TBO

[Tom-D]

What is it?

What does that magic number really mean?

 

[JHW]

an arbitrary number which means you can't put certain planes on a 135 ticket or export them to certain countries without doing expensive and unnecessary work to them first.

 

[poadeleted21]

It's what the buyer of a plane looks at and calculates how long it's going to be before he "needs" an overhaul. While I wouldn't be scared to run an engine past TBO, especially had I been the one who's been flying behind it for the past 1700 hours. I'd probably consider it mandatory when buying a plane near TBO, if for no other reason than my fear of the unknown. If I bought a plane with an engine near or past TBO, a MOH was figured into the purchase price and will be done soon after the purchase.

 

[wabower]

It means the closer to the number that your engine has achieved hours in service, the lower the offer you can expect to receive.

What is it?

What does that magic number really mean?

 

[Tom-D]

an arbitrary number which means you can't put certain planes on a 135 ticket or export them to certain countries without doing expensive and unnecessary work to them first.

That's the result, how did the manufacturer arrive at that number?

 

[Capt. Geoffrey Thorpe]

What is it?

What does that magic number really mean?

I've never been able to find out. Is it a B-10 or a B-90 or something in between, or something else altogether? How is it determined?

Without that kind of information the number means absolutly nothing.

 

[Tom-D]

It means the closer to the number that your engine has achieved hours in service, the lower the offer you can expect to receive.

There again, that is a result of the number, how was the number derived.

 

[Tom-D]

It's what the buyer of a plane looks at and calculates how long it's going to be before he "needs" an overhaul. While I wouldn't be scared to run an engine past TBO, especially had I been the one who's been flying behind it for the past 1700 hours. I'd probably consider it mandatory when buying a plane near TBO, if for no other reason than my fear of the unknown. If I bought a plane with an engine near or past TBO, a MOH was figured into the purchase price and will be done soon after the purchase.

Rusty, what if that engine had been overhauled to service limits prior to your 1700 hours of happy flying?

Would you go past TBO?

 

[Tom-D]

I've never been able to find out. Is it a B-10 or a B-90 or something in between, or something else altogether? How is it determined?

Without that kind of information the number means absolutly nothing.

It really does mean something when you how the manufacturer determined the number.

 

[JHW]

There again, that is a result of the number, how was the number derived.

I've worked at enough engine manufacturers to have an idea that aviation engine manufacturers probably aren't any different. I've seen advertised parts life predicated on B50 failures when the part was first made, magically evolved to B10 failures, between the 1970's and the 90's. Nothing changed except the marketing.

 

[Tom-D]

TBO is a number of hours that the manufacturer has determined that a new engine can be run and all parts in that engine will meet service limits.

That is why 43.2 is written the way it is.

 

[poadeleted21]

Rusty, what if that engine had been overhauled to service limits prior to your 1700 hours of happy flying?

Would you go past TBO?

When I lived in MT, I trusted my A&P/IA. He flew right seat in the soup with me over the montana rockies, he's been an A&P/IA/CFII for nearly 4 decades, does his own engine overhauls and had nary an engine problem during that time, on engines that he personally had MOHed 3 times. If he says it's time for an overhaul, it's time, if he says it can go another 200, it can go another 200. Unless it's him telling me or someone else I might gain that much trust in over the course of 5 years, we're overhauling near TBO. Maybe it's just me being overly cautious, but it's what I'd have to do to not worry. I may be irrational, but i'm your typical buyer.

 

[wabower]

And so? What do we know now that makes any difference compared
to what we knew before?

TBO is a number of hours that the manufacturer has determined that a new engine can be run and all parts in that engine will meet service limits.

That is why 43.2 is written the way it is.

 

[Tom-D]

When I lived in MT, I trusted my A&P/IA. He flew right seat in the soup with me over the montana rockies, he's been an A&P/IA/CFII for nearly 4 decades, does his own engine overhauls and had nary an engine problem during that time, on engines that he personally had MOHed 3 times. If he says it's time for an overhaul, it's time, if he says it can go another 200, it can go another 200. Unless it's him telling me or someone else I might gain that much trust in over the course of 5 years, we're overhauling near TBO. Maybe it's just me being overly cautious, but it's what I'd have to do to not worry. I may be irrational, but i'm your typical buyer.

Ask your mechanic which he would rather buy, an engine that has 1755 hours TT (on a 1800 TBO), or an engine that has 4500+ hours and 1750 SMOH.(on a TBO of 1800)

Ask which engine he would expect to buy more new parts for.

