Aerospace & DefenseFree referencePDF, Excel, Word exportAerospace MRO

TSN, TSO, CSN, TBO and Other MRO Time-in-Service Terms

Component life and repair status in aviation maintenance are recorded with a tight set of abbreviations. This glossary explains the ones planners, stores staff and ERP consultants meet most often, including the one pair (TSO) that has two meanings.

Download:

Term	Meaning	Category	How it is used
TSN	Time Since New	Life tracking	Total hours accumulated by a part since manufacture
CSN	Cycles Since New	Life tracking	Total landings, pressurisation or engine cycles since manufacture
TSO	Time Since Overhaul	Life tracking	Hours since the last overhaul, resets after a full overhaul
CSO	Cycles Since Overhaul	Life tracking	Cycles since the last overhaul
TSR	Time Since Repair	Life tracking	Hours since the last repair event
CSR	Cycles Since Repair	Life tracking	Cycles since the last repair event
TSI	Time Since Inspection	Life tracking	Hours since the last inspection of a given type
TSLV	Time Since Last Visit	Life tracking	Hours since the last shop visit for an engine or module
TBO	Time Between Overhaul	Life limits	Manufacturer recommended hours between overhauls, often used for piston engines
LLP	Life Limited Part	Life limits	Part with a mandatory retirement limit in hours, cycles or calendar time
FH	Flight Hours	Utilisation	Airborne time, usually from wheels up to wheels down
FC	Flight Cycles	Utilisation	One takeoff and landing
AFL	Aircraft Flight Log	Records	Record of flights, defects and fuel for each sector
MTBUR	Mean Time Between Unscheduled Removals	Reliability	Average operating time between unplanned removals of a component
MTBF	Mean Time Between Failures	Reliability	Average operating time between confirmed failures
NFF	No Fault Found	Repair status	Unit tested in the shop with no defect reproduced
BER	Beyond Economical Repair	Repair status	Repair cost exceeds the limit set against replacement cost
RTS	Return to Service	Repair status	Release of a unit or aircraft after maintenance
CRS	Certificate of Release to Service	Records	Signed statement that maintenance was done to approved data
AOG	Aircraft on Ground	Operations	Aircraft cannot fly until a part or action is completed
MEL	Minimum Equipment List	Operations	Lists equipment that may be inoperative for dispatch under stated conditions
MPD	Maintenance Planning Document	Planning	OEM source for scheduled maintenance tasks and intervals
MRB	Maintenance Review Board	Planning	Regulatory body process that approves the initial scheduled maintenance programme. Not the same as a manufacturing Material Review Board
MSG-3	Maintenance Steering Group 3	Planning	Logic used to develop scheduled maintenance tasks
AD	Airworthiness Directive	Compliance	Mandatory regulatory instruction to inspect or modify
SB	Service Bulletin	Compliance	Manufacturer recommendation or instruction, mandatory only if an AD refers to it
ARC	Airworthiness Review Certificate	Compliance	EASA continuing airworthiness certificate for an aircraft
PMA	Parts Manufacturer Approval	Parts	FAA approval for a replacement part made by a non-OEM producer
TSO (standard)	Technical Standard Order	Parts	FAA minimum performance standard for an article. Same letters as Time Since Overhaul, read the context
PN	Part Number	Parts	Identifier for a part design
SN	Serial Number	Parts	Identifier for one unit of a part number
LRU	Line Replaceable Unit	Parts	Unit that can be changed on the flight line
SRU	Shop Replaceable Unit	Parts	Sub-assembly replaced inside the repair shop
Rotable	Rotable component	Parts	Serialised part that is repaired and returned to stock repeatedly
Repairable	Repairable component	Parts	Part that can be repaired, often tracked by serial number
Consumable	Consumable part	Parts	Part used once and discarded, such as a fastener or seal
Expendable	Expendable part	Parts	Part not normally repaired, may be life limited or shelf life limited
PO	Purchase Order	Supply	Order to a vendor for parts or repair
ILS	Inventory Locator Service	Supply	Online parts search used by aviation buyers
ATA	Air Transport Association chapter	Records	Two digit system code used to file manuals and defects

Terms are general industry usage. Operators and MRO shops sometimes define variants in their own maintenance programme or exposition, and those definitions take priority inside that organisation.

Component life data review

Optional review of how time and cycle counters are held across your systems.

Optional. The tool and every export work without this. No spam.

How to use the glossary

Search a term such as TSN or BER, or filter by category using the search box. Use the Category column to find all life tracking terms together or all repair status terms together. Export to Excel if you are building a pick list for a maintenance system, or to PDF to give new staff in the planning office or stores a one page reference on their first day.

Life tracking terms show how a unit age is counted
Repair status terms drive what happens to a returned unit
Compliance terms feed tracking of mandatory actions

Reading the life tracking terms

A rotable carries several counters at once. TSN and CSN count from manufacture and never reset. TSO and CSO reset when the unit is fully overhauled, while TSR and CSR reset after repair, so the repair level defined in the shop manual matters. A part may be limited by hours, cycles or calendar time, whichever comes first. Life limited parts also need their back to birth trace, because a limit is only provable with an unbroken record from new.

Common mistakes

Teams often store only the latest counters and lose history, which breaks the trace for life limited parts. Another common error is confusing the Maintenance Review Board used for programme approval with a Material Review Board used for nonconforming product, and a third is reading TSO as the FAA standard when the record means Time Since Overhaul. State the full term in your data dictionary and use consistent field names across stores, planning and finance.

Do not overwrite TSN when a unit is overhauled
Record the unit of the counter (hours or cycles)
Keep a separate field for calendar limits

Frequently Asked Questions

What is the difference between TSN and TSO?

TSN counts hours since the part was new and never resets. TSO counts hours since the last overhaul and resets to zero after a full overhaul. A unit can be 12,000 hours TSN and 1,500 hours TSO at the same time.

What does BER mean in aviation maintenance?

BER means Beyond Economical Repair. A unit is declared BER when the estimated repair cost passes a threshold the operator or owner has set against replacement value. It is then scrapped, scavenged for parts or returned.

Is a Service Bulletin mandatory?

Not by itself. A Service Bulletin is a manufacturer document. It becomes mandatory when an Airworthiness Directive or a regulator requirement refers to it and makes compliance compulsory.

What is a life limited part?

A life limited part has a retirement limit in flight hours, cycles or calendar time set in the type certificate data or airworthiness limitations. When the limit is reached it must be removed from service and cannot be restored by repair.

If your component records are spread across a maintenance system and an ERP, we can help set up a private question layer that shows life used and life remaining on request.

Talk to an Expert Explore More Free Tools