MAINTAINABILITY: The relative ease and economy of
time and resources with which an item can be retained in, or restored to, a
specified condition when maintenance is performed by personnel having
specified skill levels, using prescribed procedures and resources, at each
prescribed level of maintenance and repair. Also, the probability that an item
can be retained in, or restored to, a specified condition when maintenance is
performed by personnel having specified skill levels, using prescribed
procedures and resources, at each prescribed level of maintenance and
Maintainability as measured when maintenance is performed during the course of
a specified mission profile. A mission-related system maintainability
MAINTENANCE: All actions
necessary for retaining an item in or restoring it to a specified
MAINTENANCE ACTION: An
element of a maintenance event. One or more tasks (i.e., fault localization,
fault isolation, servicing and inspection) necessary to retain an item in or
restore it to a specified condition.
MAINTENANCE, CORRECTIVE: See
MAINTENANCE EVENT: One or
more maintenance actions required to effect corrective and preventive
maintenance due to any type of failure or malfunction, false alarm or
scheduled maintenance plan.
MAINTENANCE, MANNING LEVEL:
The total number of authorized or assigned personnel to support a given system
at specified levels of maintenance.
MAINTENANCE, PREVENTIVE: See
MAINTENANCE RATIO: A measure
of the total maintenance manpower burden required to maintain an item. It is
expressed as the cumulative number of labor hours of maintenance expended in
direct labor during a given period of the life units divided by the cumulative
number of end item life units during the same period.
MAINTENANCE, SCHEDULED: See
MAINTENANCE, UNSCHEDULED: See
MAINTENANCE TASK: The
maintenance effort necessary for retaining an item in, or changing/restoring
it to a specified condition.
MAINTENANCE TIME: An element
of downtime which excludes modification and delay time.
MEAN DOWNTIME (MDT): The
average time a system is unavailable for use due to a failure. Time includes
the actual repair time plus all delay time associated with a repair person
arriving with the appropriate replacement parts.
MEAN MAINTENANCE TIME: A
basic measure of maintainability taking into account maintenance policy. The
sum of preventive and corrective maintenance times, divided by the sum of
scheduled and unscheduled maintenance events, during a stated period of
MEAN TIME BETWEEN DEMAND
(MTBD): A measure of system reliability related to demand for logistic
support. The total number of system life units divided by the total number of
system demands on the supply system during a stated period of time.
MEAN TIME BETWEEN DOWNING
EVENTS: A measure of system reliability related to readiness and
availability. The total number of system life units divided by the total
number of events which cause the system to be unavailable to initiate its
mission(s), over a stated period of time.
MEAN TIME BETWEEN CRITICAL FAILURE
(MTBCF): A measure of mission or functional reliability. The mean number
of life units during which the item performs its mission or function within
specified limits, during a particular measurement interval under stated
MEAN TIME BETWEEN FAILURE
(MTBF): A basic measure of reliability for repairable items. The mean
number of life units during which all parts of the item perform within their
specified limits, during a particular measurement interval under stated
MEAN TIME BETWEEN MAINTENANCE
(MTBM): A measure of the reliability taking into account maintenance
policy. The total number of life units expended by a given time, divided by
the total number of maintenance events (scheduled and unscheduled) due to that
MEAN TIME BETWEEN MAINTENANCE
ACTIONS (MTBMA): A measure of the product reliability parameter related
to demand for maintenance labor. The total number of product life units,
divided by the total number of maintenance actions (preventive and corrective)
during a stated period of time.
MEAN TIME BETWEEN REMOVALS
(MTBR): A measure of the product reliability parameter related to demand
for logistic support: The total number of system life units divided by the
total number of items removed from that product during a stated period of
time. This term is defined to exclude
removals performed to facilitate other maintenance and removals for product
MEAN TIME TO FAILURE (MTTF):
A basic measure of reliability for non-repairable items. The total number of
life units of an item population divided by the number of failures within that
population, during a particular measurement interval under stated
MEAN TIME TO REPAIR (MTTR): A
basic measure of maintainability. The sum of corrective maintenance times at
any specific level of repair, divided by the total number of failures within
an item repaired at that level, during a particular interval under stated
MEAN TIME TO RESTORE SYSTEM
(MTTRS): A measure of the product maintainability parameter, related to
availability and readiness: The total corrective maintenance time, associated
with downing events, divided by the total number of downing events, during a
stated period of time. (Excludes time for off-product maintenance and repair
of detached components.)
