Achieved availability

Similarly to inherent availability, assumption is made of an ideal support infrastructure, i.e. logistics delays and other waiting times are ignored but downtime is composed of corrective and preventive downtime.


Availability is the proportion of time (%) for which the system is operational, i.e. available for use. It is directly affected by reliability and maintainability parameters. Mathematically speaking, this is expressed by the ratio of the Uptime and the total time (Uptime + Downtime). In systems engineering, specific availability metrics are differentiated: Inherent, Achieved and Operational Availabilities. See definitions.

Reliability Block Diagram is a common tool for availability analysis.

Condition Based Maintenance

See predictive maintenance

Condition Monitoring

Methods to measure parameters which indicate the equipment condition. For example: vibration analysis, thermography, oil analysis, ultrasound emission.

Corrective Maintenance (CM)

Ensemble of activities related to the restoration of an item after its failure, with the purpose of bringing it back to its standard operational state.


The criticality of a failure mode is a measure of the probability of the failure mode occurrence. Criticality analysis is part of FMECA.

Derating (electronics)

Derating is a process by which the allowed stress on an electronic component (Power, Voltage or Current) is updated based on a set of parameters.

For example: a derating curve can define the allowed power dissipation of an IC based on the case temperature.


Downtime otherwise known as “outage duration” is a time interval during which the equipment is not operational. Downtime has several meanings, depending on the definition of the start and end points of the outage.

End Of Life (EOL)

The term “End Of Life” (EOL) relates to a product whose production will soon end. Component EOL is important for asset management and spare part policies.
Example: Company A has a production line which uses special pumps. The pumps are manufactured by company B. When company B issues an EOL notice regarding the pumps, company A needs to quickly place a last order and/or to consider a redesign of the production line.

Failure Rate

The number of failures (according to a specific failure mode) per unit of time for a specific item. Failure rate is usually stated in either FPMH (failures per million hours) or FIT (failures in time i.e. failures per billion hours).

Failure Mode

Describes the manner in which an item may fail to perform its intended function. A single item may have several failure modes.

Failure Modes are essential for Failure Mode and Effects Analysis.

Failure Mode and Effects Analysis FMEA / FMECA

Failure mode and effect analysis (FMEA) is a systematic technique for failure analysis. FMEA includes an identification of as many failure modes as possible for each system component, the failure causes and effects. This is usually the first step in any system reliability study.
The FMEA method is part of standard programs such as Six Sigma, RCM, MSG-3.
Failure Mode, Effects and Criticality Analysis (FMECA) is an extended version of FMEA which includes evaluation of failure mode criticality (see Criticality for more details).

Failure reporting, analysis, and corrective action system (FRACAS)

FRACAS refers to a method for documenting failures and corrective actions of a system. FRACAS is usually implemented as a software.

FRACAS is required by international standards for critical systems. For example, EN 50126-1 standard for RAMS (Reliability, Availability, Maintainability and Safety) in railway applications requires the use of FRACAS as part of the system Reliability and Maintainability life cycle.

Functional Safety

Functional safety is the part of the overall safety of a system or that depends on the system operating correctly in response to its inputs, including the safe management of likely operator errors, hardware failures and environmental changes.

Integrated Logistics Support (ILS)

The management and technical process through which supportability and logistic support considerations are integrated into the design and taken into account throughout the life cycle of systems/equipment and by which all elements of logistic support are planned, acquired, tested, and provided in a timely and cost-effective manner. From: Dictionary of Military and Associated Terms. US Department of Defense 2005.

Inherent availability

Assumption is made of an ideal support infrastructure, i.e. logistics delays and other waiting times are ignored and downtime is essentially composed of corrective downtime. This is a design-level availability.

ISO 55000

ISO 55000 is an international standard for physical asset management, based on PAS 55. The standard covers topics such as: Alignment of organizational objectives, life cycle asset management, risk management and risk-based decision-making, as well as information management.

Lead Time

Lead time usually describes the time interval between the point in which a new component is ordered – to the point when the component arrives and is available for use.


Expected lifetime of an equipment, from design to disposal. Also called “womb to tomb” time.

Level Of Repair Analysis (LORA)

Methodology originating in the US Army whose purpose is to determine the optimal elements of item maintenance: to replace the item or not, repair or discard it and where to perform maintenance (facilities). Decision is made with the objective of minimizing overall costs, and while accounting for numerous parameters like: facilities, utilities, support equipment, test equipment, personnel qualifications…


Maintainability refers to the ability of the system to be properly maintained. It is related to design issues such as: accessibility, ease of test and diagnosis, requirements for calibration and lubrication.


A Metric is a parameter that provides a valuable measurement of a system performance and is used for comparisons and follow-up.


The MIL-HDBK-217 title is “Reliability prediction of electronic equipment”. It contains failure rate models for various part types used in electronic systems, such as ICs, transistors, diodes, resistors, capacitors, relays, switches, connectors, etc.

