What happens during Engine Performance Restoration
An engine performance restoration is a scope of work ("workscope") for a turbofan engine shop visit, where the engine is removed from the aircraft and sent to a engine maintenance facility and the engine case is opened.

An engine performance restoration involves the inspection, rebalancing or replacement of specified parts in the core gas flow path, that is, HPC, combustor, and HPT. These parts are most critical for the fuel burn and EGT margin of the engine. If any of these parts are life-limited, they will be changed at this point.

It also should include any mandatory inspections or replacements, covering the whole engine, dictated by the manufacturer or regulators.

It excludes any optional refurbishment or inspection of the fan, LPC, LPT, and gearbox. It may also exclude inspection of parts deemed non-performance critical in the core.

Because of this, the cost of the visit is reduced, but the engine will not be as efficient and potentially require another visit sooner than a "full overhaul". Thus, an engine performance restoration is the most basic scheduled engine shop visit. It is common that a performance restoration be alternated with a full overhaul.

See Engine Maintenance Concepts or Basics of Aircraft Maintenance Reserve Development and Management for a more detailed overview.