failure pattern.
Figure 1.8 Percentages of Failure Modes that conformed to each failure pattern
Collectively, only 11 percent of aircraft system Failure Modes behaved according to failure patterns A, B, and C, where the likelihood of failure rises with increased operating age. Failure patterns A and B have a well-defined wearout zone; it makes sense that Failure Modes conforming to these failure patterns could effectively be managed with a fixed interval overhaul or replacement. Failure patterns A, B, and C are typically associated with simple items that are subject to, for example, fatigue or wear such as tires, brake pads, and aircraft structure.
However, the remaining 89 percent of aircraft system Failure Modes occur randomly. They correspond to failure patterns D, E, and F. After the short increase in the conditional probability of failure in pattern D, as well as the infant mortality period present in failure pattern F, the Failure Mode has the same likelihood of occurring at any interval in the equipment’s expected service life. Therefore, for 89% of Failure Modes, it makes no sense to perform a fixed interval overhaul or replacement because the probability of failure is constant. These failure patterns are typically associated with complex equipment such as electronics, hydraulics, and pneumatics.
Two most notable issues
1.Only two percent of the Failure Modes conformed to failure pattern B as shown in Figure 1.9, yet this was the failure pattern that defined the way they believed equipment failure behaved!
Figure 1.9 Percentage of Failure Modes that conformed to Failure Pattern B
2.After the short increase in the conditional probability of failure in pattern D, as well as the infant mortality period present in failure pattern F, 89 percent of Failure Modes occur randomly, as depicted in Figure 1.10.
What was astonishing was that the maintenance plans in use were derived assuming nearly all Failure Modes behaved according to failure pattern B. Yet only two percent of the Failure Modes actually behaved that way. Furthermore, it was shown that most Failure Modes occur randomly. Therefore, fixed interval overhaul or replacement technically made no sense. That is, if an item is replaced today, it has the same chance of failing tomorrow as it does one year later.
Figure 1.10 Percentage of Failure Modes that conformed to Failure Patterns D, E, and F
Figure 1.11 Percentage of Failure Modes that conformed to Failure Pattern F
Figure 1.12 Reintroducing infant mortality
More important, not only were the vast majority of scheduled overhauls and replacements senseless, their efforts to control the failure rate with fixed interval overhaul and replacement were counterproductive. Their study showed that 68 percent of Failure Modes behaved according to failure pattern F, as depicted in Figure 1.11.
Infant mortality (e.g., component installed backwards, tool left behind, poor operating procedures) played a significant role in the high unreliability rates. Therefore, these weaknesses were making things worse with scheduled overhauls and replacements. As depicted in Figure 1.12, each time a scheduled overhaul or replacement was performed, infant mortality was reintroduced into an otherwise stable system.
This research conclusively proved that fixed interval overhaul or replacement is technically not the right action to take when failure is not a function of operating age. In fact, in most cases, scheduled over-haul and replacement hurt reliability. Because most Failure Modes occur randomly, the failure rate could not be controlled by performing more scheduled overhauls and replacements. Armed with these facts, a new way of deriving scheduled maintenance tasks needed to be developed, setting the stage for the birth of RCM principles.
1.6 The Development of RCM Principles
From this research, RCM principles were first conceived within the commercial airline industry. MSG-1, Handbook: Maintenance Evaluation and Program Development was prepared by the 747 Maintenance Steering Group and published in 1968. This document contained the first use of decision diagram techniques to develop a prior-to-service scheduled maintenance program.
Improvements to MSG-1 led to the development of MSG-2: Airline/Manufacturer Maintenance Program Planning Document, which was published in 1970. MSG-2 was used to develop the scheduled maintenance programs for the Lockheed 1011 and the Douglas DC-10. It was also used on tactical military aircraft McDonnell F4J and the Lockheed P-3.
In the mid-1970s, the Department of Defense was interested in learning more about how maintenance plans were developed within the commercial airline industry. In 1976 the Department of Defense commissioned United Airlines to write a report that detailed their process. Stanley Nowlan and Howard Heap, engineers at United Airlines, wrote a book on the process and called it Reliability-Centered Maintenance. Their book was published in 1978. To many, Stanley Nowlan and Howard Heap are considered two of the most significant pioneers of the RCM process. Their book remains one of the most important documents ever written on equipment maintenance.
Using Nowlan and Heap’s book as a basis for update, MSG-3, Operator / Manufacturer Scheduled Maintenance Development was published in 1980. Since then, MSG-3 has gone through many updates. MSG-3 continues to be used within the commercial airline industry today, but is still intended to develop a scheduled maintenance program for prior to service aircraft.
Since Nowlan and Heap’s book was published, there have been various updates to the RCM process, namely the identification of environmental issues. The late John Moubray was another great pioneer of the RCM process; he did a great deal to advance RCM throughout commercial industry. His book RCM II was first published in the United Kingdom in 1991 and in the United States in 1992.
Streamlined RCM and SAE JA1011
Although RCM is a resource intensive process, analyses can be completed efficiently if the process is used correctly with the right people. However, in the mid 1990s, streamlined versions of RCM started to appear. These versions often omit key steps in the process and differ significantly from what Nowlan and Heap originally intended. As a result, the Society of Automotive Engineers (SAE) published SAE JA1011, Evaluation Criteria for Reliability-Centered Maintenance (RCM) Processes in 1999. This internationally-recognized standard outlines the criteria that any RCM process must embody in order to be called RCM. SAE JA1011 was updated in 2009.
The RCM process defined in this book complies with SAE JA1011. More important, it remains true to what the original
