Maintenance Scheduling Based on Accumulated Crack Growth

Abstract

Currently United States Air Force aircraft structures are inspected and maintained in accordance with a Force Structural Maintenance (FSM) plan. Because actual aircraft mission usage is different for each individual aircraft structure in a force of aircraft, the schedule for individual structural inspections and modifications is adjusted to account for differences in the crack growth accumulation rate. Scheduling adjustments are based on continuous direct or indirect measurements of a potential crack length through individual aircraft tracking. The purpose of this paper is to describe (1) the logistics/maintenance environment in which the tracking program operates, (2) the crack length projection procedures embedded within the tracking program, and (3) the accuracy requirements for the procedures employed. Accuracy requirements are studied through an analysis of potential errors that are possible in the projections of calendar time to critical crack length. The analysis isolates the effect of potential errors in the estimate of current crack length and compares this component of the total error with those due to the unknown future flying rate and the unknown future stress environment severity. The method permits comparison of prediction precisions for competing tracking systems, and conversely, provides realistic accuracy goals for the development of new systems. Numerical examples are presented based on data from current tracking programs.