Ok, time for a third, second opinion. If you're running a piston single with a Lycoming engine and your compressions are 80/80 75/80 74/80 72/80 then 15hours later the engine is vibrating on it's mounts a bit. So you send it for maint.
Now the compressions are 80/80 74/80 60/80 60/80. Would you fly it? I'm sending it for a TOH possibly even a major depending on what my boss wants.
What do you guys/gals think of those numbers? Keeping saftey in mind, we fly on the west coast...over water, mountains the like.
RJ
Low compression engine Q
Moderators: North Shore, sky's the limit, sepia, Sulako, lilfssister, I WAS Birddog
The $64 question is, where is the leak?
Put the 80 psi in the plug at TDC, then listen for air hissing at:
1) intake
2) oil breather
3) exhaust
It will never leak at the intake - the valve is cooled by incoming fuel/air mixture.
If it's hissing at the oil breather, you've got worn or broken rings. Really no big deal, honestly. Keep an eye on the oil burn rate and the top plug. Oil on the bottom plug is no big deal, if you see oil on the top plug, or you're burning 1 qt of oil per hour, or if the compression is less than 60 it's time to pull the jug.
If it's hissing at the exhaust stack, DON"T FLY IT unless you're happy to force-land it. It MIGHT just be a bit of carbon stuck in the exhaust valve, but it might be a crack in the exhaust valve head. If it goes, you will not be happy. Also, over time, exhaust valves "neck" or erode just behind the valve face, and if it gets too thin, it may break off, which also won't make you very happy.
Regardless of what the regulations say, the exhaust valve is what you want to eyeball for cracks or necking. You can fly with broken rings for quite some time.
Put the 80 psi in the plug at TDC, then listen for air hissing at:
1) intake
2) oil breather
3) exhaust
It will never leak at the intake - the valve is cooled by incoming fuel/air mixture.
If it's hissing at the oil breather, you've got worn or broken rings. Really no big deal, honestly. Keep an eye on the oil burn rate and the top plug. Oil on the bottom plug is no big deal, if you see oil on the top plug, or you're burning 1 qt of oil per hour, or if the compression is less than 60 it's time to pull the jug.
If it's hissing at the exhaust stack, DON"T FLY IT unless you're happy to force-land it. It MIGHT just be a bit of carbon stuck in the exhaust valve, but it might be a crack in the exhaust valve head. If it goes, you will not be happy. Also, over time, exhaust valves "neck" or erode just behind the valve face, and if it gets too thin, it may break off, which also won't make you very happy.
Regardless of what the regulations say, the exhaust valve is what you want to eyeball for cracks or necking. You can fly with broken rings for quite some time.
Hedley wrote:The $64 question is, where is the leak?
Put the 80 psi in the plug at TDC, then listen for air hissing at:
1) intake
2) oil breather
3) exhaust
It will never leak at the intake - the valve is cooled by incoming fuel/air mixture..
I have issues with the statement for never leaking at the intake. Continental has had lots of problems with the intake valves not being properly lapped. This is on the 520 550 series. The loss of compression is usually found in the first 200 hous but I have seen it occur at 5-600.
This would only apply if you have had a cylinder chage recently.
If you do decide to top a 520 or 550 i would have the shop re-lap the intake valves on the factory new jugd before intalling them. I realise you said you have a lycoming but the intake valve lap is a commmon problem.
It could be the rings lining up and causing leakage. If the pressure loss it not at the exhaust stack, try flying it for a couple of hours and then re-test the compression.
Given that you have 3000 hrs on this block and we don't know when the last top o/h or cylinder chage was. I would lean towards the full overhaul.
Also I have to note that if your boss is running 3000 hrs on an engine, flying in that enviroment - he is likely going to opt for just a top o/h
I would be looking for the full rebuild. including any accessoires components like mags and vacuum pumps that are past half life.
By getting to 3000 hrs with not top o/h the owner has saved enough $$ to pay for a new motor. If the accessories have less than half time change them and keep for spares and trouble shooting.
