what is TBO

[rchamble]

"The other expense is in reduced TBO. The TSIO-520UB on the Bonaza has a 1600 hour TBO and most need top overhauls way before that (check out the specs on most of the planes for sale.) A lot of the TBO issues are related to the high operating temperatures that most of the components have to withstand."

 

[jsstevens]

"The other expense is in reduced TBO. The TSIO-520UB on the Bonaza has a 1600 hour TBO and most need top overhauls way before that (check out the specs on most of the planes for sale.) A lot of the TBO issues are related to the high operating temperatures that most of the components have to withstand."

Time Before Overhaul. Manufacturer of the engine specifies a TBO. The most commn ones are in the 2000 hour range. I think there's a small lycoming (the O-235 maybe?) that has a 2400 TBO. If you're using a plane commercially, you must rebuild at or before TBO. If you're using it under Part 91 it's optional and done on condition. How an engine is used and maintained has a lot to do with how likely it is to make TBO or even go beyond.

John

 

[rchamble]

Thanks a lot, you cleared up muddy water lol!!

 

[Capt. Geoffrey Thorpe]

"The other expense is in reduced TBO. The TSIO-520UB on the Bonaza has a 1600 hour TBO and most need top overhauls way before that (check out the specs on most of the planes for sale.) A lot of the TBO issues are related to the high operating temperatures that most of the components have to withstand."

There is the manufacturer recommended Time Between Overhauls (TBO) that is based on engine testing, and there is the actual time that any engine will survive before an overhaul. The two will be different.

Given a number identical engines running under identical conditions with identical criteria for deciding on "failure", you will find that every engine fails at a different time. Look up the Weibul distribution for the equations commonly used to model the life of systems.

Given real life operating conditions where every engine is operated and maintained differently you get into a guessing game.

Frequently operated engines will last longer (in engine hours). An engine in a hangar queen will die an early death. Excessive high power/ high temperature operation is not a good thing. Lots of very cold operation / starting without preheat is not good.

You pays yer money and you takes yer chance.

 

[poadeleted20]

Given a number identical engines running under identical conditions with identical criteria for deciding on "failure", you will find that every engine fails at a different time. Look up the Weibul distribution for the equations commonly used to model the life of systems.

I might also suggest reference to Mike Busch's recent article "Is Engine TBO a Myth" in the Feb 2010 Sport Aviation, which suggests the actual distrbution of engine failure over time is not a Weibull distribution, but rather has a high incidence of early failure, a long stretch of low incidence, and rising incidence again out by book TBO.

 

[Capt. Geoffrey Thorpe]

I might also suggest reference to Mike Busch's recent article "Is Engine TBO a Myth" in the Feb 2010 Sport Aviation, which suggests the actual distrbution of engine failure over time is not a Weibull distribution, but rather has a high incidence of early failure, a long stretch of low incidence, and rising incidence again out by book TBO.

That is a typical failure model:

http://en.wikipedia.org/wiki/Bathtub_curve

 

[sba55]

"The other expense is in reduced TBO. The TSIO-520UB on the Bonaza has a 1600 hour TBO and most need top overhauls way before that (check out the specs on most of the planes for sale.) A lot of the TBO issues are related to the high operating temperatures that most of the components have to withstand."

Basically, it's an arbitrary number that manufacturers come up with.

Yes, high operating temps and high pressures make a huge difference in regards to TBO. Ironically, manufacturers generally recommend operating settings that will result in early failures and that are bad for your engine.

The *-520 engine is very reliable. If operated correctly (ie. operated very differently than the incorrect instructions that the manufacturer gives you), it will last way past TBO. If operated poorly in line with mfr recommendations, it probably won't make TBO.

-Felix

 

[Ted]

Basically, it's an arbitrary number that manufacturers come up with.

Yeah, because millions of hours of service use over decades constitutes an arbitrary number...

Whether an engine requires replacement before, during, or at TBO has a tremendous amount to do with how you treat it and frequency of use. It's a good reference guideline that will help you estimate how long you can go prior to. If you're operating Part 91, though, it doesn't mean you necessarily need to replace your engine when it hits that magic number.

