past TBO engine, great compressions, what does that mean

WannFly

WannFly

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Priyo
bored at work....

i see some planes on controlled TaP with engine well past TBO, like 800 hrs - 1000 hrs past TBO but with compression of 76-79 range on all 4 cylinders. how does that work? do u just keep flying that thing till the compression comes down or till it stops running before you OH?
 
The best I can offer is that manufacturers are very conservative in setting TBO recommendations. Under Part 135 those TBO's are mandatory but under Part 91 they are simply recommendations. Having said that, the best bang for your buck may well be to fly it way beyond TBO as long as engine parameters, compression, oil consumption and others are within specified limits and the engine is making full power. IOW, overhaul on condition, not time. They will eventually wear out.
 
Compression is just a way to check if valves or rings leak. Even with perfect compression, your crankshaft can snap in half the next time you take off.
To be fair, I'd rather have poor compression and a good camshaft, than the other way round. head jobs (sic) are easy and cheap.
What I'd like to see in a high time engine is a trend monitored with oil analysis. That gives you some idea about the internals, but even then your crankshaft can still snap in half.
 
Tom would know much better than me, but I am sure there are many cranks and cams that have thousands of hours on them still flying around. When you O/H an engine, the cam and crank are checked for cracks, checked for roundness and thickness, they are also visually inspected for strange wear marks. So...in theory if the cam and crank check out and are still in spec, they go right back into the engine riding on new bearings.
 
mtuomi said:
Compression is just a way to check if valves or rings leak. Even with perfect compression, your crankshaft can snap in half the next time you take off.
To be fair, I'd rather have poor compression and a good camshaft, than the other way round. head jobs (sic) are easy and cheap.
What I'd like to see in a high time engine is a trend monitored with oil analysis. That gives you some idea about the internals, but even then your crankshaft can still snap in half.
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:yeahthat: If you're flying under 91 regs, then there is no mandatory TBO. An engine that is maintained and operated properly should be able to exceed TBO. If it's not making metal or burning through oil, let it fly.
 
Every engine I know of except a new one is a Pandora's box.
Recently we found a 0-470 that was running great, had 3 teeth missing from the timing gears. Another 0-300 Has 6000 hours since major, but was torn down every year inspected and repaired as required. I seen logs that say the overhaul was completed, but really wasn't, I've seen engines that were signed off as a repair when in fact the engine was overhauled.

You just never know, it is a crap shoot.
 
Here is what TBO means to part 91 operators.
This is an hours of service number that the manufacturer believes with normal care that each part in the engine will meet service limits.
Running past this point means you have a lessor change of using these parts again.
When I buy any engine, I want to know how many times it has been overhauled and what was replaced at each overhaul, that way I can anticipate what will be required to be replaced at the next overhaul.
 
Tom-D said:
Here is what TBO means to part 91 operators.
This is an hours of service number that the manufacturer believes with normal care that each part in the engine will meet service limits.
Running past this point means you have a lessor change of using these parts again.
When I buy any engine, I want to know how many times it has been overhauled and what was replaced at each overhaul, that way I can anticipate what will be required to be replaced at the next overhaul.
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NO Tom....it doesn't. I have an engine with 200 hrs remaining till TBO.....and new bearings and crank with a few hundred hours. IMHO It's just been OH'd with fresh mags and new hoses.... in the FAA's eyes its just about spent.

TBO is a meaningless number......especially when crankshafts and cases can go thousands and thousands of hours before replacement.
 
Checkout_my_Six said:
NO Tom....it doesn't. I have an engine with 200 hrs remaining till TBO.....and new bearings and crank with a few hundred hours. IMHO It's just been OH'd with fresh mags and new hoses.... in the FAA's eyes its just about spent.

TBO is a meaningless number......especially when crankshafts and cases can go thousands and thousands of hours before replacement.
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I believe you need to read my statement again, The Number posted as TBO is what the manufacturer believes, not what it may really be.

They used a new engine to set this number, not what you repaired or stuff you replaced.
 
Checkout_my_Six said:
especially when crankshafts and cases can go thousands and thousands of hours before replacement.
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Many cranks and cams are reused at service limits, they are not always re-ground.
 
Tom-D said:
I believe you need to read my statement again, The Number posted as TBO is what the manufacturer believes, not what it may really be.

They used a new engine to set this number, not what you repaired or stuff you replaced.
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do you know how many hours those 14CFR33 endurance tests are performed before they determine the magic numbers?

hint:.....it's a fraction of the final TBO. It's not a reliability test, nor is it a failure test, nor is it statistical. It's purely a judgement call....otherwise known as a WAG.
 
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Checkout_my_Six said:
do you know how many hours those 14CFR33 endurance tests are performed before they determine the magic numbers?

hint:.....it's a fraction of the final TBO. It's not a reliability test, nor is it a failure test, nor is it statistical. It's purely a judgement call....otherwise known as a WAG.
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Remember, it is their engine and their opinion that set that number. Doesn't matter one tinker's damn what we want or would believe.
 
You need to work with your mechanic,after you reach the magic number. It's how comfortable you feel flying the airplane.
 

Exactly :)

(based on the beaching marks, that's a machining crack that propagated? Was it ever inspected for that?)
 
mtuomi said:
Exactly :)

(based on the beaching marks, that's a machining crack that propagated? Was it ever inspected for that?)
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I don't know what your talking about?
This was a modified Lycoming 540 in my Christen Eagle. I bought the plane with 400 hours on the airframe and engine. It broke in half about the 10th time I flew it. Engine never quit running, it quit making power but was still idling when I landed.
 
pigpenracing said:
I don't know what your talking about?
This was a modified Lycoming 540 in my Christen Eagle. I bought the plane with 400 hours on the airframe and engine. It broke in half about the 10th time I flew it. Engine never quit running, it quit making power but was still idling when I landed.
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Based on the marks in the crankshaft that I can see in your picture, that looks like a poorly machined crankshaft with an initial fracture that caused it to fail. The concentric lines originating from the point of initial crack are called beach marks, and are often a telltale of poor machining work.
 
