Time Between Overhauls (TBO)

TBO is the operating time (in hours, and sometimes calendar years) a manufacturer recommends between major engine overhauls. Time remaining to overhaul is a big driver of a piston or turboprop's value and of your near-term cost of ownership — an engine near TBO implies a large, foreseeable expense.

Why it matters: TBO is the manufacturer's recommended interval between engine overhauls, and it anchors engine economics: an engine near TBO carries a large, imminent cost, and the market prices it accordingly. The difference in value between a fresh engine and a runout one is a substantial share of many aircraft's total worth.

Two practical notes: for many private operations TBO is a recommendation rather than a hard limit, and well-monitored engines sometimes run beyond it on condition — while commercial rules are typically stricter. And calendar time matters too: an engine that sat idle for years can need attention long before the hour limit, which is why time since overhaul is read alongside how consistently the aircraft flew.