Oil Drain tube

Small holes or slits in crankcase breather tubes are very common in most reciprocating aircarft engines (e.g., Lycs, Continentals, Franklins, etc.). In fact, they are required in most certified aircraft that I am aware of.  Their purpose is to provide a way for the crankcase to ventilate in the event that the outlet end of the breather tube becomes blocked by a frozen plug of vented crankcase moisture.  If the end of the tube is blocked by ice, it is likely that the resulting crankcase internal pressure will blow out the crankshaft seal and loss of oil will likely ensue.  

However, this situation would appear to not be directly applicable to the 912 -- I would think that a total blockage of the 912's oil tank breather tube could result in a reduction (?) of the flow of crankcase oil from the crankcase sump back onto the oil tank -- it would upset the engineered balance of oil flow?? I am curious as to ROTAX's response in this regard.

ps -- The "excessive oil overflow" issue you mention seems unlikely to me.

In case of a complete blockage, I would think that crankcase blowby pressure would quickly build up in the oil tank, which should easily be high enough to blow out any blockage of ice or dirt.

Also, considering that the tank vent fitting on the tank cover doesn't extend into the tank and thus into the oil, no oil could possibly flow out of the tank via the vent hose/tube. Provided, of course, that the aircraft is in a normal flight attitude. No negative Gs allowed. 

Alan

Thank you for your replies concerning this issue, nevertheless i would be glad to hear from Rotax Specialists about the posed question.

Please see the manual reference.

The venting line must be protected from any kind of ice formation for condensation by using e.g. isolation protection or routing in a hose with hot airflow. Additionally, a bypass opening should be foreseen before the cowling outlet.