Do any parts get refurbished at OH, or only replaced?

Are you saying the shops you recommend are punitive? That is, they make it a point to charge your more because you went over TBO?


If you reuse the same parts at TBO, why are they not hurting when they get to 4000 hours total time? if you aren't reusing them, then the overhaul was expensive and you should have flown to 4000 hours anyway.

What makes the 4000 hour life part suddenly able to do a full second TBO period if you overhaul? Do they spray magic "life renewal elixir" on them or something? They are pieces of metal, you can't erase their operational history at overhaul.


So you advocate operating on condition? Well, so do I.


That will make any unplanned engine event that much more painful since you paid so much for the engine value.

If one of my engines goes belly up, and it is 92% the way through a TBO period, I haven't lost much value. I go get it overhauled and I'm back in the air. I bought a plane with the high time engines priced in properly. A had a full borescope on them, and a very good 5 point run ground check, made book speeds (actually a few knots over), and made it to FL450 at the max step weight in the manual. Everything checked out. So far, they have been fine.


This is consistent with your theory that cost equals value. It does not work out much of the time. It is easy to spend other people's money. If you were writing the checks, you might have a different take on this. The most expensive airplane is almost never the best value, either.

Paying more for an airplane has a lot of residual costs. Sales taxes are higher. Insurance is higher and at some hull value, becomes harder to get. Property taxes are higher. Cost of capital is higher. Your market downside risk is higher. And so forth. It isn't just the up front price difference you have to consider.

Mike C.

Most major parts, the bulk of the engine value, are reused. Some parts are replaced, like seals, gaskets, etc.

For example, the major parts are good for 12000 to 14000 cycles on my JT15D. This includes turbine wheels, impeller, etc. Some parts are "on condition" like HT blades, they can be used for as long as they pass inspection.

So my parts are rated to last several TBO periods depending on what your cycle to hour ratios are.

Modern airliners average about 20,000 hours to the first overhaul.

Mike C.

But those parts that are reused, are they refurbished in any way? Ground down, etc. I can see the argument in favor of overhaul, IF parts are refurbished in a way that may extend their life, or I can buy into the argument that if parts fail due to foregoing an OH that they may cause damage to other components that otherwise would have been fine thereby costing more in the long run. Collateral damage so to speak.

With respect to seals and gaskets, I assume those can be pretty important to the longevity of an engine. I tend to think of those items being susceptible to drying out or becoming brittle with (calendar) time. So perhaps calendar time between OH should be considered as well (just as a reasonable operator would do on piston engines).

I'm not advocating one way or the other, just trying to learn more so I can make my own judgment.

Mostly they are inspected to see if they are within overhaul limits so they can be put back into the engine. None of that reverses the aging of the metal.

An overhaul description is here:

https://www.elliottaviation.com/aircraf ... -upgrades/

"During an engine overhaul, all portions of the engine are disassembled, cleaned, inspected, reassembled, tested (ensure the facility you will be using has an appropriate test cell to run the engine off-wing through all parameters) and shipped back to the installation agency."

Nothing in that list adds life to the part.


Mostly they are replaced because they are disturbed during disassembly and are one time use.

If a gasket or seal leaks, than that's an on condition repair.

Mike C.

An overhaul may not add life to a part, but it can prevent it from being shortened.

If you have a bearing seal that’s near the end of life and it gets replaced during an overhaul but not during a hot section, then you have extended the life of the bearing by doing the overhaul.

Waiting until the seal fails risks ending the life of the bearing prematurely, and possibly damaging other parts as well when the bearing fails.

That risk gets run every time you start the engine. If the seal is at the end of its life at a magic hourly overhaul time, that part would then be life limited and replacement would be mandatory at every interval regardless of the overhaul period. Or the manufacturer did a really poor job in selecting the seal for the application and they are punitively requiring every engine owner pay for their poor engineering.

Nothing lasts forever and parts do wear out. Usually not at some magic number. Performance based engine analysis like the MORE program and others continue to dispel the belief that fixed TBO intervals are based on technical merits.

Believing in fixed time TBO's is like telling your doctor to schedule heart bypass surgery because you have achieved the average age to do so and going past that age will guarantee bypass and valve replacement.

