Definitions

The terms used in the reliability and logistic industry can often contradict and be confusing.   Where possible common definitions should be used.  Below is a collection of international and military standard definitions.

Reliability:

  • Reliability. (IEC 60050-192  Item 01-24) The ability to perform a required, without failure, for a given time interval, under given conditions.

    • NOTE 1:  The time interval duration may be expressed in units appropriate to the item concerned, e.g. calendar time, operating cycles, distance run, etc., and the units should always be clearly stated.
    • NOTE 2: Given conditions include aspects that affect reliability, such as: mode of operation, stress levels, environmental conditions, and maintenance.
    • NOTE 3: Reliability may be quantified using measures defined in Section 192-05, Reliability related concepts: measures.
  •  Reliability. (IEC 60050-192  Item 05-05) The probability of performing as required for the time interval (t1, t2), under given conditions.

    • NOTE 1: Given conditions include aspects that affect reliability, such as: mode of operation; stress levels; environmental conditions; and maintenance, where applicable.
    • NOTE 2: It is usually assumed that the item is in a state to perform as required at the beginning of the time interval.
    • NOTE 3: When t1 = 0 and t2 = t, then R(0, t) is denoted simply as R(t) and termed the reliability function, or survival function of the item. See IEC 61703, Mathematical expressions for reliability, availability, maintainability and maintenance support terms, for more details.
    • NOTE 4: See also reliability, <of an item> (192-01-24).
  • Reliability. (Defence Standard 00-40 Part 7 Issue 1) The ability of an item to perform a required function under stated conditions for a specified period of time.

    • NOTE: The term reliability is also used as a reliability characteristic denoting a probability of success, or a success ratio.
  • Reliability. (MIL-STD-721C) (1) The duration or probability of failure-free performance under stated conditions. (2) The probability that an item can perform its intended function. (For non-redundant items this is equivalent to definition (1). For redundant items this is equivalent to definition of mission reliability)

  • Reliability. (ISO 13372:2004). Probability that a machine will perform its required functions without failure for a specified time period when used under specified conditions.

  • Basic Reliability. (Defence Standard 00-40 Part 7 Issue 1) The ability of an item to perform its required functions without failure or defect for the duration of its life profile.

    • NOTE: Reliability is deemed to include Durability.
  • Mission Reliability(MIL-STD-721C) The ability of an item to perform its required functions for the duration of a specified “mission profile”.

  • Mission Reliability. (Defence Standard 00-40 Part 7 Issue 1) The probability that an item will perform its required functions for the duration of a specified mission profile.

  • Inherent Reliability. (Defence Standard 00-40 Part 7 Issue 1) The reliability potential present in a design i.e the reliability which is dependent solely on the quality of design and assumes perfect quality of manufacture and correct use in the field.

Failure Types:

  • Design Failure. (IEV 191-04-07) A failure due to inadequate design of an item.

  • Degradation Failure. (IEV 191-04-22) A failure which is both a gradual failure and a partial failure.

  • Degradation (of performance). (IEV 161-01-19)  An undesired departure in the operational performance of any device, equipment or system from its intended performance.

  • Critical Failure (IEV 191-04-02) A failure which is assessed as likely to result in injury to persons, significant material damage or other unacceptable consequences.

  • Complete Failure. (IEC 192-03-04) Failure characterized by the loss of all required functions. 

  • Common Mode Failure. (IEC 192-03-19) <within a system> failures of different items characterized by the same failure mode.

    • NOTE 1: Common mode failures may have different causes.
    • NOTE 2: Common mode failures can also be common cause failures (192-03-18).
    • NOTE 3: The potential for common mode failures reduces the effectiveness of system redundancy.
  • Catastrophic Failure. (IEV 191-04-12). A sudden failure which results in a complete inability to perform all required functions of an item.

  • Aging Failure / Wearout Failure. (IEC 192-03-16) failure whose probability of occurrence increases with the passage of calendar time due to cumulative deterioration.

    • NOTE 1: Ageing is a physical or chemical phenomenon that involves changes in characteristics of the material with time, in some circumstances related to interaction with its environment.
    • NOTE 2: In some instances, it may be difficult to distinguish between wear-out and ageing phenomena.
  • Fault.  (IEC 192-04-01) <of an item> inability to perform as required, due to an internal state.

