Air / Oil Separator -- Pros / Cons

[MisfitSELF]

I currently have an Air / Oil Separator on my airplane but was wondering what the tribal knowledge was on this topic.  I'm sure it has to have been hashed out in detail so if you have link to the thread, please link it.

BTW:  M20J, Lycoming IO-360-A3B6

Bruce

[MikeOH]

I'm happy with mine on my M20F (IO-360A1A) Nice clean belly!  I haven't been aware of any cons but this is my first airplane so I have nothing to compare to.

[mike_elliott]

Cons are blow by bi products (combustion acids, etc) are dumped back into your crankcase to magnify contamination of oil supply, and it can disguise a ring issue, Pros are you don't have to clean the belly as often.

 

[A64Pilot]

Sometimes they are necessary, like for example if you have a wet vacuum pump, the pump being oil lubricated discharges an oil mist that either oils the belly or is separated and put back into the sump.

Most often an oil sep is installed to clean up a belly, they aren’t necessarily an indicator of a worn engine but could possibly be.

I used to build my own, this is a knock off copy of the M20 separator, it works but not as good as an Air Wolf which is a copy of the Walker oil separator originally used on marine GM Diesels I believe.

In my opinion it doesn’t hurt to remove them every other annual or so and let one sit overnight in mineral spirits to clean one out, maybe over time they could accumulate some sludge, or maybe not. As far as I know mine never did but I’ve heard reports of them that have, perhaps it’s due to the temp differences where they are used, I fly in warm weather almost exclusively.

 

[jlunseth]

I live in the frozen north so I have an issue maybe most others don’t. I intended to do a landing at our international airport, MSP, and in preparation decided to practice some fast approach landings, thinking MSP might want me to “keep the speed up.” It was a cold day here, 12 dF if I recall. I did my usual run up, flew out to a rural airport about 25 miles away, did a rapid approach and landed. On takeoff for another I saw very low oil temp so I immediately circled back to land. There was heavy oil a good two feet wide and about six feet long on both sides of the aircraft, coming out of the back of the cowling. The eventual diagnosis was that the air/oil separator had frozen up so the engine could not breath and was forcing oil out of every available orifice. Fortunately no harm was done.

Apparently, the air/oil separator collects moisture, quite a bit of it sometimes, and it is the exit from the crankcase for the breather tube. Now on cold days I will always spend time on the ramp letting the engine warm up everything in the engine compartment. That episode was years ago, close to a decade, and I haven’t had a problem since then even though, in a turbo and in winter, the temps at altitude are sometimes so cold that the windows fog over despite the defroster going full blast. Like -50’s dF. As long as I have warmed the engine compartment before takeoff everything is good.

[A64Pilot]

(in my opinion) the proper way to handle crankcase blow by is to duct it into the exhaust, done properly it would form a slight negative crankcase pressure which is good, it also of course couldn’t freeze up and it’s my belief that the belly would stay completely oil free as surely the exhaust gas is hot enough to at least vaporize the oil mist.

I don’t know why this isn’t done, it would seem to be to be an easy money maker for an STC to do this. 

[LANCECASPER]

On 10/12/2023 at 8:11 AM, A64Pilot said:

(in my opinion) the proper way to handle crankcase blow by is to duct it into the exhaust, done properly it would form a slight negative crankcase pressure which is good, it also of course couldn’t freeze up and it’s my belief that the belly would stay completely oil free as surely the exhaust gas is hot enough to at least vaporize the oil mist.

I don’t know why this isn’t done, it would seem to be to be an easy money maker for an STC to do this. 

I like that design and it was used on the M20TN (Acclaim). Some owners didn't change oil often enough and/or keep the hoses cleaned often and some sludge did build up and some coking occurred..

There is a service bulletin that eliminates it going to the exhaust. But for the time being I'm making sure all hoses are in good shape and cleaned regularly and I’m leaving mine just the way it is. 

[A64Pilot]

Why would you need a separator if the gasses go into the exhaust? Surely the exhaust could handle the small amount of oil? By handle I mean eliminate it, exhaust can of course “handle” large amounts of oil, smoke like you see at air shows is just oil injected into the exhaust of course and other than an oily belly there is no harm. I assume oily belly, I know a Huey smoke bird’s tail boom was covered in oil to the point of it dripping off, but that was a HUGE amount of oil.

 

[jlunseth]

They did exactly that with the Acclaim in the beginning, ducted the breather into the exhaust. It would coke up and there were a number of engines needing a top at 400 hrs. Don’t have an Acclaim so don’t know exactly what they did to fix it but they did fix it.

[A64Pilot]

I could see it coking up, and I assume this coking could be handled with an inspection at oil change time, that is pull the hose off and clean the opening as necessary, but probably less exposure if you will to simply revert back to open to atmosphere. I can see how that could lead to blown seal, most dangerous one being front crankshaft, but I don’t see how it could lead to cylinder wear?

