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Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 10:00 am
by 3Dreaming
FastEddieB wrote: As an aside, I only recall getting carb ice once in my flying career. I was flying some students around on short flights in a J3 Cub. South Florida, so not cold at all. On one flight, I got the impression the throttle was creeping. I had to keep nudging it forward to get the same rpm. When I finally had full throttle and still had lowered rpm and performance, carb ice dawned on me and I applied carb heat. The engine stumbled, then died momentarily before coming back. Whew! On landing, there was actually frost on the outside of the carb and it was quite cold to the touch.


My most memorable carb ice incident was also in a J3. The day was April 1st, 2006. I had just purchased the airplane in southwestern Michigan. The temps that day were right around the freezing level with snow showers. It was my first flight in a J3, and I was all bundled up for the cold, and strapped snuggly in the back seat with a lap belt and shoulder harnesses. Like you I was flying along and the RPM would drop off a little bit. Being new to the airplane I thought the throttle was working its way back, so I kept pushing it forward a little at a time. Flying along all fat dumb and happy the engine coughed, and I reached for carb heat. The only problem was I couldn't reach the carb heat. I finally hooked the knob with the sole of my shoe and pulled on carb heat. The RPM dropped of a little more and the engine ran rough for a few seconds and then smoothed out. When I turned the carb heat off the engine RPM increased by about 200.

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 11:10 am
by MrMorden
The Rotax 912 series is very resistant to carb heating. That's not to say it can't happen, but it's rare. When I was training at Lockwood, I was told the biggest danger zone for carb ice in the 912 is in the 60°F temperature range with high humidity. I believe this to be true.

I've never had a for sure carb ice event, but there was one possible time it started to happen in the conditions I mentioned above. I have an analog tach only, and it doesn't even twitch when I apply carb heat. I do verify with each condition inspection that the carb heat flapper plate is operational in my CTSW.

I have noticed that the seal at the bottom of the metal flap is deteriorated and so there is a little leakage of warm air into the air intake. I'm okay with that, it probably just makes carb ice less likely. Many 912 installations like the RV-12 just draw warm cowl air with filters attached right to the back of the carbs and forego any type of carb heat installation, so this is no big deal.

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 1:40 pm
by WDD
I had thought carb icing had less to do with it being cold outside, but rather how humid it was. The cold in the carb is created by the decrease in air pressure in when the air rushes through the carb body. The pressure drop is greater when you close the throttle, as the engine is initially still running hard, and thus pulling/sucking air just as hard but through a narrower carb opening - such as when you close throttle for landing. Decreases pressure even more, dropping temp even more.

Pressure X Volume = Temp

If the air is humid, and if the carb temperature drops below freezing that water vapor changes to ice.

Hotter air can hold more water than colder air. Thus, colder air is likely less humid to start with. A 20 degree day is less likely for carb icing than 50 and humid.

At least that's my understanding of how it all works.

(Footnote: I know lower pressure lowers the temperature at which water changes to gas - boiling, so does a lower pressure also lower the temperature at which water changes to sold - freezing? )

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 1:47 pm
by FastEddieB
Looks good to me!

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 1:50 pm
by Wm.Ince
3Dreaming wrote:
Wm.Ince wrote:
ShawnM wrote:I use it during my run up checklist to confirm that it works. :mrgreen:
What are the indications that verify it is actually working?
Bill, on your CTSW it is more of an alternate air rather than carb heat. Most carb heat set ups pull heated air from around the exhaust. This heated air is normally enough to cause enough change in air density to see a drop in engine RPM. The CTSW just pulls air from inside the engine compartment. While it is certainly warmer than outside air the temperature differential is not enough to indicate a drop in RPM when tested.

Thank you and I concur. That has been my experience also.

I was wondering if Shawn was seeing something different, since his airplane has the same Rotax engine.

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 5:26 pm
by Jim Hardin
Nice chart FastEddie! May I ask the source?

