RCM Tasks

A draft standard for Reliability-Centered Maintenance (RCM) provides detailed requirements for proactive tasks and task intervals and for default actions intended to manage the consequences of functional failures of physical assets.

Reliability-Centered Maintenance (RCM) is a process to help people determine the best policies for managing the functions of physical assets, and for managing the consequences of their failures. A technical committee sponsored by the International Society of Automotive Engineers (SAE) has developed a draft standard, Evaluation Criteria for Reliability-Centered Maintenance (RCM) Processes, to provide criteria that can be used to evaluate proposed maintenance-program-development processes and determine whether they are RCM processes.

An RCM process systematically identifies all of the asset's functions and functional failures, and identifies all of its reasonably likely failure modes (or failure causes). It then proceeds to identify the effects of these likely failure modes, and to identify in what way those effects matter. Once it has gathered this information, the RCM process then selects the most appropriate asset management policy. The seven-step process, outlined in the accompanying section Essential Elements of RCM, was discussed at length in a previous article that also covered the evolution of RCM.

Unlike some other maintenance development processes, RCM considers all asset management options: on-condition tasks, scheduled restoration tasks, scheduled discard tasks, failure-finding tasks, and one-time changes (to hardware design, operating procedures, personnel training, or other aspects of the asset outside the strict world of maintenance), as well as default actions. Criteria for resolving these issues, covered in RCM Questions 6 and 7, are explained in the discussion that follows.

Question 6: Proactive tasks
What should be done to predict or prevent each failure (proactive tasks and task intervals)? This is a complex topic, and so its criteria are presented in two groups. The first group pertains to the overall topic of selecting failure management policies. The second group of criteria pertains to scheduled tasks and intervals, which comprise proactive tasks as well as one default action (failure-finding tasks).

Failure management policies
The standard's criteria for a process that selects failure management policies are:

  • The selection of failure management policies shall be carried out as if no specific task is currently being done to anticipate, prevent, or detect the failure.
  • The failure management selection process shall take account of the fact that the conditional probability of some failure modes will increase with age (or exposure to stress), that the conditional probability of others will not change with age, and the conditional probability of yet others will decrease with age.
  • All scheduled tasks shall be technically feasible and worth doing (applicable and effective), and the means by which this requirement will be satisfied are set out under scheduled tasks in the failure management section.
  • If two or more proposed failure management policies are technically feasible and worth doing (applicable and effective), the policy that is most cost-effective shall be selected.

There are three things worth noting about these criteria.
First, RCM is a thorough process. Some analytic processes assume that the conditional probability of all failure modes increases with age (or exposure to stress), that everything is ruled by a wearout failure process. In fact, studies by United Airlines and the U.S. Navy show that a distinct minority of complex items experiences wearout. Most complex items experience infant mortality (conditional probability of failure high right after work is performed, then settles out at some lower level), while most of the remaining items fail randomly with respect to age. An RCM process will recognize that any of these patterns may be seen for each failure mode, and will not assume that one pattern fits all.

Second, the SAE group found that the exact definition of technically feasible (or applicable) and worth doing (or effective) proved to be more challenging than anticipated. These terms represent specific criteria to be applied to proposed scheduled tasks. Everyone in the group agreed upon the set of criteria that should be applied. However, some said that certain criteria were part of the technically feasible group of criteria, while others said that the same criteria were part of the worth doing group. After some discussion, it became clear that these differences reflected a real and acceptable diversity within the RCM community about various technical details of the RCM process. The SAE group agreed that the standard should not take sides with one portion of the RCM community or the other. Accordingly, its standard says that all scheduled tasks shall be technically feasible ... and worth doing. The criteria to be satisfied are found in the subsequent section. However, the standard deliberately does not say specifically which criteria belong to which category.

Third, the SAE standard recognizes that some RCM processes stop as soon as they find a task that is technically feasible and worth doing, while others keep looking in case they find another task that is also technically feasible and is more worth doing. The SAE standard does not exclude either kind of RCM process. However, it does offer acceptance criteria for the RCM processes that do keep looking.