MIL-HDBK-217 includes two types of models:

  • Parts Count – A rough assessment that does not account for component stresses (Power, Current and Voltage)
  • Parts Stress – A more accurate method that accounts for component stresses

MSG-3 (Maintenance Steering Group - 3rd session)

Methodology that recommends maintenance scheduling and tasks for new commercial / business airplanes, in absence of in-service operational data. It consists in Considering the consequences of functional failures and their impact on flight Safety / Operational / Economics, and possibly leads to redesign of critical items.


Mean Time Between Failure – mean equipment operation time between failure under specified conditions. For more details regarding MTBF, MTTF and MTBCF, click here.


Mean Time To Repair / Restore. MTTR does not include logistic time. Repair / Restore time including logistic time is referred to as “Mean Down-Time”.

Overall Equipment Effectiveness (OEE)

Factor that measures Asset productivity, by multiplying the availability, performance rate and the quality rate. Commonly used in production plants.

On Condition Maintenance

See Predictive Maintenance

Operational Availability

The percentage of planned operation time in which the system is indeed operational.

Predictive Maintenance

Activities performed on an item with the purpose of measuring the condition of the item, in order to efficiently schedule an appropriate preventive action. This usually involves installing measuring devices and developing prediction models that help forecast the equipment behavior.  Similar to condition-based maintenance, or on-condition maintenance.

Preventive Maintenance (PM)

Activities performed on an operational item with the objective of reducing the probability of a functional failure. It includes preventive replacement of the item, inspection of the item (visual, measurement…). Usually performed at fixed intervals of time, therefore it is also called scheduled maintenance.

Proactive Maintenance

Extension of predictive maintenance concept that incorporates the effort to identify, understand and correct the failure root causes and mechanisms, for example by proposing design modifications.


Acronym for: Reliability, Availability, Maintainability and Safety. RAMS analyses are a well-established standard in safety-critical fields like oil & gas, nuclear power, but also in other fields like renewable energies. These analyses are helpful during design stages for identification of reliability bottlenecks, reliability improvements, performance prediction and maintenance strategy planning, and during operations when historical records become available for analysis.

Reactive Maintenance

See corrective maintenance


Reliability is the capability of an equipment not to break down during operation. Mathematically speaking, reliability is a function of time that describes the probability that an item will successfully operate up to a certain time t. For that reason it is often called survival function. When time t tends to infinity, the reliability function tends to 0.

Reliability Block Diagram (RBD)

Reliability Block Diagram is a diagrammatic method for describing how system functionality is affected by behavior of its sub-systems. System functionality is preserved as long as a path exists from the diagram input to output nodes. RBD is an excellent tool for modeling systems with redundancies such as Stand-By, Parallel, K of N, and even networks and nested networks.

Reliability Centered Maintenance (RCM)

RCM is a process to ensure that assets continue to do what their users require in their present operating context. RCM includes a set of methods for defining, reviewing and improving the asset maintenance policy in order to achieve the required goals (safety, reliability, efficiency and profitability).

Restoration time

Time required to bring an assembly back to its operational state through replacement of one of its components, excluding the removal and assembly times of the component into the assembly.

Risk Based Inspections (RBI)

Maintenance strategy based on the identification of potentially hazardous failure modes and the assessment of a related risk level. The risk level is defined by the multiplication of the likelihood of failure and the consequence of failure. Appropriate inspections schedule is defined in accordance with the obtained risk levels.

Risk Matrix

A matrix of severity columns and occurrence rate rows. A Risk color is assigned to each cell in the matrix.

Risk Matrix

Risk Matrix

Risk Matrices are used for various risk analyses including FMEA / FMECA.

Root Cause Analysis (RCA)

The primary goal of Root Cause Analysis (RCA) is to identify the “root cause” (or root causes) related to a system failure or other undesirable effect. A “root cause” is the most basic factor which triggers the chain of events that lead to failure.
RCA methodology was developed in the 1950s be NASA. RCA is a critical step for achieving systematic reliability improvement as well as effective maintenance.


Approach consisting in deliberately not maintaining an equipment until it fails. While this practice is not common, it is usually considered for items with low value and low impact on asset availability.


The state of technical system freedom from unacceptable risk of harm. From: EN50126 (1999)

Safety Integrity Level

Safety integrity level (SIL) is defined as a relative level of risk-reduction provided by a safety function. SIL is a central part of IEC 61508 standard for functional safety. Automotive Safety Integrity Level (ASIL) is the version of SIL that applies to the automotive industry (ISO 26262)

Six Sigma

Six sigma is a set of techniques and tools for process improvement. It was developed by Motorola in 1986. Today, it is used in many industrial sectors.
Six Sigma seeks to improve the quality output of processes by identifying and removing the causes of defects (errors) and minimizing variability in manufacturing and business processes. Each Six Sigma project carried out within an organization follows a defined sequence of steps and has expertized value targets, for example: reduce process cycle time, reduce pollution, reduce costs, increase customer satisfaction, and increase profits.

Scheduled Maintenance

See preventive maintenance

Turnaround time (TAT)

In the context of logistics, turnaround time (TAT) is the time interval measured between the moment when a spare is ordered from the stock to the point when the stock is replenished. Stock renewal can occur either by ordering new components or by repairing the failed components.

Unscheduled Maintenance

See corrective maintenance