We often get asked how, when, and why to burp the Rotax 912ULS and 912iS Sport engines used on the RV-12 and RV-12iS. You should always read, be familiar with, and follow the recommendations in the Rotax engine manual as your primary guide. But here is a little additional context from Scott McDaniels, the manager of our maintenance and prototype shop.
Rotax specifies the 912 burping procedure for only one purpose… To accurately check the oil level.
It’s not to prevent hydraulic lock. If you look at the shape of the engine case, you will see that even if the majority of the oil drained from the tank into the engine (it can’t all drain because the pick-up tube doesn’t go all the way to the bottom of the tank), it would not be enough to fill the case volume that is below the cyl, so the chance of oil filling a cyl bore enough to cause a hydraulic lock is extremely remote (I’m not going to say “zero chance,” because I have seen instances of people severely overfilling with oil).
There is also no danger of “wiping cyl bores clean” of protective oil by moving the prop. If the engine isn’t burning any oil in normal operation, there isn’t any oil remaining on the cyl walls to be wiped off, after it is shut down.
One major benefit of doing an oil level check when the engine is warm is it gives you the most accurate oil level reading because of the much lower oil viscosity making it flow much easier when pumping the oil from the engine to the canister where the level is measured.
A common oil-level check mistake is doing it in the cold of winter after the airplane has been sitting for a while and some oil drain-back to the engine has occurred. Because the oil is very thick when it’s cold, once you have gotten it to burp there will still be a thick coating of oil on the inside of the engine case that hasn’t been returned to the tank. If you add oil based on the appearance of it being slightly low, it is common to have it overfilled after just a couple of instances. So, this is one scenario where checking the oil after a flight is a good procedure to use. You don’t have to do it when it is scalding hot….. open the oil door to vent the engine compartment while you put the airplane away. Once you have everything else taken care of and the door closed, it is usually cooled enough that you can do an oil level check as the last thing.
Then, when arriving for the next flight and as long as there is no puddle of oil on the floor, you know precisely what the oil level is. Of course, this may not work as well with shared-usage airplanes.
And there’s nothing wrong with checking both after (engine warm) and before (engine cold) a flight.