 

[Tom-D]

And so? What do we know now that makes any difference compared
to what we knew before?

That depends upon what you knew before.

 

[JHW]

Ask your mechanic which he would rather buy, an engine that has 1755 hours TT (on a 1800 TBO), or an engine that has 4500+ hours and 1750 SMOH.(on a TBO of 1800)

Ask which engine he would expect to buy more new parts for.

not enough information. If it's a lycoming I'd base my decision on the decade the crankshaft was forged.

 

[Tom-D]

I can see you do not understand the real meaning of TBO, and thus drag the thread off topic.

a first run engine will have parts in it that can be returned to service with minimal rework. Any other engine will be a guessing game after you open and inspect.

 

[JHW]

I can see you do not understand the real meaning of TBO, and thus drag the thread off topic.

a first run engine will have parts in it that can be returned to service with minimal rework. Any other engine will be a guessing game after you open and inspect.

all engines are a guessing game until you open and inspect. That's why you need to have a collection of other engines to pull parts from. But you know that and are trying to make a point that escapes me.

 

[wabower]

I'd say it depends more one what usable or pertinent information was presented by your answer to your own question. Which was nothing.

That depends upon what you knew before.

 

[Jaybird180]

 

[Geico266]

Good stuff Tom, I appreciate the info.

I would pick the first run engine any time over the one over hauled 3-4 times.

However, in the experimental world there are several engine overhaul shops that have exceptional reputations for rebuilding engines from parts they themselves inspect and approve. I have flown many hours behind some of these engines (Aerosport for one) that has been amazingly smooth, reliable, and trouble free. Their reputation in the RV world is second to none.

Check out their website. http://www.aerosportpower.com/

I know Bart & Sue, owners and they are top knotch for tech info and builder assistance.

 

[Tom-D]

Good stuff Tom, I appreciate the info.

I would pick the first run engine any time over the one over hauled 3-4 times.

However, in the experimental world there are several engine overhaul shops that have exceptional reputations for rebuilding engines from parts they themselves inspect and approve. I have flown many hours behind some of these engines (Aerosport for one) that has been amazingly smooth, reliable, and trouble free. Their reputation in the RV world is second to none.

Check out their website. http://www.aerosportpower.com/

I know Bart & Sue, owners and they are top knotch for tech info and builder assistance.

The Key to knowing what went into the engine is in the shops ability to document the overhaul on a work order or log book sign off.

 

[Ted]

TBOs on piston aircraft engines are more black magic and marketing than true engineering data. From my days working for one of the groups, this is how it went.

Back when an engine was first introduced (we're talking in the range of 40 years ago now), it had a low TBO. The CFRs talk about this, I forget the exact section. Over the years, various improvements as well as field service indicated that these engines could last longer than they were initially certified to. Petitions were made to the FAA, who either approved or disapproved the TBO extension. There is coordination with the FAA on this.

Most piston engines nowadays are certified based on similarity to previous engines, since the changes are typically pretty minimal. The TBO therefore ends up being petitioned to be equal to whatever the previous engine was. Sometimes marketing attempts to push it higher, and engineering/FAA either will agree or disagree. The goal is safe operation without concern over major failures prior to the time period. Top overhauls are considered acceptable.

There's not a ton of science to it, and it's pretty hard for there to be much. Piston engines have such a huge range of operation styles among operators (power settings, ROP/LOP/etc.), many engines sit for long periods of time, and there is a large variety with quality of logging maintenance work done. Plus you have such a wide variety of parts quality that it's hard to make a definitive number. Marketing wants to make TBO longer, engineering wants to be more conservative and make it shorter.

Then you get PMA parts in, which are supposed to be equal but really aren't. Sometimes they're better, sometimes they're worse. This is not factored into TBO, since from a certification perspective they're equal.

As a result, the numbers that exist are conservative in many cases, and many people won't make TBO with their engines. I talked to one person a few weeks ago whose Apache engines lasted to 4300 SMOH without an overhaul, and were still running well. He also was flying the plane a ridiculous amount (even more than I flew the Aztec). Then you hear about the people who have trouble making TBO with their Navajo engines.

So, that comes down to it is a semi-arbitrary number (not fully arbitrary, but somewhat) that gives you a rough indication of how long your engine should last. Unfortunately, you don't see much in the way of reliability improvements anymore, and you don't see anything for performance improvements. This is primarily because piston guys don't know how to approach the FAA with a plan on how to introduce those improvements to a legacy fleet.

