Do Piston Engine TBOs Make Sense?

[ipengineer]

Me and a partner are looking into purchasing a 2005 SR22 and it has got me looking into engine overhaul time. Do most people here follow manufactures TBO? Some of the planes we are looking at are over TBO and others are getting close to it.

I came across this post on the AOPA blog which questions if Engine TBO's make sense. From looking at the graph it seems like the majority of engine failures are from engines recently overhauled. This makes sense to me; I can think of a lot of things that become less stable with change.

http://blog.aopa.org/opinionleaders/2014/03/13/do-tbos-make-sense/

 

[Henning]

TBO makes little sense as a stand alone edict of overhaul for GA operators. It makes much more sense to operate 'on condition', but this requires advanced monitoring of condition.

 

[PaulS]

If an engine averages 2,400 rpm while it is running, it will spin 288,000,000 times at 2,000 hours. That's a lot of spins. I haven't had to consider this, but I think TBO would be as far as I would run an engine. I know others would disagree.

 

[ipengineer]

TBO makes little sense as a stand alone edict of overhaul for GA operators. It makes much more sense to operate 'on condition', but this requires advanced monitoring of condition.

What do you think is the best approach to achieving "advanced monitoring of condition"? For example, TBO is 2000 hours and you exceed that, what intervals should you be concerned with monitoring the condition and what all is involved?

 

[Henning]

What do you think is the best approach to achieving "advanced monitoring of condition"? For example, TBO is 2000 hours and you exceed that, what intervals should you be concerned with monitoring the condition and what all is involved?

Constant monitoring of operational conditions vis-a-vis multi point EGT/CHT/TIT, temp, and fuel flow parameters against performance, and oil analysis as well as filter cutting/inspection on oil changes. Also run compression and take a peek at your valves with a scope every 100 hrs along with checking/resetting points and timing. If you run well LOP, you can really extend the life of your cylinders as well as save fuel and introduce fewer contaminants into your oil. Keeping a good dynamic balance on the prop is a low cost way to lower wear and tear to the whole airframe, not just the firewall forward.

Most important factor to going beyond TBO is to fly frequently.

 

[F01LA]

Need better (normalized) stats. Of course there are few failures 2500-2999; Relatively few airplanes with that number of hours on engines.

 

[Henning]

Need better (normalized) stats. Of course there are few failures 2500-2999; Relatively few airplanes with that number of hours on engines.

Not really, lots of flight schools run their planes over 4000hrs.

 

[RyanB]

Being that it is a recommendation, some people do run engines off condition. Oil analysis is a good clue as to how healthy the engine is. If the engine is drinking oil or making metal, its probably about time for a tear down. But if its healthy, it should last beyond tbo and be just fine. Have a mechanic take a look just to be sure the current owner isnt wanting to sell right before a major overhaul is needed.

 

[ja_user]

If an engine averages 2,400 rpm while it is running, it will spin 288,000,000 times at 2,000 hours. That's a lot of spins. I haven't had to consider this, but I think TBO would be as far as I would run an engine. I know others would disagree.

You don't replace anything on your car based upon a # of rotations. You replace them all on condition or fault. Tires, Wheels, brakes, etc...

Do you replace your 80k mile tire at 80k miles or when they are worn? What about your brakes?

Do you replace your engine when the warranty is up, or do you wait to see if it is burning oil, running rough, etc?

 

[StinkBug]

Major difference there being that when the engine on your car decides to take a dump you just pull over. Comparing to auto maintenance is a bit of a silly and dangerous thing to do. Not to mention the fact that most people do a horrible job of maintaining their cars to begin with.


You also fail to notice that most people determine age or condition of a car based on number of miles, which IS a measure of rotations, though a bit less than accurate.

 

[Cpt_Kirk]

Not really, lots of flight schools run their planes over 4000hrs.

I've flown a 172 with over 5k on the engine. Brought me to TLH and back with no squawks.

It seems to fly sideways a bit, but the engine was fine.

 

[mrkbbd]

Not really, lots of flight schools run their planes over 4000hrs.

But those are not making 50+hp / cylinder, they are making more like 40hp / cylinder, big difference in longevity

Never assume you are going to make TBO, especially in a used airplane, you don't know how that engine was run its whole life

 

[JHW]

It's a concern. Recommended tbo for the lycoming 540 in my pawnee is 12 years or 2000 hours. When it ticked over 3600 hours and 20-something years I thought, hey that thing is getting to be mid time, I'd better pick up a spare engine in case it goes south someday. So we bought an aztec to part out and kept a cherry 1800ish hour engine from it, to be used sometime in the next decade as needed.

 

[mrkbbd]

It's a concern. Recommended tbo for the lycoming 540 in my pawnee is 12 years or 2000 hours. When it ticked over 3600 hours and 20-something years I thought, hey that thing is getting to be mid time, I'd better pick up a spare engine in case it goes south someday. So we bought an aztec to part out and kept a cherry 1800ish hour engine from it, to be used sometime in the next decade as needed.

