TBO
Time Between Overhauls
The recommended engine operating hours between major overhauls, set by the engine manufacturer.
Time Between Overhauls is a manufacturer recommendation (not a Part 91 legal limit) for how many engine hours a piston aircraft engine can operate before being overhauled. Lycoming and Continental publish TBO figures by model. A common value is 2,000 hours for a Lycoming O-360 or IO-360 series; an IO-550 may be 1,700–2,000 hours depending on variant.
TBO is measured in tachometer hours, not Hobbs. An aircraft with high taxi time can accumulate Hobbs hours noticeably faster than tach, and tracking the wrong one can give a misleading picture of how close the engine is to overhaul.
Under Part 91, exceeding TBO is not illegal — many engines run well past published TBO with oil analysis, careful operation, and engine monitor data. Under Part 135, TBO compliance is mandatory.
When it matters
TBO drives the engine reserve component of per-hour ownership cost. Divide expected overhaul cost by hours-to-TBO to get the per-hour reserve.
In Aloft360
Calculate engine reserve