Oil Drain tube

Thank you for your support. So, regarding the document shown and considering my "painted" picture below, it is not acceptable to have an opening on zone 1 instead of being on zone 2. Therefore, it wouldn't be foreseen before the cowling outlet as required by the manual, am I correct?

The main points are that the tubing should flow continuously downward if possible (which is not happening in your drawing) and that there also be a way for moisture to escape at a place inside the cowling, where things are warm, should the end of the tube, which is outside the cowling, become frozen.

It would be best to place the opening closer to the end of the tube as per the Rotax installation manual.

And while it's not specifically stated in the section cited above, that "notch" vent should face AFT, so that there is no possibility of it creating a "ram air" effect into the vent line and thus pressurizing the line and oil tank.  The whole intent is for that vent line to operate at "neutral" pressure.  This is a very common design for oil breather tubes in certified aircraft, and every one I've seen specifies to have that "notch" facing aft.

As for your concerns about "draining the oil tank" through the vent line – it would be physically impossible.  If you remove the top of your oil tank, you will see that the vent line opens into the tank at the very top of the tank, well above the normal oil level.  Tryng to "suck" oil from the tank through that opening would be like holding a straw a few centimeters above the water level in a glass, and trying to suck the water through the straw.  It just can't be done!

The Rotax call out clearly shows the notch inside the cowl, not likely it will have ram air at that area.  In an open engine installation I would agree to face it aft of any incoming airflow.

As for "sucking" out the oil you are right, to a point.  The oil tank is not just a neat bunch of oil sitting in the can.  The return oil coming from the crankcase is mixed perhaps as much as 50% with air and this air has to be extracted in the centrifuge type tank entry where it is pushed to the outside of the screen at a steep angle so that the foam will be separated as it flows into the centre for the suction tube at the bottom.  There is considerable turbulence within the can while this transpires.  Oil vapors are captured in the mist that exists inside the tank and if the vent tube is in a low pressure zone it is common to see oil streaks on the aircraft underside.  Additionally the reason we do not want a low pressure zone for the vent outlet is that the pressure on the top of the oil within the tank is part of your design to help drive the oil down into the end of the suction tube pickup.  Indeed in high altitude applications in drones and some gliders we see lower oil pressures for this very reason.  

I have seen very high altitude designs on Rotax where they actually will add pressure to the oil tank top to maintain pressure in extreme conditions.  Normally only necessary above 20,000+ ft in my experience.  

Just for fun, look at what  pressure you have in atmospheric sea level vs your altitude....your oil pressure will drop by the amount you lose at altitude.  So for 10,000 ft, no problem, you drop 4 to 5 psi, no big deal...but as you continue up at some point you may get concerned...so let's not start out with the vent in a low pressure zone.