TBO - Love This Guy!

flyingriki

flyingriki

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flyingriki
Always has a sensible approach. Fun to see the ancient ones bellyache about a modern, and completely successful, approach to engine management!! :hairraise:

What Makes an Engine Airworthy?
We can safely ignore manufacturer's TBO as a thoroughly discredited concept, but how do we assess whether a piston aircraft engine continues to be airworthy and when it’s time to do an overhaul? Compression tests and oil consumption are part of the story, but a much smaller part than most owners and mechanics think, writes Mike Busch's in his*latest AOPA Opinion Leader's blog.

http://
blog.aopa.org/opinionleaders/2014/07/02/what-makes-an-engine-airworthy/
 
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I like his approach! What gets me is a car can sit for 30 years in a field and be locked up. You can break it free and start it up and it will run many more thousnads of miles and years. Why is it aircraft engines seem to be not as stout? Now I understand in a car you can just pull over when things go south but still. But is a sitting engine really that bad in planes more so than cars? And I also understand mechanical things like to be used and used often but still...
 
How modern is it ? Caterpillar issued the same type of guidance for its engines in the 1920's.
 
What I always find entertaining about these threads is that everyone seems to think that TBO is nonsense and that their engine is going to exceed those numbers. I've probably overhauled more engines that didn't make TBO (hours wise) than ones that have.
 
mondtster said:
What I always find entertaining about these threads is that everyone seems to think that TBO is nonsense and that their engine is going to exceed those numbers. I've probably overhauled more engines that didn't make TBO (hours wise) than ones that have.
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Very true. And you don't always get a warning that you're about to have an overhaul come up.
 
mondtster said:
What I always find entertaining about these threads is that everyone seems to think that TBO is nonsense and that their engine is going to exceed those numbers. I've probably overhauled more engines that didn't make TBO (hours wise) than ones that have.
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I think that in many cases TBO is indeed nonsense but that doesn't mean I expect every engine to run well past it. IME the biggest factor in engine life is usage. I replaced an engine with about 1400 hrs with a bad cam that had a TBO of 1500 but the engine spent most of it's life flying about 30 hours per year and then sat for 3 months while some work was being done to the airframe. I ran two other engines on the same airplane to 1800 and 2200 hours and both were still running well when I decided to have them overhauled. But those engines were seeing 150-180 hrs per year.

These were IO470-Ls and IMO the TCM TBO spec is much lower than it would be if the same engines were being sold to an airplane manufacturer for use on new airplanes. Otherwise there's no incentive for TCM to raise the TBO after enough usage data is available to justify an increase.

I also find it amusing that some folks see TBO hours as a hard limit but think nothing of running an engine for 25 years without a peek inside when the usage is low.
 
Where is TBO on this graph?
768px-Bathtub_curve.jpg
 
The article mentions an engine with 40/80 compression was still making "rated power". Does that mean the same engine with higher compression would make more than rated power?

And another question about compression.. From what I understand the cylinders are pressurized and then a reading is taken. How much time goes by before the reading is taken? Are we talking around 5 seconds or 15 minutes?
 
Fred, you got there ahead of me.
I respect Mike Busch for his experience and forthright manner.
However, saying that a high leakdown rate engine that is burning oil still makes full power by observation does not make it so.
(it is NOT compression Mike - so adjust your typewriter)
Unless that engine is put on a dyno then we do not know what percentage of rated power it is making - and it might even be close to rated, we just don't know.
Even a cylinder with a good size hole in the piston will fire and make power - and blow by - because of the rapid motion of the piston.

But there is a flip side.
My Fat Albert The Apache, with original 150 hp O-320's had recently overhauled cylinders on when I bought it with 25 and 125 STOH respectively. I knew those cylinders were at least 3rd run - because I knew the frugal A&P FBO who had owned it since 1962. It started well (due to the mags and carb now being 100% courtesy of my wallet immediately after purchase) and ran somewhat smoothly. At around 700 hours STOH one cylinder was marginal and another was looking poorly on leakdown. On general principles I decided to put 8 new cylinders on.
Well, in spite of the engines passing the leakdown and being considered airworthy there was a night and day difference. The increase in power was noted in the first 100 feet of the first takeoff and the engines ran noticeably smoother
(more evenly matched power from each cylinder I assume)
This is a plane that I was flying 2 to 4 times a week so it wasn't buyers pride coloring my observations after installing the new cylinders. I was breaking ground a full runway light sooner than usual.

Yes, you can continue to run high leakdown, high oil burning cylinders. Just don't kid yourself that there is no difference between them and frsh cylinders :D
 
mondtstr, my observations also.
Nothing makes an engine run longer than running frequently.

edit: Fred. It is how fast the airpressure in the cylinder leaksdown (not a compression test like we do on cars, etc.) Look it up online.
 
simtech said:
I like his approach! What gets me is a car can sit for 30 years in a field and be locked up. You can break it free and start it up and it will run many more thousnads of miles and years. Why is it aircraft engines seem to be not as stout? Now I understand in a car you can just pull over when things go south but still. But is a sitting engine really that bad in planes more so than cars? And I also understand mechanical things like to be used and used often but still...
Click to expand...

