The short answer
TBO (Time Between Overhauls) is the engine manufacturer's recommended interval, in flight hours and calendar years, between major overhauls — commonly 1,800–2,200 hours or 12 years for piston engines. For most privately operated (Part 91) aircraft, TBO is a recommendation, not a legal limit.
What TBO is and isn't
Manufacturers publish TBO in service documents — for example, 2,000 hours or 12 years, whichever comes first, for many Lycoming and Continental models. It reflects the point where the manufacturer recommends complete disassembly and overhaul.
For Part 91 private operations, exceeding TBO is legal: the engine must be maintained and in condition for safe operation, but there is no regulation forcing an overhaul at the TBO number. Many well-maintained engines run past TBO on condition, monitored through compression checks, oil analysis, and borescope inspections. Commercial operators under Part 135 may be held to TBO or an approved program.
Why buyers and owners track it
Engine overhauls are the single largest recurring cost in piston aircraft ownership — often $25,000–$60,000+ depending on the engine. Where the engine sits relative to TBO drives valuation, insurance conversations, and budgeting. An engine's SMOH against its TBO is the standard shorthand for "how much engine is left".
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.
Frequently asked questions
Is it legal to fly past TBO?
Under Part 91 (typical private ownership), yes — TBO is a manufacturer recommendation, not a regulatory limit. The engine still has to be maintained and in condition for safe operation. Commercial operations may be required to observe TBO under their operating rules or approved maintenance programs.
What is a typical TBO for a piston aircraft engine?
Most general aviation piston engines carry a TBO between 1,800 and 2,200 hours, paired with a calendar recommendation (commonly 12 years), whichever comes first.