The life of an aircraft is determined by its operational capabilities and maintenance costs rather than its initial design life goal. The guidelines of MIL-HDBK-1530 call for a Force Structural Maintenance Plan (FSMP) that is the basis of planned maintenance actions for a fleet. If unanticipated structural problems are identified due to design deficiencies or unplanned usage, the FSMP is updated using the deterministic damage tolerance methods of MIL-HDBK-1530. However, the effects of usage and time will cause fatigue cracks and corrosion damage to initiate and grow, compromising structural integrity of the fleet. Because of the uncertain nature of the sizes of the cracks that are in the fleet and the need to evaluate the interaction of cracks in multiple elements, the assessment of the effect of a population of fatigue cracks is typically made using probabilistic risk analysis. When such widespread fatigue cracking, corrosion, or use beyond the original life goals cause the deterministically based maintenance plan to be changed to ensure adequate structural integrity, the fleet is considered to be aging [Lincoln, 2000].

Sustainment is the process by which an aging aircraft fleet is maintained in an operational state. Sustainment encompasses both the actual maintenance of the structures and the analyses and tests needed to plan the maintenance tasks. As such, damage tolerance analyses are an integral part of sustainment.