Reliability – The duration or probability of
failure-free performance under stated conditions. (Compare with Logistics
Reliability and Mission Reliability)
Actions that are available or can be taken to negate or mitigate the effect of
a failure on a system.
Probability – The probability of an event, given that
another event is known to have occurred.
Action – An identification or actions, automatic or
manual, which can be taken to circumvent a failure. Also, a documented design,
process, procedure, or materials change implemented and validated to correct
the cause of failure or design deficiency.
or Sympathetic Failure – The inability of two or more
items to perform their function as the result of some single event, thus
possibly negating redundancy and acting as a single point failure mode.
– A relative measure that combines both the consequences (i.e., severity) of a
particular failure mode and its frequency of occurrence.
Analysis (CA) – A procedure by which each potential failure
mode is ranked according to the combined influence of severity and probability
Matrix – A graphical representation of the failure mode and
effects, usually graphed as probability of occurrence vs. severity level.
Degradation Analysis –
Analysis involving the measurement and extrapolation of degradation or
performance data that can be directly related to the presumed failure of the
product in question.
A method of altering the failure rate of a component based on stresses caused
by operating the component beyond its specifications.
– A measure of useful life.
– The percentage of time that a system, device or component is active.
End Effect –
In a FMEA, the consequence(s) of a failure mode on the operation, function, or
status of the highest indenture level. (Compare with Local Effect and Next
Higher Level Effect).
Distribution – The most widely known and used distribution in
reliability evaluation of systems, most often used where the rate at which
events occur does not vary. A Weibull Distribution reduces to an exponential
distribution when the beta (slope) parameter is set to 1.0.
– The ability to sustain a failure and retain sufficient
operational capability for save mission continuation.
– The ability to sustain a failure and retain the capability to successfully
terminate the mission.
– The loss of ability of a system, device or process to perform a required
function. The manifestation of a fault. (See also Hardware Failure and Software
Analysis – The logical, systematic examination of a system to
identify the probability, causes, and consequences of potential failures.
Cause– The circumstances during design, manufacturing or use which
have induced or activated a failure mechanism. The basic reason(s) for a
failure. (Compare with Failure Mechanism).
– A mathematical model that describes the probability of failure
ocurring over time. Also known as a probability density function (pdf), this
function can be utilized to determine the probability that a failure takes
place in a given time interval
Effect – The consequences a failure has on the operation,
function, or status of a device. (See also Object Failure Effect and Design
Mechanism– The physical, chemical, electrical, thermal or other
process that results in failure. For a system, the failure mechanism is the
process of error propagation following a component failure which leads to a
system failure. (Compare with Failure Cause).
– The characteristic manner in which a failure occurs. Within a failure mode
diagnostic model, failure modes represent specific ways in which a system,
device or process can fail. (See also Object Failure Mode).
Modes and Effects Analysis (FMEA) – An inductive, bottom-up method
of analyzing the system effects of individual failure modes.
Modes, Effects and Criticality Analysis (FMECA) – A FMEA that
also includes criticality calculations for each failure mode and effect.
– A function that describes the number of failures to a system, device
or component that can be expected to take place over a given unit of time, most
often expressed as the number of failures per million hours. It can be computed
as the inverse of MTBF.
Reporting and Corrective Action System (FRACAS) – A
process by which failures are identified and analyzed so that corrective
actions can be implemented back into the design and/or manufacturing process.
– Actions taken to assure a fault cannot occur.
Tolerance – The built-in capability of a system to provide
continued correct execution in the presence of one or more failures.
– A tree-like representation of failure causes that can be used to
determine the probability of the outcomes of those failures.
Analysis – An analysis approach in which each potential system
failure is traced back to all faults that could cause the failure. It is a
top-down approach, whereas the FMEA is a bottom-up approach.
Failure Mode Effects Analysis (Functional FMEA or FFMEA) – A FMEA
that identifies the potential impact of each functional failure mode on mission
Hazard Analysis (FHA) – An analysis of the effects, risk,
severity and probability of potential faults that is performed during the
specification and design stages, typically before failure modes are identified.
The FHA then becomes the baseline for the FMECA's developed later.
Level – a designation which identifies an item's relative
complexity as an assembly or function. In a system, the first indenture level
is the system. Examples of lower indenture levels could be system segments
(level 2), prime items (level 3), subsystems (level 4), components (level 5),
subassemblies or circuit cards (level 6), and parts (level 7).
Effect – In a FMEA, the consequence(s) of a
failure mode on the operation, function, or status of the specific item being
analyzed. (Compare with Next Higher Level Effect and End Effect).
Related Reliability – The probability that no corrective (or
unscheduled) maintenance, unscheduled removals, and/or unscheduled demands for
spare parts will occur following the completion of a specific mission profile.
Reliability – The ability of a system to perform failure free,
under specified operating conditions and time without demand on the support
system, measured as a mean time between maintenance actions. Also, a measure of
a system's ability to operate without logistics support. All failures, whether
the mission is or can be completed, are counted.