MEAN TIME TO SERVICE (MTTS):
A measure of an on-product maintainability characteristic related to servicing
that is calculated by dividing the total scheduled crew/operator/driver
servicing time by the number of times the item was serviced.
MISSION RELIABILITY: The
measure of the ability of an item to perform its required function for the
duration of a specified mission profile. Mission reliability defines the
probability that the system will not fail to complete the mission, considering
all possible redundant modes of operation.
MISSION PROFILE: A
time-phased description of the events and environments experienced by an item
during a given mission. The description includes the criteria for mission
success and critical failures.
MISSION TIME: That element of
up time required to perform a stated mission profile.
(MTTRF): A measure of mission maintainability: The total corrective
critical failure maintenance time, divided by the total number of critical
failures, during the course of a specified mission profile.
MODIFICATION TIME: That time
during which a product is being modified to enhance or expand functionality,
correct a design deficiency, improve safety or reliability through design
changes, or to bring the product up to the latest configuration.
NON-DEVELOPMENTAL ITEM (NDI): Any previously
developed item used exclusively for governmental purposes by a Federal agency,
a State or local government, or a foreign government with which the U.S. has a
mutual defense cooperation agreement; any such item with minor modifications;
and any item fully developed and in production but not yet in use. (See
“Buying Commercial and Non-Developmental Items: A Handbook [SD-2, Apr 1996,
OUSD/A&T]” or the Federal Acquisition Regulation Parts 6, 10, 11, 12 and
14, for a complete definition and criteria.)
(NDI): Any method used for inspecting an item without physically,
chemically, or otherwise destroying or changing the design characteristics of
the item. However, it may be necessary to remove paint or other external
coatings to use the NDI method. A wide range of technology is usually
described as nondestructive inspection, evaluation, or testing (collectively
referred to as non-destructive evaluation or NDE). The core of NDE is commonly
thought to contain ultrasonic, visual, radiographic, eddy current, liquid
penetrant, and magnetic particle inspection methods. Other methodologies,
include acoustic emission, use of laser interference, microwaves, magnetic
resonance imaging, thermal imaging, and so forth.
Failures at the component, equipment, subsystem, or system (product) level
that are identifiable by analysis but cannot be identified through periodic
testing or revealed by an alarm or an indication of an anomaly.
NOT-OPERATING TIME: That time
during which the product is operable according to all indications or the last
functional test, but is not being operated.
OPERABLE: The state in which an item is able to
perform its intended function(s).
OPERATIONAL ENVIRONMENT: The
aggregate of all external and internal conditions (such as temperature,
humidity, radiation, magnetic and electric fields, shock vibration, etc.)
either natural or man made, or self-induced, that influences the form,
operational performance, reliability or survival of an item.
OPERATIONAL R&M: A
measure of reliability and maintainability that includes the combined effects
of design, installation, quality, environment, operation, maintenance, etc. on
OPERATIONAL READINESS: The
ability of a military unit to respond to its operation plan(s) upon receipt of
an operations order. (A function of assigned strength, item availability,
status, or supply, training, etc.).
OPERATIONAL TEST AND EVALUATION
(OT&E): Test and evaluation which focuses on the development of
optimum tactics, techniques, procedures, and concepts for products and items,
evaluation of reliability, maintainability and operational effectiveness, and
suitability of products and items under realistic operational
PERCENT ISOLATION TO A GROUP OF RIs: The percent
of time that detected failures can be fault isolated to a specified ambiguity
group of size n or less, where n is the number of replaceable items
PERCENT ISOLATION TO A SINGLE
RI: The percent of time that detected failures can be fault isolated to
exactly one replaceable item (RI).
(PS): A design document stating the functional requirements for an
(SPECIFICATION): Requirements that describe what the product should do,
how it should perform, the environment in which it should operate, and
interface and interchangeability characteristics. They should not specify how
the product should be designed or manufactured.
