TBO (time between overhauls)
TBO is the manufacturer's recommended interval, in flight hours or cycles, between complete engine overhauls, at which the engine is stripped, inspected and rebuilt with new life-limited parts. Business jet turbofans typically have TBOs of 3,500–8,000 hours and some modern engines are 'on condition' with no fixed limit. An overhaul costs from around $500,000 for a small engine to several million for a large one, so the hours remaining to TBO drive a used aircraft's price. Part 135 operators must follow TBO limits; private Part 91 owners have more latitude but their aircraft are then harder to put on a charter certificate.