Nasa Icing courses

Jim Macklin wrote:
There is also the fact that the oil pump is at one end of
the engine and the cam shaft lobe that fails is probably at
the other [a guess] and the oil takes some time to reach the
journal and establish a full oil bearing.

Add a little sludge, maybe some cold oil and an over-revved
engine, and you get cam lobe /journal failure or the lifter.

Pilot error due to poor operation.



This is on an O-540. The cam lobe that wore is in the center of the shaft.

So how long is too long to sit? The longest my airplane sat between
flights since the O/H was one stretch of 7 weeks while it was in the
shop for corrosion repair on the belly. Second longest was also in the
shop for 4 weeks for a new interior. Other than that it has been flown
a minimum of every 20 days.

So if it is pilot error, I am all ears as to what I can do to improve my
technique. So far, no one has been able to tell me anything I did
wrong, including Penn Yan.

Strictly my opinion -

Your bad lobe is one of the cam lobes that operates two lifters, so it,
or its companion will be the first to go in a marginal lubrication
situation. The lobes depend on oil thrown from the crankshaft cheeks -
some thing that I don't think happens for a couple of minutes after a
cool start - and especially with a cold engine.

It isn't a question of how quickly does the oil pump get oil get to the
cam bearings as journal bearings will retain enough oil to allow them
to be starved for a minute or two and oil gets there in a few seconds
anyway.

More critical though is how quickly does the crankcase develop that
general fog/spray of oil to finally lubricate all the miscellaneous
(and expensive) surfaces such as cam lobes and cylinder bores. The cam
is at the top of the engine & the lobes are the last thing to get lube.
Possibly if the crank is turning too slow, gravity is such that thrown
blobs of oil can't even make it to the lobes until the oil; gets really
thin. Remember that a new engine will inherently have considerably
less oil leakage from the bearings.

That's why preheat is so important to a Lycoming engine. It isn't as
though it is someting that should be done (usually). It is something
that must be done religiously every time (especially with summer weight
oil) as once cam surfaces are scratched, failure isn't far away. The
longer an engine has been sitting, the more the need for preheat. The
newer the engine, the more the need for preheat too. When and how
much? Who knows.

Also - thick oil reduces the flow demand of the engine, and more of it
will simply blow over the relief valve. Oil that is bypassed this way
has no access to the heat of the engine. Oil warmup will be slow even
if the CHT is getting into an operating range.

My guess is that there was a cold start or cool start with summer oil
combination somewhere in its history although you indicated you always
preheated. Or maybe the cam which is supposed to be case hardened
isn't as hard as it should be. You might check the cam hardness with a
new file on another lobe. It should not be able to bite into it. If
it does, get a hardness test done on it.

Strictly my opinion -

Your bad lobe is one of the cam lobes that operates two lifters, so it,
or its companion will be the first to go in a marginal lubrication
situation. The lobes depend on oil thrown from the crankshaft cheeks -
some thing that I don't think happens for a couple of minutes after a
cool start - especially with a cold engine.

It isn't a question of how quickly does the oil pump get oil get to the
cam bearings as journal bearings will retain enough oil to allow them
to be starved for a minute or two and oil gets there in a few seconds
anyway.

More critical though is how quickly does the crankcase develop that
general fog/spray of oil to finally lubricate all the miscellaneous
(and expensive) surfaces such as cam lobes and cylinder bores. The cam
is at the top of the engine & the lobes are the last thing to get lube.
Possibly if the crank is turning too slow, gravity is such that thrown
blobs of oil can't even make it to the lobes until the oil; gets really
thin. Remember that a new engine will inherently have considerably
less oil leakage from the bearings.

That's why preheat is so important to a Lycoming engine. It isn't as
though it is someting that should be done (usually). It is something
that must be done religiously every time (especially with summer weight
oil) as once cam surfaces are scratched, failure isn't far away. The
longer an engine has been sitting, the more the need for preheat. The
newer the engine, the more the need for preheat too. When and how
much? Who knows.

Also - thick oil reduces the flow demand of the engine, and more of it
will simply blow over the relief valve. Oil that is bypassed this way
has no access to the heat of the engine. Oil warmup will be slow even
if the CHT is getting into an operating range.

My guess is that there was a cold start or summer oil combination
somewhere in its history although you indicated you always preheated.
Or maybe the cam which is supposed to be case hardened isn't as hard as
it should be. You might check the cam hardness with a new file on
another lobe. It should not be able to bite into it. If it does, get
a hardness test done on it.

: Your bad lobe is one of the cam lobes that operates two lifters, so it,
: or its companion will be the first to go in a marginal lubrication
: situation. The lobes depend on oil thrown from the crankshaft cheeks -
: some thing that I don't think happens for a couple of minutes after a
: cool start - especially with a cold engine.

: It isn't a question of how quickly does the oil pump get oil get to the
: cam bearings as journal bearings will retain enough oil to allow them
: to be starved for a minute or two and oil gets there in a few seconds
: anyway.

It's quite unfortunate that the regulations make adding an oil
accumulator/pre-oiler cost-prohibitive for most. Pushing 60-80psi oil from the last
run into the system 30-60 seconds before hitting the starer would help a lot of this.

-Cory

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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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