Doesn't the pawnee only turn like 2400RPM? big difference between that and the Cirrus motor

 

[Iv4nK]

I haven't had to consider this, but I think TBO would be as far as I would run an engine.

I thought most new design engine models start with a ridiculously conservative TBO hours for a few years, then have it progressively increased to almost double the value without ever changing anything with the actual design of the engine? This increase is most likely based on condition monitoring of a large data set, whether a proven engine model will have a 2000TBO or a 1600TBO for instance, right?

Doesn't that sort of imply that TBO likely has very little to do with actual capabilities of an engine and more to do with liability mitigation? An engine manufacturer wants to advertize as high of a TBO as they possibly can (since it makes their engine cheaper to operate per hour) but at the same time, they have to minimize liability in case of accidents where mechanical failure plays a role.


Here's another thought - two IO-360 engines (IO-360-HB and IO-360-ES) have different TBOs (500 hours difference) Now, of course - those two are not identical engines and I don't have the knowledge about the exact differences between the two - but they produce the same horsepower at the same RPM, have the same compression ratio and use the same fuel - so I have to ask myself - does the "HB" model really require a MAJOR OVERHAUL 500 hours earlier than the "ES" model? Will everything on the "HB" model really be worn out beyond tolerances 500h earlier?

N.B. I'm not a mechanic nor am I claiming any of the above is 100% factually correct, I'm merely pointing out to what the available information is and trying to logically analyze it to the best of my abilities

 

[mrkbbd]

I thought most new design engine models start with a ridiculously conservative TBO hours for a few years, then have it progressively increased to almost double the value without ever changing anything with the actual design of the engine? This increase is most likely based on condition monitoring of a large data set, whether a proven engine model will have a 2000TBO or a 1600TBO for instance, right?

Doesn't that sort of imply that TBO likely has very little to do with actual capabilities of an engine and more to do with liability mitigation? An engine manufacturer wants to advertize as high of a TBO as they possibly can (since it makes their engine cheaper to operate per hour) but at the same time, they have to minimize liability in case of accidents where mechanical failure plays a role.


Here's another thought - two IO-360 engines (IO-360-HB and IO-360-ES) have different TBOs (500 hours difference) Now, of course - those two are not identical engines and I don't have the knowledge about the exact differences between the two - but they produce the same horsepower at the same RPM, have the same compression ratio and use the same fuel - so I have to ask myself - does the "HB" model really require a MAJOR OVERHAUL 500 hours earlier than the "ES" model? Will everything on the "HB" model really be worn out beyond tolerances 500h earlier?

N.B. I'm not a mechanic nor am I claiming any of the above is 100% factually correct, I'm merely pointing out to what the available information is and trying to logically analyze it to the best of my abilities

Isn't the "HB" for helicopters?

 

[JHW]

Doesn't the pawnee only turn like 2400RPM? big difference between that and the Cirrus motor

Mine is a 260hp and turns up to 2700 give or take. Maybe a little more, give than take, depends on how i squinted at the tach when setting the governor. It's an underpowered POS and needs all the help it can get.

But you're right there is a big difference. The pawnee has to run flat out at low altitude in the hottest weather. It has to run long days every day for a few weeks and then sit neglected rusting away for 10+ months.

 

[mrkbbd]

Mine is a 260hp and turns up to 2700 give or take. Maybe a little more, give than take, depends on how i squinted at the tach when setting the governor. It's an underpowered POS and needs all the help it can get.

what do you normally run it at? I've heard the 182RGs lasting forever because they are only making 235hp out of the same motor (not as good of an induction system though)

 

[Iv4nK]

Isn't the "HB" for helicopters?

Haven't thought about it, it might be, in which case I chose the wrong two engines to compare

There's a number of engines from the same series that have quite different TBOs, though. Another big difference is with the addition of turbo normalization. Adding a turbo seems to decrease the engine's "proposed life" even though all it does is make it operate as if it was at 3000ft even when it's at 15.000ft. How much is this different than a weekend flyer who does local area flights at 3000ft all year?

 

[JHW]

what do you normally run it at? I've heard the 182RGs lasting forever because they are only making 235hp out of the same motor (not as good of an induction system though)

to stagger off the ground and limp to the field takes all she's got plus some leaning forward and swearing

 

[mrkbbd]

to stagger off the ground and limp to the field takes all she's got plus some leaning forward and swearing

So it's a no go on high density altitude days

 

[JHW]

How much is this different than a weekend flyer who does local area flights at 3000ft all year?

the weekend flyer flying over his house is probably throttled back to <50% power.

But it's a moot point. Most GA piston engines corrode away before they wear out, whether run hard or easy.

 

[JHW]

So it's a no go on high density altitude days

no, generally that describes the only time it flies.

 

[Checkout_my_Six]

Maintenance induced failures and or infant mortality and or manufacturing defects are a concern for the first couple "hundred" hours after an overhaul/zero time/SMOH.

Keep in mind....the majority of effort involved during an OH is "inspection"....not replacing with new parts.

Crank shafts....and sometimes cams....can have thousands and thousands of hours on them before replacement.

 

[JHW]

A word to the OP for what it's worth.