For reasons of the above variables, I would rather buy an aircraft with engines at TBO with the intent of overhauling them myself. I will likely be able to fly it a few hundred more hours (the 310 we flew for 400 hours past TBO), and then I can do the overhaul the way I want it done.

 

[Henning]

I can see you do not understand the real meaning of TBO, and thus drag the thread off topic.

a first run engine will have parts in it that can be returned to service with minimal rework. Any other engine will be a guessing game after you open and inspect.

Yep, that's why it's smart to do first overhaul near TBO, then run that engine to condition rejection if you are allowed.

 

[docmirror]

That's the result, how did the manufacturer arrive at that number?

Useful life of the #3 exhaust valve, guide, and seat.

 

[X3 Skier]

When I was an Engineer in the USAF, turbine engines used to have a TBO. We could overfly it by 10% but it was a "hard" number that required pulling the engine and shipping it back to the Depot for overhaul and was based mostly on ignorance and superstition with a pinch of real information on service experience thrown in. GA Piston engines follow the same approach AFAIK.

In the 70's with the advent of borescopes, oil analyis, performance trending and other techniques, everything went to on condition with no TBO limit.

Piston engines could operate the same way and in fact there are some guys who can set up such a program for an engine and suggest when and if it needs service. One such source publishes articles in the EAA Magazine but his name escapes me. If I owned a plane I would have no reservation going "on condition".

Cheers

 

[Ted]

When I was an Engineer in the USAF, turbine engines used to have a TBO. We could overfly it by 10% but it was a "hard" number that required pulling the engine and shipping it back to the Depot for overhaul and was based mostly on ignorance and superstition with a pinch of real information on service experience thrown in. GA Piston engines follow the same approach AFAIK.

See my previous post for how it's done at the piston GA manfacturers. You're pretty close.

 

[denverpilot]

For those who've already braved the rapids... I've always found the most useful information to be stories of how you knew it was "time" on an over-TBO engine. Various people have different tolerances for how much they'll push it, and eventually "cave in" and do it. Others do it on specific criteria.

Everyone I've talked to about the O-470 says she'll give plenty of warning if filters are being checked properly and oil analysis is being done. Heck, our oil analysis caught a small rip in the air filter a couple of years ago that wasn't easy to see. Silicon went up by a tiny margin. Which meant we were sucking dirt.

What are your "we knew it was time" stories?

 

[docmirror]

Well, when bits of the rod exited the bottom of the case, I knew right then it was time for it to come out.

 

[Ted]

We decided at 2100 hours (400 past TBO) that the 310's engines were time. We based it on the following:

-Oil pressure in cruise had decreased for both engines (bottom of green where they used to be mid/top of green)
-Compressions dropping on a number of cylinders
-Left engine having a decent amount of oil coming out of the breather
-Poor starting characteristics for left engine

We could have pulled and done a few jugs on the left engine and gotten some more time, but with the lower oil pressure, didn't see any benefit there. If oil pressure was good and we weren't seeing the oil coming out of the left engine, we would have kept running the pair. The right engine realistically could have run probably another few hundred hours, but there's not much point in splitting the engines like that.

 

[denverpilot]

The right engine realistically could have run probably another few hundred hours, but there's not much point in splitting the engines like that.

Not being a twin guy, why? Seems like it'd be easier on cash flow to have them split, as long as they're producing correct power. Is it the down-time of doing it more than the split itself or?

Just curious. Won't ever really be a factor for me.

 

[Ted]

Not being a twin guy, why? Seems like it'd be easier on cash flow to have them split, as long as they're producing correct power. Is it the down-time of doing it more than the split itself or?

Just curious. Won't ever really be a factor for me.

Well, I should say that for us it didn't make much sense. The Aztec had its engines split 1000 hours apart when I bought it. If we were looking at 1000 hours, it might make more sense. But I doubt we'd be able to push the right engine to 3,000 hours.

Yes, it is easier on cash flow to have to do one engine. But then that means you have the plane down for ~6 weeks twice not too far apart from eachother. For how we fly, it doesn't make a ton of sense, because we don't like having the plane down for that long. If you're someone that flies for pleasure and don't mind the downtime, it probably doesn't matter as much. There are also ways to speed up the overhaul process in the form of doing engine exchanges and the like, which may make sense in many cases. In our case, we wanted to overhaul the engines we had as they were factory Continental remans that we knew were in good shape. We didn't want to risk exchanging for cores in worse shape.