    • NOTE 1: A fault of an item results from a failure, either of the item itself, or from a deficiency in an earlier stage of the life cycle, such as specification, design, manufacture or maintenance. See latent fault (192-04-08).
    • NOTE 2: Qualifiers, such as specification, design, manufacture, maintenance or misuse, may be used to indicate the cause of a fault.
    • NOTE 3: The type of fault may be associated with the type of associated failure, e.g. wear-out fault and wear-out failure.
    • NOTE 4: The adjective “faulty” designates an item having one or more faults.
  • Fault.(ISO 13372:2004). Condition of a component that occurs when one of its components or assemblies degrades or exhibits abnormal behavior, which may lead to the failure of the machine.

    • NOTE 1 - Fault is often the result of a failure of an item itself, but may exist without prior failure.
    • NOTE 2 - In English, the term "Fault" is also used in the field of electric power systems with the meaning as given in 604-02-01; then the corresponding term in French is "default".
  • Fault Mode. (IEV 191-05-22) One of the possible states of a fault item, for a given required function.

    • NOTE 1: A fault may be the result of a failure, but can exist without a failure.
    • NOTE 2: Planned actions or lack of external resources are not a fault.
  • Failure Mode. (IEC 192-03-17) DEPRECATED: fault mode, manner in which failure occurs. Note 1: A failure mode may be defined by the function lost or other state transition that occurred.

  • Failure Mode. (ISO 13372:2004). Effect by which a failure is observed.

  • Failure Mode. (DEFSTAN 00-45 Part 1 Issue 1) A single event that causes a functional failure.

  • Failure Mode. (MIL-STD-721C). The consequence of the mechanism through which the failure occurs. I.e. short, open, fracture, excessive wear.

  • Failure Mode. (ISO 14224:2006). Effect by which a failure is observed on the failed item.

  • Failure Mode. (AS IEC 60300.3.11). One of the possible states of a failed item, for a given required function.

  • Failure Mode. (AS IEC 60812-2008). Manner in which an item fails.

  • Failure Mode. (SAE JA1011). A single event, which causes a functional failure.

  • Failure. (ISO 13372:2004) Termination of the ability of an item to perform a required function.  Note: Failure is an event as distinguished from fault which is a state.

  • Failure. (DEFSTAN 00-45 Part 1 Issue 1) The inability of an item to meet a desired standard of performance. (Acuitas Comment: Note that this does not mention function. For example if the item failed a stated wear limit, but was before functional failure, then by this definition it is a failure)
    • NOTE 1 - After failure an item has a fault.
    • NOTE 2 - "Failure" is an event, as distinguished from "Fault" which is a state.
    • NOTE 3 - This concept as defined does not apply to items consisting of software only.
    • (Relken Comment: Note that this is more closely aligned to functional failure and does not include a definition where the item fails a specification, such as brake pads being past limits.)
  • Failure. (IEV 191-04-01) The termination of the ability of an item to perform a required function

  • Evident Failure. (JA1011 2009) A failure mode whose effects will on their own eventually and inevitably become evident to the operating crew under normal circumstances.

  • Hidden Failure. (JA1011 2009) A failure mode whose effects will not on their own become evident to the operating crew under normal circumstances.
    • NOTE - A gradual failure may be anticipated by prior examination of monitoring and can sometimes be avoided by preventive maintenance.
  • Gradual Failure. (IEV 191-04-11) A failure due to a gradual change with time of the given characteristics of an item.

  • Functional Failure. (DEFSTAN 00-45 Part 1 Issue 1) A state in which a physical asset or system is unable to perform a specific function to a desired level of performance.

  • Functional Failure. (JA1011 2009) A state in which a physical asset or system is unable to perform a specific function to a desired level of performance.
    • NOTE - The use of the term "failure mode" in this sense is now deprecated.
  • Random Failure. (IEC 61511 ISA 84) failure, occurring at a random time, which results from a variety of degradation mechanisms in the hardware

  • Random Failure. (IEC 62280-1 Ed 1.0) A failure that occurs randomly in time.

  • Random Failure. (DEF STAN 00-40 Part 7 Issue 1 2003) A failure whose time of occurrence is predictable only in a probabilistic sense (i.e not deterministic).

