of a proactive maintenance plan, new operating procedures, updates to technical publications, modifications to training programs, equipment redesigns, supply changes, enhanced troubleshooting procedures, and revised emergency procedures.
In the context of RCM, these other solutions are referred to as default strategies, as depicted in Figure 1.3.
Figure 1.2 Examples of solutions that RCM can yield
Figure 1.3 RCM can yield a scheduled maintenance program and default strategies
In the context of RCM, together, scheduled maintenance tasks and default strategies are referred to as failure management strategies, as depicted in Figure 1.4. These solutions are designed to manage failure.
Figure 1.4 Failure management strategies
1.5 The Evolution of RCM Principles
It is important to understand the evolution of RCM in order to appreciate the majesty of its principles. RCM’s evolution is best told as a story, as it was told to me.
The story starts in the mid 1950s in the commercial airline industry where, at the time, it was believed that nearly all failures were directly related to operating age. In other words, failure was more likely to occur as operating age increased. Figure 1.5 illustrates this point.
The x-axis represents age, which can be measured in any units such as calendar time, operating hours, miles, and cycles. The y-axis represents the conditional probability of failure. The philosophy associated with the failure pattern is that, assuming an item stays in service and reaches the end of the useful life, the probability of failure greatly increases if it remains in service. In other words, as stated by United Airlines’ Stanley Nowlan and Howard Heap, it was believed that “every item on a complex piece of equipment has a ‘right age’ at which complete overhaul is necessary to ensure safety and operating reliability.” Therefore, it was believed that the sensible thing to do was to overhaul or replace components before reaching the end of the useful life with the belief that this would prevent failure.
The mindset that failure was more likely to occur as operating time increased was deeply embedded in the maintenance programs. At the time, approximately 85% of aircraft components were subject to fixed interval overhaul or replacement. The maintenance programs were very high in scheduled overhauls and scheduled replacements.
Figure 1.5 Traditional view of failure
Time marched on. By the late 1950s, new aircraft emerged that included brand new and more technologically advanced equipment such as electronics, hydraulics, pneumatics, pressurized cabins, and turboprop engines. Because the equipment was new, there was no operational experience or any historical failure data available. Therefore, the useful life of the new equipment components was unknown. However, a maintenance plan still had to be developed. As a result, the new plans were mirrored from the old plans. For the new equipment where there was no current maintenance to mirror, they took their best educated guess. The aircraft were sent into service and maintained using maintenance plans formulated in this manner.
By the early 1960s, failure data had been accumulated. Worldwide, the crash-rate was greater than 60 crashes per million takeoffs, and two-thirds of these crashes were due to equipment failure. To put this crash rate into perspective, that same crash rate in 1985 would be the equivalent of two Boeing 737s crashing somewhere in the world every day.
The increased crash rate became an issue for operations, management, government, and regulators, so action was taken in an attempt to increase equipment reliability. Consistent with the philosophy at the time—that failure was directly related to operating age (as depicted in Figure 1.6)—the overhaul and replacement intervals were shortened, thereby increasing the amount of maintenance that was performed and increasing maintenance downtime. An example of a shortened overhaul interval is depicted in Figure 1.6.
Figure 1.6 Example of a shortened overhaul interval
The new maintenance plans were put into service. After a period of time, they noticed that three things happened.
1.In very few cases things got better.
2.In very few cases things stayed the same.
3.But, for the most part things got worse.
The Federal Aviation Administration (FAA) and industry were frustrated by their inability to control the failure rate by changing the scheduled overhaul and replacement intervals. As a result, a task force was formed in the early 1960s. This team of pioneers was charged with the responsibility of obtaining a better understanding of the relationship between operating reliability and policy for overhaul and replacement.
They identified that two assumptions were embedded in the current maintenance philosophy.
Assumption 1: The likelihood of failure increases as operating age increases.
Assumption 2: It is assumed we know when those failures will occur.
The team identified that the second assumption had already been challenged. In an attempt to decrease the failure rate, the overhaul and replacement intervals were shortened, as depicted in Figure 1.6. But when the intervals were shortened, the failure rate increased. It was then identified that the first assumption—the likelihood of failure increases as operating age increases—needed to be challenged.
As a result, an enormous amount of research was performed. Electronics, hydraulics, pneumatics, engines, and structures were analyzed. What was discovered rocked the world of maintenance at the time. The research showed that there wasn’t one failure pattern that described how Failure Modes behave. In fact there are six failure patterns, as seen in Figure 1.7.
Failure patterns A, B, and C all have something in common. They exhibit an age-related failure phenomenon. Likewise, failure patterns D, E, and F have something in common. They exhibit randomness.
Figure 1.7 Six patterns of failure
What was especially shocking was the percentage of Failure Modes that conformed to each failure pattern. Figure 1.8 summarizes the percentage of Failure Modes conforming to
