REFLECTOR: Oil Leaks, Plennum Air Pressures, and the True Confessions of a new mechanic.

Terry Miles terrence_miles at hotmail.com
Thu Aug 27 14:23:42 CDT 2009


Hi all

I have another report to make.  The factory supplies a stainless steel oil
line with an inner teflon lining in the engine install kit.  I was slow
build wings and body with Hangar 18 in SC.  In the course of the years it
took to get the up to the point of engine installation,  those lines got
move a million times and stuffed into different boxes.  I am guessing in the
processs at some point I kinked the outer SS sheath trying to bend it in two
to fit in a box.  I only vaguely recall this now, because I saw it and
believed it to be a non issue.  It's not.  If you damage that outer metal
sheath on the line, think about replacing it.  What happened to me is that
over the course of about 50 hours and with vibration of flight,  that metal
was able to chaff a hole in the line.    (Side issue: I am also finding
chaff marks on my electrical here and there and particularly out of the
access holes in the utility duct.) 

 

So here is the picture:   The leak was on the oil outflow line on a LY 540.
It routes downward from the aft face of the engine to a coupling right at
the entrance to the LH utililty channel.  The line has orange heat shroud
(fire sleeve) on it so the oil was coursing down between the SS and
firesleeve.   My guess is the leak started at about 50 hours right when I
switched from mineral oil to ashless disp oil. 

 

I never had any thing in the way of blow by oil in those first 50 hours.
Then when I switched to the ashless, I started having an oil trace out of my
air/oil Separator belly drain, which I located to the LH of the belly
centerline near the LH duct entrance.    I took the leak to be a ring seat
issue because the viscosity of the oil being thinner.  

 

I was watching all this.  I inspect very carefully, but I was
mis-interpreting in several ways.  The oil was not coming out of the
sparator mast, it was coming out of the hole I cut in the cowl for that
mast.   I have true NACA ducts (Hangar 18) on my bird that are a
considerably different shape and power than the factory NACA-esque openings.
The net result is I have a lot of positive pressure in my engine
compartment.  Before these oil drips could actually land on the floor of the
cowling, they were being sucked out of the separator mast opening.  I was
investigating for a clogged  separator.  It wasn't the separator.  Then I
thought I had developed a bad ring.  It was never much oil.  When the
separator proved to be working right, I should have kept looking.  

 

My wake up call was when oil started coming out of the LH aft edge of my
nose gear door.  

 

My first thought was it was the front oil cooler, but it was dry.  Then my
heart sank to think I had a rupture somewhere in the LH utility duct.  It
was only then that noticed the slightest droplet of oil on the
oil-line-coupling attach bracket in the engine compartment which is of
course RIGHT at the entrance to the LH utility duct.  When I went to pull
back the fire sleeve for a better look, I found the interior fibers all
soaked with engine oil.  I yanked the line and it failed a pressure test big
time, right where the small distortion  was in the SS mesh.

 

In summary the engine outlet oil line was leaking due to chaff from the
kinked (crushed in handling) SS sleeve that gouged into the internal poly
line.  The leaking oil was not dripping anywhere for me to see in the way
you would expect,  but rather being blasted out the near by cowl opening for
the oil/air separator drain mast.  Oil was also being blasted into the LH
utility duct all by means of positive air pressure going from high pressure
engine compartment to low pressure end of the duct in the nose.  The drips
were coming off right at the entry to the duct.   

 

I bought a new hunk of line from the facory.  She is all back together and
has 15 plus hours since repair.  I drilled a 3/32 drain hole in the LH util
duct just aft of the entry door, and a small amount ran out just stactic on
the ramp, the rest blew out in the next few flights .  Now there is no trace
anywhere.  

 

I thought this was a discovery worth the sharing.  As a side comment, I know
some guys plug the utility ducts at the fire wall.  I am not inclined to do
that myself so that I can smell a little of what is going on back there, but
I am sure opinions vary.

 

Regards,

Terry

 

 

 

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