse Reactor Pressure-Vessel Head Corrosion Event — When the space shuttle Columbia launched on January 16, 2003, there were 3,233 Criticality 1/lR critical item list hazards that were waived. Hazards that result in Criticahty 1/lR component failures are defined as those that will result in loss of the orbiter and crew. In both, the Challenger and Columbia accidents The machine was talking to us, but nobody was hstening. (US DOE, 2005)... 

---