Klim, Z. and Czarnecki, H., "Maintenance Action Based on the Time Dependent Failure Rate for Safety–Critical Components," SAE Technical Paper 2003-01-2984, 2003, doi:10.4271/2003-01-2984.
The aircraft components' lifetime is a key decision–making metric for the performance of safety–critical items. The piece–part degradation and age–related changes are critical from the perspective of design and continued airworthiness. The most obvious issue during design development is to establish the need for planned replacement for components that are known to have a limited life. During investigation of an airworthiness issue, it is necessary to determine if the anomaly is time–dependent. If it is, then the anticipated failure probability as a function of time must be estimated such that a decision regarding corrective action can be made. For both cases, an analysis must be performed to determine if and when planned replacement is necessary. Because unanticipated retrofits are costly and difficult, credible and accurate lifetime prediction is essential. The purpose of the maintenance is to prevent the component degradation and loss of its initial capacity to perform an intended function. All maintenance actions could be classified into two main categories: on–condition and hard time removal. The effect of these actions on the component reliability could be different depending on the component failure rate. The maintenance category and the component reliability assessed by the exponential and Weibull model are discussed. The results show that to restart the “clock at zero” by maintenance action for the latent failures is not a necessary equivalent to restarting the component cumulative probability of failure at zero. The current reliability and safety assessment approach is based upon a constant failure rate and the latency is a key–parameter to establish maintenance intervals for components without limited lifetime. However, the latency is not a unique parameter required for the maintenance action. Some maintenance tasks have to be introduced based on the time dependent failure rate and the component lifetime, regardless if the item failure is active or latent. The System Safety Assessment procedure could include this analysis in order to preclude the failure probability increase and maintain aircraft safety at the required level.