Why and when must the preventive maintenance be performed?

In this work we will illustrate the criteria useful to identify simple and quick operations through which “the why and the when” a scheduled maintenance can be performed in order to avoid excessive checks and controls, often difficult and expensive to run. The question that in fact arises in the preparation of preventive maintenance program is if it is possible to satisfy the reliability requirements compatible with the economic and safety needs of the user. The answers can only be given following a correct heuristic method. Experimental data, resulting from repetitive measurements, should only serve to build simulation models that can predict what the future will hold and then as subsequent control measurements to confirm the above-mentioned model. This is particularly useful in the planned maintenance schedule, in which you get to answer the question: "how often"? Fit mathematical models allow to calculate, with precision and accuracy, the convenient time at which to intervene. To do this, it is therefore necessary to understand the mechanisms that dominate the evolution of the processes (prognostics) that may lead to failures. We present two examples of applications: one concerning a railway structure and the other relative to an aircraft structure.