O boy! What did I start!!

Sent from my SM-G935V using Tapatalk
 
Tom-D said:
Every engine I know of except a new one is a Pandora's box.
Recently we found a 0-470 that was running great, had 3 teeth missing from the timing gears. Another 0-300 Has 6000 hours since major, but was torn down every year inspected and repaired as required. I seen logs that say the overhaul was completed, but really wasn't, I've seen engines that were signed off as a repair when in fact the engine was overhauled.

You just never know, it is a crap shoot.
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There's been some stories of new ones being Pandora's Boxes, too. :-(
 
When I bought my plane, I bought it based on the recent very well documented engine overhaul, and I was happy I got an airplane to go with it.
After the first couple of hundred hours, I know the engine well and trust it 100%. And that is a great feeling to have as an owner.
But I still know the crank can snap in half the next time I start it up.
 
denverpilot said:
There's been some stories of new ones being Pandora's Boxes, too. :-(
Click to expand...

:yeahthat:

Tom failed to note that a new engine is also a Pandora's box...but with a warranty.
 
Checkout_my_Six said:
you say that like it's a good thing?...:eek:
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your choice.. I have never had a new engine fail. and I also knew the when it was ready for re-build, I could use the parts as cores.

It's just a case of "pay me now or pay me later" Some one has to fly the first hours.
 
Tom-D said:
your choice.. I have never had a new engine fail. and I also knew the when it was ready for re-build, I could use the parts as cores.

It's just a case of "pay me now or pay me later" Some one has to fly the first hours.
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you never heard of a freshly OH'd engine failing? ...cause they do. Maintenance/Manufacturing Induced Failures. :eek:
 
Checkout_my_Six said:
you never heard of a freshly OH'd engine failing? ...cause they do. Maintenance/Manufacturing Induced Failures. :eek:
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We were talking NEW engines. If you are going to change the subject, let us know.
Try and buy a CubCrafter's Cub with a used engine.
I wished I knew how to build a fresh overhaul with 200 hours on it.
 
Tom-D said:
We were talking NEW engines. If you are going to change the subject, let us know.
Try and buy a CubCrafter's Cub with a used engine.
I wished I knew how to build a fresh overhaul with 200 hours on it.
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so....how is the TBO affected by a fresh OH?
 
mtuomi said:
When I bought my plane, I bought it based on the recent very well documented engine overhaul, and I was happy I got an airplane to go with it.
After the first couple of hundred hours, I know the engine well and trust it 100%. And that is a great feeling to have as an owner.
Click to expand...
mtuomi said:
Based on the marks in the crankshaft that I can see in your picture, that looks like a poorly machined crankshaft with an initial fracture that caused it to fail. The concentric lines originating from the point of initial crack are called beach marks, and are often a telltale of poor machining work.
Click to expand...

More likely it was from an unaddressed prop strike.
 
For an answer that might actually be helpful instead of another argument:

Often times, people price the plane (the buyer and/or seller) as if it is needing an overhaul. So every hour you fly before you actually overhaul is a little bonus. A buddy of mine is on 4000 hours on an O-320. That is not uncommon for planes that fly frequently. Oil analysis, compressions, and some other stuff can probably help you and a mechanic determine when an overhaul will actually be necessary. But the number is kind of arbitrary for our purposes, again, except for how people may price things.
 
wrbix said:
Surprised no one has invoked the name of Mike Busch here:
http://www.avweb.com/news/savvyaviator/savvy_aviator_45_how_risky_is_going_past_tbo_195241-1.html

....and full discussion of related issues (like maintenance induced failures and infant mortality in engines) in his book Manifesto.
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yup.....Mike's concepts in his book are very common in other industries. RCM has been very good to me....:D

With the advent of engine monitors in GA RCM and "on condition" maintenance is now possible. Other industries have been doing this for decades....to include airlines, industrial machinery, power plant (nuke industry)....and many others. The concepts are not new.....just new to GA.
 
PilotRPI said:
For an answer that might actually be helpful instead of another argument:

Often times, people price the plane (the buyer and/or seller) as if it is needing an overhaul. So every hour you fly before you actually overhaul is a little bonus. A buddy of mine is on 4000 hours on an O-320. That is not uncommon for planes that fly frequently. Oil analysis, compressions, and some other stuff can probably help you and a mechanic determine when an overhaul will actually be necessary. But the number is kind of arbitrary for our purposes, again, except for how people may price things.
Click to expand...

Exactly how I bought my Saratoga. The engine had 1950 hours on it and was being "marketed" by the seller as TBO and the seller simply assumed whoever bought it was going to fly it to an engine shop and drop $50k for a factory reman io-540...

I had a thorough prebuy that checked out great. Last OH was 1998 and the plane solidly averaged 100 hours / year of mostly cross country IFR flying (they had a log). The plane never sat according to the logs (oil change tach times).

I've ran 3 oil analysis since buying and they are great. All compressions are high 70's. I installed a jpi830 shortly after purchase and it helps reassure me all parameters are where they should be. Oil consumption is about 1qt per 4-5 hours which my mechanic says is perfectly fine. He says I would be a complete idiot to touch this engine at this point. I run camgaurd with every change as well.

My plan is to keep flying on this "free" engine and simply watch it closely (like you should any engine). I'll OH when necessary. Who knows might be another 1,000 hours ?

Healthy TBO engines are some of the best deals and safest purchases.
 
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