Center on Reliability

The commercial airline industry was the first to realize the benefits of a maintenance decision-making process. According to a white paper by Aladon, this led to the development of the MSG3 process in the aviation industry; in manufacturing, it's just called RCM. (For a complete description and multiple articles on RCM, go to www.aladon.co.uk.)

One of the most startling developments to come out of RCM studies involves preventive maintenance. "Many people still believe that the best way to optimize plant availability is to do some kind of proactive maintenance on a routine basis," says Aladon. This assumes one traditional view of failure (Figure 2), where devices fail as they enter a wear-out zone after a certain period of time.

This may have been true 30 years ago, but equipment is much more complex these days. Now, we identify six patterns of failure to deal with (Figure 3). According to Aladon, studies on commercial aircraft showed only 4% of failures conformed to pattern A, which contradicts another widely held belief that most products have a "bathtub" failure curve. Only 2% conformed to B, 5% to C, 7% to D, and 14% to E.

But here's the startling development: a whopping 68% conformed to pattern F--high infant mortality followed by random failures.

"These findings contradict the belief that there is always a connection between reliability and operating age," says Aladon. "Nowadays, this is seldom true. Unless there is a dominant age-related failure mode, age limits do little or nothing to improve the reliability of complex items. In fact, scheduled overhauls increase overall failure rates by introducing infant mortality back into otherwise stable systems."

Commercial aircraft and process control systems both use similar systems: pneumatics, electro-hydraulics, servomotors, networks, control valves, pumps, miles of wire and cables, networks, computers, electronic controls, and flow, temperature, level, and pressure sensors.