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Maintenance Terminology

Availability – The degree to a piece of equipment or a weapon system works properly when it is required. Computed as uptime divided by both uptime plus downtime. Availability can be most easily improved by increasing Reliability or decreasing the Maintenance Turn Time.

Backlog Maintenance – Preventative Maintenance that is necessary to prevent the deterioration of the asset or its function, but which has not been carried out.

Block Replacement – A method of repairing an isolated ambiguity group by replacing all suspected repair items as a block.

Can Not Duplicate (CND) – A fault indicated by BIT, or another sensor or health management reasoner, which cannot be confirmed at the first level of maintenance. (Compare with Retest OK).

Condition-Based Maintenance – A form of Predictive Maintenance initiated as a result of knowledge of the condition of an item from routine or continuous monitoring.

Corrective Maintenance – A Maintenance Event performed in response to a failure. Also called Fault-Induced or Reactive Maintenance. Although this "run it until it breaks" maintenance mode is still practiced, it is very inefficient and contributes to not only a high life cycle cost and low operational availability, but also to the risk of possible damage through secondary failures.

Deferred Maintenance – Preventative or Corrective Maintenance that has been deferred until a future budget cycle, or postponed until funds or parts become available. This leaves the system, by definition, in a partially degraded state, but one in which the system is Partially Mission Capable.

Degradation – The decline in a system or subsystem’s performance.

Degraded Mode – A mode in which the system is Partially Mission Capable, due to either complete or partial failure of one or more components. This may be due to a range of performance parameters that fall outside the normal acceptance value but have not resulted in a fault condition. This degraded mode covers the parameter range of a time domain failure mode from the beginning of normal wear degradation to the limit of the operational acceptance. A degraded mode may be used both as a simulation parameter and to provide design knowledge for prognostics development.

Dependability – The probability that a system can be used to perform task when desired (including the impact of downtime due to any type of maintenance).

Depot Level Maintenance– Maintenance in which the faulty SRU is tested to determine the cause of the malfunction. Faulty components are removed and replaced, and the SRU is verified ready for service by successful performance tests at the depot or sent to the factory for test and repair. In both two and three-level approaches to maintenance, D-Level Maintenance typically constitutes the last maintenance level.

Downtime – The amount of time a system is unable to perform its expected function due to scheduled or unscheduled maintenance.

Environment– A set of elements outside of the system, which may influence or may be influenced by the system.

Emergency Maintenaince – Preventative or Corrective Maintenance carried out at the highest priority to prevent a critical situation from occuring or continuing to occur.

Fault-Induced Maintenance – A Maintenance Event performed in response to a failure. Also called Corrective or Reactive Maintenance. Although this "run it until it breaks" maintenance mode is still practiced, it is very inefficient and contributes to not only a high life cycle cost and low operational availability, but also to the risk of possible damage through secondary failures.

Fully Mission Capable (FMC) – The system is in full working order, capable of performing any of its assigned missions.

Inherent Availability (AI) – The probability a system will be ready for operational use when required, based on the design characteristics only. Calculated as the ratio of mean time between downing events and the mean downtime (not including Logistics Delay Time). Compare with Operational Availability, which takes Logistics Delay Time into account.

Intermediate Maintenance (I-Level Maintenance) – Maintenance in which the malfunctioning assembly is tested to determine the cause of the malfunction and isolate the failure to a SRU. Repair is effected by removal and replacement of the faulty SRU and successful performance of the LRU to verify that it is ready for service. In three-level approaches to maintenance, I-Level Maintenance (which is sometimes called shop-level maintenenance) is typically the second level of maintenance. In two-level approaches to maintenance, This level is sometimes eliminated.

Life Unit A generic term for a standard time-based or event-based unit of measure, against which operational conditions are evaluated. Typical life units include flying hours, operating hours, sorties and calendar or clock time.

