When shopping for an airplane, you will see many terms and acronyms used in describing the condition of the airplane listed for sale. Here are some of the most commonly used condition codes and terms used in ads along with their corresponding meanings.
“Overhaul” - 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. Overhauls can be done on engines, propellers, cylinders, crankshafts, engine mounts, avionics, and many other aircraft components and sub-components.
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 serviceable standards. It implies that many parts are reused instead of being replaced. This also applies to other components such as magnetos, carbs etc.
“TSOH” – Means Time Since Overhaul, which is the number of flight hours since the overhaul was performed on that particular part. Again, this can be an entire engine or any part or an engine. It can also be a propeller or avionics.
“TBO” or “TBOH” – This means Total Basic Operating time or Time Between Overhaul. All engine manufacturers set recommended overhaul interval in hours, which is a rough and not guaranteed guide to life expectancy of an engine before it will need overhaul. In a commercial aircraft, one used for hire, the operator cannot exceed the TBO hours without overhauling or replacing the part. Private operators have no legal requirement to follow TBO, but safety and frequent mechanical inspections are prudent things to do when using any component after its TBO has been reached.
“SMOH” or “TSMOH” – This is 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. A 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. This does not zero out the time on an engine, in the way that a factory overhaul would.
“TTSN,” “TSN,” or “TT” – This refers to Total Time Since New or Time Since New or Total Time, and this is usually an airframe time reference for the total number of flight hours on a used aircraft.
“SFRM” or “SFRMN” - This means time Since Factory Re-Manufactured, and it is the time flown since the engine was re-manufactured by Lycoming, Continental, or some other approved engine shop. Having an aircraft engine re-manufactured zeros out its operation time and give you a fresh TBO limit.
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