There's always the question of whether or not it makes sense to overhaul it, though. For example, I have one friend at Williamsport who has 1900 hours on his factory overhauled engines in his Aztec. The engines themselves don't leak oil, have good compression, good oil pressure, etc. The fuel injection servos are wearing out, along with a number of other little things on them. To some people, especially if dispatchability is important, it makes sense to just overhaul the thing and get all the new parts. It's a more complicated question, and one you have to figure out the proper answer for yourself.

 

[JeffDG]

Yeah, because millions of hours of service use over decades constitutes an arbitrary number...

But TBO is determined before the first engine is installed in a production airframe.

 

[Ted]

But TBO is determined before the first engine is installed in a production airframe.

And where do you think that number comes from? An engine with a couple of minor changes is still effectively the same engine. It's not arbitrary, and if you note most engine families have the same TBO numbers. That's not accidental.

 

[ron22]

Yeah, because millions of hours of service use over decades constitutes an arbitrary number...

But what do engineers at the manufactures really know?
I mean really who would trust anything an engineer said.

 

[Ted]

But what do engineers at the manufactures really know?
I mean really who would trust anything an engineer said.

I learned a long time ago on here that nothing said on here is ever trusted when said by someone with credibility...

 

[sba55]

But what do engineers at the manufactures really know?
I mean really who would trust anything an engineer said.

Ron, it's not engineers. It's marketing and financials....

 

[JeffDG]

Ron, it's not engineers. It's marketing and financials....

Bingo...not an engine expert, but computer hard drives publish a "Mean Time Between Failures" that is generally longer than when hard drives were invented, so they were not tested to that number of hours.

 

[Trapper John]

Bingo...not an engine expert, but computer hard drives publish a "Mean Time Between Failures" that is generally longer than when hard drives were invented, so they were not tested to that number of hours.

If you apply the methods used to calculate MTBF to a human in their 30s, you get a MTBF of around 900 years...


Trapper John

 

[Ted]

Bingo...not an engine expert, but computer hard drives publish a "Mean Time Between Failures" that is generally longer than when hard drives were invented, so they were not tested to that number of hours.

Ok, here's some education then (since aircraft engines are not hard drives):

- Aircraft engines have not changed significantly in the past 40 years
- TBOs HAVE changed in that same time period (so sayeth the old engineers I've spoken with)

Why? Millions of hours and decades of service history that has provided information that allows these numbers to be expanded.

Yeah, let's trust an economics guy with a couple hundred hours of flying time to tell us the significance of TBO, and ignore people who actually run these engines for a living.

 

[poadeleted20]

Ok, here's some education then (since aircraft engines are not hard drives):

- Aircraft engines have not changed significantly in the past 40 years
- TBOs HAVE changed in that same time period (so sayeth the old engineers I've spoken with)

Why? Millions of hours and decades of service history that has provided information that allows these numbers to be expanded.

That's not the only reason. While engine designs may not have changed, materials technology and process/quality control have. We've made improvements in the materials used in the engines (e.g., the high-chrome valve guides now used in Lycoming cylinders), as well as in the processes used to make the parts and even the metal itself, so we have better assurance that the parts will perform up to spec longer. So even if the design is unchanged, the engine's reliability and longevity have. It's like Papa John's says -- "better ingredients make a better pizza" even if the recipe is the same.

 

[gismo]

Yeah, because millions of hours of service use over decades constitutes an arbitrary number...

Whether an engine requires replacement before, during, or at TBO has a tremendous amount to do with how you treat it and frequency of use. It's a good reference guideline that will help you estimate how long you can go prior to. If you're operating Part 91, though, it doesn't mean you necessarily need to replace your engine when it hits that magic number.

There's always the question of whether or not it makes sense to overhaul it, though. For example, I have one friend at Williamsport who has 1900 hours on his factory overhauled engines in his Aztec. The engines themselves don't leak oil, have good compression, good oil pressure, etc. The fuel injection servos are wearing out, along with a number of other little things on them. To some people, especially if dispatchability is important, it makes sense to just overhaul the thing and get all the new parts. It's a more complicated question, and one you have to figure out the proper answer for yourself.

