TABLE OF CONTENTS Traditionally, commercial air transport RCM reserves the term "critical" for those failures that have an immediate and direct safety impact. "A critical failure is any failure that could have a direct effect on safety." (Nolan & Heap) Note the word direct imposes specific qualifying criteria not imposed elsewhere. This qualification excludes failures that aren't immediately evident, or weren't single failures. For "non-evident failures," the absence of "evidence" means there is no direct impact on safety. The "immediate" requirement screens out multiple, train-redundant failures.
The first major permutation of RCM's definition of "critical" occurred in the transition from aerospace to nuclear power. NRC defi-nitions for what are now called "essential" components those whose failure could affect fission product environmental releases gave "critical" a new dimension. Since many nuclear components occupy this category, critical applications grew by default. The tendency to associate the term with specific components, rather than failures, compounded confusion.
The final application of the term "critical" to non-nuclear units, produced a flow process that divided analysis into critical and non-critical. Dividing components in this way (a la nuclear) proved confusing as fossil plants struggled to abandon their historical "critical" interpretation associated with production impact. By failing to rigorously apply the original criteria, and indiscriminately applying it on the basis of production impact, "critical" scope grew. The R engineering definition from FMECA diverged, as well. "Critical" was based on a numerical value that only had meaning as a relative ranking in the context of all other equipment in...