One subtle point about ice. The Humidity level is irrelevant to icing :shock: It is all about the dew point...

Here is a really amusing story about one such incident. Keep in mind the "witness" is not a pilot.

Carb Heat 1.jpg
Carb Heat 1.jpg (103.16 KiB) Viewed 172 times

Carb Heat 2.jpg
Carb Heat 2.jpg (121.22 KiB) Viewed 172 times


Carb Heat on sale in aisle 4...

Note that the airplane was flying near the clouds, actually between them. Where do the clouds start to form? At the point where Temperature and Dew Point come together.

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 5:33 pm
by FastEddieB
I found it via a Google image search.

Credit should be given to “Boldmethod”, though many similar charts were found as well.

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 8:27 pm
by ShawnM
Wm.Ince wrote:I was wondering if Shawn was seeing something different, since his airplane has the same Rotax engine.


I do have he Rotax but as 3Dreaming wrote I have actual carb heat that is pulled from around the muffler when I activate "carb heat" on the panel so I see the rpm drop.

Interesting how your CT is.

Re: Carb Heat in cold weather

Posted: Thu Jan 11, 2018 9:45 pm
by 3Dreaming
ShawnM wrote:
Wm.Ince wrote:I was wondering if Shawn was seeing something different, since his airplane has the same Rotax engine.


I do have he Rotax but as 3Dreaming wrote I have actual carb heat that is pulled from around the muffler when I activate "carb heat" on the panel so I see the rpm drop.

Interesting how your CT is.


The CTSW pulls air from inside the cowling. The CTLS has actual carb heat that pulls air from around the muffler using a standard Rotax airbox.

One of the neatest set ups I've seen was on a Aeroprakt. It had the cone type filters on the back of the carbs, with an enclosure around with an opening to the outside for fresh air, and valve with ducting for hot air from the muffler. The enclosure almost made me think that it was built by Flight Design, because it had the grey speccled paint that they used to use.

Re: Carb Heat in cold weather

Posted: Fri Jan 12, 2018 9:41 am
by ShawnM
3Dreaming wrote:
ShawnM wrote:
Wm.Ince wrote:I was wondering if Shawn was seeing something different, since his airplane has the same Rotax engine.


I do have he Rotax but as 3Dreaming wrote I have actual carb heat that is pulled from around the muffler when I activate "carb heat" on the panel so I see the rpm drop.

Interesting how your CT is.


The CTSW pulls air from inside the cowling. The CTLS has actual carb heat that pulls air from around the muffler using a standard Rotax airbox.

One of the neatest set ups I've seen was on a Aeroprakt. It had the cone type filters on the back of the carbs, with an enclosure around with an opening to the outside for fresh air, and valve with ducting for hot air from the muffler. The enclosure almost made me think that it was built by Flight Design, because it had the grey speccled paint that they used to use.


Very interesting indeed. Wonder why they have it two different ways. The Aeroprakt setup does sound like a great idea since it uses the cone filters.

Re: Carb Heat in cold weather

Posted: Fri Jan 12, 2018 10:17 am
by 3Dreaming
ShawnM wrote: Very interesting indeed. Wonder why they have it two different ways. The Aeroprakt setup does sound like a great idea since it uses the cone filters.


I think they made the change for a number of reasons. First the CTSW air box was a Flight Design part, that used an adapted automotive filter from a VW. It was something that was being used by Flight Design before LSA and ASTM standards. It didn't provide an adequate temperature increase when carb heat was applied.

The CTLS had many improvements over the CTSW when it was introduced. With regards to carb heat they decided to use a Rotax air box instead of their own design. With the Rotax box and ducting from around the exhaust you do get a drop in RPM when testing the carb heat.

One thing worth noting is that most all Rotax installations that I am familiar with you don't get much RPM drop when checking carb heat. In my opinion the reason for this is because the carbs automatically adjust the mixture based on pressure, where other aircraft carburetors don't.