Posted by: Orlando Web Services | December 15, 2011

Terms for Hours and Overhaul Alphabet Soup!

Ever look at ads for aircraft and see a bunch of alphabet soup in the description concerning engine time, overhaul time, total time…etc? So what does 4280 TT;  920 SMOH; 1203 STOH; 1234 LMNOP stand for? To help explain this alphabet soup without going into too many details, I created a list for you to review that was based on a conversation I had with a guy who does annuals. Since I am not mechanically inclined I found this very helpful in understanding just what these ads were saying and NOT saying about an aircraft. BTW…LMNOP doesn’t mean a thing unless we use it for: Low Maintenance No Owner Present – many owners avoid keeping their plane current and legal.

Terms for Hours and Overhaul Alphabet soup!

Below are terms used in aircraft-for-sale advertisements showing the engine hours since the last overhaul were carried out are quoted. Hopefully this information will clarify what some of these terms mean and why it’s important not to confuse them.


Overhaul is a term used by the general aviation industry when an aircraft engine is cleaned, carefully inspected, and repaired or has parts replaced to meet service limits.

An overhaul is an overhaul as per the manufacturers specifications. There is no such thing as a major overhaul, just an overhaul, even though you will see the word “major” used to describe them.

Most overhaul’s are defined by the manufacturer with supporting documentation (usually Service Bulletins) that define what must be done and what parts must be replaced.

If an engine, for example, is advertised as overhauled, you have the right to ask how it was done. Was it done to factory new standards or to factory serviceable standards?

Only the very lower quality overhauls are done to factory servicable standards. It implies that many parts are reused instead of being replaced. This also applies to other components such as magnetos, carbs etc.

TSOH (Time Since Over Haul)

Time Since Over Haul is the number of flight hours since an Overhaul was performed.

TBO or TBOH (Time Between Over Haul)

Time Between Overhauls, an engine manufacturer’s recommended overhaul interval in hours, a rough and not guaranteed guide to life expectancy of an engine before it will need overhaul.

SMOH or TSMOH (Since Major Over Haul)

Since the overhaul process requires the engine to be taken apart, it is typically an expensive process. The value of a used engine decreases if it is close to requiring an overhaul, so used engines (and aircraft) typically list their time since overhaul or TSOH.

STOH (Since Top Over Haul)

Top overhaul is a term used by the general aviation industry when all the cylinders on the engine are overhauled or replaced with new, possibly due to corrosion.

TTSN, TSN, TT (Total Time Since New) or AFTT (Air Frame Total Time)

Total Time Since New is usually an airframe time reference for the total number of flight hours on a used aircraft.

TTAF/E (Total Time Air Frame/Engine)

Total Time Airframe and Engine(s) is usually an airframe time reference for the total number of flight hours on a used aircraft.

SFRM or SFRMN (Since Factory Re-Manufactured)

References to the time since the engines were remanufactured.

Blue skies!!!


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