Line Replaceable Unit (LRU) – A component within a system where all of the maintenance actions required to replace component can be performed without having to return the system to a maintenance facility. Typically this requires that the maintenance actions do not require heavy industrial tools.

Logistics Requirements – Equipment, facilities, personnel, technical data or other resources required to support the maintenance concept.

Logistics Delay Time – The downtime incurred as a result of waiting for equipment, facilities or other logistics resources to become available in order to support a maintenance event.

Maintainability– Describes the ease with which an item to be retained in, or restored to, a specified condition when maintenance is performed by personnel having specified skills using prescribed procedures and resources at each prescribed level of maintenance and repair. See also Software Maintainability.

Maintenance – All actions necessary for retaining an item in or restoring it to a specified condition.

Maintenance Action – One, of possibly many, elements of a maintenance event.

Maintenance Event – The performing of those maintenance actions, including troubleshooting and diagnostics, required to restore a system to working order.

Maintenance Event Time – The sum of unscheduled and scheduled maintenance action times spent on a specific maintenance event.

Maintenance Hours per Life Unit The maintenance hours required divided by the appropriate life unit.

Maintenance Ratio – A measure of the total maintenance manpower burden required to maintain a system. The ratio is expressed as the cumulative number of manhours of maintenance expended divided by the cumulative number of end item operating hours during the same time.

Maintenance Schedule– A predetermined schedule, set of intervals, or by which maintenance events are carried out.

Maintenance Turn Time The time required to service and return to a system to mission-ready. This includes any setup required to prepare the system for its next mission.

Mean Downtime (MDT)  – The average elapsed time between losing Mission Capable status and restoring the system to at least partially mission capable status. Calculated as the ratio of total downtime over the number of downing events–most often, the total maintenance time over the number of maintenance events.

Mean Time to Repair  (MTTR) – The reliability weighted mean of repair times for an operational end item This includes test time, access time, fault isolation time, remove and replace or repair time, checkout time, and access secure time.

Mean Time Between Maintenance Actions (MTBMA) – A ratio of the total uptime over the number of scheduled and unscheduled maintenance events.

Mean Time Between Preventative Maintenance (MTBPM)– A ratio of the total uptime over the number of preventative maintenance events.

Mean Time Between Scheduled Maintenance (MTBSM) – A ratio of the total uptime over the number of scheduled maintenance events.

Mean Time Between Unscheduled Maintenance (MTBUM) – A ratio of the total uptime over the number of unscheduled maintenance events.

Mean Repair Time (MRT)– The average corrective maintenance time in an operational environment, calculated as the total corrective maintenance time over the number of corrective maintenance events.

Mission Capable (MC) – An assessment of a system’s ability to perform its required tasks, in either a Fully Mission Capable (FMC) or Partially Mission Capable (PMC) status.

Operating & Support (O&S) – The phase of a system's life cycle during which it is operated and maintained.

Operational Availability (Ao) – The probability a system will be ready for operational use when required in an operational environment. Calculated as the ratio of mean time between downing events and the mean downtime (including Logistics Delay Time). Compare with Inherent Availability, which does not take Logistics Delay Time into account.

Operational Dependability (Do) – The ratio of the Mean Time Between Critical Failures (MTBCF) over the sum of the MTBCF and the Mean Time to Restore Function (MTTRF). This measure is used to determine the degree to which the system satisfies the need for critical management information.

Operational Effectiveness – The ratio of sucessful missions and the total number of missions attempted.

Operational Maintenance (O-Level Maintenance) – Maintenance in which the repair action consists of the removal and replacement of the malfunctioning assembly (LRU, black box, equipment). In both two and three-level approaches to maintenance, O-Level Maintenenace typlically constitutes the first level.

Operational Sustainability– A measure of the degree to which a system can continue to maintain the necessary level of support for a specified duration of operations beyond its initial deployment period.

Operational R&M – The reliability and maintainability achieved in actual use.

Operational Readiness– The probability that the system is operating satisfactorily at any point in time, excluding downtime for scheduled maintenance or training.

