Maintenance
TBO (Time Between Overhauls)
The manufacturer's recommended engine operating time between major overhauls, commonly 1,800-2,200 hours for piston engines.
In practice
TBO is a planning number, not a legal limit for most Part 91 operations - engines routinely run past it on condition. Financially, though, TBO is the denominator that turns a future overhaul into a per-hour cost today: overhaul price divided by TBO is the reserve every flying hour should be funding. Groups that skip that arithmetic meet the overhaul as a five-figure surprise instead of a funded line item.
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