Rotax safety question

[Ecoloqua]

This may have been covered before but In the interest of safety I need to ask about it. I am currently training in a LSA with a Rotax 912. This particular aircraft was involved in a incident 2 yrs ago. The NTSB report states that the while at level cruise the pilot experienced a significant drop in fuel pressure followed by the engine sputtering. He activated the fuel pump and it helped a little but soon after the engine just quit. He was forced to make an emergency landing in a cow pasture and hit a berm which tore off the left side of the landing gear. The final report stated that the likely cause was that the carb vent tube was incorrectly routed directly into the air filter which influenced the air fuel mixture and caused a lack of fuel due to the pressure in the float bowl exceeding the normal operating range. After the accident a representative from Rotax came out to the airplane and verified this as the cause.

Fast forward to last week as I was pre-flighting the airplane for a lesson I decided to look inside the engine cowling and sure enough the vent tube from the carb IS STILL being routed directly into the air filter. There is no airbox and it is my understanding that some people vent the carb tube directly into the airfilter. Being a new student pilot I just need to know if this is safe or what? I deffer to you experts who fly these engines everyday to tell me if I am worrying for no reason or if this is a legitimate concern. I told my flight instructor about it and he doesnt seem concered at all.

[drseti]

The routing of the carb vent line (and, in fact, the choice of installing a Rotax air box, or dual K&N air filters, or some other induction system), is a responsibility of the airframe manufacturer, not the engine maker. Rotax covers these considerations thoroughly in their installation manual, and has issued service bulletins regarding proper venting of the carbs. But they are powerless to compel airframe manufacturers to comply. Caveat emptor!

[3Dreaming]

On the Rotax powered airplanes that I work on I have seen 3 different ways of routing the vent line.

On the Flight Design CTSW the vent lines are routed to inside the air filter box. If the line becomes disconnected then you get an unbalance between the carbs.

The Flight Design CTLS uses a Rotax air box. the vent lines are connected to the air box. This is a standard Rotax installation.

On the Tecnam aircraft that I have worked on just have an air filter mounted on the rear of each carb. The vent lines on these are normally tucked under the bail that holds the float bowl in place.

[3Dreaming]

BTW having improper vent pressure in the carb float bowl would not cause a drop in fuel pressure. Fuel pressure is measured in the lines before the fuel gets to the carbs.

[Ecoloqua]

Thanks for the responses guys. I understand now that those vent hoses need to be in a static enviroment with no ram air effect. Being that this engine has no airbox and the tubes are currently being run directly into the air filter, Is it really just a matter of pulling the tube out of the air filter and simply routing the tube underneath the float bowl clip? I will bring it up again to my CFI and if they dont correct it then I will have to respectfully ask to train in a different aircraft. I beleive they have a Peregrine and possibly a Tecnam.

Indicidentally if this were my airplane I would have a proper airbox installed with the vent lines routed corecclty into it.
Why take a chance? Especially after they already had a renter crash-land the plane becuase of this. I just dont get it.

[3Dreaming]

Thanks for the responses guys. I understand now that those vent hoses need to be in a static enviroment with no ram air effect. Being that this engine has no airbox and the tubes are currently being run directly into the air filter, Is it really just a matter of pulling the tube out of the air filter and simply routing the tube underneath the float bowl clip? I will bring it up again to my CFI and if they dont correct it then I will have to respectfully ask to train in a different aircraft. I beleive they have a Peregrine and possibly a Tecnam.

Indicidentally if this were my airplane I would have a proper airbox installed with the vent lines routed corecclty into it.
Why take a chance? Especially after they already had a renter crash-land the plane becuase of this. I just dont get it.

You have to realize that sometimes they have to come up with a possible cause for an accident. The routing of the vent hoses into the air filter may not have been the actual cause.

Also simply adding an air box may not be an option. You have to have approval from the aircraft manufacture to do so.

The next time you preflight snap a picture of what you are talking about and post it up here.

[Ecoloqua]

Still working on figuring out how to post pics but here is the direct link to the photo taken by the NTSB of the actual carb showing the vent tube installed directly into the K and N filter. I looked the other day and it is still hooked up exactly like this photo.

https://dms.ntsb.gov/public/55500-55999 ... 570396.pdf

[roger lee]

Vent tube inequality usually only cause a rough running engine and of all the ones I have seen do this none have done nothing more than be a little rough. Then some of the fuel in one of the carbs pukes out the vent tube. That vent tube is open to the atmosphere through the carb.
Even the roughness caused isn't that terrible and nothing there should have shut down an engine.
Putting the vent tubes inside an air filter isn't the best option and would be better served out alongside the carb bowl bail wire.
The Rotax supplied air boxes do indeed have that vent tube routed from the carb to the airbox.

