Evolutionary Maintenance for Aircraft Engines

Maintenance and, in particular, maintenance policy plays a major -*art in achieving systems operational effectiveness at minimkm cost. The traditional approach to maintenance planning involves selecting optimal policies from known maintenance strategies such as scheduled inspection, preventive maintenance, corrective maintenance, oncondition maintenance etc. Using a traditional approach one may get the optimal policy as either one or combination of the above mentioned strategies, usually derived using well known optimization techniques that try to optimize the cost of maintaining the system constraint on availability, reliability etc [ref. 1,2]. However, for complex systems such as aircraft engines that have thousands of parts it is not practical to find an optimal maintenance policy under realistic assumptions. Many operators derive their maintenance plan based on the acceptable number of failures over a period of time and the expected life (contractual requirement). In many mechanical systems, components wear out as they get older (i.e., their times to failure are age-related). Also, the failure of one part very often causes damage to many other parts in the system so it is often cost-effective to replace parts before they fail. At the same time, removal of items before they fail results in loss of useful life. During maintenance, many failed parts may be replaced with parts that are recovered from previous repair (hence will have different time-to-failure distribution and wear out characteristics). Also, most of the safety critical items of the engine are hard lived (age based preventive replacement) and hence will be removed from service as soon as the age of the part reaches the specified hard life. For these reasons, it is quite likely that an initial maintenance policy may need to be adjusted with new policy after every maintenance event. This paper looks at the concept of evolutionary maintenance. This approach is especially useful for expensive, complex systems, such as aircraft engines. In evolutionary maintenance, the maintenance and inspection schedule is adjusted after every service activity. The procedure requires knowledge of the time-to-failure distribution of each module of the engine and the corresponding hard lives of any of its components if applicable. After every maintenance and inspection activity, the hard life and preventive maintenance schedule are revised by adjusting the maintenance interval without increasing the risk of failure (risk initially committed by selecting a maintenance policy). The paper compares the effectiveness of the evolutionary maintenance against the traditional approach based on the required reliability or acceptable number of failures between maintenance.