Even with a coking possibility I’d rather have a clean belly if I could, but that’s just me. Every airplane I’ve ever had the belly gets oily, add in a little dust, and it’s nasty.

[jlunseth]

I am not a mechanic. All I can tell you is that when the coking problem was discovered the early tops magically stopped. Not a simple inspection though. You have to go in and take hoses off, and look for coking before the aperture in the exhaust tube. https://www.mooney.com/wp-content/uploads/2020/12/SBM20-312.pdf

[Pinecone]

My 252 has an oil separator.  It appears to be factory.

I had an issue with all of a sudden higher oil consumption.  My local FBO checked and nothing was leaking.  But said they had seen this on another TSIO-360.  The breather connection to the crankcase is forward starboard, and the oil separator is aft port.  There is a hose that crosses from right to left near the front of the engine then back to the baffle where the oil separator is mounted.

They removed the hose and oil separator.  They flushed the hose with mineral spirits and compressed air.  And flushed the oil separator with mineral spirits.

The oil consumption went back to normal.

[LANCECASPER]

On 10/12/2023 at 3:08 PM, jlunseth said:

I am not a mechanic. All I can tell you is that when the coking problem was discovered the early tops magically stopped. Not a simple inspection though. You have to go in and take hoses off, and look for coking before the aperture in the exhaust tube. https://www.mooney.com/wp-content/uploads/2020/12/SBM20-312.pdf

It's actually much easier than it looks in the service bulletin.

The yellow Tygon hose, which is spec'd out in the Mooney parts manual you buy by the foot and it's relatively inexpensive. I replaced it as soon as I bought the airplane and now I clean it out by taking it off and flushing it with mineral spirits. If it was just changed at every annual it would be fine. Also the hose that runs down to the tailpipe is just a piece of Gates hose, inexpensive and not hard to remove and clean,  or just change. I look in the opening with a borescope and it's clean. Personally I think they over-reacted and threw out a good design too early. Maintaining the hoses is not difficult, but I agree if they plug up that could change the cylinder pressure.

I have the new tailpipe called out in the Service Bulletin if I ever decide to do the service bulletin, but running it into the exhaust keeps the underneath cleaner, more of just a tan dust near the exhaust pipe.

 

[jlunseth]

That's great. I don't know much about the Acclaim. What I do know I learned sitting around the stove at Willmar (figuratively, there's no stove). Sounds like a plane born with everything you need to fly LOP and fly fast.

[Steve Dawson]

If the OP doesn't mind that I'm using his thread for a M20 installation question: I just bought a M20 air/oil separator and will be installing it on my M20K 231. Where does the oil line drain back into? Is there anything wrong with having it drain into the back of the accessory case by removing the nut that's below the mags and replacing it with a nut with a barbed or threaded fitting? 

Thanks 

[Will.iam]

42 minutes ago, Steve Dawson said:

If the OP doesn't mind that I'm using his thread for a M20 installation question: I just bought a M20 air/oil separator and will be installing it on my M20K 231. Where does the oil line drain back into? Is there anything wrong with having it drain into the back of the accessory case by removing the nut that's below the mags and replacing it with a nut with a barbed or threaded fitting? 

Thanks 

If i recall my oil separator drains back into the oil filler tube as there is a small side tube going into it from the bottom of the separator on my 252. I would take a pic but it’s in annual at the moment. 

[Will.iam]

On 10/12/2023 at 8:22 PM, Pinecone said:

My 252 has an oil separator.  It appears to be factory.

I had an issue with all of a sudden higher oil consumption.  My local FBO checked and nothing was leaking.  But said they had seen this on another TSIO-360.  The breather connection to the crankcase is forward starboard, and the oil separator is aft port.  There is a hose that crosses from right to left near the front of the engine then back to the baffle where the oil separator is mounted.

They removed the hose and oil separator.  They flushed the hose with mineral spirits and compressed air.  And flushed the oil separator with mineral spirits.

The oil consumption went back to normal.

Interesting that cleaning it and the beather hoses helped lower oil consumption. Unless it was the drain back hose that was clogged and all the oil was going out the seperator’s overboard hose instead of draining back i just don’t see how it would help but would be easy to try and clean to see if my oil consumption would go lower. 

[philiplane]

the only separator worth using is the Airwolf. It solves the problems of freezing, and of water in the oil, by a superior design. The M20 brand separators are worthless. 

[Steve Dawson]

22 hours ago, Will.iam said:

If i recall my oil separator drains back into the oil filler tube as there is a small side tube going into it from the bottom of the separator on my 252. I would take a pic but it’s in annual at the moment. 

Thanks William, The oil filler tube is in front of the engine baffle so it's inaccessible for the drain.  