By getting to 3000 hrs with not top o/h the owner has saved enough $$ to pay for a new motor. If the accessories have less than half time change them and keep for spares and trouble shooting.
I imagine your company has based their rates on TBO of that particular engine any thing after that is money in there pocket. Running on condition will work for a few hundred hours but TBO's are there for a reason and 3000 hours is way over my limit. They are now making engine costs at your risk. With the parameters you have stated you need a new engine end of story.
The engine was warm the second compression check with two cyls at 60/80. Warm on the prev. one too I believe. On it's last flight home, the a/c didn't produce more than 2300 static RPM so I'm told. Thus I sent it for maint.
As for the prop...it's an average mid-time prop. Nothing special.
Why, what are you thinking Dash 27?
RJ
As for the prop...it's an average mid-time prop. Nothing special.
Why, what are you thinking Dash 27?
RJ
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Mitch Cronin
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- Location: Right beside my dog again...
Personally, I'm not so alarmed by the compression all by itself (Hedley gave you some excellent tips on how to consider that) as I am by the sudden parameter shift, combined with vibration that I think you're saying is new....
Sounds to me like a shop visit was a good choice.
Sounds to me like a shop visit was a good choice.
My concern isn't so much the low compression as the vibration. Bent valve or sticking valve possibly or cracked cylinder...wild guess here without picking up the tools. Pulling the two low cylinders and replacing with new or overhauled ones is my first move. With the cylinders removed you are in a better position to assess the health of the engine with a good look at the internals...ie camshaft lobes, wear on the piston skirts, ball ends on the pushrod tubes, wear on the wrist pins etc. Pull the oil filter. Any metal in the screens? Have a soap test done. (oil analysis) The results should give you indications of any abnormal wear. How is the oil pressure? If the bottom end is in reasonable condition the oil pressure should be too. If after a good poke around in the engine you find you're flying a basketcase then pull the engine. The cost of splitting the case for a full overhaul vs changing a bad cylinder or two is huge. Was the engine good and warm for the compression tests that came in low? A bad magneto, spark plug or lead can cause a vibration too.
If the low cylinders have 3000hrs on them, they're due. The only history on them is what is written in the engine log. That log stays with the case so there may be many more hours on those cylinders than 3000.
I've seen cylinders fail after 4hrs, fresh from overhaul!!! They had probably been through several engines in their component life.
Hope this helps.
If the low cylinders have 3000hrs on them, they're due. The only history on them is what is written in the engine log. That log stays with the case so there may be many more hours on those cylinders than 3000.
I've seen cylinders fail after 4hrs, fresh from overhaul!!! They had probably been through several engines in their component life.
Hope this helps.
I think too many people may be obsessing about the 3,000 hours.
Honestly, with correct operation, hours are almost irrelevant.
For example .. factory engine with 50 hours in first month. Then parked for 3 years without pickling. It will need an overhaul because of internal corrosion .... but it's only got 50 hours (and 3 years) since new!
Other extreme ... let's say you have an aircraft that doesn't see cold weather, so no bottom end wear on startup. It flies 2 four hour legs each day with CHT never over 350F and oil temp never over 180F.
If there are no manufacturing defects, you can probably fly 5000 (or more) hours on that engine, if you keep changing accessories and jugs as required along the way. If you want a really reliable engine, swap the accessories every 500 hours, and the jugs every 1,000 hours.
Honestly, with correct operation, hours are almost irrelevant.
For example .. factory engine with 50 hours in first month. Then parked for 3 years without pickling. It will need an overhaul because of internal corrosion .... but it's only got 50 hours (and 3 years) since new!
Other extreme ... let's say you have an aircraft that doesn't see cold weather, so no bottom end wear on startup. It flies 2 four hour legs each day with CHT never over 350F and oil temp never over 180F.
If there are no manufacturing defects, you can probably fly 5000 (or more) hours on that engine, if you keep changing accessories and jugs as required along the way. If you want a really reliable engine, swap the accessories every 500 hours, and the jugs every 1,000 hours.
