jetcareers - View Single Post - How often are you required to have a major overhaul?

ananoman · February 25th, 2003, 23:48

Some terminology for the curious:

Remanufactured: This is a factory rebuild, it can only be done by the original manufacturer and the engine is overhauled to new limits and is given a new log book. Since it has a new log book it is "zero timed" and none of the previous hours carry over.

Rebuilt, Overhaul: This can be done by any certificated mechanic although I would only have this done by an experienced shop that specializes in engines. The engine is torn down and all parts are measured and inspected. There are 2 types of overhauls. The engine can be rebuilt to "new limits" which is the same as a new engine, or it can be restored to "service limits" where worn parts are reused if they measure within a certain range. Obviously a service limit overhaul is cheaper. If you ever buy an aircraft you want to know what you are getting. This is why a factory reman. adds more value to a used plane since it is obvious what you are getting. With an overhaul you don't know unless it is from a good shop. Rebuilt engines carry over the same log book.

TBO: Time between/before overhaul. As others have said this is a factory number and is a recommentation only for most people. Since it comes from the factory it may not be entirely realistic as they have an incentive to make it as high as possible. Many are not aware of it, but TBO often has a calendar component as well. An unused engine can be killed by corrosion just as easily as wearing out from use. This is why it is important to preserve an engine that is not going to be flown for a while using special oils and procedures. This is often referred to as "pickling" an engine. It is often worse to let an engine sit and the engines that are run hard and often will usually live a long and happy life. As a side note part 135 operators can exceed TBO with the FAA's permission. This is based on operator experience and is usually done in 5-10% increments. At overhaul the engines are inspected and if ok, an extension can be granted up to a certain point. Usually it ammounts to 400-500 hours at most. The TBO for small engines used in trainers is usually in the order of 2000-2400 hrs. Big engines are usually around 2000 and anything with a turbo is usually 1400-1600 hours. Old and problem engines can be 1200-1500 hours.

Top overhaul: This is the replacement of a cylinder assembly or "jug". It can be the result of burned valves, a cracked head, etc. Certain engines have valve problems and you will see alot of the big Turbo Continentals that are topped at 800 hours or half way to TBO.

When buying cylinders there are several types. Most of the factory cylinders are nitrided. This is a very hard surface treatment where the metal is heated to a high temperature and exposed to amonia gas. Some claim that their cylinders are better since they are through hardened instead of nitrided, but the bottom line is they are not as wear resistant as a nitrided cylinder, so I would avoid these. Although not common anymore some cylinders are also overhauled by chrome plating the cylinder bores. Chrome is very hard and wear resistant but does not hold oil well. It was most common in radial engines. For a modern flat engine I would avoid these as well. Other cylinders can be found that have been plated with Nickel that has been impregnated with carbide. This is common in motorcycles and high performance car engines. These might be a good choice.

February 25th, 2003, 23:48	#15
ananoman Senior Member Join Date: Dec 2002 Posts: 916	Re: Show me the reg... Some terminology for the curious: Remanufactured: This is a factory rebuild, it can only be done by the original manufacturer and the engine is overhauled to new limits and is given a new log book. Since it has a new log book it is "zero timed" and none of the previous hours carry over. Rebuilt, Overhaul: This can be done by any certificated mechanic although I would only have this done by an experienced shop that specializes in engines. The engine is torn down and all parts are measured and inspected. There are 2 types of overhauls. The engine can be rebuilt to "new limits" which is the same as a new engine, or it can be restored to "service limits" where worn parts are reused if they measure within a certain range. Obviously a service limit overhaul is cheaper. If you ever buy an aircraft you want to know what you are getting. This is why a factory reman. adds more value to a used plane since it is obvious what you are getting. With an overhaul you don't know unless it is from a good shop. Rebuilt engines carry over the same log book. TBO: Time between/before overhaul. As others have said this is a factory number and is a recommentation only for most people. Since it comes from the factory it may not be entirely realistic as they have an incentive to make it as high as possible. Many are not aware of it, but TBO often has a calendar component as well. An unused engine can be killed by corrosion just as easily as wearing out from use. This is why it is important to preserve an engine that is not going to be flown for a while using special oils and procedures. This is often referred to as "pickling" an engine. It is often worse to let an engine sit and the engines that are run hard and often will usually live a long and happy life. As a side note part 135 operators can exceed TBO with the FAA's permission. This is based on operator experience and is usually done in 5-10% increments. At overhaul the engines are inspected and if ok, an extension can be granted up to a certain point. Usually it ammounts to 400-500 hours at most. The TBO for small engines used in trainers is usually in the order of 2000-2400 hrs. Big engines are usually around 2000 and anything with a turbo is usually 1400-1600 hours. Old and problem engines can be 1200-1500 hours. Top overhaul: This is the replacement of a cylinder assembly or "jug". It can be the result of burned valves, a cracked head, etc. Certain engines have valve problems and you will see alot of the big Turbo Continentals that are topped at 800 hours or half way to TBO. When buying cylinders there are several types. Most of the factory cylinders are nitrided. This is a very hard surface treatment where the metal is heated to a high temperature and exposed to amonia gas. Some claim that their cylinders are better since they are through hardened instead of nitrided, but the bottom line is they are not as wear resistant as a nitrided cylinder, so I would avoid these. Although not common anymore some cylinders are also overhauled by chrome plating the cylinder bores. Chrome is very hard and wear resistant but does not hold oil well. It was most common in radial engines. For a modern flat engine I would avoid these as well. Other cylinders can be found that have been plated with Nickel that has been impregnated with carbide. This is common in motorcycles and high performance car engines. These might be a good choice.