Time between overhaul

One important measure of an aircraft engine's overall economics is how often it has to be overhauled, the so-called time between overhaul, typically seen as TBO or TBOH.

The time between overhauls is generally a function of the complexity of the engine. Piston-based engines are much more complex than their turbine-powered cousins, and generally have TBO's on the order of 1,200 to 2,000 hours of running time. They tend toward the lower number if they are new designs, or include boosting options like a turbocharger. In comparison, jet engines and turboprops often have TBO's on the order of 3,000 to 5,000 hours.

Since the overhaul process requires the engine to be taken apart, it is typically an expensive process. The value of a used engine decreases if it is close to requiring an overhaul, so used engines (and aircraft) typically list their time since overhaul or TSOH.

It should be noted that the TBO is a time 'recommended' by the manufacturer and depending upon how the aircraft is being operated, overhauling the engine at this time is not necessarily mandatory. For aircraft used for non-commercial purposes overhauls are not mandatory, but highly recommended. Likewise, the TBO time recommended doesn't guarantee that the engine will last that long.