Oil Breather Tube Orientation

Hi Sean,

ROTAX explains what to do in IM 79-00-00 on pages 20 and 21 of the January 2021 issue, but does not explain why: "NOTE The profile clamp of oil tank should be between +40 mm (1.57 in) and -360 mm (-14.17 in.) on the z-axis."

My explanation: If the tank is too low, all the oil will run out of the crankcase into the oil tank after the engine has been shut down. When checking the oil level of the cold engine, it only takes a few turns because there is almost no oil left in the crankcase. This has consequences for lubrication when starting the cold engine. My new 912, which now has 250 hours on the clock, has almost no oil consumption, so I don't have to top it up between 100 hours, and the gurgling requires 25 to 40 compressions, depending on the temperature. Until then, the oil pump delivers sufficient oil to all parts of the cold engine. For me, the height of the oil tank according to Rotax specifications makes sense because it ensures a balanced oil level in the crankcase and oil tank, and the gurgling occurs so late (even if it is annoying) that all parts are definitely lubricated before the cold engine is started for the first time.

If the oil tank is too high, the crankcase will be flooded. Perhaps there are specialists who can describe the consequences of too high an oil level in the crankcase in detail. In any case, it doesn't seem to be a good thing, and the gurgling may occur much later or not at all.

In both cases, I see an indirect connection to oil consumption. Either the engine is started too dry, which can damage the cylinder liner, piston, piston rings, and basically all moving parts. Or a flooded crankcase can cause high oil consumption due to excessively wet cylinder and too much pressure in the crankcase (seals, etc.), especially when starting. I am not an engine specialist, but my opinion is that oil consumption, especially during start-up, is influenced by the incorrect height of the oil tank. During further engine operation, I do not see any significant influence on oil consumption, unless the engine is already damaged.

What I don't understand in your case is the low compression rev. until the gurgling starts. I can't explain this because you wrote that the oil tank is mounted at the correct height relative to the propeller shaft (x-ayis), which doesn't add up. If the oil tank is at the correct height, there must be more oil in the crankcase that cannot be transported to the oil tank with just a few compressions. What does the dipstick show? Regardless of the height at which the oil tank is mounted, when the propeller is turning and the subsequent gurgling occurs, you should be able to see the oil level on the dipstick. If the entire dipstick is oily, you cannot tell whether the oil is overfilled.

After the first oil change, I added a little too much oil (probably 1/4 liter) so that the dipstick was completely oily. Now, after about 100 hours, the oil level is just below the upper mark—which is a good feeling.

The height and position of the end of the oil vent hose seems to be of secondary importance to me.

I think that during the start-up process, the starter motor is activated without ignition for about 1 to 2 seconds. Only then does it start with ignition. This allows the mechanical oil pump to pre-lubricate properly, which shouldn't do any harm. Perhaps there are specialists who can explain this or have a different opinion.

Greetings

Paul Wieler wrote:

Rotax Wizard, I would be curious to know if you would have a recommendation for oil tanks that are embedded in the nacelle... like those Tecnam P2006ts?   I know for a fact they never came with notches, but I do something like that with all the other birds I work on because our temps get downright chilly (-30C is not unusual) so breathers freezing is a concern especially as that is key for oil return.

As for the OP if you would like a preview of the page and I am sure you have seen it, you can technically have low spots, but it recommends a drain bore to prevent "condensate"

Hi Paul

The tank in or out of the nacelle is not really the issue.  The pressure above the oil in the tank should be ambient air, not ram air or air under low pressure.  Rotax does not supply the vent line so any "notches" would be as per the installation manual and every OEM should do this.  i cant speak to Tecnam and the fact you do not have them, that would be a question for them. 

At -30C the ambient air will not have much, or perhaps no, moisture.  We all know in Canadian winter your skin cracks and dries due to this fact.  Air however that has moisture, when we heat it up as with a running engine, will tend to develop droplets in the top of the oil tank.  This is not really an issue as long as you run your oil in the correct running range to evaporate such moisture accumulations while flying.  

The issue with moisture and droplets comes when it accumulates enough to start to drain from the engine and into the colder outside air.  If your vent tube exists into a LOW PRESSURE area, for example into the high speed air stream, this can cause freezing of the condensate into an ice ball that may block the breathing of the outlet.  The method to address this is typical and you can find the exact same guidance in AC 43-13 current issue on drains.  This is nothing new and it will address the low pressure draw also as you then can place the notch inside your nacelle where there is no low pressure suction.  (normally for most aircraft) 

Contrary to what some believe very cold temperatures are not really an issue with ice formation, the problem is like carb icing, it is worse in high humidity and in the 15 to 20 C range.  The temperature drop from air flow, just like a venturi in the carb, will cause ice to form.  At -30 C, that should not be an issue.  I would however put in a notch near the vent line perhaps near the oil tank and not at the bottom of the nacelle.

Cheers

Quote:

"i cant speak to Tecnam and the fact you do not have them, that would be a question for them."

I don't see notches at times from different MFG's. When I see the plane for its annual I always look for the notch and cut one in if it's not there. I always trim back the vent tubes if they are hanging out the bottom of the aircraft so they are not in a low pressure area and keep it up in the cowl to help with a slightly better warm air flow vs out the bottom of the cowling.