Burping the Rotax

[Paul_G]

Thank you so much for all of your replies. I'm trying to come to a conclusion with all of this information.

Here's my question: If I'm confident that there is sufficient oil in my airplane, does it do any damage by not burping the Rotax. I'm concerned that maybe it needs to have some pre-circulation prior to starting. I certainly could use some exercise - and winding up the rubber band could provide that, but I'm concerned that burping is a pre-lubrication need prior to starting. Yes? or No?

[drseti]

I'm concerned that burping is a pre-lubrication need prior to starting. Yes? or No?

IMHO, no. The only reason I can think of for using crankcase pressure to return oil to the tank is to get an accurate dipstick reading. OTOH, how else can you be confident about your oil level?

EDIT - actually, there is another good reason for burping the engine. It gives you a direct feel for the compression in each cylinder. There is great benefit to knowing before each flight that all valves are sealing properly at TDC, and nothing is leaking past the rings.

[3Dreaming]

From a lubrication standpoint there is no reason to turn the engine through if the oil is on the flat of the dipstick. Personall I prefer to pull it through at least 4 compression to make sure there is no hydraulic lock of the cylinders, especially with the carbureted engine.

[FastEddieB]

I'm concerned that maybe it needs to have some pre-circulation prior to starting. I certainly could use some exercise - and winding up the rubber band could provide that, but I'm concerned that burping is a pre-lubrication need prior to starting. Yes? or No?

My understanding as a non-engineer...

The worst thing for wear is to drag metal over metal in the absence of an oil film. And plain bearings need oil pressure to provide that oil film, and rings need splash from the crankshaft to lubricate them. And the fastest and best way to achieve those ends is just to start the darn thing. I don’t think there’s evidence that turning an engine over by hand “limbers up” the oil or has any other long-term benefit.

On my Sky Arrow with its high-mounted engine, I rotate the prop one time to bring each blade into reach/view. That would also catch hydraulic lock. But I can’t envision how that could happen on our engines. If you had a gravity feed plane with a leaky needle and seat, the float bowl would just overflow into the overflow tube - I can’t see how it could fill a cylinder instead. Has anyone heard of such a thing in a ROTAX?

Again, I know we have “real” engineers here, so I wouldn’t mind being corrected if any of the above is on error.

[3Dreaming]

That would also catch hydraulic lock. But I can’t envision how that could happen on our engines. If you had a gravity feed plane with a leaky needle and seat, the float bowl would just overflow into the overflow tube - I can’t see how it could fill a cylinder instead. Has anyone heard of such a thing in a ROTAX?

I am not sure what overflow tube you are talking about. There is a vent tube, but it is well above the inlet and outlet of the carburetor. I have a CTLS sitting in the shop, with the Rotax intake box that is installed it sure looks like a needle that is not seated would allow fuel to flow into the intake.
I also remember from Rotax training they warned us about hydraulic lock, ans showed bent rods to drive the point home.

[ShawnM]

I'm interested in checking it hot after my last flight of the day and then comparing that reading to my next burp during my preflight. Curious if there is a difference of not.

Oh, there's definitely a difference. Oil expands when hot, so the post-flight reading will be higher on the dipstick that the cold engine reading.

Well then if this is true, you could then see oil on the lower half of the flat when in actuality, and the engine is cold, you are below the flat. Not good. I'll stick with my method of burping a cold engine before my first flight of the day as it works for me.

[MrMorden]

No, regardless of who's been flying the plane, if the oil is anywhere on the flat of the dipstick, there's no reason to burp the engine. But again, no matter who's been flying the plane, it is essential to check the dipstick before every flight.

I agree with this. Burping is only an exercise to return oil to the tank to get a good level check. If the oil is on the flat of the stick, it's in the acceptable range, and burping will only make the level go up, so it serves no purpose at that point. In fact, it just means that your oil pump has to move more oil from the tank to the engine where it's actually doing some good.

The oil level needs to be checked before each flight. There might not be a puddle of oil on the floor, but what if you you had a failure somewhere that cause 99% of the oil to leak out on your last flight? Unlikely, but then you don't know what you don't know...

