Oil level hot / cold accuracy

[seagull]

I usually check oil level post-flight. The oil is warm and easier to burp.

Before I get blasted, my plane is hangared and I am the only one with a key. I fly it 3-4 times a week and check for leaks before and after flying.

The question;
Today I burped and checked level before flying (cold engine). The level was below the “flat” so I added oil, it took 10 oz to be at 1/3 flat. When I landed I checked it and it was 1/4 flat above the top of the flat. The level should be in the middle of the flat. Is that on a cold or warm burp / oil? It is significantly different.

[RFSchaller]

I have had a few burps that caused me to add oil only to find it too high on next preflight. I always attributed it to an incomplete burp. When it’s too high it usually shows up on the bottom of the fuselage from the oil tank vent line.

[Piper J3]

Cold oil doesn't scavenge well to the drain port on the bottom of the crankcase because the pitch angle on the bottom of the case is very flat.

[seagull]

Originally Posted by Piper J3

Cold oil doesn't scavenge well to the drain port on the bottom of the crankcase because the pitch angle on the bottom of the case is very flat.

Exactly! This is not from an “incomplete burp”.
So the question remains what is the correct time to measure oil level. My vote goes for when it is warm. Is that what Rotax intended?

[NinerBikes]

i burp mine when warm, usually before I close the hangar door.

You will get a more complete burp and slightly higher oil levels indicated when you burp the motor hot.

I will also burp the engine when cold before the next flight.

With being in a small T hangar and it being winter, it will take quite a few turns to burp the oil if I just shut the plane off and cold burp everything up and back into the sump tank right before the next time I fly.


How many hours now on this oil and filter change

[PaulvS]

The Rotax 912 operators manual specifies to check the oil level before starting... so when the oil will be cold. The level should be between the middle and the top of the flat part of the dipstick. Presumably the oil tank has been designed to allow for the increased volume of the oil when it is hot.

I agree that it is easier to burp when it is hot, but with multigrade oil it is not difficult to burp when the engine is cold.

[seagull]

I was looking at Van’s recommendation. They recommend when it is warm, but also say you can do it warm and cold. Back to my original question of getting two distinctly different readings.
http://https://www.vansaircraft.com/...eck-oil-level/

[NinerBikes]

Originally Posted by seagull

I was looking at Van’s recommendation. They recommend when it is warm, but also say you can do it warm and cold. Back to my original question of getting two distinctly different readings.
http://https://www.vansaircraft.com/...eck-oil-level/

The burp when cold is for Rotax engines mounted in other planes, such as taildraggers with Bush tires on them. Sometimes, this configuration causes problems with the fuel floats in the carbs, also, since the engine is not close to being level.

In some situations, such as on some Kitfox builds, the sump tank is mounted so low on the firewall, such that the oil can syphon back into the engine or back cylinder heads and cause hydraulic lock.

So turning the prop after sitting or in your hanger, ensures that there's no hydraulic lock that could bend connecting rods or do other damage to your engine, before you use the electric starter.

I do get two different readings, if I only burp when cold, it's shows up lower on the dip stick, sometimes as much as 3/16 to 1/4 " difference.

[PaulvS]

Originally Posted by seagull

I was looking at Van’s recommendation. They recommend when it is warm, but also say you can do it warm and cold. Back to my original question of getting two distinctly different readings.
http://https://www.vansaircraft.com/...eck-oil-level/

I'm sure the dipstick readings will be different between cold and hot because of expansion. I know on my car that the dipstick for the steering fluid has two marks, one for hot and one for cold.

Perhaps it would make sense to make note of the hot oil level on the dipstick which corresponds to the full level when the engine is cold. That way you can confidently check the level when it is hot and know that there is enough oil in the tank per Rotax.

If the oil level is too high it will blow out the vent tube, but if it's too low... best to avoid! Vans FAQ does give guidance on burping and checking oil level warm or cold, but they also do say to use the Rotax manual as the primary reference, which says to burp and check before engine start.

[rsr3]

The Rotax takes a significant amount of blades before burping when cold, and often the first burp isn't enough. It'll take a while longer but keep going more than you would with a warm engine and you'll find the oil level will come back up to more or less where you expect it.

Always be sure before topping up as the difference between min and max oil quantity is not very much at all.