  • Random Failure. Can also refer to a constant failure rate as referenced in the following publications.  (Relken Comment:  The definition of a random failure being a constant failure rate is at odds with the definitions above which talk more about the failure event being non-deterministic):

    • Dr. Abernethy The New Weibull Handbook
    • DEF-STAN 00-45 Para Figure 6
    • JA1012 Para 13.1.4.1 and other paragraphs
  • Primary Failure. (IEV 192-03-06) failure not caused either directly or indirectly by a failure or a fault of another item. 

  • Potential Failure (DEFSTAN 00-45 Part 1 Issue 1). An identifiable condition of an asset that indicated a functional failure will occur if no preventive action is taken.

  • Potential Failure (JA1011 2009) An identifiable condition that indicates that a functional failure is either about to occur or is in the process of occurring.

  • Partial Failure. (IEC 192-03-05) failure characterized by the loss of some, but not all, required functions. Note 1: Partial failure may lead to a degraded state (192-02-25).
  • Non-Relevant Failure. (IEV 191-04-14). A failure that should be excluded in interpreting test or operational results or in calculating the value of a reliability performance measure.

  • Non-Critical Failure. (IEV 191-04-03). A failure which is assessed as not likely to result in injury to persons, significant materials damage or other inacceptable consequences.

  • Multiple Failure. (JA1011 2009 / DEFSTAN 00-45 Part 1 Issue 1) An event that occurs if a protected function fails while its protective device or protective system is in a failed state.

  • Misuse Failure (IEV 191-04-04) A failure due to the application of stresses during use which exceed the stated capabilities of the item.

  • Mishandling Failure (IEV 191-04-05) A failure caused by incorrect handling or lack of care of the item.

  • Manufacturing Failure. (IEV 191-04-08) A failure due to non-conformity during manufacture to the design of an item or to specified manufacturing processes.

  • Relevant Failure. (IEV 191-04-13) A failure that should be included in interpreting test or operational results or in calculating the value of a reliability performance measure.

  • Secondary Failure. (IEC 192-03-07) A failure caused by a failure or a fault of another item.

  • Sudden Failure. (IEV 191-04-10)  A failure that could not be anticipated by prior examination or monitoring.

  • Systematic Failure / Reproducible Failure. (IEC 192-03-10) A failure that consistently occurs under particular conditions of handling, storage or use.

    • NOTE 1: A systematic failure can be reproduced by deliberately applying the same conditions, although not all reproducible failures are systematic.
    • NOTE 2: The cause of a systematic failure originates in the specification, design, manufacture, installation, operation or maintenance of the item.
  • Weakness Failure. (IEV 191-04-06) A failure due to a weakness in the item itself when subjected to stresses within the stated capabilities of the item.

Maintenance Task Types:

  • Breakdown Maintenance.  (ISO 13372:2004).  Maintenance performed after a machine has failed.

  • Corrective Maintenance. (IEC 192-03-10) Maintenance carried out after fault detection to effect restoration. Note 1: Corrective maintenance of software invariably involves some modification.

  • Corrective Maintenance. (DEFSTAN 00-45 Part 1 Issue 1) Maintenance actions taken to restore the functional capability or condition of an item after a potential failure is discovered or a functional failure has occurred.

  • Corrective Maintenance. (SMRP Body of Knowledge) Corrective work that was identified through preventive and/or predictive maintenance tasks and completed prior to failure in order to restore the function of an asset.

  • Condition Based Maintenance. (ISO 13372:2004) Maintenance performed as governed by condition monitoring programs.

  • Condition Based Maintenance. (IEC 192-06-07) Preventive maintenance based on the assessment of physical condition.

    • NOTE 1: The condition assessment may be by operator observation, conducted according to a schedule, or by condition monitoring (192-06-28) of system parameters.
  • Condition Monitoring. (DEFSTAN 00-45 Part 1 Issue 1) Any technique used to monitor the condition of an asset. (Acuitas Comment. 

    • Note that under JA1011 On-Condition Tasks include Condition Monitoring is discussed more as a specialist technique.)

  • Condition Monitoring. (IEC 192-06-28) Obtaining information about physical state or operational parameters.