They are, however you have to remember it's putting out a lot more power per cubic inch the vast majority of the time. I agree to run on condition and just keep repairing the engine parts that need it. However, the TBO schedule is not a bad one for doing an IRAN.
 
FredFenster said:
The article mentions an engine with 40/80 compression was still making "rated power". Does that mean the same engine with higher compression would make more than rated power?

And another question about compression.. From what I understand the cylinders are pressurized and then a reading is taken. How much time goes by before the reading is taken? Are we talking around 5 seconds or 15 minutes?
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Its not really a specific time. It usually does not change over time. You turn it on and look at what kind of leak down you have.
 
Henning said:
They are, however you have to remember it's putting out a lot more power per cubic inch the vast majority of the time. I agree to run on condition and just keep repairing the engine parts that need it. However, the TBO schedule is not a bad one for doing an IRAN.
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But are they not designed to run that way? I can see they have a higher load and run at higher RPM's than a car but they are designed for that. Unless it's just a mechanical limitation which I can see. All the same principles happen for combustion so I guess it all comes down to materials or is it the fact of being horizontally opposed and critical placement of major components? It just seems odd that an engine sitting for a year has a shortened death sentence but a car sitting can keep going. My truck engine has over 3600 hours on it and still going strong and I use the fool out of it for towing, and frequent short trips. A plane going for that long is waiting for something to happen.

I'm looking to buy an aircraft and most planes have been sitting, hence why they are for sale and to top it off most have been overhauled in the 90's. it sure is discouraging. I guess it's time to read up on engines more.
 
This lycoming 540 in my Eagle let me know it wanted a overhaul at 500 hours. Made a safe landing at the airport and the engine never quit running.
 

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pigpenracing said:
This lycoming 540 in my Eagle let me know it wanted a overhaul at 500 hours. Made a safe landing at the airport and the engine never quit running.
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Yours was also not exactly stock, as I recall. Or am I thinking of someone else's engine?
 
gismo said:
I also find it amusing that some folks see TBO hours as a hard limit but think nothing of running an engine for 25 years without a peek inside when the usage is low.
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Yep, if anything the years are more important than the hours, but neither is a hard and fast limit in reality.

The 310's last set of engines made it to 2100 hours and about 15 years on 1700 hour TBOs. I could've kept the right one on going just fine, and could've topped the left one and kept it going, but the timing made sense for doing the overhauls. Pushing 300 on this set now, hoping for 3,000.
 
FredFenster said:
The article mentions an engine with 40/80 compression was still making "rated power". Does that mean the same engine with higher compression would make more than rated power?

And another question about compression.. From what I understand the cylinders are pressurized and then a reading is taken. How much time goes by before the reading is taken? Are we talking around 5 seconds or 15 minutes?
Click to expand...
The "compression" numbers we're talking about really have very little to do with compression as in compression ratio or the compression of the fuel air mixture on the compression stroke.

The test consists of placing the piston at TDC and pressurizing the cylinder through one of the spark plug holes. The source of pressurized air is fed through a regulator set to maintain approximately 80 PSI and then through an orifice that's 40 thousandths of an inch in diameter. Two pressure gauges read the pressure on each side of the orifice. If the leakage past the piston rings and valves equals the what 40 PSI can pass through a 0.040" diameter hole the gauges would read 80 and 40 respectively and this would be recorded as a "compression" reading of 40/80 (all assuming the gauges are perfectly accurate.

The volume of air passing through a .040" orifice during a high power combustion event is probably a thousand times less (educates guess) than the expanded volume of burnt gasses. If true that would mean that the power loss due to escaping gasses would amount to about one tenth of one percent of the power produced.

BTW, the only reason this testing is performed at TDC is so the prop can easily be prevented from rotating. It would be easier and just as effective to test at BDC except that one of the valves in the cylinder would be open. Unfortunately TDC is also the point where the piston rings reverse their travel on the cylinder wall and as a result there's usually a "wear step" that compromises the seal between rings and cylinder. If you think about it, a cylinder could leak badly at TDC yet seal very well at any piston position below that in which case the "compression" reading would be poor but the loss of combustion gasses past the rings would be minimal since the piston doesn't spend much time at TDC. Conversely if the cylinder walls were badly scored you might get a great "compression" test result but have considerably leakage of combustion gasses during the power stroke.

Also the pressure that exists in the cylinder during a combustion event is 10-30 times higher than the pressure in the cylinder during a differential compression test. And the extra pressure pushes the rings into the cylinder walls creating a much tighter seal.

Finally, cylinder leakage is often associated with high oil consumption but a "well oiled" cylinder often produces very good compression test readings because the oil that gets past the rings helps the rings seal against the test pressure.
 
simtech said:
I like his approach! What gets me is a car can sit for 30 years in a field and be locked up. You can break it free and start it up and it will run many more thousnads of miles and years. Why is it aircraft engines seem to be not as stout? Now I understand in a car you can just pull over when things go south but still. But is a sitting engine really that bad in planes more so than cars? And I also understand mechanical things like to be used and used often but still...
Click to expand...

I ask the same question to the bold old timer that got me into aviation. He explained that after so many years with the crank resting on the bearings it create a flat spot. This is what causes problems once you start running the engine. I don't know how true this is but it's what I was told about aviation engines that have been sitting for many years.
 
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