The second group of criteria pertains to scheduled tasks and intervals. The specific criteria fall into two overall groups: those that pertain to all scheduled tasks, and those that pertain to specific kinds of scheduled tasks.

Identifying tasks
Scheduled tasks are tasks that are performed at fixed, predetermined intervals, including continuous monitoring (where the interval is effectively zero). The standard requires a process that identifies scheduled tasks to make the following decisions:

  • In the case of an evident failure mode that has safety or environmental consequences, the task shall reduce the probability of the failure mode to a level that is tolerable to the owner or user of the asset.
  • In the case of a hidden failure mode where the associated multiple failure has safety or environmental consequences, the task shall reduce the probability of the hidden failure mode to an extent which reduces the probability of the associated multiple failure to a level that is tolerable to the owner or user of the asset.
  • In the case of an evident failure mode that does not have safety or environmental consequences, the direct and indirect costs of doing the task shall be less than the direct and indirect costs of the failure mode when measured over comparable periods of time.
  • In the case of a hidden failure mode where the associated multiple failure does not have safety or environmental consequences, the direct and indirect costs of doing the task shall be less than the direct and indirect costs of the multiple failure plus the cost of repairing the hidden failure mode when measured over comparable periods of time.

When considering actions to be taken to address a failure with safety or environmental consequences, RCM will reject a task that does not reduce the probability of failure to a level tolerable to the owner or user of the asset. This implies that an RCM process will provide for finding out what probability of failure is tolerable to the asset's owner or user, and will not leave that decision to a vendor.

When considering actions to be taken to address a failure with no safety or environmental consequences, RCM will compare the costs of doing the task against the costs of not doing the task. Sometimes people will compare the cost of a single task against the cost of a single failure, forgetting that the task may be performed many times before a failure might be encountered. This is especially true of on-condition tasks, which look for warnings that a failure is about to occur. The SAE standard therefore calls upon the process to compare costs over comparable periods of time. For example, if the failure is expected to occur every 5 years, on average, then the costs of the task might be considered over a 5-year time span.

The criteria that pertain to specific kinds of scheduled tasks are broken down by type of scheduled task: on-condition, scheduled discard, scheduled restoration, and failure-finding.

On-condition tasks
An on-condition task is a scheduled task used to detect a potential failure. Such a task has many other names in the maintenance community. Some call them predictive tasks (in contrast to preventive tasks, a name that these people apply to scheduled discard and scheduled restoration tasks). Some call them condition-based tasks, referring to condition-based maintenance or CBM (again, in contrast to time-based maintenance or scheduled discard and scheduled restoration tasks). Some call them condition-monitoring tasks, since the tasks monitor the condition of the asset.

The SAE standard does not attempt to settle this diversity in vocabulary. The name of the task is less important than what it does. The standard requires a process that identifies an on-condition task (or predictive or condition-based or condition-monitoring task) to use the following criteria before accepting the task:

  • There shall exist a clearly defined potential failure.
  • There shall exist an identifiable P-F interval (or failure development period).
  • The task interval shall be less than the shortest likely P-F interval.
  • It shall be physically possible to do the task at intervals less than the P-F interval.
  • The shortest time between the discovery of a potential failure and the occurrence of the functional failure (the P-F interval minus the task interval) shall be long enough for predetermined action to be taken to avoid, eliminate, or minimize the consequences of the failure mode.

Some say that RCM does not offer a way to assign task intervals to on-condition tasks. However, Stan Nowlan and Howard Heap offered a straightforward way to do so, in 1978, and that approach is used in the SAE standard. An on-condition task is looking for a warning sign, an indication that failure is going to occur. It is called an on-condition task because the repair work that follows it is performed on the condition that a warning sign exists.

