Rotax Carb Ice

I have never heard of carb ice being an issue on the RV-12, however, there was a recent NTSB report of an accident involving a Rotax powered CTLS brought down by carb ice near Chicago IL. The stated cause was the pilots failure to use carb heat during extended glide in icing conditions. I was wondering how they do that, and why we don't need carb heat.

Tom

When they say in "icing conditions" I think that's code for moist humid air that can build up as ice in aircraft induction systems.

I had a Kitfox with the 912S setup very similar to how the RV-12's are (just two air filters in the low pressure side of the cowl. The theory is that many / most 912 installs always were setup this way, with no provision for carb heat and over many years they have never had a problem.

One day I was on a long descent from 10,000 feet after circling our local Volcano, it was about 30 min of very low power / gliding descent to landing. I did a straight in to the runway and elected to do a touch and go. When I gave it the power, it started coughing and sputtering and wouldn't make but half power... Limped around the pattern and landed. Pulled cowl and looked at everything I could find. Nothing found and it started right up and ran just fine. I "suspect" carb ice, but don't have confirmation.

Many say that the Rotax carb being a straight slide carb does not accelerate the air like a marvel schebler, but there do seem to be isolated incidents. May be dependent on install setup and other factors.

Rotax does make an intake plenum that can be fit with carb heat in place of the normal air filters.

Clear you engine!

As a general practice its always a good idea to clear your engine occasionally when making long idle descents. Many a Cessna and Piper have crashed in a training environment while practicing emergency landings when they tried to power up to find their engine uncooperative for one reason or another. This would be the same for Rotax as well.

Jetguy said:

As a general practice its always a good idea to clear your engine occasionally when making long idle descents. Many a Cessna and Piper have crashed in a training environment while practicing emergency landings when they tried to power up to find their engine uncooperative for one reason or another. This would be the same for Rotax as well.

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John could you be more specific about what you mean by "clear your engine"?

WingedFrog said:

John could you be more specific about what you mean by "clear your engine"?

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Run your throttle from Idle to full power then back to Idle while in a long descent.

fuel pressure drop

Not sure how this would relate to carb icing, but last February we had a couple of weeks of cold weather, (around 30-35F).

I had been flying until then without covering the oil or water radiators, and my fuel pressure was pretty stable in the 5 psi range.

That cold morning I took off at approx 5 psi, but 10-15 min later, fuel pressure dropped almost in half, to 2.7 psi, (whilst my blood pressure doubled!), but did not experienced any rough engine behavior.

I landed and covered 1/3 of the oil cooler and about 2 inch in every extreme of the water radiator. Back in the air, pressure moved up to usual levels again.

Godo

I have a fair bit of maintaining and operating experience now with both certified and uncertified Rotax 912 motors in the UK.

When the engines are cold and the air humid, the certified engines (with exhaust muffler fed hot air to the single air box) can be seen to pick up a bit of ice. I have never seen this in the air - only during maintenance and pre take off ground runs.

On our RV-12 we fitted an excellent (non-certified) carb heat system:

Skydrive Rotax 912 Carb Heat

The great thing with this set up is its simplicity. Coolant is fed to jackets mounted on the outside of the carb outlets. Carbs are kept warm permanently when the engine is running but there is no loss of power as the air remains cold.

This is a very popular system in the UK where we do get the occasional damp/humid carb ice inducing weather....

aerofurb said:

I have a fair bit of maintaining and operating experience now with both certified and uncertified Rotax 912 motors in the UK.

When the engines are cold and the air humid, the certified engines (with exhaust muffler fed hot air to the single air box) can be seen to pick up a bit of ice. I have never seen this in the air - only during maintenance and pre take off ground runs.

On our RV-12 we fitted an excellent (non-certified) carb heat system:

Skydrive Rotax 912 Carb Heat

The great thing with this set up is its simplicity. Coolant is fed to jackets mounted on the outside of the carb outlets. Carbs are kept warm permanently when the engine is running but there is no loss of power as the air remains cold.

This is a very popular system in the UK where we do get the occasional damp/humid carb ice inducing weather....

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The Governor (Dave Valcik) has this system on his and it seems to work.

I had an HKS 700 running with Bing carburetors. HKS offered an optional electric heater probe for the throats of those carburetors for carb heat.

The hot water coolant jacket system works very well as Aerofurb mentioned. The electric system is the worst as it pulls power when used or all the time. The hot air system is the most common, but does affect the fuel/air mixture and makes it run rich. The hot water jacket doesn't need activating and is always on and doesn't affect the mixture.
The weather in the UK is certain conducive for carb ice.
The variable throat carb is somewhat less susceptible to carb ice over a standard carb, but it certainly does happen and in some climates worse that others.