PREDICTED: That which is
expected at some future time, postulated on analysis of past experience and
PROCESS ACTION TEAM (PAT): A
group of individuals with complementary skills, committed to a common purpose,
set of performance goals, and approach for which they hold themselves
accountable, who work together using shared knowledge and capabilities to
improve business processes.
This type of Government specification, also called a system specification,
establishes requirements for items used for a particular weapon system or
program. Little potential exists for the use of the document in other programs
or applications. It is written as a performance specification, but it may
include a blend of performance and detail design type requirements.
PREPARATION TIME: The time
spent obtaining, setting up, and calibrating maintenance aids; warming up
PREVENTIVE MAINTENANCE (PM):
All actions performed to retain an item in specified condition by providing
systematic inspection, detection, and prevention of incipient
QUALIFICATION TEST: A test conducted under
specified conditions, by or on behalf of the customer, using items
representative of the production configuration, to determine if item design
requirements have been satisfied. Serves as a basis for production approval.
Also known as a Demonstration Test.
REACTION TIME: The time between the instant a
product is required to perform a function or mission and the time it is ready
to perform that function or mission. It is the time needed for a product to be
transitioned from a non-operating state to an operating state.
REASSEMBLY: Assembling the
items that were removed during disassembly and closing the reassembled
REDUNDANCY: The existence of
more than one means for accomplishing a given function. Each means of
accomplishing the function need not necessarily be identical. The two basic
types of redundancy are active and standby.
Active Redundancy - Redundancy in
which all redundant items operate simultaneously.
Standby Redundancy - Redundancy
in which some or all of the redundant items are not operating continuously
but are activated only upon failure of the primary item performing the
RELEVANT: That which
can occur or recur during the life of an item.
The duration or probability of failure-free performance under stated
conditions. (2) The probability that an item can perform its
intended function for a specified interval under stated conditions. (For
non-redundant items this is equivalent to definition (1). For redundant items
this is equivalent to definition of mission reliability.)
MAINTENANCE (RCM): A disciplined logic or methodology used to identify
preventive and corrective maintenance tasks to realize the inherent
reliability of equipment at a minimum expenditure of resources.
The improvement in reliability that results when design, material, or part
deficiencies are revealed by testing and eliminated or mitigated through
REPAIR TIME: The time
spent replacing, repairing, or adjusting all items suspected to have been the
cause of the malfunction, except those subsequently shown by interim test of
the system not to have been the cause.
probability that a failed item will be restored to operable condition within a
specified time of active repair.
REPAIRABLE ITEM: An
item which, when failed, can be restored by corrective maintenance to an
operable state in which it can perform all required functions
REPLACEABLE ITEM (RI) or
REPLACEABLE UNIT (RU): An item, unit, subassembly, or part which is
normally intended to be replaced during corrective maintenance after its
REQUEST FOR PROPOSAL
(RFP): A letter or document sent to suppliers asking to show how a
problem or situation can be addressed. Normally the supplier's response
proposes a solution and quotes a price. Similar to a Request for Quote (RFQ),
although the RFQ is usually used for products already developed.
SCHEDULED MAINTENANCE: Periodic prescribed
inspection and servicing of products or items accomplished on the basis of
calendar, mileage or hours of operation. Included in Preventive
SCREENING: A process
for inspecting items to remove those that are unsatisfactory or likely to
exhibit early failure. Inspection methods includes visual examination,
physical dimension measurement, and functional performance measurement under
specified environmental conditions.
relative ease with which an item can be serviced (i.e., kept in operating
performance of any act needed to keep an item in operating condition, (i.e.
lubricating, fueling, oiling, cleaning, etc.), but not including preventive
maintenance of parts or corrective maintenance tasks.
A failure of an item that causes the system to fail and for which no
redundancy or alternative operational procedure exists.
ANALYSIS: An analytical procedure for identifying latent paths that
cause occurrence of unwanted functions or inhibit desired functions, assuming
all components are operating properly.
