-----Original
Message-----
From: Frederick
Moreno [mailto:frederickmoreno@bigpond.com]
Sent: Thursday,
12 March 2009 9:45
PM
To: Lancair Mail (lml@lancaironline.net)
Subject: Air Oil Separator
Woes
I now have about 60 hours on my
IO-550 in my Lancair IV.
We headed off for a big air show
near Melbourne, about 1800 miles east. Hot day departure,
high power climb to 9500 feet, lots of pork, baggage, and fuel on board.
First stop, oil all over the belly. Uh-oh. This problem had
occurred earlier on a few occasions, but with only a few drops of oil.
This was a gusher.
Did some trouble shooting around the
air oil separator, checked lines to assure they were clear, cleaned up the mess,
added two quarts, put tail between legs, turned around and headed home
cautiously: lower power slower flatter climb, lower cruise manifold pressure and
RPM.
Arriving home, only a trace of oil
caught in the cup of the air oil separator gas vent which had previously
overflowed and spilled all the oil inside the cowl and along the belly.
Hmmmm…..
I wrote ECI who made the cylinders
providing a fairly comprehensive report and asked for guidance. I and some
friends did a lot of research. The ECI reply and our research all pointed
to excessive blow by carrying too much oil to the air oil separator, more than
it could handle, and when its maximum flow rate for oil return was exceeded, it
filled and overflowed. Cause: possibly lousy break in, possibly cylinders
running too cold and thus retaining choke which caused ring flutter, other
possibilities. All pointed to pulling jugs and inspecting. UGH.
I returned to the hangar today with
some bright and technically astute friends and we had about two hours of debate
and finally concluded that we should first PROVE there was a lot of blow by
before turning a single cylinder nut.
There were a lot of red herrings and
a lot of ground tests and finally a couple of fairly abusive flight tests to
check the fix. We found our way through the maze with only a few
dead ends.
The problem? Captured oil is
supposed to drain from the air oil separator to the crankcase via a pushrod
tube. The drainage is supposed to be assisted with a bit of extra air
blast to push it along using a bit of flow from the top of the cowl.
In fact, by pulling tubes one by one
and testing, we found that the oil was running up the pushrod tube, up the
drain tube and INTO the bottom of the separator, filling it and finally
overflowing it. It occurs primarily at high power high RPM and
climb. The raised nose decreases the drop to the engine and tips the
flow into reverse, it appears. Reduce power and lower the nose and the
pressure balances change and the reverse flow stops.
We finally put a rag in the breather
line and did a ground full power run up (after sinking the bird up to the axles
in the grass to keep it from skidding away) and there was virtually NO
BLOWBY. The rag stayed in the breather discharge line, and did not even
pick up any oil. The blow by that was occurring was small enough to seep
through the rag without pushing it out the hole.
We plugged the drain return line
feeding into the pushrod tube, put the air oil separator oil drain into a cup,
and then blasted off on some abusive high power sustained climbs to
10,000. Only a trace of oil was captured, and I guess that is to be
expected with everything pushed up to the firewall and the nose up 12
degrees.
Morals of the story:
1) Modern engines apparently do not
need air oil separators. My old Lycoming 540 sure did! That was
then. This is now.
2) If you have enough pressure drop
across the engine due to lots of ram pressure at the cooling air inlets, a tight
fitting plenum, and open cowl flaps while climbing to create a lot of suction,
the large pressure drop created across the engine baffles can screw up the
pressure distribution in the air oil separator and its various air and oil lines
causing oil to flow backwards.
3) Be suspicious of oil going
overboard. It may not be blow by at all.
Other than being shot at and missed,
there is nothing quite as gratifying as avoiding a premature top overhaul.
Whew.
Fred
Moreno