[Ecoloqua]

I agree with that 100%. Besides that I would also think that the ocassional "vent tube burp" from the carb would dump fuel directly onto the filter element compromising it and creating a nice gas-soaked fire hazard. On our last training flight while we were doing some steep manuevers the CFI and I both detected a strong fuel smell inside the cockpit. He attributed it to overfilling the tanks before we took off. Could that fuel smell actually have been fuel emitted from the carb tubes?

Anyway I heard from my CFI about tonight and he agrees its something that should be brought to the owners attention.

Thanks again for the help. This forum and the people here are amazing!

[roger lee]

You problem may have nothing to do with a vent tube placement. You may just need the carb float level set. It may be too high in the carb and it will then puke out the vent tube. Poor carb sync causing the engine to shake will cause fuel to puke out the tube because the floats can't control the level.

[bstrachan]

I don't think the function of the float bowl vent tube is well understood. True, you want to have ambient atmospheric pressure in the float bowl chamber, and a properly installed vent tube (ie, one that is open to static pressure, like the static port on your pitot-static system) will do that. But a major reason for the vent tube's existence is in case the float/needle valve fails. If this happens, fuel pressure at the carb inlet will fill the float bowl (fuel level in the float bowl is critical BTW) and then, if the fuel has nowhere else to go, it will pressurize the fuel metering jets in the carb and flood the engine. You don't want that. The float bowl vent is there to prevent this. The vent tube (just like the vent tube on the oil tank) should be routed so that if the float/needle valve DOES fail, fuel will be discharged overboard in such a way that it will do as little harm as possible. Routing the float bowl vent tube into the air filter or air box is nuts, in my opinion. But then so is using an engine-mounted mechanical fuel pump, in this day and age. That is 1940s technology and is a sure recipe for vapor-locking problems. In a high-wing airplane fuel is pressurized all the way from the tank to the pump inlet... those of us with low-wing airplanes don't have this advantage. But I digress.

Next time I have the cowl off my StingSport I'll see how they routed the float bowl vent tubes and maybe even post a picture.

"Bing carburetors belong on motorcycles."

[bstrachan]

Before somebody flames me over my last post, let me just point out a couple of things.

1. I DID NOT advocate connecting the float bowl vent tube to the static line that feeds the ASI, VSI and altimeter. That would be even nuttier than putting it in the airbox.

2. I know for a fact that the vent tubes on my airplane do NOT go into the airbox (I don't have one) nor into the K&N air filters (I take them off at annual to clean them, that's how I know). I fact, I "think" they go out the bottom of the cowl forward of the firewall, along with the main tank vent and the oil tank vent, but I will have to verify this.

3. It isn't as common as it once was, we have better materials and better filters now, but float valves still do fail occasionally. a) the needle and/or the seat can wear out b) the float can sink c) a bit of debris can get into the seat orifice. If this happens, the fuel level in the float bowl will be way high but the engine will still run (probably not well, but enough to get you on the ground). If there were no float bowl vent, the engine would flood and that would be that.

Fly safe!

[3Dreaming]

Your vents are likely tucked under the bail that holds the float bowl on. The reason for having the vents routed into the airbox or tucked under the bail when you have K&N filters, is that the vent and the air coming into the carb need to be matching pressures. If the pressures aren't matching it can cause the engine to run poorly.

As for the engine driven fuel pump, the engine needs to be able to run independently of the electrical system. If all you had in your low wing airplane were an electric pump and it fails you will be on the ground. By having the engine driven pump, in the event of an electrical system failure your engine will stay running.

[drseti]

"Bing carburetors belong on motorcycles."

Pretty strong generalization. There are advantages (and limitations) to fuel injection. There are advantages (and limitations) to carburetors. One size does not fit all.

[Wm.Ince]

I don't think the function of the float bowl vent tube is well understood. True, you want to have ambient atmospheric pressure in the float bowl chamber, and a properly installed vent tube (ie, one that is open to static pressure, like the static port on your pitot-static system) will do that. But a major reason for the vent tube's existence is in case the float/needle valve fails. If this happens, fuel pressure at the carb inlet will fill the float bowl (fuel level in the float bowl is critical BTW) and then, if the fuel has nowhere else to go, it will pressurize the fuel metering jets in the carb and flood the engine. You don't want that. The float bowl vent is there to prevent this. The vent tube (just like the vent tube on the oil tank) should be routed so that if the float/needle valve DOES fail, fuel will be discharged overboard in such a way that it will do as little harm as possible. Routing the float bowl vent tube into the air filter or air box is nuts, in my opinion. But then so is using an engine-mounted mechanical fuel pump, in this day and age. That is 1940s technology and is a sure recipe for vapor-locking problems. In a high-wing airplane fuel is pressurized all the way from the tank to the pump inlet... those of us with low-wing airplanes don't have this advantage. But I digress.

Next time I have the cowl off my StingSport I'll see how they routed the float bowl vent tubes and maybe even post a picture.

"Bing carburetors belong on motorcycles."

Interesting opinion.