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Reliability-Centered Maintenance (RCM)

By: Ned Criscimagna

Introduction

Reliability-Centered Maintenance, or RCM, is a logical, structured framework for determining the optimum mix of applicable and effective maintenance activities needed to sustain the operational reliability of systems and equipment while ensuring their safe and economical operation and support. Although RCM focuses on identifying preventive maintenance (PM) actions, corrective actions are identified by default. That is, when no PM action is effective or applicable for a given item, the item is run to failure (assuming safety is not an issue). From that perspective, RCM identifies all maintenance. RCM focuses on optimizing readiness, availability, and sustainment through effective and economical maintenance.



Overview of Concept

Prior to the development of the RCM approach, it was widely believed that everything had a right time for some PM, usually replacement or overhaul. Many maintenance personnel believed that by replacing parts of a product or overhauling the product (or reparable portions thereof), the frequency of failures during operation could be reduced.

Despite this commonly accepted view, the results seemed to tell a different story. In far too many instances, PM seemed to have no beneficial effects. Indeed, in many cases, PM actually made things worse by providing more opportunity for maintenanceinduced failures. The RCM approach provides a logical way of determining if PM makes sense for a given item and, if so, selecting the appropriate type of PM based on the following:

The objective of maintenance is to preserve an items function(s). RCM seeks to preserve system or equipment function, not just operability for operabilitys sake. Redundancy improves functional reliability but increases life cycle cost in terms of procurement and life cycle cost.

RCM focuses on the end system. RCM is more concerned on maintaining system function than individual component function.

Reliability is the basis for decisions. The failure characteristics of the item in question must be understood to determine the efficacy of preventive maintenance. RCM is not overly concerned with simple failure rate; it seeks to know the conditional probability of failure at specific ages (the probability that failure will occur in each given operating age bracket).

RCM is driven first by safety and then economics. Safety must always be preserved. When safety is not an issue, PM must be justified on economic grounds.

RCM acknowledges design limitations. Maintenance cannot improve the inherent reliability it is dictated by design. Maintenance, at best, can sustain the design level of reliability over the life of an item.

RCM is a continuing process. Differences between the perceived and actual design life and failure characteristics are addressed through age (or life) exploration.

The RCM concept has completely changed the way in which PM is viewed. It is now a widely accepted fact that not all items benefit from PM. Moreover, even when PM would be effective, it is often less expensive (in all senses of that word) to allow an item to run to failure rather than to do PM.



Origins of RCM

Nowhere was the traditional philosophy of PM challenged more than in the airline industry. By the late 1950s, airline maintenance costs had increased to a point where they had become intolerable. Meanwhile, the Federal Aviation Agency (FAA) had learned through experience that changing either the frequency or content of scheduled fixed-interval overhauls had no effect on the failure rate of certain types of engines. A task force consisting of representatives of the airlines and aircraft manufacturers was formed in 1960 to study the effectiveness of airline PM as then implemented.

The task force developed a basic logic technique for developing a PM program. Subsequently, a Maintenance Steering Group (MSG) was formed to manage the development of the PM program for the new Boeing 747 (B747) jumbo jet. The PM program developed by the steering group, documented in a report known as MSG-1, resulted in an affordable PM program that ensured the safe and profitable operation of the aircraft. For example, the requirement for structural inspections for the B747 was kept to 66,000 manhours, compared to 4 million man-hours for the DC-8.

The FAA was so impressed with MSG-1 that they requested that the logic be generalized, so that it could be applied to other aircraft. So in 1970, MSG-2, Airline Manufacturer Maintenance Program Planning Document, was issued. MSG-2 defined and standardized the logic for developing an effective and economical maintenance program. In 1978, the DoD contracted with United Airlines to conduct a study into efficient maintenance programs. The study produced MSG-3, a decision logic called Reliability-Centered Maintenance (RCM) that:

  • Emphasized the detection of Hidden Failures
  • Moved from a process-oriented to a task-oriented concept
Although created by the aviation industry, RCM quickly found applications in many other industries. RCM is used to develop PM programs for public utility plants, especially nuclear power plants, railroads, processing plants, and manufacturing plants. It is no overstatement to say that RCM is now the preeminent method for evaluating and developing a comprehensive maintenance program for an item.



The RCM Process

Figure 1 summarizes the steps in the RCM process. Ideally, RCM analysis begins in the design and development of a new system. However, it can also be used quite effectively to modify and optimize the maintenance program for an existing system. Whether the system is being developed or has been in the field for some time, the basic steps in the RCM process remain the same:

  1. Obtain necessary input information
  2. Perform the RCM analysis
    1. Implement logic tree
    2. Determine effectiveness
    3. Determine economical impact
    4. Identify PM tasks
    5. Package Tasks
  3. Monitor the maintenance program
  4. Update and refine the maintenance program as warranted by analysis of operational data
Figure 1. The Shape in the RCM Process
Figure 1. The Shape in the RCM Process (Click to Zoom)


The logic tree consists of a series of questions intended to determine where preventive maintenance or design changes are required (due to safety), or where preventive maintenance is economically preferable to a purely corrective maintenance approach (i.e., perform maintenance only upon failure). Figure 2 is an excerpt of the logic tree.

Figure 2. Excerpt from RCM Logic Tree
Figure 2. Excerpt from RCM Logic Tree (Click to Zoom)



The RAC RCM Application Guide

Many commercial standards and textbooks are available on the subject of RCM. The standards are very general in nature and rely on referencing specific activities associated with RCM (Weibull analysis, for example). The textbooks are very detailed and excellent for a formal course in RCM. However, they are not application-focused and not intended to serve as easy-to-use references.

The RAC RCM Application Guide was designed to fill the gap between textbooks and standards. Figure 3 shows the Table of Contents for the guide. As can be seen from Figure 3, in addition to describing the principles of and process for RCM, the guide provides information needed to perform some key analyses and to determine the intervals for scheduled PM tasks.

Figure 3. RCM Application Guide Table of Contents
Figure 3. RCM Application Guide Table of Contents (Click to Zoom)



Conclusion

RCM provides a disciplined way to determine the optimum mix of applicable and effective maintenance activities needed to sustain the operational reliability of systems and equipment while ensuring their safe and economical operation and support. The new RAC Application Guide on RCM provides the information and steps needed to develop and implement a successful RCM program.