The International Cessna^® 170 Association

Carburetor ice has got to be one of the most weird phenomenons in aviation. I have been flying my C-170B for over twenty years and over three thouand hours, and I don't believe I've ever encountered carburetor ice even once. Guys flying with me used to get uncomfortable when I failed to apply carburetor heat when reducing power in the pattern, but I never had any indication of ice. This experience has been in all kinds of atmospheric conditions and relative humidity conditions. Maybe I just don't live right.

I think it may be that carburetor icing is more prevalent in certain parts of the country. In the Twin Cities area, I've seen it pretty routinely in my J3 when conditions are right. In my 170B it's less common, but it has happened a few times. Not at all in the Swift, so far. Both the Swift and the 170 have O-300A engines, and I think the carburetor is better protected from Ice in the O-300 than in the Cub's C90 or A65.

My typical experience is to get ice within the first ten or fifteen minutes of flight when the temperature is in the 60s or 70s with high humidity and visible haze. In the 170 it shows up as roughness and loss of RPM, which clears up after applying carb heat for a few seconds.

I bought my J3 while living in Hawaii, and I don't remember experiencing icing there at all. Totally different climate. You're right, it is weird stuff.

It brought up and interesting visit at our local airport yesterday. The other idea was that maybe it could have been "induction" icing which I had not thought of, although possible, conditions were right, I don't think it was.
We had and old Alice Chalmers that would ice up every once in awhile during the summer if the conditions were just right. I always thought it funny that it could be 75 degree, humid, and uncomfortable out and that tractor would spudder and you look and here was the carb all nice and white with ice.
In any case it was good to be there with my son at the time, he did good at handling the situation


W.

W-
If you had visible moisture/ice in the air, the air cleaner might have been getting plugged. Just a thought. It would not clear up with carb heat.

I got what must have been a case of carb ice a couple weeks ago, it was -14F and clear, I forget the dewpoint. taxi and runup fine, once around the pattern and felt like it was "chugging" a bit as we went, used carb heat like normal and it was fine, on landing I started putting the power to it and it died. another runup and it would definately ice up without the carb heat, I could find no other problems that would affect things - even inspected the venturi the next day in the hangar which is fine.

I'm wondering if the reduced power on base and final with carb heat wasn't enough to keep it warm enough, since it was kinda cold outside.

jrenwick wrote:...I think the carburetor is better protected from Ice in the O-300 than in the Cub's C90 or A65....

Exactly! The smaller Continentals have seperate oil tanks and the carbs are isolated from the warmth of engine heat. They ice up pretty regularly.
But the C-145/O-300 carbs are mounted directly to the oil sump which is full of hot oil and the carb and induction passages are immediately warmed by that hot oil. (And the oil is cooled by that cold, vaporized fuel.)

The only time I can ever remember encountering carburetor ice was back in about May of 1950. I was flying my Stinson L-5 home to San Francisco on leave from Ladd AFB in Fairbanks, Alaska. I was flying at about 2500 AGL down the Sacramento Valley south of Red Bluff on a lovely sun shiney day. I noticed that my RPM had dropped off about a hundred from 2100 and nuddged it back up. After a bit I noticed I'd lost aout a hundred again. What the hell? I reset it and noticed that the throttle was set at about the second "T" on the word "Throttle" on the quadrant. Sure as hell the RPM slowly dropped town to 2000 again. I had never before knowingly encountered carburetor ice on that Lycoming O-435 engine In the years I'd flown it. The old pucker string began to snug up. I thought "Oh, JC. Here I am 3000 miles from home base with engine problems". I had worked up to about full throttle when I wondered about the possibility of carburetor ice. I pulled on the heat and the engine belched a couple of times and took off like a scalded dog. Apparently a real high humidity day. Live and learn.
I used to fly a Noorduyn Norseman C-64 in Alaska and it was a SOB for sucking up water and icing the carburetor when it was taking off on floats. I used to start the take off run wirh carbretor heat on, then dump the heat to get full power as speed was gained and the prop vortex trailed back.

I have been flying a 170B for about 100 hours now, and experience carb ice on a regular basis when the temp is less than 80F and high humidity. My plane is based in very humid MD, but I have not experienced this carb ice on this regular of basis in any other type. Generally when I perform the runup, the engine loses about 600 RPM when I apply carb heat, then regains as the ice mets. After I take off it's the same senario. I have resorted to flying around with the carb heat cracked while flying to avoid ice. Does anyone else commonly have carb ice in high humidity situations.

600 RPM LOSS!!!??? During preflight run up?
And again INFLIGHT?


You are suffering from collapsed induction scat tubing or other restriction, in the carb heat circuit.
(not likely carb ice)

George,

Amen!

That certainly doesn't sound like Carburetor ice to me either.

Do you park your plane on the ramp? Is rain, or moisture, collecting in the scat tubing where your plane is parked? Your problem sounds similar to a friends based in Santa Barbara, CA that was parked on ramp.

My plane does have to live outside. I have not checked the scat tubing for moisture, but it sounds logical. Now that you mention it, I had the problem back in the spring (lots of rain), then the problem went away for most of the summer (hot and dry). The problem showed up again over the past few weeks when we have been getting a lot of rain and high humidity. I will check the scat tubing for moisture after it rains next time. Did your friend find a way to keep the moisture out of the scat tubing?

A-HA!

Some time back, I posted in the MX Library a Special Alert Bulletin addressing this very issue (and the solution.)

http://www.cessna170.org/forums/viewtop ... cial+alert

(Thanks, Jim! Your memory is much better than mine.)

George's link to the solution solved my friends problem. Thanks for providing the link George . Jim

blueldr wrote: I have been flying my C-170B for over twenty years and over three thouand hours, and I don't believe I've ever encountered carburetor ice even once. Guys flying with me used to get uncomfortable when I failed to apply carburetor heat when reducing power in the pattern, but I never had any indication of ice. This experience has been in all kinds of atmospheric conditions and relative humidity conditions. Maybe I just don't live right.

Must be all that ethanol in your mogas keeps it melted.