A carb heat trick

[A and C]

Not as clear as it might seem

First a few words of caution, carb ice is not only a function of the engine and atmospheric conditions it is also a function of intake design my PA28 exhibits less inclination to carb ice than my DR400 but both have the same engine and carb.

I would strongly recommend that cab heat is applied just before the power is reduced to make the approach to the runway ( top of base leg or equivalent position) because at point the heat exchanger has the most heat in it and is capable of doing the most good in terms of ice removal and as the carb is now clear of ice the engine will continue to run and put some heat into the system. The real danger if you don't do this being that if the carb should ice during a glide approach you won't know it has iced until you try to go around when opening the throttle gives you no engine response.

EDIT

Backpacker I agree with you, the most likely time for a carb to ice is with the butterfly valve closed, this position gives a very large acceleration of the air ( so large PX & TX drop ) as it try's to get through the small gap, ad the fuel from the idle jet evaporating and you have the perfect conditions for carb ice.

A major ice encounter !
A few years back when in the cruise at FL090 in solid IMC OAT + 2 the DR400 engine started loosing power I applied carb heat and this slowed the power loss but it was clear that the aircraft was going to have to descend to increase the OAT being as the power was reducing the throttle was left in the cruise position because it would not move and as is the training mantra the mixture was set full rich...............l BIG MISTAKE !!!!! The rpm dropped and the EGT fell rapidly, my next action was to lean the mixture to give me max EGT this gave me the best RPM ( power ) and the most heat in the carb heat exchanger.
The mixture was set a long way lean of normal optimum for the ( rapidly decreasing ) altitude because the carb was so restricted that any more fuel would result in a massively over rich mixture and a decrease in the heat in the carb heat exchanger.
Eventual at about 3500 ft the engine power was restored and the settings retuned to the normal values................. A little later during the final approach to the airfield as we broke out of the overcast I was thinking that the Irish Sea looked very cold and uninviting !

After this incident the carb heat system was checked and a very well hidden crack found in the carb heat valve, the result of this crack was that the engine was only getting partial heat and so the systems ability to clear ice much impaired.