Written by Nicholas Kamuda
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One of the major requirements of physical asset management is the ability to assess the reliability of assets in scientific terms. Since the middle of the last century, popular ideas regarding reliability have changed dramatically. Notably, in the mid 1970s, there was a sudden shift in views of equipment failure that corresponded to the release of a study in the American commercial aviation industry by the Maintenance Steering Group Task Force.

An earlier shift in people's perspective on maintenance occurred in the early 1950s. Prior to that point, the conventional model of physical asset Reliability involved a more or less constant failure rate that culminated in a "wear-out" period at the end of the asset's operational life. In the 50s, that view shifted to one that incorporated new, second generational ideas of an "infant mortality" period (an initial period involving high failure rates). These two reliability models combined to produce what is known as the "bathtub" model of equipment failure.

Current Models of Asset Reliability

In general, production equipment now is much more complicated than it was even 30 years ago. The civil aviation industry studies in the 60s identified five failure patterns in addition to the traditional view, bringing the total to six. The first pattern is the so-called bathtub curve described above. The first, second, and third failure patterns exhibit wear out characteristics. However, of the components on civil aircraft that were assessed in these studies, only 11% fell into these 3 patterns.

The remainder of the components fell into either a pattern of steady reliability, or a pattern with high infant mortality that drops to a constant conditional probability of failure. In fact, 68% of the aircraft components studied fell into the final category, a number that indicates that for most complex assets, there is no direct correlation between operating age and reliability. New maintenance techniques, including RCM2, or Reliability Centered Maintenance, undertake to incorporate this new thinking when developing failure management policies.

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