The Intelligent Maintenance
By Manny Gdalevitch
The modern aircraft, although extremely complex when compared to its older predecessors, requires less and less maintenance. This aspect is particularly puzzling when one observes the changes in maintenance costs that occurred in the last 30 years. Maintenance used to be the biggest cost in the aircraft operating expenses. Nowadays, maintenance costs represent only a fraction of total operating costs. (Please see Figure 1.)
What really happened in the last four decades that caused such a drastic reduction in maintenance costs? One would logically assume that with advances in technology and the increased complexity of aircraft equipment, maintenance would become more intensive, resulting in increased maintenance costs. How then can it be explained that aircraft maintenance needs are decreasing?
There are two phenomena, which took place simultaneously, that can explain this change. The first relates to advances in Reliability and Maintainability science, which resulted in new maintenance philosophy. The second relates to the development of the Maintenance Steering Group (MSG) process. However, one should not forget the technological advances and stricter regulatory design requirements (such as damage tolerant structures, systems safety etc.), which are variables that have an influence on maintenance, but to a lesser degree. This article will address the Reliability and MSG developments only. It will also attempt to clarify the on-going confusion regarding the various maintenance philosophies.
Advances in Reliability and Maintainability
In the early days of aviation, the initial basic maintenance program provided to the operator was developed by the manufacturer. The basic maintenance philosophy utilized was to overhaul everything at a specified time. One underlying assumption in this philosophy was an intuitive belief that because mechanical parts wear out, the reliability of any equipment is inversely related to operating age. It therefore followed that the more frequently equipment was overhauled, the better protected it was against the likelihood of failure.
For years to follow, scheduled maintenance was a craft learned through experience and rarely examined analytically. However, over the years it was found that certain types of failures could not be prevented, no matter how intensive were the maintenance activities. This was not because the intervals were not short enough, nor because the teardown inspections were insufficiently thorough. Moreover, findings also showed that contrary to expectations, for many items, the likelihood of failure did not increase with increasing operating age. In fact, the overhauls and teardowns resulted in a reduction in reliability due to high infant mortality associated with after-installation of equipment.
The Reliability and Maintainability advances that followed took a new approach toward scheduled maintenance both from analytical and practical points of view. The new reliability approach produced studies that had examined age reliability patterns. Studies published in the Reliability Centered Maintenance (Nowlan and Heap, 1978) have concluded that about 89 percent of items cannot benefit in terms of reliability from a limit on operating age (hard time overhaul). Consequently, only about 11 percent might benefit from a limit on operating age. (See Figure 2 on page 55).
As one can see, a well defined wearout region is not uniform in the six curves and only A & B curves show clear wearout characteristics (6 percent of all items).
Although it is generally assumed that the reliability bathtub curve is representative of most items, only 4 percent of items fit into this pattern (curve A). Moreover, most complex items have been shown to have conditional probability curves represented by curves C to F.
Conclusively, these reliability findings indicate that for the majority of aircraft equipment a hard-time overhaul is an ineffective maintenance practice. Notably, the overhaul practice is extremely expensive and in fact, reduces the reliability of the equipment.
Value for Money Choosing the right maintenance program for your operations By Bill de Decker September 2000 Aircraft maintenance is expensive. On average it consumes about 25...
Scheduled Maintenance Tasks Working through the development process with the Maintenance Steering Group By Jack Hessburg March 2000 Jack Hessburg has over 40 years experience in...
What's This 'A' Check, 'C' Check Stuff? Aircraft inspections defined By Jack Hessburg April 2000 Jack Hessburg has over 40 years experience in aircraft maintenance. He is...