For a warning sign to be practical, there must be some identifiable time, or period, or interval, between the appearance of the warning sign and the onset of the failure. Someone must look for the warning sign often enough to be able to see the warning sign before the failure occurs. And once the warning sign is discovered, there must be enough time remaining before failure to take the appropriate action.

Some may be tempted to restrict the predetermined action to those actions that are taken by the maintenance department. RCM offers a wider range of options. Under RCM, the appropriate action may be any of several things. It may be, schedule repair work and order parts. It may be, establish plant conditions so that operations can continue while this asset is off-line for repairs. It may be, land the aircraft at the nearest airport. If the warning sign and the task (and its associated interval) do not offer enough time to take some action to avoid, eliminate, or minimize the consequences of the failure mode, then an RCM process should reject it.

Scheduled discard tasks
The next kind of task is a scheduled discard task, defined as a scheduled task that entails discarding an item at or before a specified age limit regardless of its condition at the time. The standard requires a process that identifies a scheduled discard task to use the following criteria before accepting the task:

  • There shall be a clearly defined (preferably a demonstrable) age at which there is an increase in the conditional probability of the failure mode under consideration.
  • A sufficiently large proportion of the occurrences of this failure mode shall occur after this age to reduce the probability of premature failure to a level that is tolerable to the owner or user of the asset.

RCM selects scheduled discard only when there is clear evidence that the asset experiences wearout. However, if the asset is likely to have a large number of failures before the clearly-defined wearout age, probably from some other failure mode, then the task will not do what it is intended to do: prevent premature failures.

Scheduled restoration tasks
The next kind of task is a scheduled restoration task, defined as 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 specified interval. The standard requires a process that identifies a scheduled restoration task to use the following criteria before accepting the task:

  • There shall be a clearly defined (preferably a demonstrable) age at which there is an increase in the conditional probability of the failure mode under consideration.
  • The task shall restore the resistance to failure (condition) of the component to a level that is acceptable to the owner or user of the asset.
  • A sufficiently large proportion of the occurrences of this failure mode shall occur after this age to reduce the probability of premature failure to a level that is tolerable to the owner or user of the asset.

RCM selects scheduled restoration only if there is clear evidence that the asset experiences wearout. RCM adds one criterion to those of scheduled discard: if a scheduled restoration task is to be practical, it must do an acceptable job of restoring the component.

Question 7: Default actions
What should be done if a suitable proactive task cannot be found (default actions)? This question pertains to unscheduled failure management policies: the decision to let an asset run to failure, and the decision to change something about the asset's operating context (such as its design or the way it is operated).

Failure-finding tasks
A failure-finding task is defined as a scheduled task used to determine whether a specific hidden failure has occurred. Failure-finding tasks usually apply to protective devices that fail without notice. This task represents a transition from the sixth question (proactive tasks) to the seventh question (default actions, or actions taken in the absence of proactive tasks). Failure-finding tasks are scheduled tasks like the proactive tasks. However, failure-finding tasks are not proactive. They do not predict or prevent failures. They detect failures that already have happened, in order to reduce the chances of a multiple failurethe failure of a protected function while a protective device is already in a failed state.

The standard requires a process that identifies a failure-finding task to use the following criteria before accepting the task:

  • The basis upon which the task interval is selected shall take into account the need to reduce the probability of the multiple failure of the associated protected system to a level that is tolerable to the owner or user of the asset.
  • The task shall confirm that all components covered by the failure mode description are functional.
  • The failure-finding task and associated interval selection process should take into account any probability that the task itself might leave the hidden function in a failed state.
  • It shall be physically possible to do the task at the specified intervals.

Different members of the RCM community have different ways of calculating the best failure-finding task intervals. Some use a mathematical formula that includes an exponential expression. Others use an approximation that requires only simple arithmetic. The SAE standard does not prescribe one way or another, so long as the process being evaluated uses a way that does account for the appropriate issues.

