Time Between Overhauls (TBO)
Time Between Overhauls (TBO)
Definition
Time Between Overhauls (TBO) is a manufacturer's recommended number of running hours or calendar time before an aircraft engine or other component must be overhauled. Depending on how the aircraft is used, TBO could be a recommended limit or a mandatory limit.
Description
Under U.S. Federal Aviation Administration (FAA) regulations, TBO is mandatory for commercial operators. An engine cannot be used beyond the manufacturer's recommended service life without an overhaul. In noncommercial operations, an aircraft may fly beyond TBO as long as a qualified mechanic certifies airworthiness. However, an engine past its TBO time, often referred to as a "run-out engine," reduces the market worth of an aircraft.
According to FAA Advisory Circular 43-11, the general aviation industry makes a distinction between two types of overhaul on a reciprocating engine:
Major Overhaul - The complete disassembly of an engine. The overhaul facility inspects the engine, repairs it as necessary, reassembles, tests, and approves it for return to service within the fits and limits specified by the manufacturer's overhaul data.
Top Overhaul - Top overhaul consists of repair to parts outside of the crankcase and can be accomplished without completely disassembling the entire engine. It can include the removal of cylinders, inspection and repair to cylinders, inspection and repair to cylinder walls, pistons, valve-operation mechanisms, valve guides, valve seats, and the replacement of pistons and piston rings.
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