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Metal fatigue in Lycoming engines limiting its TBO?



 
 
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  #11  
Old November 5th 04, 03:03 AM
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The limiting factor isn't generally crank/rod/ or other major
component fatigue (especially in those bullet-proof O-320s)but is
probably corrosion and cam/lifter spalling. Oil analysis might catch
either one. I doubt the factory has any info beyond that they would
or even could release.

From a strictly cumulative fatigue analysis standpoint there could
have been an innocent relatively short term event in the engine life
such as simply max throttle on a bitterly cold winter day that has
done most of the fatigue damage. Or it could simply be long term
operation at a wrong RPM where the crankshaft & prop are resonant.

This type of "abuse" exists, but the wise ones (i. e. the certifiers)
have decided and found that there is enough metal in this engine that
it seems to be able to handle it. Hence for example there is no
prohibited RPM range. Certainly the operating experience of a large
fleet helps reinforce this. Whether that is true for precisely all
supposedly identical engines and all conditions is impossible to say,
but the history on this engine series is probably the best in the
business. But engineering is an art, not a science. As one
(mechanical) I can understand why the factory is reluctant to say to
keep operating it.

I have an O-320 E2D in a 172M last assembled over 29 years ago, & now
at 1700 hrs TTSN. It has been opened only slightly for the oil pump
gear AD, but otherwise it has shown no deterioration in compression,
smoothness, or oil consumption over its lifetime. I'd like to think I
have run it pretty carefully, but I realize I am running a slight
chance. On the other hand, seeing the occasional AD show up on newly
manufactured parts scares me too. The comfort I have is that I
personally know the entire history of its operation. My decision is
to keep operating it.

Our IA once mentioned a Citabria in fish spotting duty out over the
Atlantic that had 3800 hrs on it without overhaul.

Unfortunately the limits of technology has to make it your call. But
others are doing it too.
  #12  
Old November 7th 04, 02:46 PM
Roger Long
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Oil analysis will NOT alert you to lifter spalling. The pieces are too big.
You will see those in a careful inspection of the oil filter element. Once
the first pin head, or smaller, size piece shows up, the lifters will go
fast. It's like taking the first brick out of a brick wall.

If you have one of the engines where the lifters can be pulled easily, you
should have them looked at every 100 hours after about the first 500. Catch
the problem early in an engine where the lifters can be pulled any you may
save the cam and an engine tear down. If you find a single speck of
metallic metal in your oil filter, pull the lifters and check.

Rocker arms are a potential fatigue failure point. In an O-320, the rocker
arm is fairly thin, it may even be stamped, and there is a dimple to form
the recess that the pushrod goes in. I've seen that dimple simply popped
out of the rocker arm in a way over TBO engine.

There were evidently a lot of lifters with sub standard metallurgy released
into the supply stream in the late 90's. We'll probably hear something from
the FAA about that before the end of the year. Despite compulsive care of
our O-320 H2AD and awareness that even a single cold start can the spalling
process in a lifter, two of ours came apart and destroyed the cam at 1000
hours.

You will so performance evidence of a cam that is grinding down due to
lifter spalling until it is worn to a startling degree. There will be a
gradual loss of horsepower but it will progress so slowly that you won't
notice unless you monitor and record performance very compulsively.

--

Roger Long



wrote in message
m...
The limiting factor isn't generally crank/rod/ or other major
component fatigue (especially in those bullet-proof O-320s)but is
probably corrosion and cam/lifter spalling. Oil analysis might catch
either one. I doubt the factory has any info beyond that they would
or even could release.

From a strictly cumulative fatigue analysis standpoint there could
have been an innocent relatively short term event in the engine life
such as simply max throttle on a bitterly cold winter day that has
done most of the fatigue damage. Or it could simply be long term
operation at a wrong RPM where the crankshaft & prop are resonant.

This type of "abuse" exists, but the wise ones (i. e. the certifiers)
have decided and found that there is enough metal in this engine that
it seems to be able to handle it. Hence for example there is no
prohibited RPM range. Certainly the operating experience of a large
fleet helps reinforce this. Whether that is true for precisely all
supposedly identical engines and all conditions is impossible to say,
but the history on this engine series is probably the best in the
business. But engineering is an art, not a science. As one
(mechanical) I can understand why the factory is reluctant to say to
keep operating it.

I have an O-320 E2D in a 172M last assembled over 29 years ago, & now
at 1700 hrs TTSN. It has been opened only slightly for the oil pump
gear AD, but otherwise it has shown no deterioration in compression,
smoothness, or oil consumption over its lifetime. I'd like to think I
have run it pretty carefully, but I realize I am running a slight
chance. On the other hand, seeing the occasional AD show up on newly
manufactured parts scares me too. The comfort I have is that I
personally know the entire history of its operation. My decision is
to keep operating it.

Our IA once mentioned a Citabria in fish spotting duty out over the
Atlantic that had 3800 hrs on it without overhaul.

Unfortunately the limits of technology has to make it your call. But
others are doing it too.



 




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