Reliability-Centered Maintenance

Clearing Up some of the Misconceptions

John E. Skog P.E.

Maintenance and Test Engineering Co. LLC

Introduction

Over the past few years I have heard the phrase Reliability-Centered Maintenance or RCM used in a variety of context. Sometimes the term has been applied correctly, sometimes in-correctly and most of the time semi-correctly. Unfortunately, its improper use has lead to many misconceptions and false expectations when it comes to RCM, consequently some people curse the term while others hold it in much higher regard.

In order to accurately describe Reliability-Centered Maintenance one must trace the development of RCM. Fortunately for all us, this is not a long journey but one that is less than 40 years.

The Evolutionary History of RCM Analysis

Reliability-Centered Maintenance had its official birthplace in the U.S. airline industry. As air travel became more popular, aircraft operators and designers began to question the premise that all equipment had a "right age" at which complete overhaul was necessary in order to ensure proper operating safety and reliability. They began to discover that many types of failures could not be prevented effectively by any type of maintenance while other types of failures could be greatly reduced with the proper amount and types of maintenance. Those types of failures which appeared to be unaffected by maintenance were examined closely and design changes were made to make the airplanes "failure-tolerant". Those types of failures that responded well to maintenance continued to be maintained on a "age based" or "time based" frequency.

There still remained the question concerning the relationship of preventive maintenance to reliability. By the late 1950’s, the airline industry had compiled enough detailed information to examine the effectiveness of preventive maintenance. At the same time, the FAA who regulated aircraft maintenance, was becoming frustrated with the fact that they were unable to control the failure rate of certain aircraft engines no matter what maintenance practices they mandated. As a result, a joint task force was formed to investigate the capabilities of preventive maintenance.

The work of this joint group led to the establishment of the FAA/Industry Reliability Program. Their published findings stated "the Committee is convinced that reliability and overhaul time are not necessarily directly related topics; therefore, these subjects are dealt with separately." Two discoveries of the group were extremely surprising to the industry:

• "Scheduled overhaul has little effect on the overall reliability of a complex item unless the item has a dominant failure mode."
• "There are many items for which there is no effective form of scheduled maintenance."

In 1965, a rudimentary preventive maintenance decision-diagram technique was developed. This technique was improved upon and presented for the first time at an AIAA Commercial Aircraft Design and Operations meeting in June of 1967. Subsequent refinements of this technique were made and published in July of 1968 in a document titled: "Maintenance Evaluation and Program Development Handbook (MSG-1)". Further improvements were made in the process and documented in: "MSG-2, Airline/Manufacturer Maintenance Program Planning Document".

Finally in 1975, United Airlines was contracted by the U.S. Department of Defense to document the basic concepts, principles, definitions and applications of a logical discipline for the development of efficient scheduled preventive maintenance programs for complex equipment, and the on-going management of such programs. Such programs are called Reliability-Centered Maintenance (RCM) programs because they are centered on achieving the inherent safety and reliability capabilities of equipment at a minimum cost. The document was completed in 1978.

So what is RCM?

Is RCM condition monitoring?

Is RCM time directed maintenance?

Is RCM operations based maintenance?

Is RCM a process?

Reliability-Centered Maintenance is all of the above! RCM refers to a scheduled-maintenance program designed to obtain the equipment’s full inherent designed reliability and safety characteristics. An RCM program is developed through a structured logical process, which rigorously examines the questions of:

• How does a failure occur?
• What are the consequences of the failure?
• Can preventive maintenance prevent the failure?

RCM programs focus not on preventing the failure of any given item but managing the consequence of that failure for the given equipment as a whole. Failures, which jeopardize safety, must be prevented. Fortunately, under most circumstances, modern design practices do no allow a single failure to result in a significant safety consequence by either not allowing important safety functions to be provided by only one source or by protecting operating safety in some other way. On the other hand, redundant systems must be protected by scheduled maintenance to ensure reliability and avoid the risk of a multiple failure. All other failures result in economic consequences, and the value of preventive maintenance can be measured in economic terms.

In short, the driving element in all non-safety maintenance decisions is the economic consequence of the failure. Within this economic context, it is then possible to develop an efficient scheduled-maintenance program. The challenge of developing an optimum maintenance program is generally significant due to the lack of solid operating and failure information. RCM provides a process for developing an optimum maintenance program even if equipment-operating experience is limited or unavailable. The structured approach identifies when condition monitoring is effective, when time or operations based maintenance is appropriate and even when failures are acceptable.

RCM will allow one to obtain the full design operating ability of the equipment. It does not necessarily identify a new series of maintenance tasks. It identifies which tasks are most applicable, which are ineffective and provides a framework for developing an optimal preventive maintenance program.

In the Next Doble Exchange: The Essentials of RCM

Editors Note: John Skog is a Doble Consultant in the area of Maintenance Management. He introduced the Client Group to RCM in 1993. John performs RCM training and consulting services through Doble and is available to assist clients in the refinement of their current maintenance programs.