10.3 System R&M
In this section we are concerned with those system effectiveness
submodels, e.g., availability, dependability, operational readiness, which can
be exercised to specify, predict, allocate, optimize, and measure system
Four types of parameters and examples of specific R&M terms
applicable to their specification and measurement, are shown in Table 10.3-1.
Each will be discussed in more detail in the following
R&M Parameters - These parameters will define the R&M contribution
to the readiness measurement of the system or unit. R&M by itself does not
define readiness; there are many other factors relating to personnel,
training, supplies, etc., that are necessarily included in any real measure of
readiness. The context of readiness includes many factors beyond the realm of
equipment capability and equipment R&M achievements. R&M parameters of
this type concern themselves with the likelihood of failures occurring that
would make a ready system no longer ready and with the effort required to
restore the system to the ready condition. Examples of this type of parameter are “mean time between
downing events” for reliability and “mean time to restore system” for
Mission Success R&M
Parameters - These parameters are similar to the classical reliability
discussion that is found in most reliability text books. They relate to the
likelihood of failures occurring during a mission that would cause a failure
of that mission and the efforts that are directed at correcting these problems
during the mission itself. Examples would be “mission time between critical
failures (MTBCF)” for reliability and “mission time to restore function” for
Maintenance Manpower Cost
R&M Parameters - Some portion of a system's maintenance manpower
requirement is driven by the system's R&M achievement. This category of
system R&M parameters concerns itself with how frequently maintenance
manpower is required and, once it is
required, how many man-hours are needed. Examples of this type of parameter
are “mean time between maintenance
actions” for reliability and “direct man-hours to repair” for maintainability.
Note that the maintainability example does not address the clock hours to
complete the repair. Time to restore the system, i.e., the system downtime, is
not as significant to the people concerned with manpower needs as the total
Logistic Support Cost R&M
Parameters - In many systems, this type of R&M parameter might be
properly titled as “material cost” parameters. These parameters address the
aspect of R&M achievement that requires the consumption of material.
Material demands also relate to the readiness or availability of the system.
Examples are “mean time between removals” for reliability and “total parts
cost per removal” for maintainability.
Let us examine some of the
techniques for using reliability data, reduced to parameters such as those
just discussed, for making reliability predictions.