I agree that the factory TBO can be a "good reference guideline" and like you said, that needs to be considered along with the operating conditions the engine has experienced. The first engine I ever replaced on my Baron was done almost 100 hrs short of the (arbitrarily short) 1500 hr TBO. That engine spent a considerable part of it's life getting about 30 hrs of use per year and operated in a manner (recommended by the engine and airplane manufacturer) that's not conducive to long life). It also sat idle for almost two months during my first year of ownership due to an unrelated maintenance issue and during the second annual inspection three cylinders and the camshaft were needing attention so the decision was made to replace it. Since then I've run two engines well past that 1500 number, one to 1800 and one to 2000+. Both were running fine at the point they were replaced.

BTW, there's also a nearly equally important calendar TBO in addition to the one for operating hours. IIRC this is typically 12 years and something that most part 91 owners ignore without giving it any concern while worrying about running an extra few hundred hours past the factory designated hour limit. And while for the most part I agree with Ted that TBO numbers aren't "arbitrary", IMO some engines (particularly TCM) have limits that one can only conclude are at the very least extremely conservative as well as inconsistent across engine families which share bottom end components. A good example of this is the 1500 hr TBO on the IO470-L's in my airplane. Put larger pistons and cylinders on the 470 crankshaft and case gives you a 25-40 HP increase and a 1800 hr TBO.

And TBO extension isn't just for us cheap a$$ part 91 types, many part 135 operators have regularly operated some engines to 3000 hrs (with FAA approval). I suspect that this kind of engine life is primarily the result of a high usage rate which I think we all can agree helps limit wear and corrosion.

FWIW, I believe that Mike Busch's comments about the long end of the failure curve for aircraft piston engines has a lot of validity, especially the part where he points out that there's not a lot of data there because many owners replace or overhaul perfectly good engines because they get nervous about running beyond the factory's hourly TBO recommendation.

 

[gismo]

"The other expense is in reduced TBO. The TSIO-520UB on the Bonaza has a 1600 hour TBO and most need top overhauls way before that (check out the specs on most of the planes for sale.) A lot of the TBO issues are related to the high operating temperatures that most of the components have to withstand."

A side comment on the "most need top overhauls before that" part of your quote: There's a lot of data showing that premature top overhauls have been mostly due to poor QC of the cylinder build process at TCM coupled with the high stress operation the airplane manufactures recommend for engine management. And a common issue is that while the typical CHT redline is 460°F or higher, there's plenty of evidence which shows that keeping CHTs at or below 380° at all times significantly improves cylinder life. IIRC, Lycoming at least recommends that 400°F be the limit for continuous operation. With some enlightenment many Bonanza operators have managed to eliminate the mid TBO top overhaul in recent years. YRMV.

 

[Capt. Geoffrey Thorpe]

BTW, there's also a nearly equally important calendar TBO in addition to the one for operating hours. IIRC this is typically 12 years and something that most part 91 owners ignore without giving it any concern while worrying about running an extra few hundred hours past the factory designated hour limit.

Aint it the truth.

FWIW - I recently tracked down the current owner of an aircraft my dad used to own that got a "field overhaul" (by the old man himself with A&P supervision) in the early 1970's (or so). The current owner said he thought it hadn't been overhauled since and was still running fine.

 

[Dean]

BTW, there's also a nearly equally important calendar TBO in addition to the one for operating hours. IIRC this is typically 12 years and something that most part 91 owners ignore without giving it any concern while worrying about running an extra few hundred hours past the factory designated hour limit.

When I go shopping, that is more important to me than the SMOH hrs. I found several planes that had low time engines, but the OH was 8-10 years ago. A higher time engine that was OH less than five years ago has a better chance of being a more reliable engine than the hangar queen engine.

 

[Ted]

And while for the most part I agree with Ted that TBO numbers aren't "arbitrary", IMO some engines (particularly TCM) have limits that one can only conclude are at the very least extremely conservative as well as inconsistent across engine families which share bottom end components. A good example of this is the 1500 hr TBO on the IO470-L's in my airplane. Put larger pistons and cylinders on the 470 crankshaft and case gives you a 25-40 HP increase and a 1800 hr TBO.

I'd agree that conservative may be the case in a number of instances, but as you know the entire aviation industry tends to be fairly conservative across the board.

Of course, I also can't speak anything as to Continental TBOs.