That risk gets run every time you start the engine. If the seal is at the end of its life at a magic hourly overhaul time, that part would then be life limited and replacement would be mandatory at every interval regardless of the overhaul period. Or the manufacturer did a really poor job in selecting the seal for the application and they are punitively requiring every engine owner pay for their poor engineering.

Nothing lasts forever and parts do wear out. Usually not at some magic number. Performance based engine analysis like the MORE program and others continue to dispel the belief that fixed TBO intervals are based on technical merits.

Believing in fixed time TBO's is like telling your doctor to schedule heart bypass surgery because you have achieved the average age to do so and going past that age will guarantee bypass and valve replacement.

This! 100%

Considering we helped clients buy two airplanes with past TBO engines this year, and helped Jim buy his, I think it is fair to say we're not against it either! Though Mike has tried to shape the argument as such... my point is exactly what Jim just said "As long as an owner is as educated as possible on the risks, then they can make intelligent decisions. I've made mine, and others get to make theirs."

The thing I have a problem with is people making statements like "you don't have to do overhauls, they'll be just fine" or basically "the greedy engine gods are just ripping you off" I think we do a severe disservice to the buying public when blanket and misleading statements are made about what you can or cannot do without providing all of the facts, and sadly the narrative is ruled by a bully who has ran off most of the industry professionals who would normally provide information based on experience instead of speculation.

The short answer is that yes you can run past TBO, and yes, just as Jim said you can unlock some real hidden value there. There are some other factors to consider, factors such as resale value, potential liability, perceived liability, what needs to be done to operate on condition correctly, what engines you should be careful to run past TBO, etc.

I think what is really lacking is an understanding of exactly what happens in an engine overhaul. Absent any engine experts, maybe Mike C. can chime in and answer that question for us.

Amen

Thanks for the interesting discussion. It’s confirmed my belief that going past TBO is quite feasible under the right conditions.
Now for the hard part, TBM, Meridian or JetProp….

And the benefits of reduced hull value when it comes to purchase price, taxes, and insurance.

Just because you are under TBO doesn't mean you can't ignore engine condition, either. Past TBO is no extra stuff to do that you shouldn't be doing under TBO as well.


Ask an engine shop. It isn't a secret, though getting an overhaul manual from Pratt is expensive.

Parts are removed, cleaned, inspected against criteria, and put back in if they pass. There is no magic life extending process in this (and indeed the life limited parts are just as life limited after the OH as before).

There are a few exotic things that can happen to extend life. There is a company that can take failing JT15D HT blades and make them pass through some regrowth process. I don't know how successful that is.

The fundamental fact is that the major engine parts last several TBO cycles under normal conditions. Turbine engines simply don't have a lot of wear, unlike piston engines. You can tell that from the oil sample results where the wear metals are hundreds of times less than piston oil. At the end of the day, the turbine engine is just a shaft turning on bearings, and that can run for a very long time.

Mike C.

And the benefits of reduced hull value when it comes to purchase price, taxes, and insurance.

Just because you are under TBO doesn't mean you can't ignore engine condition, either. Past TBO is no extra stuff to do that you shouldn't be doing under TBO as well.


Ask an engine shop. It isn't a secret, though getting an overhaul manual from Pratt is expensive.

Parts are removed, cleaned, inspected against criteria, and put back in if they pass. There is no magic life extending process in this (and indeed the life limited parts are just as life limited after the OH as before).

There are a few exotic things that can happen to extend life. There is a company that can take failing JT15D HT blades and make them pass through some regrowth process. I don't know how successful that is.

The fundamental fact is that the major engine parts last several TBO cycles under normal conditions. Turbine engines simply don't have a lot of wear, unlike piston engines. You can tell that from the oil sample results where the wear metals are hundreds of times less than piston oil. At the end of the day, the turbine engine is just a shaft turning on bearings, and that can run for a very long time.

Mike C.

Don’t forget they give you a piece of paper that says overhaul on it.

Basically correct.

The disassembly is extreme, the engine is taken apart completely and then has to be rebuilt completely.

It is actually a bit more labor than building it new since you the extra disassembly and cleaning tasks.

Many people, like yourself, think an overhaul makes the parts effectively "new" again. It doesn't.

Mike C.