[A64Pilot]

On 1/11/2024 at 10:47 PM, philiplane said:

the only separator worth using is the Airwolf. It solves the problems of freezing, and of water in the oil, by a superior design. The M20 brand separators are worthless. 

I wouldn’t say they are worthless, they do work, but probably not as well as the Airwolf, but they are a lot less expensive too.

The issue with moisture is sort of problematic, the trick is of course the separator has to be kept pretty warm if it’s cold then it will condense moisture in it and return it back to the sump, but I think it will very quickly then boil off again. Think dropping a drop of water into a hot oil filled fry pan.

One assumes if you lived in the Great White North it would be very simple to wrap a piece of hose fire shield around your M20 and secure it with safety wire to keep it above the temp moisture will condense. It of course has a constant flow of pretty hot air going through it.

I never insulated one because I never had to, anyone that does oil analysis should know if they have moisture in their oil, those that live in the cold often see it as a yellowish goop dripping from their crankcase vent and sometimes in the oil filler cap.

I suspect moisture in oil is more from very cold temps as in the oil never getting hot enough or a series of short flights more than it is from a separator, but as it’s never been a problem for me I don’t know.

[A64Pilot]

11 hours ago, Steve Dawson said:

Thanks William, The oil filler tube is in front of the engine baffle so it's inaccessible for the drain.  

I mounted mine high and drained it into an aft cylinder heads oil drain back tube (Lycoming) But I don’t think it matters where it drains back to, you could even fit a catch can if you believe it’s something that shouldn’t go back in your engine. There shouldn’t be much oil unless you have excessive blow by. Worst case if the M20 were to fill with oil it would just cease to function as a seperator and your blow by would go out the tube as if the M20 weren’t installed.

Ideally in my opinion your crankcase vent should go up hill as much as possible before going down for obvious reasons

[PeteMc]

10 hours ago, A64Pilot said:

I never insulated one because I never had to

I have a K and typically fly in the teens, so cold air even in the summer and very cold in the fall thru to spring.  There was already an M20 on my plane when I bought it and I never every thought about insulating it until I just read it hear.  Neither my original (very good) mechanic or my second very long time (very good) mechanic never mentioned it should be insulated.  And I think both would have if it was a thing. 

Your point of running the engine long enough to burn off any moisture is probably the real issue.  And how long the plane sits to allow condensation to build up again inside the engine. 

[N201MKTurbo]

I really like the separators on Cessna 310s. They are cleanable and have a multilayer metal mesh that coalesces the oil mist.

[blaine beaven]

On 10/12/2023 at 6:30 AM, jlunseth said:

I live in the frozen north so I have an issue maybe most others don’t. I intended to do a landing at our international airport, MSP, and in preparation decided to practice some fast approach landings, thinking MSP might want me to “keep the speed up.” It was a cold day here, 12 dF if I recall. I did my usual run up, flew out to a rural airport about 25 miles away, did a rapid approach and landed. On takeoff for another I saw very low oil temp so I immediately circled back to land. There was heavy oil a good two feet wide and about six feet long on both sides of the aircraft, coming out of the back of the cowling. The eventual diagnosis was that the air/oil separator had frozen up so the engine could not breath and was forcing oil out of every available orifice. Fortunately no harm was done.

Apparently, the air/oil separator collects moisture, quite a bit of it sometimes, and it is the exit from the crankcase for the breather tube. Now on cold days I will always spend time on the ramp letting the engine warm up everything in the engine compartment. That episode was years ago, close to a decade, and I haven’t had a problem since then even though, in a turbo and in winter, the temps at altitude are sometimes so cold that the windows fog over despite the defroster going full blast. Like -50’s dF. As long as I have warmed the engine compartment before takeoff everything is good.

When I bought my plane in 2014 my mechanic noticed that the overboard hose from my separator didn’t have a “Jesus hole” in it. Basically, about 6” above the bottom of the firewall, before the curve of the metal end of the hose that comes out the cowl flap, he drilled a small hole. 
 

The idea being that if the end of the hose freezes up, the pressure can still escape without blowing out any seals in the engine. The hole in the cowling shouldn’t freeze up. And there is enough vacuum/pressure that in normal conditions it doesn’t leak. 
 

I’ve never noticed any mess or residue around the Jesus hole, so I assume I have yet to freeze it up despite flying in some pretty cold temps. 

[jlunseth]

I can only tell you what happens from a cold weather pilot’s perspective, but from that perspective the separators seem to work fine if they are allowed to warm a little on the ground. The day I had a problem it was about 12 dF and the air at ground level here in the MSP area is pretty dense on those days. I was in a hurry to practice some rapid descents and landings, and I got in the air right away. As I said in the other thread, I have not had trouble at cruise in some extremely cold temps - as long as the separator has had a chance to come up to an operating temperature before takeoff.

Edited January 14, 2024 by jlunseth