My past flying experience was mostly with Piper Cherokees, but I remember that flying with a 0-200 Continental required more attention to carb icing--not just in while in descent but under some conditions, ice could form even in cruise.
So, I started reading. What a revelation!
First, carb ice is a big deal when ambient temps are between 30 F and 65 F. That is when you risk being in the "red zone" in the chart where carb ice is a "serious" risk even while cruising in clear air!
Rule of thumb:
Be really careful when flying in ambient temps between 30 F and 65 F. (Even more critical in the 40 F to 65 F range--common temps in the Fall and Spring in Mid Atlantic region.) If the Dew Point is not 15 F (8 C) LOWER than the ambient, you are in the red zone and you need to be aware that carb ice can (and is likely to) form in all flight conditions. This is more or less equivalent to 58-60% relative humidity.
Carb Heat and Diligence is the remedy. So, when flying in that 40-65 F range, I will always use Carb Heat just before takeoff and will pull Carb Heat on periodically to check for carb ice during the flight.
It is important to use Carb Heat always when reducing RPM during descent even in much warmer conditions when flying in the Mid Atlantic region where RH is almost always higher than 40%. But, remember that after long periods of "idle" throttle setting, carb heat may no longer be effective (engine has cooled down) and it is good to have carb heat off just before touchdown, so full power is available for a possible go-around.
FAA Advisory
Advisory Circular on Carb Ice
The engineer in me had to know more. Seems that one important issue is the location of the carburetor. The carburetor gets cold from ambient air, vaporization of the fuel, and induction cooling in the venturi. While the humid air can and is supercooled--and can remain as a gas to well below zero F, when it encounters the rough edges inside the carb, it condenses and immediately solidifies as ice. The solution--warm the carburetor---usually with air that is above 32 F--air directed over the exhaust heat exchanger to the carb when you pull Carb Heat on.
If carb ice is present, when Carb Heat is pulled, the ice will melt, with water and ice chunks entering the combustion chamber, making the engine run rough for some time.
Here is some additional info. Other than at full throttle, carb air temp is likely to be 26-30 (possible more under certain circumstances) degrees F BELOW ambient. Ice can collect anywhere from 32 F down to 10 F carb temperature. (40 F ambient OAT - 30 drop in carb = 10 F carb air temp; 62 F OAT - 30 = 32 F carb air temp.)
Normally before ice can form, water vapor in the air must first be changed to liquid that condenses on the carb. That were dew point comes in. So, if the drop in temp causes the air temp to fall below the dew point, then ice can grow. Ice can form without first becoming a liquid----"supercooled" water vapor can exist below the dew point (e.g in clouds). When encountering a cold metal surface, this supercooled water vapor can change into ice.
(32 deg F or 0 deg C is technically the melting point of ice, however all water liquid or vapor does not necessarily become ice at this temp, so the freezing point of water may be at a temp much lower than 32 deg F.)
In some instances, if cold enough, the water vapor changes directly from a vapor to solid crystals that do not stick to the carb. (generally at a dew point below 14 F) That is why a carb air temp gauge can be helpful to indentify if carb air temp is in the unsafe range (14-50 F = -10-+10C)--above ice crystals and below dew point--requiring carb heat. Generally full carb heat adds about 40-50 degrees F to the air--depending on EGT. Partial carb heat is seldom advisable as it may not be enough heat and may not raise carb air temp above freezing---possibly making the situation worse--ALWAYS PULL FULL CARB HEAT AND LEAVE ON FOR ENOUGH TIME TO BE SURE ALL ICE IS MELTED.
Keep in mind that temps generally drop with increased altitude-in a "standard" atmosphere. Ambient OAT's drop about 3.6 degrees per 1000 feet. And, Dew Point Temp also drops--about 1 degree per 1000 feet. So at 3000 feet, that 15 degree difference at ground level decreases to 7.2 degrees. You can see from the chart that at 50 degrees OAT with 35 DP, could become 39 OAT and 32 DP where "serious" carb ice could form even cruising under power. Be especially careful of clouds, and/or areas of precipitation and fog as the OAT to Dew Point spread is especially low.
Also be careful to remember that when carb heat is on, the fuel air mixture can be very rich. Turning carb heat off before touchdown in short final is generally a good practice. In really cold conditions however, with density altitude "minus" 2000 ft for example, the carb in full rich mode may still be on the lean side, causing rough running---in such cases, carb heat may actually improve the mixture to a more rich setting.
Keep in mind that there are MANY variables--so always be vigilant regarding the POTENTIAL for carb ice---consider the ambient conditions, your plane's particular characteristics (even different carburetors will affect ice formation), and your control inputs.
A good rule of thumb is to remember that carb ice generally causes a reduction in RPM/Power and carb heat will tend to melt carb ice if left on long enough. And the melting ice water will cause the engine to run rough for awhile and then allow the engine speed to increase with the engine running smooth again. If carb ice is POSSIBLE--use carb heat (more than for just a couple seconds) periodically to check if this "melting ice" effect occurs.
After a great deal of searching, it appears that one of the best experts on the MA3-SPA Marvel carburetor in my Sport Cub is located in Niles, MI. http://www.dgsupply.com/about-us
They rebuild these carbs and they are highly recommended in forums. I called them to get their opinion of the value of having a carb temp gauge and they indicated that it was a good idea--suggesting that some form of thread sealant on the probe was advisable, and agreed that the probe with the smaller diamter, shorter length tip was better. (The Marvel factory agreed in regards to the thread sealant, but they take "no position" pro or con regarding a temp gauge.)
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