Belly Oil

Hey Sean. Only reasons I am aware of, and no doubt you’ve checked them, are either the oil tank vent hose is positioned and/or cut as to cause air pressure to pull oil out in flight; or too much oil in the system- even a tiny bit over full on the tank dipstick can cause an oily underside.

A vented catch bottle at the bottom of the firewall would probably work well. Just have to find one that’s appropriate.

Hi Sean

Reference your installation manual.  in it you will find your answer, it is the same as found in the FAA AC-43-13 current issue.   

The vent outlet is in a low pressure zone and is sucking out the oil vapor.  Be sure that the line somewhere inside your cowling has a vent "notch" that not only prevents ice forming on the line end but will balance the low pressure sucking on the line.  Trust me, this is a very common problem and the solution is very simple.

Cheers

RW quote "Trust me, this is a very common problem and the solution is very simple."

I see this on most aircraft. Like RW stated if the hose is placed improperly or not setup properly them you can get the oily film on the bottom of the plane. That said I almost always see some oily film or discolor on the bottom of the plane from the oil tank vent and the exhaust discoloration on the bottom of the plane. I use a product called "Oil Eater" to clean and wipe all this off. It is water based and biodegradable, but boy spray it on and the oily film will be running off before you wipe it off. I've been using it for 30 years and it causes zero damage and does a great job. I buy it on Amazon now and use it to get grime and oil off my hagar floor and use it at home to wipe down any greasy residue on top of the stove and stove hood. I buy it in a 5 gallon pale, but it comes in one gallon jugs too. I use this to clean K&N air filters too. Works great and will not damage the filter.

Sam Purpura wrote:

A vented catch bottle at the bottom of the firewall would probably work well. Just have to find one that’s appropriate.

Ah! but what sort of Catch Can suits a  912ULS???

I have a home made catch can, at the bottom of the firewall. It's a small plastic 300ml drink bottle with strategically located in/out tubes. Idea good, execution poor - doesn't work! Needs more knowledgeable input to get oil to drop out of suspension.

Will give the Rotax oil tube "notch" a go.😈