Just making the point that if you bought a used machine such that if it was your turn when the overhaul came up, amortizing that cost over 12 years of ownership at 100 hours/year is quite costly. You would need to fly a lot more per year, or keep the ship for 30 years. On the other hand perhaps your capital cost was not so high with a mid or high time engine. There's a fine line between capital costs and operating costs that needs to be rationalized, though.