It also doesn't work if you want to do any STC engine upgrades/changes. Not that we're planning on it, but if you are, then what that means is you have a half-time engine that you now are either overhauling very early, or that you sell and try to buy a core (assuming an exchange program, which is how most of them work).

 

[Jim Logajan]

What is it?

What does that magic number really mean?

Are you asking about FAA or ASTM (LSA) certified engines?

My understanding is that for ASTM, a manufacturer may set the TBO to the longest time that a copy of that engine model has run properly without changing any parts.

Otherwise I think the TBO may also be determined from shorter runs by measuring the wear of parts and interpolating when the fastest wearing part will reach the acceptable size limit set by the designers.

But I can't swear to any of the above; it is just my understanding. That so many engines have nice round numbers near multiples of a thousand hours rather suggests there is a lot of conservative interpolation going on.

 

[denverpilot]

Good info, Ted. I'd forgotten about the core exchange gamble. With Continental, you also have the "light" case vs the "heavy" case issue on the O-470, if you swap a core, I think.

Maybe they won't ship light cases anymore and just lose money on those cores... Never looked into it. Just know there's two distinct cases, and one is more generally sought after, than the other.

All sorts of fun data points to consider. End of the day, I know we're all going to just crack the wallet and have a memorial service for all those Benjamins... Heh. AMU bonfire. Bring some stuff to make s'mores.

 

[Ted]

AFAIK Continental is only shiipping heavy cases on factory remans. Some engine shops do the same (Western Skyways is one), but one engine shop I talked to said their practice is to give you back whatever case they feel like. Which means if you're they're buddy, you'll get my heavy case and I'll get your crappy light case.

Lycoming has a similar comparison with the flat tappet vs roller tappet and wide deck vs narrow deck engines. All new cases from Lycoming are wide deck, roller tappet engines (except for certain engines that haven't been authorized to ship with roller tappets due to power issues). In theory, these cases leak the least and give you the best valvetrain longevity, mostly due to cold start protection. I would say that theory is typically correct. Again, caveat emptor on the overhaul shops.

 

[SCCutler]

A material change in oil pressure would be a big red flag for me.

 

[JHW]

Lycoming has a similar comparison with the flat tappet vs roller tappet and wide deck vs narrow deck engines.

That's really not the same thing. While it's true that a WD and ND lycoming are interchangeable as far as legality to install, in reality the baffling won't fit. It's not a drop-in change like the continental heavy vs light case.

 

[flyingmoose]

I can see you do not understand the real meaning of TBO, and thus drag the thread off topic.

a first run engine will have parts in it that can be returned to service with minimal rework. Any other engine will be a guessing game after you open and inspect.

We decided at 2100 hours (400 past TBO) that the 310's engines were time. We based it on the following:

-Oil pressure in cruise had decreased for both engines (bottom of green where they used to be mid/top of green)
-Compressions dropping on a number of cylinders
-Left engine having a decent amount of oil coming out of the breather
-Poor starting characteristics for left engine

We could have pulled and done a few jugs on the left engine and gotten some more time, but with the lower oil pressure, didn't see any benefit there. If oil pressure was good and we weren't seeing the oil coming out of the left engine, we would have kept running the pair. The right engine realistically could have run probably another few hundred hours, but there's not much point in splitting the engines like that.

You also had the luxury of trending the engines. That is not always the case. Those are all good reasons, but if you didn't know what the oil pressure has been in the past then its hard to come to the conclusion to keep running without other indications as you also mentioned.

Its a great idea to keep a flight log of engine parameters so you know if things are changing.

 

[Henning]

A material change in oil pressure would be a big red flag for me.

Meh, till it starts knocking, it's all good.

 

[Ted]

That's really not the same thing. While it's true that a WD and ND lycoming are interchangeable as far as legality to install, in reality the baffling won't fit. It's not a drop-in change like the continental heavy vs light case.

If you go back and re-read what you quoted, I said "similar" not "same." It is similar, even though it's not identical. The wide deck cases also have a weight increase, which is why some racing folk I've seen prefer narrow deck. I've also heard some people claim the narrow-deck cases were better, although I disagree.

Keep in mind that Lycoming is not sending out narrow-deck cases anymore. So if you do a Lycoming factory overhaul, you'll just have to modify the baffles as appropriate. Or you could keep your narrow-deck case and have someone else overhaul it. Lycoming does sell narrow-deck cylinders.