 

[gismo]

I'd agree that conservative may be the case in a number of instances, but as you know the entire aviation industry tends to be fairly conservative across the board.

No kidding?

Of course, I also can't speak anything as to Continental TBOs.

I bet you could if you tried.

It's my understanding that a common practice is to start with a fairly low number for a significantly new model and then extend that once a large enough sample space is available. And AFaIK the extension doesn't happen because no airframe manufacturer is using that particular engine model on an new production. I'm told this is the reason behind the 300 hr shorter TBO for an IO-470 vs an IO-520.

And what can you tell me about the history of the TBO on the O-320-H2AD?

 

[Ted]

No kidding?

The part I don't get is why people have an issue with it. Under Part 91, you still have freedom to pretty much do as you so please. So, what's the problem?

I bet you could if you tried.

Sure I could, but having a total of 15 logged hours behind (or next to) Continentals and not being familiar with their operating practices, it would be almost as silly as an economist with no engine experience and no more than a couple hundred hours operating these things commenting on this stuff at all. As such, I refrain from comment.

It's my understanding that a common practice is to start with a fairly low number for a significantly new model and then extend that once a large enough sample space is available. And AFaIK the extension doesn't happen because no airframe manufacturer is using that particular engine model on an new production. I'm told this is the reason behind the 300 hr shorter TBO for an IO-470 vs an IO-520.

That is my understanding as well.

And what can you tell me about the history of the TBO on the O-320-H2AD?

Zip, zilch, nada. Wasn't there, and it hasn't come up in discussions with old engineers who were there.

 

[Trapper John]

...and it hasn't come up in discussions with old engineers who were there.

"How to take a perfectly good engine and screw it up", is probably not part of the lunch and learn program...


Trapper John

 

[Ted]

"How to take a perfectly good engine and screw it up", is probably not part of the lunch and learn program...

Actually, we frequently talk about failures and other problems.

 

[SkyHog]

- Aircraft engines have not changed significantly in the past 40 years

Why are Airplane engines superior to auto engines again.....

 

[Dan Thomas]

Why are Airplane engines superior to auto engines again.....

Airplane engines fit airplane operations, auto engines fit auto operations. The auto engine has been much more refined and improved over the years because the market is big enough that they can afford to spend that money and competitive enough that they can't afford not to spend it.

I'd rather fly behind a aircraft engine rather than a car engine any day. BTDT.

Dan

 

[DutchessFlier]

I fly behind an O-320-H2AD on my 172N every time, with 1500hrs on it, pulling +76 on all four cylinders burns no oil, has put on 300 hours since it went into flight school service in May, 2009, oil changed and tested every 40-45 hrs, never any metal, and the only comment by my tech is that it is a shame that the engine has to be pulled at 2000 hrs. It's the 3rd H2AD in the plane and every issue originally found with the engine had been rectified long ago. Camguard at oil changes and Aeroshell 15-50 in the case. Runs like a top!

 

[RotorAndWing]

I fly behind an O-320-H2AD on my 172N every time, with 1500hrs on it, pulling +76 on all four cylinders burns no oil, has put on 300 hours since it went into flight school service in May, 2009, oil changed and tested every 40-45 hrs, never any metal, and the only comment by my tech is that it is a shame that the engine has to be pulled at 2000 hrs. It's the 3rd H2AD in the plane and every issue originally found with the engine had been rectified long ago. Camguard at oil changes and Aeroshell 15-50 in the case. Runs like a top!

Why is it being removed at 2000 hours?

 

[DutchessFlier]

Its in a leaseback to the flight school right now and it has to be pulled within 10% of the TBO time: The most it can run is 2200 hrs, unless I take it out of the flight school before then.

 

[Dr. O]

Local fella bought a Warrior that had been operated since new by a big name flight school.. The engine was past the 2000 TBO and was one of the reasons the school decided to sell it - and their fleet, changing to DA-40 aircraft... Local fella bought it and he flew every day, 7 days a week, weather permitting... When his mechanic finally decided it was time, the engine had 3400 hours on original cylinders, etc... Had never been apart... It finally flunked the valve wobble test, and while the bottom end was still good they decided that a top overhaul was not a good decision... I looked at the bearings when they had it apart and the bottom end would have easily gone another 1000 hours...

