The International Cessna^® 170 Association

I picked up some ice last week flying over water. I didnt notice my carb temp, but saw that my EGT was way low. Pulled carb heat, heard the ice go through and EGT came back up. That ice does a good job of leaning.

Larry

It's always instructive to fly in extremes to which you plane is not usually subjected. Hot and cold can have different effects on systems and components, and if you always fly at 40-80F, you may not know that some seal or linkage or bushing or ... won't be happy at 20F.

Even as simple as discovering that the gaps seals on doors are not so effective. I flew from NorCal to Phoenix for Thanksgiving, and was at 115/125 for the trip. Ambient was about 15 degrees and although the airplane & engine didn't mind it, my left leg went numb after a bit of exposure. And over the summer we had temps >120F at Livermore and that was a definitely learning experience for how the carb heat, throttle, engine, me responded to that extreme.

RT

Eric said:

"Use carb heat religiously, as carb ice has brought down many perfectly serviceable aircraft."

Eric is absolutely correct. What I want to add is that if you have a good case of carburetor ice, enough to kill the engine, leaning the mixture will keep it running, so that the carb heat can melt the ice. When I tell this to pilots and flight instructors they do not seem to beleive me. One good carb icing day, in my 170, over a safe landing area, I kept leaning and leaning as the carb iced up. Then I put the mixture in full rich and the engine quit. Full throttle and full carb heat did nothing. So I leaned the mixture and the engine ran real nice, making enough carb heat to melt the ice. What more proof do you need? Checking a whole bunch of carb ice articles on the AOPA website, going way back in history, I found only two that mentioned leaning to keep the engine running.

Archie

Apparently I don't live right. I've never had a carburetor ice problem with any of my C-170s. As a matter of fact, I have generally been unable to get them to ice up even when trying.

As Archie alluded, keeping the engine running so as to generate heater-muff heat is necessary in order to completely rid the carb of any ice that has built up... But it's even more important to disallow that much ever to accumulate. The use of carb heat will result in an overly rich mixture due to the hot air's ability to better vaporize the fuel. That is one of the reasons the application of carb heat (usually) causes a reduction in rpm (with a fixed pitch prop.)
If you find that you must run with carb heat constantly ON....then re-leaning your engine to accomodate the overly-rich condition is recommended.

Hmmm.... I always understood that the engine ran richer with carb heat on because the heated air is less dense, but the fuel flow rate is the same.

I've heard of people taking advantage of better vaporization by using carb heat to improve leaning performance (the EGTs differ less between cylinders because fuel distribution is better). Darryl Bolduc pointed out in a talk to our EAA chapter that if you do that, you need to be careful that the application of carb heat doesn't warm the carburetor throat up into the icing range of approximately 10 degrees C either side of zero. So he recommended not leaving the heat on unless you have a carb temperature gauge and use it.

The other argument against leaving the heat on comes from an English pilot friend of mine: if you leave it on, you're no longer able to apply heat to get rid of the ice. His philosophy is that if the icing is so bad that you have to keep applying carb heat, you should land and wait until the weather improves. We did exactly that once when I was flying with him. The result was a delightful overnight stay in a little French town that I never would have seen otherwise. A win-win solution to the problem, it seemed to me!

The air is indeed less dense. But it vaporizes fuel better. Carb heat application also changes airflow characteristics. In most installations it creates a loss of manifold pressure due to routing....another reason a loss of power usually occurs. It also is unfiltered air.

I learned about the use of carb heat and leaning from Max Conrad. For the recent pilots, Max was a pioneer in ocean crossings in light planes. He took me up in his twin Comanche. A real honor and education. For the lowest fuel burn, for long distance flights he would lean till the engines were rough, and then apply enough carb heat to richen enough till the engines ran smooth. The heat caused better vaporization of the fuel for more efficient combustion, which is, more horsepower for less fuel. This was at lower than usual power settings and airspeeds, which in most or our airplanes gives more mpg. Max would adjust his power settings by using his airspeed indicator as he had no tachs or MP gages. He said tachs are not necessary as your ear is the best synchronizer. Without manifold pressure gages, and the rudder trim centered, adjust the throttles till the airplane flies straight and you have equal power on both sides. It was strange wiith all the missing instrunemts but Max asked "What good is an oil pressure gage out in the moddle of the ocean?"

Archie

War story about carb heat. While flight instructing for Sierra Avademy back in 1972 I found myself in a bingo fuel situation in a Cessna 152. I was hopping from airplane to airplane that day and allowed the student to pre-flight the airplane out of my presence. Long story short, about 30 minutes into the flight I noticed the fuel gauges were on EMPTY. Fortunately we were within 30 minutes of the airport. I pulled carb heat on and leaned aggressively - we made it to the airport - There was no measurable fuel remaining in the tanks. Needless to say, I never allowed a student to preflight out of presence again.

Carb heat war stories should probably be put on another link. But anyway, when I was in flight school, in the late 80's a friend of mine from Britan, took up his favorite 152 for a sunday morning ride (to build time).While practicing comm. manevers, he started loosing power, he pulled carb. heat to no avale, after a short time the engine quit! He found a plowed feild and landed with no problems! (as we were instructed, land twards the farm house so you don't have far to walk) (just kidding). He walked to the farm house but beeing central IL. the people were probbaly at Church! So John walks back out to the A/C checks every thing out and was able to start the A/C! He tests everything and it all looks good! He shuts down and turns the A/C the other direction and starts and flys the A/C home! The next day when we all get to class no one belives that John did this untill we see the mud in the A/C and under the wings! (We all put together $3.00 each to buy him a wheel-barrow to carry his balls around in). It turns out that the flopper, at speed, wouldn't hold the carb. heat . When at runup everything was fine! So the carb. flopper has been one of my (must see items on any look at an A/C) not just an annual!
brad