We have found that one of the most important factors to consider when purchasing an aircraft is engine health. Engines are expensive, one of the most costly components in any aircraft – so you really don’t want to skimp on your knowledge here. Overhaul costs alone can be $20K plus depending on engine type and condition. This is why it is so important for you to have some general knowledge about aircraft engines before you buy.
Let’s dig a little deeper.
First, aircraft engines like to be run, and run often. If they are not, and depending on engine type, they will corrode from the inside out. So how do we measure run time? One of the most misunderstood terms when it comes to aircraft purchasing is SMOH. Since Major Overhaul (SMOH) is the term used in aircraft classifieds and indicates the number of hours since the last major overhaul was completed. Easy enough and most buyers will automatically assume that an aircraft with low SMOH will have a healthy engine.
But is this true?
Engine manufactures will publish a recommended amount of time that an engine can operator before it should receive an overhaul. This time varies depending on engine type and is usually around 1800-2400 hours and is referred to as time between overhauls (TBO). Hence an engine with 200 SMOH and a TBO of 2000 hours should be able to operate for another 1800 hours. Well maybe, there is also another recommended time that engine manufactures publish that is not as well known. This is calendar time, usually in years. Therefore an engine has two recommended times for overhaul; for example it could be 2000 hours or 12 years whatever comes first.
Thus when looking at an aircraft classified and you note that an aircraft engine has 200 SMOH do not assume that it is a good engine. That engine may have been overhauled in 1963 and has not run for the past 10 years! More than likely it will need an overhaul or may have to be replaced altogether.
Another item to note is that the TBO given from manufactures are recommendations and not hard and fast rules. Many engines will operate for hundreds of hours past TBO as long as they are maintained and operated often.
Best case scenario you would like an aircraft with an engine that has been recently overhauled within the last few years and run (flown) weekly.