In addition to scheduled engine inspections based on hourly usage, such as Hot Section Inspections and overhauls, aircraft engines can have major calendar inspections due under certain circumstances. These typically come into play if the engine has not had a major shop visit in 10 years or more. Depending on the make/model of engine, these LUI (Low Utilization Inspections) could come due at various intervals between 10-15 years.
Generally, these inspections involve a complete engine borescope to verify that the engine does not have any corrosion issues. Additionally, some engine components may require replacement.
Ordinarily, the costs of these LUIs are not covered by the engine warranty programs. Engine OEMs, such as Pratt & Whitney and Rolls Royce, have LUI requirements.
Once the initial LUI is completed after 10 years, it may become due again every 24 months for some engine models.
However, during years 1 thru 15, depending on model, an engine may have had a maintenance event that allows the clock to be reset back to zero once complete. For example, complying with Service Bulletins during an HSI that extends the engine overhaul intervals is an example of such an event.
Typically, maintenance tracking programs do not pick up these calendar-reset events that did not involve a complete engine overhaul and will send you a notification that you have a Low Utilization Inspection coming due, when in fact you do not.
There are other calendar-reset events. A thorough engine Logbook research is required to determine this.
Cessna CIL-21-03: Water Separator Corrosion Repair
Falcon 900: Closing the Baggage Door
Citation Sovereign Inspection Safety
Important Global Flap Spigot Bushing Service Bulletin
Falcon 900 Series: 12-Month Inspection Program