The other criteria for failure-finding tasks address points that can lessen the usefulness of a failure-finding task. Sometimes a task is proposed that checks only the electrical power supply to a protective device, without checking whether its sensor or display device has failed. Such a task will not detect all circumstances in which the protective device might be in a failed state.

Likewise, sometimes a task is proposed that might actually damage the protective device. For example, it may be possible to wear out an automatic switch by forcing it to work in order to see whether it has failed. This does not necessarily mean that such a task is inappropriate. However, a process that conforms to this SAE standard will be aware of this possibility and will take it into account as the process evaluates the task.

Run to failure
If a process offers a decision to let an asset run to failure, the SAE standard requires the process to use the following criteria before accepting the decision:

  • In cases where the failure is hidden and there is no appropriate scheduled task, the associated multiple failure shall not have safety or environmental consequences.
  • In cases where the failure is evident and there is no appropriate scheduled task, the associated failure mode shall not have safety or environmental consequences. In other words, the process must not allow its users to select run to failure if the failure mode, or (in the case of a hidden failure) the associated multiple failure, has safety or environmental consequences.

Changing the operating context
The standard's criteria for the decision to change the asset's operating context are:

  • Wherever possible, the RCM process should endeavor to extract the desired performance of the system as it is currently configured and operated by applying an appropriate scheduled task.
  • In cases where such a task cannot be found, one-time changes to the asset or system may be necessary, subject to the following criteria:
    • In cases where the failure is hidden, and the associated multiple failure has safety or environmental consequences, a one-time change that reduces the probability of the multiple failure to a level tolerable to the owner or user of the asset is compulsory.
    • In cases where the failure mode is evident and has safety or environmental consequences, a one-time change that reduces the probability of the failure mode to a level tolerable to the owner or user of the asset is compulsory.
    • In cases where the failure mode is hidden, and the associated multiple failure does not have safety or environmental consequences, any one-time change must be cost-effective in the opinion of the owner or user of the asset.
    • In cases where the failure mode is evident and does not have safety or environmental consequences, any one-time change must be cost-effective in the opinion of the owner or user of the asset.

The SAE standard expects an RCM process to leave the final decision about one-time changes to the owner or user of the asset. It is the owner or user who must decide whether a specific probability of failure is tolerable. It is the owner or user who must decide whether a one-time change is cost-effective.

The SAE standard for RCM is expected to help those who wish to apply RCM as they evaluate their own processes, or the processes proposed by vendors and consultants. By using the standard, organizations will be able to determine which processes are RCM processes, and which are not. The standard should be ready for use by September 30, 1999. MT


(A discussion of the overall standard and how to use it appeared in a previous article Standard To Define RCM.)

Dana Netherton is chairman of the SAE's RCM subcommittee, and a principal of American Management Systems, Inc. (AMS), Fairfax, VA. He has been working in the field of RCM since 1989, chiefly with surface ships in the U.S. Navy. Before joining AMS, he served in the U.S. Navy for 10 years as an officer in nuclear submarines. He can be contacted by e-mail This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Essential Elements Of RCM

The proposed standard, Evaluation Criteria for Reliability-Centered Maintenance (RCM) Processes, contains the following statement as a basis for an RCM process: Any RCM process shall ensure that all of the following seven questions are answered satisfactorily and are answered in the sequence shown below :
1. What are the functions and associated desired standards of performance of the asset in its present operating context (functions)?

2. In what ways can it fail to fulfil its functions (functional failures)?

3. What causes each functional failure (failure modes)?

4. What happens when each failure occurs (failure effects)?

5. In what way does each failure matter (failure consequences)?

6. What should be done to predict or prevent each failure (proactive tasks and task intervals)?

7. What should be done if a suitable proactive task cannot be found (default actions)?

To answer each of the above questions satisfactorily, information shall be gathered, and decisions shall be made using the critera discussed in the body of this article. All information and decisions shall be documented in a way that makes the information and the decisions fully available to and acceptable to the owner or user of the asset.

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