I know Eddie checks his oil *after* a flight, and I suspect that the reason is where his engine is located. But I think if there was an oil-related engine failure leading to an incident or accident, you'd be hard-pressed to convince the FAA & NTSB that checking the oil after you fly the airplane is a good practice.

[MrMorden]

That would also catch hydraulic lock. But I can’t envision how that could happen on our engines. If you had a gravity feed plane with a leaky needle and seat, the float bowl would just overflow into the overflow tube - I can’t see how it could fill a cylinder instead. Has anyone heard of such a thing in a ROTAX?

I am not sure what overflow tube you are talking about. There is a vent tube, but it is well above the inlet and outlet of the carburetor. I have a CTLS sitting in the shop, with the Rotax intake box that is installed it sure looks like a needle that is not seated would allow fuel to flow into the intake.
I also remember from Rotax training they warned us about hydraulic lock, ans showed bent rods to drive the point home.

Lock is really only a problem for airplanes where the oil tank is incorrectly mounted at a position higher than the engine, IIRC. This was a problem for early Kitfox airplanes and has been corrected in newer models, for example.

[Warmi]

The oil level needs to be checked before each flight. There might not be a puddle of oil on the floor, but what if you you had a failure somewhere that cause 99% of the oil to leak out on your last flight? Unlikely, but then you don't know what you don't know...

....

Indeed but if you check the oil after each flight and you are the only one flying the plane - the very next day there will be only two possibilities - either a puddle on the floor or oil being inside the engine/oil tank - so yeah, you always do need to check the oil level - the question was just when.

[drseti]

so yeah, you always do need to check the oil level - the question was just when.

And my answer to that question is: both. I have my students check all operating fluids (oil, coolant, fuel, hydraulic fluid) before every flight (as part of the preflight), to make sure the plane is ready to fly, and then again after every flight (to see if anything has changed in flight).

[MrMorden]

IMO you need to always check before, and if you are going to check after, then you need to do both. But IMO for an airplane you own that's not a trainer doing both is overkill, you'll see if anything changed on your next preflight.

[FastEddieB]

There might not be a puddle of oil on the floor, but what if you you had a failure somewhere that cause 99% of the oil to leak out on your last flight? Unlikely, but then you don't know what you don't know...

Missing the logic here. Wouldn’t that have been caught when I checked the oil after my last flight?

Edited to add: I see Warmi picked up on that as well.

[MrMorden]

There might not be a puddle of oil on the floor, but what if you you had a failure somewhere that cause 99% of the oil to leak out on your last flight? Unlikely, but then you don't know what you don't know...

Missing the logic here. Wouldn’t that have been caught when I checked the oil after my last flight?

It was unclear from my post, I meant that oil needs to be checked at some point between each flight. The before or after discussion is a separate issue.

[FastEddieB]

...but what if you you had a failure somewhere that cause 99% of the oil to leak out on your last flight? Unlikely, but then you don't know what you don't know...

War story.

In the summer of 2006 I was hired to fly an E-AB biplane from Phoenix back to Copperhill.



It had used no oil the first leg or two, then I think a quart. I was trying to get back to Copperhill on the second day, but was losing the race with the sun and overnighted in Savannah, TN.

Next morning, I checked the oil and there was nothing on the stick. It took quart after quart, until finally...



YIKES! 5 quarts to even show on the stick. Though that brings the dipstick markings into question. The plane must have been nearly out of oil when I landed. And there’s some pretty inhospitable terrain in that part of central TN. In any case, I was not comfortable flying the plane until the cause of the rapid oil loss was identified, and had the owner drive over and pick me up. Another pilot flew it the rest of the way a few days later. I don’t think the cause of the catastrophic oil loss was ever firmly identified.

[ShawnM]

And my answer to that question is: both. I have my students check all operating fluids (oil, coolant, fuel, hydraulic fluid) before every flight (as part of the preflight), to make sure the plane is ready to fly, and then again after every flight (to see if anything has changed in flight).

I also visually check ALL my fluids BEFORE each first flight of the day. Even if I flew the plane yesterday and there are no puddles on the hangar floor. It’s how I was taught and I’d prefer a surprise while on the ground and not at 4500 feet over some inhospitable terrain. Oil, coolant and fuel are all checked during my preflight. Only checking it after is like closing the barn after the horse already got out.