SPECIFICATION (SPS): A type of specification that establishes
requirements for military-unique items used in multiple programs or
STORAGE LIFE: The
length of time an item can be stored under specified conditions and still meet
specified operating requirements. Also called shelf life.
combination of sets, groups, etc. which performs an operational function
within a product (system) and is a major subdivision of the product. (Example:
Data processing subsystem, guidance subsystem).
SUPPLY DELAY TIME:
The time between the demand on the supply system for a part or item to repair
a product, or for a new product to replace a failed product, and the time when
it is available.
SYSTEM: A composite
of equipment and skills, and techniques capable of performing or supporting an
operational role, or both. A complete system includes all equipment, related
facilities, material, software, services, and personnel required for its
operation and support to the degree that it can be considered self-sufficient
in its intended operational environment.
SYSTEM DOWNTIME: The
time interval between the commencement of work on a system (product)
malfunction and the time when the system has been repaired and/or checked by
the maintenance person, and no further maintenance activity is
(a) For repairable systems and items: the probability that a system can
successfully meet an operational demand within a given time when operated
under specified conditions. (b) For "one-shot" devices and
non-repairable items: the probability that the system will operate
successfully when called upon to do so under specified conditions.
SYSTEM FINAL TEST
TIME: The time spent confirming that a system is in satisfactory
operating condition (as determined by the maintenance person) following
maintenance. It is possible for a system final test to be performed after each
correction of a malfunction.
PARAMETER: A measure of reliability or maintainability in which the
units of measurement are directly related to operational readiness, mission
success, maintenance labor costs, or logistics support costs.
TESTABILITY: A design characteristic which allows
an item's status (operable, inoperable, or degraded) be determined and faults
within the item to be isolated in a timely manner.
TEST, ANALYZE, AND FIX
(TAAF): A synonym for reliability growth in which the three main
elements (test, analyze deficiencies, and take corrective action) for
achieving reliability growth are identified.
TEST, MEASUREMENT, AND
DIAGNOSTIC EQUIPMENT (TMDE): Any product or item used to evaluate the
condition of another product or item to identify or isolate any actual or
TEST POINT: A jack or
similar fitting to which a test probe is attached for measuring a circuit
parameter or wave form.
TIME: Time is a
fundamental element used in developing the concept of reliability and is used
in many of the measures of reliability. Determining the applicable interval of
time for a specific measurement is a prerequisite to accurate measurement.. In
general, the interval of interest is calendar time, but this can be broken
down into other intervals as shown in Figure 3-1.
FIGURE 3-1: INTERVALS OF
TIME, TURN AROUND: That element of maintenance
time needed to replenish consumables and check out an item for
TOTAL SYSTEM DOWNTIME: The time interval between
the reporting of a system (product) malfunction and the time when the system
has been repaired and/or checked by the maintenance person, and no further
maintenance activity is executed.
UNDER TEST (UUT): A UUT is any product or item (system, set, subsystem,
assembly or subassembly, etc.) undergoing testing or otherwise being evaluated
by technical means.
UNSCHEDULED MAINTENANCE: Corrective maintenance
performed in response to a suspected failure.
UPTIME: That element of ACTIVE TIME during which
an item is in condition to perform its required functions. (Increases
availability and dependability).
UPTIME RATIO: A composite measure of operational
availability and dependability that includes the combined effects of item
design, installation, quality, environment, operation, maintenance, repair and
logistic support: The quotient of uptime divided by the sum of uptime and
USEFUL LIFE: The number of life units from
manufacture to when the item has an unrepairable failure or unacceptable
failure rate. Also, the period of time before the failure rate increases due
UTILIZATION RATE: The planned or actual number of
life units expended, or missions attempted during a stated interval of
VERIFICATION: The contractor effort to: (1)
determine the accuracy of and update the analytical (predicted)
data; (2) identify design deficiencies; and (3) gain
progressive assurance that the required performance of the item can be
achieved and demonstrated in subsequent phases. This effort is monitored by
the procuring activity from date of award of the contract, through hardware
development from components to the configuration item (CI).
WEAROUT: The process that results in an increase
of the failure rate or probability of failure as the of number of life units