I have heard that many people do not experience carburetor ice in their Comanche. Experience with my carburetored PA24-250 Comanche over the last 5 years (600+ hours) has been just the opposite: It is by far the worst to develop carburetor ice of the types I have regularly flown ( 3000 hours) – Taylorcraft BC12D/C152/C172/PA28-151/Citabria 7ECA/PA24-250. I do not dispute those that say they have no problems with carburetor ice, rather, I think it must have something to do with a/c induction configuration.

My Comanche is a 1959 PA24-250 of S/N 1114 vintage. Ram air is directly inducted through a Brackett air filter into the carburetor air box and is ducted via a tightly fitted seal. Any time visible precipitation is present carburetor ice is a likelihood at temperatures above minus 25 Celsius to about plus 5 Celsius. This happens whether the engine is cold or fully warm, although it seems to be worse during climb when the engine is warming up. I utilize a carburetor temperature gauge to monitor the situation but it is not a foolproof device. Many times the gauge has shown in the “green” and I have had what appears to be severe carburetor ice situations. My all cylinder engine monitor is useful in detecting ice as well. I maintain constant vigilance when in these situations and experiment with the individual situation.

Carburetor ice is suspected if the manifold pressure drops 1 inch or more or if the carburetor temperature is in the “yellow” or “red” band and visible precip is present. I can tell you from experience that your heart will race at the roughness experienced from only a 2 inch drop in manifold pressure when full heat is applied. Worthy of note, is to be sure the engine is leaned for the altitude you are at because the roughness may be due to an overly rich engine when carburetor heat is applied.

I use one of two methods to combat Carburetor Ice. On detection of Carburetor ice, I first add full heat to clear the ice, then I add partial carburetor heat such that the carburetor temperature gauge reads approximately plus 10 Celsius and after a few minutes I add full heat and listen for a transient engine roughness. If no roughness is detected then I add partial heat at the level it was at the start of the partial heat sequence. If roughness occurs then I add 5 more degrees and repeat the sequence until no roughness occurs. After the situation is stabilized I lean the engine as per manufacturer’s specs.

Method two is to modulate the carburetor heat from none to full on to none. This is done at an interval of time dependent on the severity of the carburetor ice situation and is experimentally derived. It is difficult to lean the engine properly while doing this so I usually do not bother at altitudes less than 8K feet, unless, carburetor heat is applied more than 50 % of the time. A typical cycle time for severe conditions can be as high as once per 2 minutes. There have been times when I needed full heat 100% of the time to keep the engine running. The most memorable time was while flying into a tropical storm IMC near Savannah, GA (11K ft/August) – this is a story in itself.

I once had a complete ram air induction system blockage that I think was caused by impact snow. I was IMC at an altitude of 8K ft over the mountains of Tennessee during the winter months and ran into an un-forcasted cold front. The engine was already at 2500 rpm because the engine speed linkage is the first to freeze up in icing conditions. I applied full carburetor heat for about 2 minutes and as I removed heat the engine lost power and rpm immediately. I repeated the situation 2 or 3 time and then just applied full heat and leaned while trying to climb as high as I could go – I made it to 10K feet before I was at 100 fpm. Later, when I could descend into warmer air the condition cleared and the engine ran normally, however, I drank a lot of liquid to replace all the sweat generated during the ordeal.