Distribution – A failure
distribution similar to the Normal distribution except that the logarithm of
the values of random variables, rather than the values themselves, are assumed
to be normally distributed.
Supports Analysis (LSA) – A systems engineering and design
process selectively applied during all life cycle phases of the
system/equipment to help ensure supportability objectives are met.
(LSAR) – That portion of LSA documentation consisting of
detailed data pertaining to the identification of logistic support resource
requirements of an equipment.
Duration – For on-orbit space systems, the average time the
system is operational before a missional critical failure occurs. The mean
mission duration is equivalent to mean time to failure for nonrepairable ground
Between Critical Failure (MTBCF) – The mean time between failures
of mission-essential functions, calculated as the ratio of active hours (those
excluding scheduled maintenance) and the number of critical failures.
Between Downing Events (MTBDE) – A measure calculated as the total
uptime over the number of downing events.
Mean Time Between
Mean Time To
Failure (MTTF) – A system, subsystem or device's mean time to
failure, as calculated at a specific point in time. This differs from MTBF in
that it changes over time as the system is maintained.
Mean Time To
First Failure (MTTFF) – The Mean Time to Failure starting from
when the system is first made to be Mission Capable.
Mean Time to
Repair (MTTR) – The total amount of time spent performing all
corrective maintenance repairs divided by the total number of those repairs.
Reliability – The probability that the system is operable and
capable of performing its required function for a stated mission duration or
for a specified time into a mission. (Compare with Basic Reliability and
Weibull Distribution – A variation of the Weibull Distribution
used to model data with distinct subpopulations that may represent different
failure characteristics over the lifetime of a product. Each subpopulation has
a separate Weibull parameters calculated and the results are combined in a
mixed Weibull distribution to represent all of the subpopulations in one
Simulation – A method of generating values from a know
distribution for the purposes of experimentation. This is often accomplished by
generating uniform random variables and using them in an inverse reliability
equation to produce failure times that would conform to the desired input
Level Effect – In a FMEA, the consequence(s) of a
failure mode on the operation, function, or status of the items in the next
indenture level above the indenture level under consideration. (Compare with
Local Effect and End Effect).
Distribution – A widely used distribution that is symmetric,
allowing the curve to be defined by a mean and a standard deviation.
Risk Assessment – An approach for documenting risk profiles based
on the failures.
Function – A mathematical model that describes the
probability of failure ocurring over time. This function can be utilized to
determine the probability that a failure takes place in a given time interval.
Distribution - A Weibull distribution whose beta (slope) value is
equal to 2.0.
– The existence of backup equipment that can be used to perform primary
functions, in the event that the primary equipment should fail. Also, the
existence of more than one way of performing a function.
– The probability that a system will perform satisfactorily for a given time
when used under specified operating conditions. More generally, reliability is
the capacity of parts, components, equipment, products and systems to perform
their required functions for desired periods of time without failure, in
specified environments and with a desired confidence. (See also Basic
Reliability and Mission Reliability). Also, the engineering discipline
concerned with predicting, monitoring, testing, and improving the reliability
of a system, device or process.
Analysis – A quantification of the sources of failures in a
system, with emphasis on the most significant contributors towards the overall
system unreliability, in order to correct them and therefore improve the
reliability of the fielded system.
Diagram (RBD) – A diagram the represents how the components
(represented by "blocks") are arranged and related reliability-wise in a larger
system. This is often, but not necessarily, the same as the way that the
components are physically related.
Distribution Curve – A curve that characterizes the changes to the
probability of failure over time.
– The set of design, development and manufacturing tasks by which
reliability is achieved.
Growth – The improvement in a reliability parameter caused by
successfully correcting design or manufacturing deficiencies.
Prediction – The primary calculation in Reliability Analysis,
referred to as the Failure Rate or number of failures during a period of time.
– A final study that provides proof that the system will remain acceptably safe
for a particular failure scenario.
– The worst potential consequence of a failure, determined by the degree
of injury, property damage, or system damage that could ultimately occur.
Failure (SPF) – Any single hardware failure or software error
which results in irreversible degradation of item mission performance below
contractually specified levels. The failure of an item which would result in
failure of the system and is not compensated for by redundancy or alternative
Failure Mode (SPFM) – The way or manner in which the sinlge
point failure of an item occurs.
Reliability – The probability that software will contribute to
failure-free system performance for a specified time under specified
conditions. The probability depends on information input into the system,
system use, and the existence of software faults encountered during input.
Calculated as the total CPU processing time over the number to software
Failure Rate – The constant failure rate after one year of
Failure – A postulated failure mode in a FMEA for which there
is no failure detection method by which the operator is made aware of the
Distribution – A simple failure calculation algorithm where a
random number is simply limited to a range.
– A statistical distribution that is widely used for matching field data, due
to its versatility and the fact that the Weibull probability density function
can assume different shapes based on the parameter (beta factor) values.