    • NOTE 1: Condition monitoring is used to determine when preventive maintenance may be required.
    • NOTE 2: Condition monitoring may be conducted automatically during operation or at planned intervals.
    • NOTE 3: Condition monitoring methods include: vibration analysis, tribology and thermography.
  • Condition Monitoring. (ISO 13372:2004) Detection and collection of information and data that indicate the state of a machine.

    • NOTE: The machine state deteriorates if faults or failures occur.
  • Diagnostics. (ISO 13372:2004) Examination of symptoms and syndromes to determine the nature of faults or failures (kind, situation, extent)

  • Deferred Maintenance (IEC 192-06-10) Maintenance postponed after identification of its need, in accordance with given maintenance rules. Note 1: Deferred maintenance can apply to preventive and corrective maintenance. Note 2: Maintenance may be deferred for availability, logistic, economic or other reasons.

  • Failure Finding Task: (JA1011 2009) A scheduled task used to determine whether a specific hidden failure has occurred.

  • On-Condition Task. (JA1011 2009) A scheduled task used to detect a potential failure.

  • On-Condition Task (JA1012 2011) A periodic or continuous task used to detect a potential failure.

  • On-Condition Task (DEFSTAN 00-45 Part 1 Issue 1) A maintenance task used to detect potential failures.

  • Predictive Maintenance (ISO 13372:2004) Maintenance emphasizing prediction of failure and taking action based on the condition of the equipment to prevent failure or degradation.

  • Predictive Maintenance (SMRP Body of Knowledge) Predictive Maintenance is an asset maintenance strategy based on assessing the condition of an asset to determine the likelihood of failure and then taking appropriate action to avoid failure. The condition of equipment can be measured using condition monitoring technologies, statistical process control, equipment performance indicators or through use of human senses, visual, auditory, etc.

  • Preventive Maintenance. (IEC 192-06-05). maintenance carried out to mitigate degradation and reduce the probability of failure. Note 1: See also condition-based maintenance (192-06-07), and scheduled maintenance (192-06-12).

  • Preventive Maintenance. (SMRP Body of Knowledge). Preventive Maintenance is an asset maintenance strategy based on replacing or restoring an asset on a fixed interval regardless of its condition. Scheduled restoration and replacement tasks are examples of preventive maintenance.

  • Preventive Maintenance. (ISO 13372:2004) Maintenance performed according to a fixed schedule, or according to a prescribed criterion that detects or prevents degradation of a functional structure, system or component, in order to sustain or extend its useful life.

  • Preventive Maintenance. (ISO 14224:2006) That done to prevent an item from failing (preventive maintenance); partof this can be simple the checks (inspections, tests) to verify the condition of the equipment to decide whether or not any preventive maintenance is required. (Figure 6 - Maintenance Catagorization show PM including Testing/Inspection, Condition Monitoring and Periodic Maintenance)

  • Proactive Maintenance. (ISO 13372:2004) Type of maintenance emphasizing the routine detection and correction of root cause conditions that would otherwise lead to failure.  Examples: High lubricant contamination, misalignment and unbalance.

  • Prognostics.(ISO 13372:2004)  Analysis of the symptoms of faults to predict future condition and remaining useful life.

  • Repair. (IEC 192-06-14) Direct action taken to effect restoration.

    • NOTE 1: Repair includes fault localization (192-06-19), fault diagnosis (192-06-20); fault correction (192-06-21); and function checkout (192-06-22).
  • Run-To-Failure. (JA1011 2009) A failure management policy that permits a specific failure mode to occur without any attempt to anticipate or prevent it.

  • Scheduled (JA1011 2009) Performed at fixed, predetermined intervals, including "continuous monitoring" (where the interval is effectively zero)

  • Scheduled Maintenance (IEC 192-06-14) Maintenance carried out in accordance with a specified time schedule. Note 1: Scheduled maintenance may identify the need for some corrective maintenance action.

  • Scheduled Discard (JA1011 2009) A scheduled that entails discarding an item at or before a specific age limit regardless of its condition at the time.

  • Scheduled Restoration (JA1011 2009) A scheduled task that restores the capability of an item at or before a specified interval (age limit), regardless of its condition at the time, to a level that provides a tolerable probability of survival to the end of another specific interval.

  • Unscheduled Maintenance (IEC 192-06-13) Corrective maintenance that cannot be deferred.