Major aircraft parts undergo inspection and overhaul6 every t1 flying hours, and replacement every t2. Experience shows the cost of overhauling a particular model of jet engine can be expressed as increasing nonlinear function a1t12b of the overhaul cycle, and operating costs per hour are an increasing nonlinear function c1t22d of the replacement time. Each replacement costs a fixed part cost f plus labor cost g1t12h to identify and fix associated problems. Formulate an unconstrained NLP over t1 and t2 to find an overhaul and replacement policy that minimizes total cost per flying hour.