How often are you required to have a major overhaul?


New Member
I have a question for some of you aircraft owners out there, how often are you required to have a major overhaul? Im not looking to buy yet, but on some of these classifieds they all say hours since major overhaul, do you have to get an overhaul done so many hours, or after so long? I just thought it would be neat to know and maybey useful oneday if i decide to purchase an aircraft oneday.
A major overhaul refers to the engine or engines on the aircraft. Each engine carries a TBO - Time Before Overhaul. Some can be hard time, meaning every X hours it must be overhauled, and others are recommended meaning if the engine runs OK beyond the TBO you can keep going just don't be surprised if it dies on you. Continentals generally run about 1,200 - 1,500 hour TBOs whereas Lycomings generally run about 2,000 hour TBOs. Individual models, of course, will vary. Now, TBO is really determined by the usage and care of the engine. A poorly maintainted, roughly used engine will most likely not even come close to reaching the TBO whereas a well used (engines that sit for long periods of time can actually accumulate more wear than a frequently used engine), well cared for engine could run for many, many hours beyond the TBO.

Also, a major overhaul is, essentially, a complete reconditioning of the engine and is different from a factory remaunfacture - where the engine is sent back to the factory and the factory overhauls the engine to a state that is considered "new."

A top overhaul is a kind-of a "half-way" overhaul where basically just the cylinders/pistons are reconditioned. This is used to "carry" the engine to the TBO if it's not quite going to make it without help.
Operations under part 91 generally don't require an overhaul at a certain time. The recommended TBO (time before overhaul) is just that, a recommendation. A lot of the planes value will be determined by the time since major overhaul (SMOH), though, so it's an important thing to consider.
pilot602, you own an apache, right? If you don't mind me asking, what kind of cost would one of your lycoming engines incur on a MOH? Thanks.
I think the 0-320s (160hp for those who don't know) are going for about $18-20k for a major/factory rebuild.
I have put two engines on two different airplanes that I have owned, Both running the LYC-O-360 engine.

The TBO is time between overhaul. Most modern engines will use 2000 hrs. some of the cont have 1500 but most of the 1200 guys are out of the fleet, there is an AD that is done at overhaul time to bring it up to 2000.

A-IO-360 have a 1000 hr TBO, because it is an aerobatic engine.

The reason the 2000 hrs is there for part 91 is only a suggestion. if you maintain your engine well, know how to operate it properly, live indoors (hanger, change the oil every 25 hours, not every 50, and take care of it during the start up, (preheat when under 40-50deg) it will run well past 2000hrs.

the 2000 hrs DOES however come into play at flight schools. as does the 100hr inspection etc...

in 1996/7 I paid 11k to have a Overhauled engine put on, in 1999 it was 18k, but I upgraded much of the engine and went to **THE** shop to do it. I expect my engine to make it to 2500 hrs and maintain good compressions.

Also, if you buy a 20 year old airplane that has 1000 hrs on the engine, and has only been flown 10hrs a year for 10 years, that engine is garbage, plan on getting a new one.
Flight schools?

Eagle, could you quote me the reg that states flight schools have to overhaul their engines at TBO? I agree with your statement about 100 hour inspections for flight schools but not for overhauls. Part 135 ops is a different story. I'm talking about flight training under part 61.
Re: Flight schools?

For Part 61 overhaul at TBO is reccomended, but not required. Part 141 overhaul at TBO is manditory. Both require a 100 hour inspection.
Re: Flight schools?

For Part 61 overhaul at TBO is reccomended, but not required. Part 141 overhaul at TBO is manditory. Both require a 100 hour inspection.

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What he said,

Plus, the insurance is the true instigator.
Show me the reg...

I disagree with the part 141 statement. The aircraft I just bought was ran way over TBO by a part 141 school. I looked it up myself and I don't see where part 141 has anything to do with it. Show me the reg and I'll admit I'm wrong....

Can't argue with individual insurance company requirements, however, I've never seen anything more than the aircraft must be airworthy per part 91 and I don't see where part 91 requires you to overhaul an engine at TBO. Personally, I wouldn't do business with an insurance company that requires a higher standard than the FAR's.

Sorry...but I'm still not buying it.
Re: Show me the reg...

Ack, I'd hate to shoot my boss in the foot, but we had a 141 school for a while, (now only 61), and we had many planes WELL over TBO. Our latest addition to the fleet was a plane running cargo that hit TBO, moved a plane from the school that was under TBO to charter and moved the TBO'ed plane to the school. me confused

Anyways, some schools run phase inspections which is a bit different than the 50hr/100hr/annual run of things.
Re: Show me the reg...

I disagree with the part 141 statement. The aircraft I just bought was ran way over TBO by a part 141 school. I looked it up myself and I don't see where part 141 has anything to do with it. Show me the reg and I'll admit I'm wrong....

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I spoke with the FSDO yesterday (was there for an unrelated topic) and Marty the FAA guy said the (TBO) time requirement is not specifically in the FARS, however for a school to be certified they must submit a maintenance plan for their aircraft. The maintence plan will specifically outline the procedures for that school's aircraft. (by type, not by specific airframe) The maontence Plan is requited for certification, which is required by the FARs, etc etc.... I didn't distinguish between a pt 61/141 school, and can ask Marty next time I see him if it will make you feel better.
Re: Show me the reg...

Yeah....why don't you ask Marty. I agree with you that 141 is non-specific about the question but that each 141 school must have their maintenance plan approved. Each plan could have different requirements. I don't mean to pick on Eagle but he made the statement a ways up that flight schools can't run over TBO, I thought that was wrong but we are working our way to a more specific answer. It's an answer I'm interested in because I have a 152 I'd like to use for instruction/rental. Let us know what Marty says....
Re: Show me the reg...

No problemo, I'll ask next time I see him, I will see another fsdo guy on saturday night and will ask him as well, he is a helecopter guy so we tend to talk slowwwww around him.
i'll keep you posted
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.
Re: Show me the reg...

Some have made insurance comments. If your engine is supposed to be overhauled at a certain number of hours and you run it 500 hours more because the compression is ok and then it throws a rod and kills someone you will not look good in court. The average juror is not very knowledgeable and the lawyer will say that you were negligent for not overhauling your engine at the factory recommended time. I am not saying that TBO cannot be exceeded but I would be careful and document exactly why you are.