Risk Analysis for Aging Aircraft Fleets

To date, the Air Force has applied the durability and damage tolerance requirements of MIL-STD-1530A in three areas: a) designing new aircraft; b) evaluating the durability and damage tolerance characteristics of aircraft which were designed prior to the current requirements of MIL-STD-1530A; and, c) evaluating structural repairs and modifications. These applications have used fracture mechanics principles in a deterministic manner. That is, flaw growth was predicted using a fixed potential flaw size, a fixed da/dn vs AK relationship, and a stress spectrum derived from a predicted average usage. While it was realized that there are many stochastic elements in the initiation and growth of cracks, the applied process was considered conservative. Initial flaw size assumptions were generally severe, tracking programs accounted for variations in usage severity, and inspections (if necessary) were scheduled at half the time required for the specified initial flaws to grow to a critical size. At the time of the assessments, there was a high (but unquantified) degree of assurance that fatigue failures and widespread cracking would not occur within the design operational lifetime.