Every used engine is a crap shoot. And we all fly behind a used engine. An engine is just a collection of parts. Like any machine made of parts, sometimes a part breaks. When it does, fix it and move on. You can analyze this forever and in the end you'll be right back to these simple ideas.

 

[flyingron]

Most owned planes that aren't flown to death don't make it to the TBO hour "limit" before condition mandates at least a partial overhaul. On the other hand, rentals that are flown regularly tend to blast through the TBO hour limit with no problem. We had a skyhawk up to about 2600 hours on 5000 TT still going strong before the owner relented and sent it down for OH.

 

[Iv4nK]

the weekend flyer flying over his house is probably throttled back to <50% power.

The key word here being "probably". If he's buzzing his house, he'll be doing full climb power on the climbout. If he's doing traffic pattern bashing all day long, he'll be doing full power takeoffs 12 times per hour.

The TBO doesn't differentiate between a weekend flier doing 12 takeoffs an hour in 100F weather from the traveler averaging 3 hour long flights in a climate where the highest summer temperatures don't reach 80F.

That's what makes me question the idea of reaching 1999.9h on an engine and removing it for a complete teardown without any consideration for the actual condition of the engine.


EDIT: To me, an engine that's just come out of maintenance is the biggest scare - that's the one place where a possibility for a catastrophic mechanical failure (due to mechanic error) seems most likely to occur.

A worn out engine will *slowly* start to give indications of it's deteriorating reliability. An engine that has the wrong torque on an important bolt will result in a much more exciting type of 'deterioration'

 

[JHW]

ayep. When we were ferrying piston twin cessnas from florida to south africa, if they handed me one with newish engines I'd insist those engines got ~200 hours before I made the big overwater leg. I decided the area around Rio was a good place for a 20-year-old single guy to run off any needed hours.

 

[MBDiagMan]

ayep. When we were ferrying piston twin cessnas from florida to south africa, if they handed me one with newish engines I'd insist those engines got ~200 hours before I made the big overwater leg. I decided the area around Rio was a good place for a 20-year-old single guy to run off any needed hours.

Yes, statistically, you are more likely to continue having power in flight with a properly cared for engine at or shortly after TBO, than with a freshly overhauled engine.

For this reason, the O200A in my Cessna 140 being in great condition and 50 hours past TBO will remain in service until it loses compression or shows metal.

 

[Henning]

Doesn't the pawnee only turn like 2400RPM? big difference between that and the Cirrus motor

No it's not. 2400 rpm, 2800 rpm, no biggie, the engine would be happier at 3400 rpm but it would need a gear drive or a tiny prop.

Keep the engine out of detonation and flown frequently and it will go a long way.

 

[Ted]

Lots of opinions. Best thing you can do is have a good inspection plan on the engines to monitor condition and pull when problems occur.

 

[Checkout_my_Six]

Lots of opinions. Best thing you can do is have a good inspection plan on the engines to monitor condition and pull when problems occur.

Hey bro....this is the internets....where everyone is an expert.

 

[N801BH]

Hey bro....this is the internets....where everyone is an expert.

........

 

[Ted]

Hey bro....this is the internets....where everyone is an expert.

Except the people who are actually experts.

 

[Checkout_my_Six]

Except the people who are actually experts.

hope the not so new job is working out....it's been a few years.

 

[coloradobluesky]

If the engine continues to make good power, have good compressions, and not use oil, the owner tends to keep running it. When an engine gets to 2000 for a rebuilt, 2500 for a factory reman and 3000 for a 1st run new, they start going downhill. When you decide to rebuild is not an exact science...

 

[ipengineer]

Good info.. I wasn't sure if once these engines hit TBO most people started to yank and rebuild them. Good to know its not a hard set number and mileage-may-very.

 

[Henning]

Good info.. I wasn't sure if once these engines hit TBO most people started to yank and rebuild them. Good to know its not a hard set number and mileage-may-very.

I was taught to run the GTSIO-520 in a 421 by a guy who flew it nearly 2000 a year as a company technology (avionics)test bed and business travel aircraft. These are rated IIRC as 1600hr engines. He'd replace them with a spare set at 2000 hrs and we'd do an IRAN on the other set and pickle them. We'd replace wear parts like bearings, however the wear was minimal and within service limits for an overhaul. Running hard LOP leaves the cylinders and turbos in good condition.

 

[Checkout_my_Six]

....and the standards for determining TBO aren't an exact science....nor are statistics involved....it's mainly from a tear down inspection after a test stand endurance run....and can be a "negotiated" number during engine certification.

Which isn't exactly the duty cycle nor the environment of a normally operated engine....so, TBO numbers IMHO should be taken with a grain of salt.

I'd much rather rely on condition monitoring indicators....than a number.

Oil analysis, oil screen/oil filter monitoring, bore scope inspections.....operating oil pressure and temp.

 

[RotorAndWing]

I was taught to run the GTSIO-520 in a 421 by a guy who flew it nearly 2000 a year as a company technology (avionics)test bed and business travel aircraft.

That's a rather inflated number....