My right engine is a factory zero time... I got it to 1850 hours and it began fouling the lower plug on one cylinder - right rear, classic situation... I agonized over sending the cylinder out for overhaul, but finally opted for a new set of Titans... The decision at the time was to pull the cylinders and inspect the cam and bearings and if there was a problem to overhaul the bottom end... Everything looked factory new, so on with the new cylinders at 1850 hours (200 hours ago) and we are off to the races.. I expect the bottom end to easily make the next ~1800 hours these cylinders should go... They would go longer if the airplane flew 3 to 5 hours a day like a big flight school would do, but that is not in the cards...

And finally, the pilot makes a huge difference... There are guys I know in the local area that seem to constantly have engine problems, stuck valves, broken rings, and on... I have owned my own planes for a half century... I have worn engines out but never had an engine fail prematurely... Had one stuck valve (on a fresh overhaul)... I baby my engines, never let the head temperature go over 400, I don't do extended ground running ("well gee the oil isn't up to the green yet", whilst his heads are smoking), I don't run 75%-80% power for extended periods and especially in hot weather, and so on... Be good to your engine and it will be good to you...

denny-o

 

[Ted]

Its in a leaseback to the flight school right now and it has to be pulled within 10% of the TBO time: The most it can run is 2200 hrs, unless I take it out of the flight school before then.

You'll want to consider that carefully - there's not a legal requirement to do that, so it sounds like a flight school requirement.

 

[poadeleted20]

Its in a leaseback to the flight school right now and it has to be pulled within 10% of the TBO time: The most it can run is 2200 hrs, unless I take it out of the flight school before then.

I've heard of insurers starting to demand on-time replacement of flight school engines. Guess they're concerned about what a plaintiff's attorney could lead 12 bozos pulled of the street to think if an accident engine had been run 5 seconds past the mfr's recommended TBO.

 

[chucky]

I baby my engines, never let the head temperature go over 400, I don't do extended ground running ("well gee the oil isn't up to the green yet", whilst his heads are smoking)

What's the right thing to do in cold weather, then? The school I rent from has asked us all to hold off on our runup until the oil temp comes up a little bit. I'm going to stick with what they recommend since they own the planes, but do people here think differently?

 

[dmccormack]

What's the right thing to do in cold weather, then? The school I rent from has asked us all to hold off on our runup until the oil temp comes up a little bit. I'm going to stick with what they recommend since they own the planes, but do people here think differently?

Engine manuals are available and provide some direction.

My very old Lycoming O-145 Manual states that the engine is ready for takeoff when it "does not hesitate or backfire" on runup.

I think waiting for the oil temp to indicate that the oil is warming (and thus circulating) coupled with oil pressure at some manufacturer expectation are sound operating methods.

 

[AdamZ]

What's the right thing to do in cold weather, then? The school I rent from has asked us all to hold off on our runup until the oil temp comes up a little bit. I'm going to stick with what they recommend since they own the planes, but do people here think differently?

At the very least it will add a few Hobb's clicks to the rental bill.

 

[chucky]

At the very least it will add a few Hobb's clicks to the rental bill.

And cheap ones for them, since they rent wet. Well, it's almost a moot point since these days I hardly ever get weather I can fly in, anyway.

 

[DutchessFlier]

Ron and Ted:

Yes thats right...its a flight school requirement and the insurance company has something to say about that as well. I wondered, though since the plane is in commercial use, if indeed the FAA has anything to say about going beyond the 2000 hrs TBO?

The tradeoff is always in play: a constantly running engine, excellently maintained (as is the whole aircraft) by top notch techs that will log 400 hours in the year, May 09 to May 10, with about another year to TBO, paying me back on the leaseback program to break even while I finish my IR, and then beyond.

I am probably going to pull the plane out of the program just before it reaches TBO and see how far I can go until it becomes necessary to do the replacement....and build up my engine fund with some of the leaseback $$$.

 

[RotorAndWing]

Ron and Ted:

I wondered, though since the plane is in commercial use, if indeed the FAA has anything to say about going beyond the 2000 hrs TBO?

.

Not if operated Part 91. If it is under Pt 135 then TBO has to be complied with unless the operator has an OpSpec to operate it longer.