A mathematical model has been developed and computerized that provides aerospace engineers and managers direct, accurate knowledge of how designated time between overhauls (TBO) affects the reliability, availability, and operational cost of an engine system. The model's inputs include: the Weibull parameters of the components making up the system; the average active repair time required to restore the system to its initial condition; and average cost estimates for engine overhaul. The final result is a set of curves: one depicts mean time between failure versus TBO; another depicts engine availability versus TBO; and a third depicts maintenance cost per operating hour versus TBO. From these curves the optimal TBO can be determined. Specific examples are presented.