Partially Mission Capable (PMC) – The system is operating in an impaired condition. It can perform at least one, but not all of its assigned missions.

Periodic (or Scheduled) Maintenance – A form of Planned Maintenance where the prescheduled events are predictably spaced throughout the system's life unit, typically at intervals of time or number of operations.

Planned Maintenance – A form of Preventative Maintenance characterized by maintenance events that are prescheduled that prevent one or more failure mechanisms from occuring.

Predictive Maintenance – A form of Preventative Maintenance where maintenance is only performed when a failure is predicted to be highly probable and imminent, in an attempt to achieve the maximum availability without impacting mission reliability. Predictive Maintenance typically makes use of equipment maintenance records to focus attention on key failures that lead to equipment availability and downtime. The field of Predictive Maintenance is currently being reassessed through the development of Prognostics to more accurately predict failures based on time domain failure trends.

Preventative Maintenance – A Maintenance Event performed prior to a failure in order to prevent its occurance. May be based on Scheduled or Predictive Maintenance. Preventative Maintenance is employed to increase mission reliability, at theexpense of availability and (arguably) maintenance costs.

Prioritized Replacement – Also called Serial Replacement. A method of repairing an isolated ambiguity group by replacing suspected repair items one at a time and then retesting to determine if a fault has been corrected. Prioritized replacement is used to improve maintenance costs (sometimes at the expense of a higher Mean Time To Repair).

Proactive Maintenance – Activities and actions applied to environment prior to and during operations to prevent problems, gain greatest reliability, and minimize failure. (Compare with Preventative Maintenance).

Readiness– The ability of the system to perform the tasks, for which it was designed, without unacceptable delays.

Reactive Maintenance (RM) - A Maintenance Event performed in response to a failure. Also called Fault-Induced or Corrective Maintenance. Although this "run it until it breaks" maintenance mode is still practiced, it is very inefficient and contributes to not only a high life cycle cost and low operational availability, but also to the risk of possible damage through secondary failures.

Repair Time– The corrective maintenance time, calculated as either a mean or maximum time, that is required to return a system or part to operational status. Repair time includes set-up, access, troubleshooting, disassembly, repair, reassembly, repair verification, system test, and readying procedures.

Restoral Time – The maximum maintenance downtime incurred as part of restoring the a system to Mission Capable status.

Retest OK (RTOK) – A maintenance event where the failure that triggered the event performs satisfactorily at the off-equipment maintenance level, or at the next higher level of maintenance. As a result of this event, personnel may return the item to service without taking corrective action. The RTOK rate is the percentage of items removed from an end item as a result of BIT or another sensor or health management resonser, that subsequently pass all related testing at the next maintenance level. (Compare with Can Not Duplicate).

Scheduled Maintenance – Maintenance performed according to a predetermined plan or guideline.

Serial Replacement – Also called Prioritized Replacement. A method of repairing an isolated ambiguity group by replacing suspected repair items one at a time and then retesting to determine if a fault has been corrected. Serial replacement is used to improve maintenance costs (sometimes at the expense of a higher Mean Time To Repair).

Shop Replaceable Unit (SRU) – A component where one or more maintenance actions can not be performed in the field, forcing the system to return to a maintenance facility.

Software Maintainability – The ease in which software modifications are made as enabled by documentation, manuals, source listings, and other support documents.

Software Maturity – A measure of the evolution of software to satisfy operational requirements, as indicated by the number and severity of required changes.

Subsystem Utilization Rate – The percentage of life unit time that the subsystem will operate, including time in standby mode.

Sustainability – The ability to support a system, including all Logistics Requirements, in order to maintain the necessary level and duration of operations to achieve varying mission objectives.

System Safety – The degree to which system operates in the absence of failures that contribute to injury or loss of life.

Unplanned Maintenance – Maintenance carried out to no predetermined plan.

Uptime – The total time in which the system is considered operational.